High Throughput Light Absorber Discovery, Part 2: Establishing Structure-Band Gap Energy Relationships.

PubMed

Suram, Santosh K; Newhouse, Paul F; Zhou, Lan; Van Campen, Douglas G; Mehta, Apurva; Gregoire, John M

2016-11-14

Combinatorial materials science strategies have accelerated materials development in a variety of fields, and we extend these strategies to enable structure-property mapping for light absorber materials, particularly in high order composition spaces. High throughput optical spectroscopy and synchrotron X-ray diffraction are combined to identify the optical properties of Bi-V-Fe oxides, leading to the identification of Bi 4 V 1.5 Fe 0.5 O 10.5 as a light absorber with direct band gap near 2.7 eV. The strategic combination of experimental and data analysis techniques includes automated Tauc analysis to estimate band gap energies from the high throughput spectroscopy data, providing an automated platform for identifying new optical materials.

Physics Goals for the Planned Next Linear Collider Engineering Test Facility

NASA Astrophysics Data System (ADS)

Raubenheimer, T. O.

2001-10-01

The Next Linear Collider (NLC) Collaboration is planning to construct an Engineering Test Facility (ETF) at Fermilab. As presently envisioned, the ETF would comprise a fundamental unit of the NLC main linac to include X-band klystrons and modulators, a delay-line power-distribution system (DLDS), and NLC accelerating structures that serve as loads. The principal purpose of the ETF is to validate stable operation of the power-distribution system, first without beam, then with a beam having the NLC pulse structure. This paper concerns the possibility of configuring and using the ETF to accelerate beam with an NLC pulse structure, as well as of doing experiments to measure beam-induced wakefields in the rf structures and their influence back on the beam.

Development of a Dielectric-Loaded Accelerator Test Facility Based on an X-Band Magnicon Amplifier

NASA Astrophysics Data System (ADS)

Gold, S. H.; Kinkead, A. K.; Gai, W.; Power, J. G.; Konecny, R.; Jing, C.; Tantawi, S. G.; Nantista, C. D.; Hu, Y.; Du, X.; Tang, C.; Lin, Y.; Bruce, R. W.; Bruce, R. L.; Fliflet, A. W.; Lewis, D.

2006-01-01

The Naval Research Laboratory (NRL) and Argonne National Laboratory (ANL), in collaboration with the Stanford Linear Accelerator Center (SLAC), are developing a dielectric-loaded accelerator (DLA) test facility powered by the 11.424-GHz magnicon amplifier that was developed jointly by NRL and Omega-P, Inc. Thus far, DLA structures developed by ANL have been tested at the NRL Magnicon Facility without injected electrons, including tests of alumina and magnesium calcium titanate structures at gradients up to ˜8 MV/m. The next step is to inject electrons in order to build a compact DLA test accelerator. The Accelerator Laboratory of Tsinghua University in Beijing, China has developed a 5-MeV electron injector for the accelerator, and SLAC is developing a means to combine the two magnicon output arms, and to drive the injector and an accelerator section with separate control of the power ratio and relative phase. Also, RWBruce Associates, working with NRL, is developing a means to join ceramic tubes to produce long accelerating sections using a microwave brazing process. The installation and commissioning of the first dielectric-loaded test accelerator, including injector, DLA structure, and spectrometer, should take place within the next year.

Development of new S-band RF window for stable high-power operation in linear accelerator RF system

NASA Astrophysics Data System (ADS)

Joo, Youngdo; Lee, Byung-Joon; Kim, Seung-Hwan; Kong, Hyung-Sup; Hwang, Woonha; Roh, Sungjoo; Ryu, Jiwan

2017-09-01

For stable high-power operation, a new RF window is developed in the S-band linear accelerator (Linac) RF systems of the Pohang Light Source-II (PLS-II) and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL). The new RF window is designed to mitigate the strength of the electric field at the ceramic disk and also at the waveguide-cavity coupling structure of the conventional RF window. By replacing the pill-box type cavity in the conventional RF window with an overmoded cavity, the electric field component perpendicular to the ceramic disk that caused most of the multipacting breakdowns in the ceramic disk was reduced by an order of magnitude. The reduced electric field at the ceramic disk eliminated the Ti-N coating process on the ceramic surface in the fabrication procedure of the new RF window, preventing the incomplete coating from spoiling the RF transmission and lowering the fabrication cost. The overmoded cavity was coupled with input and output waveguides through dual side-wall coupling irises to reduce the electric field strength at the waveguide-cavity coupling structure and the possibility of mode competitions in the overmoded cavity. A prototype of the new RF window was fabricated and fully tested with the Klystron peak input power, pulse duration and pulse repetition rate of 75 MW, 4.5 μs and 10 Hz, respectively, at the high-power test stand. The first mass-produced new RF window installed in the PLS-II Linac is running in normal operation mode. No fault is reported to date. Plans are being made to install the new RF window to all S-band accelerator RF modules of the PLS-II and PAL-XFEL Linacs. This new RF window may be applied to the output windows of S-band power sources like Klystron as wells as the waveguide windows of accelerator facilities which operate in S-band.

RF pulse compression for future linear colliders

NASA Astrophysics Data System (ADS)

Wilson, Perry B.

1995-07-01

Future (nonsuperconducting) linear colliders will require very high values of peak rf power per meter of accelerating structure. The role of rf pulse compression in producing this power is examined within the context of overall rf system design for three future colliders at energies of 1.0-1.5 TeV, 5 TeV, and 25 TeV. In order to keep the average AC input power and the length of the accelerator within reasonable limits, a collider in the 1.0-1.5 TeV energy range will probably be built at an x-band rf frequency, and will require a peak power on the order of 150-200 MW per meter of accelerating structure. A 5 TeV collider at 34 GHz with a reasonable length (35 km) and AC input power (225 MW) would require about 550 MW per meter of structure. Two-beam accelerators can achieve peak powers of this order by applying dc pulse compression techniques (induction linac modules) to produce the drive beam. Klystron-driven colliders achieve high peak power by a combination of dc pulse compression (modulators) and rf pulse compression, with about the same overall rf system efficiency (30-40%) as a two-beam collider. A high gain (6.8) three-stage binary pulse compression system with high efficiency (80%) is described, which (compared to a SLED-II system) can be used to reduce the klystron peak power by about a factor of two, or alternatively, to cut the number of klystrons in half for a 1.0-1.5 TeV x-band collider. For a 5 TeV klystron-driven collider, a high gain, high efficiency rf pulse compression system is essential.

Physics Goals for the Planned Next Linear Collider Engineering Test Facility

DOE Office of Scientific and Technical Information (OSTI.GOV)

Raubenheimer, Tor O

2001-10-02

The Next Linear Collider (NLC) Collaboration is planning to construct an Engineering Test Facility (ETF) at Fermilab. As presently envisioned, the ETF would comprise a fundamental unit of the NLC main linac to include X-band klystrons and modulators, a delay-line power-distribution system (DLDS), and NLC accelerating structures that serve as loads. The principal purpose of the ETF is to validate stable operation of the power-distribution system, first without beam, then with a beam having the NLC pulse structure. This paper concerns the possibility of configuring and using the ETF to accelerate beam with an NLC pulse structure, as well asmore » of doing experiments to measure beam-induced wakefields in the rf structures and their influence back on the beam.« less

Physics goals for the planned next linear collider engineering test facility

DOE Office of Scientific and Technical Information (OSTI.GOV)

Courtlandt L Bohn et al.

2001-06-26

The Next Linear Collider (NLC) Collaboration is planning to construct an Engineering Test Facility (ETF) at Fermilab. As presently envisioned, the ETF would comprise a fundamental unit of the NLC main linac to include X-band klystrons and modulators, a delay-line power-distribution system (DLDS), and NLC accelerating structures that serve as loads. The principal purpose of the ETF is to validate stable operation of the power-distribution system, first without beam, then with a beam having the NLC pulse structure. This paper concerns the possibility of configuring and using the ETF to accelerate beam with an NLC pulse structure, as well asmore » of doing experiments to measure beam-induced wakefields in the rf structures and their influence back on the beam.« less

Physics goals for the planned next linear collider engineering test facility.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bohn, C.; Michelotti, L.; Ostiguy, J.-F.

2001-07-17

The Next Linear Collider (NLC) Collaboration is planning to construct an Engineering Test Facility (ETF) at Fermilab. As presently envisioned, the ETF would comprise a fundamental unit of the NLC main linac to include X-band klystrons and modulators, a delay-line power-distribution system (DLDS), and NLC accelerating structures that serve as loads. The principal purpose of the ETF is to validate stable operation of the power-distribution system, first without beam, then with a beam having the NLC pulse structure. This paper concerns the possibility of configuring and using the ETF to accelerate beam with an NLC pulse structure, as well asmore » of doing experiments to measure beam-induced wakefields in the rf structures and their influence back on the beam.« less

Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure

DOE PAGES

Simakov, Evgenya I.; Arsenyev, Sergey A.; Buechler, Cynthia E.; ...

2016-02-10

We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic band gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. Wemore » conducted an experiment at the Argonne Wakefield Accelerator (AWA) test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Lastly, excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.« less

Design of 140 MW X-band Relativistic Klystron for Linear Collider

NASA Astrophysics Data System (ADS)

Dolbilov, G. V.; Azorsky, N. I.; Shvetsov, V. S.; Balakin, V. E.; Avrakhov, P. V.; Kazakov, S. Yu.; Teryaev, V. E.; Vogel, V. F.

1997-05-01

It has been reported at EPAC-96 on successful experimental results on achievement of 100 MW output rf power in a wide aperture (15 mm), high gain (80 dB) 14 GHz VLEPP klystron with distributed suppression of parasitic oscillations (G.V. Dolbilov et al., Proc. EPAC-96, Sitges (Barselona), 10-14 June, 1996, Vol. 3, p. 2143). This report presents design of an electrodynamic structure of the X-band klystron for linear collider with a higher efficiency up to 56 % which will be achieved at the same parameters of the electron beam (U = 1 MeV, I = 250 A, emittance 0.05 π cm\\cdotrad). Design rf output power of the klystron is 140 MW. Experimental investigations of electrodynamic structure of the klystron are planned to perform using the driving beam of the JINR LIA-3000 induction accelerator (E = 1 MeV, I = 250 A, τ = 250 ns).

Phase Transformation Synthesis of Strontium Tantalum Oxynitride-based Heterojunction for Improved Visible Light-Driven Hydrogen Evolution.

PubMed

Zeng, Weixuan; Bian, Yuan; Cao, Sheng; Ma, Yongjin; Liu, Yi; Zhu, Anquan; Tan, Pengfei; Pan, Jun

2018-06-07

Tantalum oxynitride-based materials, which possess narrow bandgaps and sufficient band energy potentials, have been of immense interest for water splitting. However, the efficiency of photocatalytic reactions is still low due to the fast electron-hole recombination. Here, a Sr2Ta2O7-xNx/SrTaO2N heterostructured photocatalyst with well-matched band structure was in situ constructed by nitridation of hydrothermal-prepared Sr2Ta2O7 nanosheets. Compared to Sr2Ta2O7-xNx and pure SrTaO2N, the Sr2Ta2O7-xNx/SrTaO2N heterostructured photocatalyst exhibited highest rate of hydrogen evolution, which is ca. 2.0 and 76.4 times of Sr2Ta2O7-xNx and pure SrTaO2N under the similar reaction condition, respectively. The enhanced performance arises from the formation of suitable band matched heterojunction accelerated charge separation. This work provides a promising strategy for the construction of tantalum oxynitride-based heterojunction photocatalysts.

Dark Currents and Their Effect on the Primary Beam in an X-band Linac

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bane, K.L.F.; Dolgashev, V.A.; Raubenheimer, T.

2005-05-27

We numerically study properties of primary dark currents in an X-band accelerating structure. For the H60VG3 structure considered for the Next Linear Collider (NLC) we first perform a fairly complete (with some approximations) calculation of dark current trajectories. These results are used to study properties of the dark current leaving the structure. For example, at accelerating gradient of 65 MV/m, considering two very different assumptions about dark current emission around the irises, we find that the fraction of emitted current leaving the structure to be a consistent {approx} 1%. Considering that {approx} 1 mA outgoing dark current is seen inmore » measurement, this implies that {approx} 100 mA (or 10 pC per period) is emitted within the structure itself. Using the formalism of the Lienard-Wiechert potentials, we then perform a systematic calculation of the transverse kick of dark currents on a primary linac bunch. The result is {approx} 1 V kick per mA (or per 0.1 pC per period) dark current emitted from an iris. For an entire structure we estimate the total kick on a primary bunch to be {approx} 15 V. For the NLC linac this translates to a ratio of (final) vertical beam offset to beam size of about 0.2. However, with the assumptions that needed to be made--particularly the number of emitters and their distribution within a structure--the accuracy of this result may be limited to the order of magnitude.« less

Physics of the saturation of particle acceleration in relativistic magnetic reconnection

NASA Astrophysics Data System (ADS)

Kagan, Daniel; Nakar, Ehud; Piran, Tsvi

2018-05-01

We investigate the saturation of particle acceleration in relativistic reconnection using two-dimensional particle-in-cell simulations at various magnetizations σ. We find that the particle energy spectrum produced in reconnection quickly saturates as a hard power law that cuts off at γ ≈ 4σ, confirming previous work. Using particle tracing, we find that particle acceleration by the reconnection electric field in X-points determines the shape of the particle energy spectrum. By analysing the current sheet structure, we show that physical cause of saturation is the spontaneous formation of secondary magnetic islands that can disrupt particle acceleration. By comparing the size of acceleration regions to the typical distance between disruptive islands, we show that the maximum Lorentz factor produced in reconnection is γ ≈ 5σ, which is very close to what we find in our particle energy spectra. We also show that the dynamic range in Lorentz factor of the power-law spectrum in reconnection is ≤40. The hardness of the power law combined with its narrow dynamic range implies that relativistic reconnection is capable of producing the hard narrow-band flares observed in the Crab nebula but has difficulty producing the softer broad-band prompt gamma-ray burst emission.

Multi-Mode Cavity Accelerator Structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jiang, Yong; Hirshfield, Jay Leonard

2016-11-10

This project aimed to develop a prototype for a novel accelerator structure comprising coupled cavities that are tuned to support modes with harmonically-related eigenfrequencies, with the goal of reaching an acceleration gradient >200 MeV/m and a breakdown rate <10 -7/pulse/meter. Phase I involved computations, design, and preliminary engineering of a prototype multi-harmonic cavity accelerator structure; plus tests of a bimodal cavity. A computational procedure was used to design an optimized profile for a bimodal cavity with high shunt impedance and low surface fields to maximize the reduction in temperature rise ΔT. This cavity supports the TM010 mode and its 2ndmore » harmonic TM011 mode. Its fundamental frequency is at 12 GHz, to benchmark against the empirical criteria proposed within the worldwide High Gradient collaboration for X-band copper structures; namely, a surface electric field E sur max< 260 MV/m and pulsed surface heating ΔT max< 56 °K. With optimized geometry, amplitude and relative phase of the two modes, reductions are found in surface pulsed heating, modified Poynting vector, and total RF power—as compared with operation at the same acceleration gradient using only the fundamental mode.« less

RF design for the TOPGUN photogun: A cryogenic normal conducting copper electron gun

DOE PAGES

Cahill, A. D.; Fukasawa, A.; Pakter, R.; ...

2016-08-31

Some recent studies of rf breakdown physics in cryogenic copper X-band accelerating structures have shown a dramatic increase in the operating gradient while maintaining low breakdown rates. The TOPGUN project, a collaboration between UCLA, SLAC, and INFN, will use this improvement in gradient to create an ultra-high brightness cryogenic normal conducting photoinjector [16]. The brightness is expected to be higher by a factor of 25 relative to the LCLS photogun [9]. This improvement in the brightness will lead to increased performance of X-Ray free electron lasers (FELs) and ultrafast electron diffraction devices [16]. Here, we present the rf design formore » this S-band photogun, which will be a drop-in replacement for the current LCLS photogun.« less

Measurement of Thermal Dependencies of PBG Fiber Properties

DOE Office of Scientific and Technical Information (OSTI.GOV)

Laouar, Rachik

Photonic crystal fibers (PCFs) represent a class of optical fibers which have a wide spectrum of applications in the telecom and sensing industries. Currently, the Advanced Accelerator Research Department at SLAC is developing photonic bandgap particle accelerators, which are photonic crystal structures with a central defect used to accelerate electrons and achieve high longitudinal electric fields. Extremely compact and less costly than the traditional accelerators, these structures can support higher accelerating gradients and will open a new era in high energy physics as well as other fields of science. Based on direct laser acceleration in dielectric materials, the so calledmore » photonic band gap accelerators will benefit from mature laser and semiconductor industries. One of the key elements to direct laser acceleration in hollow core PCFs, is maintaining thermal and structural stability. Previous simulations demonstrate that accelerating modes are sensitive to the geometry of the defect region and the variations in the effective index. Unlike the telecom modes (for which over 95% of the energy propagates in the hollow core) most of the power of these modes is located in the glass at the periphery of the central hole which has a higher thermal constant than air ({gamma}{sub SiO{sub 2}} = 1.19 x 10{sup -6} 1/K, {gamma}{sub air} = -9 x 10{sup -7} 1/K with {gamma} = dn/dT). To fully control laser driven acceleration, we need to evaluate the thermal and structural consequences of such modes on the PCFs. We are conducting series of interferometric tests to quantify the dependencies of the HC-633-02 (NKT Photonics) propagation constant (k{sub z}) on temperature, vibration amplitude, stress and electric field strength. In this paper we will present the theoretical principles characterizing the thermal behavior of a PCF, the measurements realized for the fundamental telecom mode (TE{sub 00}), and the experimental demonstration of TM-like mode propagation in the HC-633-02 fiber.« less

NICER Packaging for SpaceX CRS-11

NASA Image and Video Library

2017-04-06

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, technicians prepare the Neutron star Interior Composition Explorer, or NICER, payload for final packaging. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

NICER Packaging for SpaceX CRS-11

NASA Image and Video Library

2017-04-06

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, the Neutron star Interior Composition Explorer, or NICER, payload is secured on a special test stand. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

NICER Packaging for SpaceX CRS-11

NASA Image and Video Library

2017-04-06

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, a technician prepares the Neutron star Interior Composition Explorer, or NICER, payload for final packaging. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

NICER Packaging for SpaceX CRS-11

NASA Image and Video Library

2017-04-06

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, the Neutron star Interior Composition Explorer, or NICER, payload is being prepared for final packaging. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

NICER Transfer (for SpaceX CRS-11)

NASA Image and Video Library

2017-04-12

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, the Neutron star Interior Composition Explorer, or NICER, payload is secured inside a protective container. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

High throughput light absorber discovery, Part 2: Establishing structure–band gap energy relationships

DOE PAGES

Suram, Santosh K.; Newhouse, Paul F.; Zhou, Lan; ...

2016-09-23

Combinatorial materials science strategies have accelerated materials development in a variety of fields, and we extend these strategies to enable structure-property mapping for light absorber materials, particularly in high order composition spaces. High throughput optical spectroscopy and synchrotron X-ray diffraction are combined to identify the optical properties of Bi-V-Fe oxides, leading to the identification of Bi 4V 1.5Fe 0.5O 10.5 as a light absorber with direct band gap near 2.7 eV. Here, the strategic combination of experimental and data analysis techniques includes automated Tauc analysis to estimate band gap energies from the high throughput spectroscopy data, providing an automated platformmore » for identifying new optical materials.« less

High throughput light absorber discovery, Part 2: Establishing structure–band gap energy relationships

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suram, Santosh K.; Newhouse, Paul F.; Zhou, Lan

Combinatorial materials science strategies have accelerated materials development in a variety of fields, and we extend these strategies to enable structure-property mapping for light absorber materials, particularly in high order composition spaces. High throughput optical spectroscopy and synchrotron X-ray diffraction are combined to identify the optical properties of Bi-V-Fe oxides, leading to the identification of Bi 4V 1.5Fe 0.5O 10.5 as a light absorber with direct band gap near 2.7 eV. Here, the strategic combination of experimental and data analysis techniques includes automated Tauc analysis to estimate band gap energies from the high throughput spectroscopy data, providing an automated platformmore » for identifying new optical materials.« less

X-Ray Variability of BL Lac Objects

NASA Astrophysics Data System (ADS)

McHardy, Ian

I present an overview of the X-ray temporal and spectral variability of BL Lacs on both short and long timescales. The previously observed behaviour of short (~days) flares superimposed on a relatively steady `quiescent' level is still broadly correct. However, for the brighter BL Lacs, the well sampled lightcurves from the RXTE ASM show that the `quiescent' level also varies considerably on timescales of ~100 days in a manner similar to that seen in Optically Violently Variable Quasars (OVVs) such as 3C279 and 3C273. Possible reasons for this behaviour are discussed. For the large majority of BL Lacs the soft and medium energy X-ray bands are dominated by synchrotron emission and, unlike the case of OVVs, the emission mechanism is not in doubt. Most interest then centres on the structure of the emitting region, and the electron acceleration processes, particularly during outbursts. That structure, and the acceleration processes, can be investigated by consideration of the spectral variability during flares, which is not simple. I review the observations of spectral variability and consider the evidence for and against homogeneous models. I also briefly compare the X-ray spectral variability of BL Lacs with that of OVVs such as 3C273.

Structural and Visible-Near Infrared Optical Properties of Cr-Doped TiO2 for Colored Cool Pigments

NASA Astrophysics Data System (ADS)

Yuan, Le; Weng, Xiaolong; Zhou, Ming; Zhang, Qingyong; Deng, Longjiang

2017-11-01

Chromium-doped TiO2 pigments were synthesized via a solid-state reaction method and studied with X-ray diffraction, SEM, XPS, and UV-VIS-NIR reflectance spectroscopy. The incorporation of Cr3+ accelerates the transition from the anatase phase to the rutile phase and compresses the crystal lattice. Moreover, the particle morphology, energy gap, and reflectance spectrum of Cr-doped TiO2 pigments is affected by the crystal structure and doping concentration. For the rutile samples, some of the Cr3+ ions are oxidized to Cr4+ after sintering at a high temperature, which leads to a strong near-infrared absorption band due to the 3A2 → 3 T1 electric dipole-allowed transitions of Cr4+. And the decrease of the band gap causes an obvious redshift of the optical absorption edges as the doping concentration increases. Thus, the VIS and near-infrared average reflectance of the rutile Ti1 - x Cr x O2 sample decrease by 60.2 and 58%, respectively, when the Cr content increases to x = 0.0375. Meanwhile, the color changes to black brown. However, for the anatase Ti1 - x Cr x O2 pigments, only the VIS reflection spectrum is inhibited by forming some characteristic visible light absorption peaks of Cr3+. The morphology, band gap, and NIR reflectance are not significantly affected. Finally, a Cr-doped anatase TiO2 pigment with a brownish-yellow color and 90% near-infrared reflectance can be obtained.

An X-Band Gun Test Area at SLAC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Limborg-Deprey, C.; Adolphsen, C.; Chu, T.S.

The X-Band Test Area (XTA) is being assembled in the NLCTA tunnel at SLAC to serve as a test facility for new RF guns. The first gun to be tested will be an upgraded version of the 5.6 cell, 200 MV/m peak field X-band gun designed at SLAC in 2003 for the Compton Scattering experiment run in ASTA. This new version includes some features implemented in 2006 on the LCLS gun such as racetrack couplers, increased mode separation and elliptical irises. These upgrades were developed in collaboration with LLNL since the same gun will be used in an injector formore » a LLNL Gamma-ray Source. Our beamline includes an X-band acceleration section which takes the electron beam up to 100 MeV and an electron beam measurement station. Other X-Band guns such as the UCLA Hybrid gun will be characterized at our facility.« less

Design, fabrication, and high-gradient testing of an X -band, traveling-wave accelerating structure milled from copper halves

NASA Astrophysics Data System (ADS)

Argyropoulos, Theodoros; Catalan-Lasheras, Nuria; Grudiev, Alexej; Mcmonagle, Gerard; Rodriguez-Castro, Enrique; Syrachev, Igor; Wegner, Rolf; Woolley, Ben; Wuensch, Walter; Zha, Hao; Dolgashev, Valery; Bowden, Gorden; Haase, Andrew; Lucas, Thomas Geoffrey; Volpi, Matteo; Esperante-Pereira, Daniel; Rajamäki, Robin

2018-06-01

A prototype 11.994 GHz, traveling-wave accelerating structure for the Compact Linear Collider has been built, using the novel technique of assembling the structure from milled halves. The use of milled halves has many advantages when compared to a structure made from individual disks. These include the potential for a reduction in cost, because there are fewer parts, as well as a greater freedom in choice of joining technology because there are no rf currents across the halves' joint. Here we present the rf design and fabrication of the prototype structure, followed by the results of the high-power test and post-test surface analysis. During high-power testing the structure reached an unloaded gradient of 100 MV /m at a rf breakdown rate of less than 1.5 ×10-5 breakdowns /pulse /m with a 200 ns pulse. This structure has been designed for the CLIC testing program but construction from halves can be advantageous in a wide variety of applications.

NICER Packaging for SpaceX CRS-11

NASA Image and Video Library

2017-04-06

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, technicians assist as a crane is used to lift the Neutron star Interior Composition Explorer, or NICER, payload up from its carrier. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

NICER Transfer (for SpaceX CRS-11)

NASA Image and Video Library

2017-04-12

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, the Neutron star Interior Composition Explorer, or NICER, payload is secured inside a protective container and loaded onto a truck outside the high bay. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

Development of a dual-pulse RF driver for an S-band (= 2856 MHz) RF electron linear accelerator

NASA Astrophysics Data System (ADS)

Cha, Sungsu; Kim, Yujong; Lee, Byeong-No; Lee, Byung Cheol; Cha, Hyungki; Ha, Jang Ho; Park, Hyung Dal; Lee, Seung Hyun; Kim, Hui Su; Buaphad, Pikad

2016-04-01

The radiation equipment research division of Korea Atomic Energy Research Institute has developed a Container Inspection System (CIS) using a Radio Frequency (RF) electron linear accelerator for port security. The primary purpose of the CIS is to detect nuclear materials and explosives, as well country-specific prohibited substances, e.g., smuggled. The CIS consists of a 9/6 MeV dualenergy electron linear accelerator for distinguishing between organic and inorganic materials. The accelerator consists of an electron gun, an RF accelerating structure, an RF driver, a modulator, electromagnets, a cooling system, a X-ray generating target, X-ray collimator, a detector, and a container moving system. The RF driver is an important part of the configuration because it is the RF power source: it supplies the RF power to the accelerating structure. A unique aspect of the RF driver is that it generates dual RF power to generate dual energy (9/6 MeV). The advantage of this RF driver is that it can allow the pulse width to vary and can be used to obtain a wide range of energy output, and pulse repetition rates up to 300 Hz. For this reason, 140 W (5 MW - 9 MeV) and 37 W (3.4 MW - 6 MeV) power outputs are available independently. A high power test for 20 minutes demonstrate that stable dual output powers can be generated. Moreover, the dual power can be applied to the accelerator which has stable accelerator operation. In this paper, the design, fabrication and high power test of the RF driver for the RF electron linear accelerator (linac) are presented.

Compact x-ray source based on burst-mode inverse Compton scattering at 100 kHz

DOE PAGES

Graves, W.  S.; Bessuille, J.; Brown, P.; ...

2014-12-01

A design for a compact x-ray light source (CXLS) with flux and brilliance orders of magnitude beyond existing laboratory scale sources is presented. The source is based on inverse Compton scattering of a high brightness electron bunch on a picosecond laser pulse. The accelerator is a novel high-efficiency standingwave linac and rf photoinjector powered by a single ultrastable rf transmitter at X-band rf frequency. The high efficiency permits operation at repetition rates up to 1 kHz, which is further boosted to 100 kHz by operating with trains of 100 bunches of 100 pC charge, each separated by 5 ns. Themore » entire accelerator is approximately 1 meter long and produces hard x rays tunable over a wide range of photon energies. The colliding laser is a Yb:YAG solid-state amplifier producing 1030 nm, 100 mJ pulses at the same 1 kHz repetition rate as the accelerator. The laser pulse is frequency-doubled and stored for many passes in a ringdown cavity to match the linac pulse structure. At a photon energy of 12.4 keV, the predicted x-ray flux is 5 × 10¹¹ photons/second in a 5% bandwidth and the brilliance is 2 × 10¹² photons/(sec mm² mrad² 0.1%) in pulses with rms pulse length of 490 fs. The nominal electron beam parameters are 18 MeV kinetic energy, 10 microamp average current, 0.5 microsecond macropulse length, resulting in average electron beam power of 180 W. Optimization of the x-ray output is presented along with design of the accelerator, laser, and x-ray optic components that are specific to the particular characteristics of the Compton scattered x-ray pulses.« less

Effectiveness of compressed sensing and transmission in wireless sensor networks for structural health monitoring

NASA Astrophysics Data System (ADS)

Fujiwara, Takahiro; Uchiito, Haruki; Tokairin, Tomoya; Kawai, Hiroyuki

2017-04-01

Regarding Structural Health Monitoring (SHM) for seismic acceleration, Wireless Sensor Networks (WSN) is a promising tool for low-cost monitoring. Compressed sensing and transmission schemes have been drawing attention to achieve effective data collection in WSN. Especially, SHM systems installing massive nodes of WSN require efficient data transmission due to restricted communications capability. The dominant frequency band of seismic acceleration is occupied within 100 Hz or less. In addition, the response motions on upper floors of a structure are activated at a natural frequency, resulting in induced shaking at the specified narrow band. Focusing on the vibration characteristics of structures, we introduce data compression techniques for seismic acceleration monitoring in order to reduce the amount of transmission data. We carry out a compressed sensing and transmission scheme by band pass filtering for seismic acceleration data. The algorithm executes the discrete Fourier transform for the frequency domain and band path filtering for the compressed transmission. Assuming that the compressed data is transmitted through computer networks, restoration of the data is performed by the inverse Fourier transform in the receiving node. This paper discusses the evaluation of the compressed sensing for seismic acceleration by way of an average error. The results present the average error was 0.06 or less for the horizontal acceleration, in conditions where the acceleration was compressed into 1/32. Especially, the average error on the 4th floor achieved a small error of 0.02. Those results indicate that compressed sensing and transmission technique is effective to reduce the amount of data with maintaining the small average error.

NICER Transfer (for SpaceX CRS-11)

NASA Image and Video Library

2017-04-12

Inside the Space Station Processing Facility high bay at NASA's Kennedy Space Center in Florida, the Neutron star Interior Composition Explorer, or NICER, payload is secured inside a protective container. A technician uses a Hyster forklift to pick up the container and move it outside of the high bay. NICER will be delivered to the International Space Station aboard the SpaceX Dragon cargo carrier on the company’s 11th commercial resupply services mission to the space station. NICER will study neutron stars through soft X-ray timing. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena and the mechanisms that underlie the most powerful cosmic particle accelerators known.

Hybrid functional study of band structures of GaAs1-xNx and GaSb1-xNx alloys

NASA Astrophysics Data System (ADS)

Virkkala, Ville; Havu, Ville; Tuomisto, Filip; Puska, Martti J.

2012-02-01

Band structures of GaAs1-xNx and GaSb1-xNx alloys are studied in the framework of the density functional theory within the hybrid functional scheme (HSE06). We find that the scheme gives a clear improvement over the traditional (semi)local functionals in describing, in a qualitative agreement with experiments, the bowing of electron energy band gap in GaAs1-xNx alloys. In the case of GaSb1-xNx alloys, the hybrid functional used makes the study of band structures possible ab initio without any empirical parameter fitting. We explain the trends in the band gap reductions in the two materials that result mainly from the positions of the nitrogen-induced states with respect to the bottoms of the bulk conduction bands.

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films.

PubMed

Talbert, Eric M; Zarick, Holly F; Boulesbaa, Abdelaziz; Soetan, Naiya; Puretzky, Alexander A; Geohegan, David B; Bardhan, Rizia

2017-08-24

In this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I 1-x Br x ) 3 ) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I 1-x Br x ) 3 from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, we observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I 1-x Br x ) 3 samples exhibit a lower Burstein-Moss shift of 0.07-0.08 eV compared to pure MAPbI 3 (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron-phonon coupling and phonon propagation in the lattice.

Effect of p–d hybridization, structural distortion and cation electronegativity on electronic properties of ZnSnX{sub 2} (X=P, As, Sb) chalcopyrite semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mishra, S.; Ganguli, B., E-mail: biplabg@nitrkl.ac.in

2013-04-15

Significant effects of p–d hybridization, structural distortion and cation-electro-negativity are found on band gap in ZnSnX{sub 2} (X=P, As, Sb). Our study suggests these compounds to be direct band gap semiconductors with band gaps of 1.23, 0.68 and 0.19 eV respectively. Lattice constants, tetragonal distortion (η), anion displacement, bond lengths and bulk moduli are calculated by Density Functional Theory based on Tight binding Linear Muffin-Tin orbital method. Our result of structural properties is in good agreement with the available experimental and other theoretical results. Calculated band gaps also agree well with the experimental works within LDA limitation. Unlike other semiconductorsmore » in the group II–IV–V{sub 2}, there is a reduction in the band gap of 0.22, 0.20 and 0.24 eV respectively in ZnSnX{sub 2} (X=P, As, Sb) due to p–d hybridization. Structural distortion decreases band gap by 0.20, 0.12 and 0.10 eV respectively. We find that cation electronegativity effect is responsible for increasing the band gap relative to their binary analogs GaInP{sub 2}, InGaAs{sub 2} and GaInSb{sub 2} respectively and increment are 0.13, 0.04 and 0.13 eV respectively. - Graphical abstract: One unit cell of ZnSnX{sub 2} (X=P, As, Sb) chalcopyrite semiconductor. Semiconductors ZnSnX{sub 2} (X=P, As, Sb) are found to be direct band gap semiconductors with band gaps 1.23, 0.68 and 0.19 eV respectively. The quantitative estimate of effects of p–d hybridization, structural distortion and cation electronegativity shows band gaps change significantly due to these effects. Highlights: ► ZnSnX{sub 2} (X=P, As, Sb) are direct band gap semiconductors. ► These have band gaps of 1.23 eV, 0.68 eV and 0.19 eV respectively. ► The band gap reduction due to p–d hybridization is 13.41%, 18.51% and 40% respectively. ► Band gap reduction due to structural distortion is 12.12%, 11.11% and 16.66% respectively. ► Band gap increases 8.38%, 3.70% and 21.31% respectively due to cation electronegativity.« less

950 keV X-Band Linac For Material Recognition Using Two-Fold Scintillator Detector As A Concept Of Dual-Energy X-Ray System

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Kiwoo; Natsui, Takuya; Hirai, Shunsuke

2011-06-01

One of the advantages of applying X-band linear accelerator (Linac) is the compact size of the whole system. That shows us the possibility of on-site system such as the custom inspection system in an airport. As X-ray source, we have developed X-band Linac and achieved maximum X-ray energy 950 keV using the low power magnetron (250 kW) in 2 {mu}s pulse length. The whole size of the Linac system is 1x1x1 m{sup 3}. That is realized by introducing X-band system. In addition, we have designed two-fold scintillator detector in dual energy X-ray concept. Monte carlo N-particle transport (MCNP) code wasmore » used to make up sensor part of the design with two scintillators, CsI and CdWO4. The custom inspection system is composed of two equipments: 950 keV X-band Linac and two-fold scintillator and they are operated simulating real situation such as baggage check in an airport. We will show you the results of experiment which was performed with metal samples: iron and lead as targets in several conditions.« less

Tunnel optical radiation in In{sub x}Ga{sub 1−x}N

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alexandrov, Dimiter; Skerget, Shawn

2014-02-21

An investigation of tunnel optical radiation in epitaxial layers of n-type In{sub x}Ga{sub 1−x}N grown on p-type GaN by novel plasma based migration enhanced epitaxy is presented. Experimental results of electro-luminescence spectra for In{sub x}Ga{sub 1−x}N/p−GaN hetero-junctions were obtained and they show two well expressed optical bands - one in range 500-540 nm and other in range 550-610 nm. An interesting detail is that each band begins and ends by sharp drops of the radiation, which nearly approach zero. A theoretical investigation of the unusual behavior of these spectra was done using LCAO electron band structure calculations. The optical rangesmore » of these bands show that the radiation occurs in the In{sub x}Ga{sub 1−x}N region. In fact, substitutions of In atoms in Ga sites creates defects in the structure of In{sub x}Ga{sub 1−x}N and the corresponding LCAO matrix elements are found on this basis. The LCAO electron band structures are calculated considering the interactions between nearest-neighbor orbitals. Electron energy pockets are found in both the conduction and the valence bands at the Γ point of the electron band structures. Also it is found that these pockets are separated by distances, for which there is overlapping between the electron wave functions describing localized states belonging to the pockets, and as a result tunnel optical radiation can take place. This type of electron transition - between such a pocket in the conduction band and a pocket in the valence band - occurs in In{sub x}Ga{sub 1−x}N, causing the above described optical bands. This conclusion concurs with the fact that the shapes of these bands change with change of the applied voltage.« less

Soft X-ray photoemission study of Co2(Cr1-xFex)Ga Heusler compounds

NASA Astrophysics Data System (ADS)

Tsunekawa, Masanori; Hattori, Yoshiro; Sekiyama, Akira; Fujiwara, Hidenori; Suga, Shigemasa; Muro, Takayuki; Kanomata, Takeshi; Imada, Shin

2015-08-01

We have performed soft X-ray photoemission spectroscopy (SXPES) and X-ray absorption spectroscopy (XAS) of the Co-based Heusler compounds Co2(Cr1-xFex)Ga (x = 0.0, 0.4, and 1.0) in order to study their electronic structures. Band-structure calculation was carried out and compared with the experimental results. SXPES spectra show hν-dependence, revealing the contributions of the Co, Cr, and Fe 3d electronic states in the valence band. The band width observed by the SXPES seems to be narrower than that predicted by the band-structure calculation. XAS spectra depend strongly on the the value of x in Co2(Cr1-xFex)Ga. The electron correlation effects are found to be stronger as x changes from 0.0 to 1.0.

Self-Shielding Analysis of the Zap-X System

PubMed Central

Schneider, M. Bret; Adler, John R.

2017-01-01

The Zap-X is a self-contained and first-of-its-kind self-shielded therapeutic radiation device dedicated to brain as well as head and neck stereotactic radiosurgery (SRS). By utilizing an S-band linear accelerator (linac) with a 2.7 megavolt (MV) accelerating potential and incorporating radiation-shielded mechanical structures, the Zap-X does not typically require a radiation bunker, thereby saving SRS facilities considerable cost. At the same time, the self-shielded features of the Zap-X are designed for more consistency of radiation protection, reducing the risk to radiation workers and others potentially exposed from a poorly designed or constructed radiotherapy vault. The hypothesis of the present study is that a radiosurgical system can be self-shielded such that it produces radiation exposure levels deemed safe to the public while operating under a full clinical workload. This study summarizes the Zap-X system shielding and found that the overall system radiation leakage values are reduced by a factor of 50 compared to the occupational radiation limit stipulated by the Nuclear Regulatory Commission (NRC) or agreement states. The goal of self-shielding is achieved under all but the most exceptional conditions for which additional room shielding or a larger restricted area in the vicinity of the Zap-X system would be required. PMID:29441251

Nitrogen-related intermediate band in P-rich GaN xP yAs 1-x-y alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zelazna, K.; Gladysiewicz, M.; Polak, M. P.

The electronic band structure of phosphorus-rich GaN xP yAs 1-x-y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. Here, it is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate bandmore » (E - band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E + band using contactless electroreflectance spectroscopy.« less

Nitrogen-related intermediate band in P-rich GaN xP yAs 1-x-y alloys

DOE PAGES

Zelazna, K.; Gladysiewicz, M.; Polak, M. P.; ...

2017-11-16

The electronic band structure of phosphorus-rich GaN xP yAs 1-x-y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. Here, it is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate bandmore » (E - band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E + band using contactless electroreflectance spectroscopy.« less

Towards band structure and band offset engineering of monolayer Mo(1-x)W(x)S2 via Strain

NASA Astrophysics Data System (ADS)

Kim, Joon-Seok; Ahmad, Rafia; Pandey, Tribhuwan; Rai, Amritesh; Feng, Simin; Yang, Jing; Lin, Zhong; Terrones, Mauricio; Banerjee, Sanjay K.; Singh, Abhishek K.; Akinwande, Deji; Lin, Jung-Fu

2018-01-01

Semiconducting transition metal dichalcogenides (TMDs) demonstrate a wide range of optoelectronic properties due to their diverse elemental compositions, and are promising candidates for next-generation optoelectronics and energy harvesting devices. However, effective band offset engineering is required to implement practical structures with desirable functionalities. Here, we explore the pressure-induced band structure evolution of monolayer WS2 and Mo0.5W0.5S2 using hydrostatic compressive strain applied in a diamond anvil cell (DAC) apparatus and theoretical calculations, in order to study the modulation of band structure and explore the possibility of band alignment engineering through different compositions. Higher W composition in Mo(1-x)W(x)S2 contributes to a greater pressure-sensitivity of direct band gap opening, with a maximum value of 54 meV GPa-1 in WS2. Interestingly, while the conduction band minima (CBMs) remains largely unchanged after the rapid gap increase, valence band maxima (VBMs) significantly rise above the initial values. It is suggested that the pressure- and composition-engineering could introduce a wide variety of band alignments including type I, type II, and type III heterojunctions, and allow to construct precise structures with desirable functionalities. No structural transition is observed during the pressure experiments, implying the pressure could provide selective modulation of band offset.

MULTICOLOR OPTICAL MICROVARIABILITY IN S5 0716+714

DOE Office of Scientific and Technical Information (OSTI.GOV)

Carini, M. T.; Walters, R.; Hopper, L., E-mail: mike.carini@wku.edu

We report the results of a study of multicolor optical microvariability in the Blazar S5 0716+714. S5 0716+714 was observed in the B and I bands continuously for five consecutive nights in 2003 March. This is the first study to apply color analysis, structure function, and cross-correlation analysis on an optical data set with the temporal coverage and photometric quality that characterizes this data. The source displayed variability on timescales from days to tens of minutes in both bands. Discrete events in the light curves lead to the determination of a range in the size of the emission regions ofmore » R {<=} 1.07 x10{sup 15} cm to R {<=} 9.8 x10{sup 15} cm, adopting {delta} = 20 from Nesci et al. Hysteresis loops were found in plots of the B-band flux (F{sub B}) and the ratio of the B to the I-band flux (F{sub B/I}); these loops were found to be clockwise for dips in the light curves and counterclockwise for bursts. Their directionality and the symmetric nature of the flares are consistent with the flux variations being dominated by the light crossing time of the emission region and not its intrinsic electron cooling or acceleration timescales. The variability between B and I bands is highly correlated with no significant lags between the B and I-band flux variations detected. Significant lags were detected between the flux in the B band (F{sub B}) and the B/I flux ratio (F{sub B/I}). A structure function analysis shows similar slopes in both bands, lying in the range of -1.0 to -2.0, indicating that the observed variability is the result of a fractional noise process, consistent with the variations arising from a turbulent process. A differential I-band calibration for comparison star 4 from the sequence of Villata et al. is also provided.« less

Ferroelectric switch for a high-power Ka-band active pulse compressor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hirshfield, Jay L.

2013-12-18

Results are presented for design of a high-power microwave switch for operation at 34.3 GHz, intended for use in an active RF pulse compressor. The active element in the switch is a ring of ferroelectric material, whose dielectric constant can be rapidly changed by application of a high-voltage pulse. As envisioned, two of these switches would be built into a pair of delay lines, as in SLED-II at SLAC, so as to allow 30-MW μs-length Ka-band pulses to be compressed in time by a factor-of-9 and multiplied in amplitude to generate 200 MW peak power pulses. Such high-power pulses couldmore » be used for testing and evaluation of high-gradient mm-wave accelerator structures, for example. Evaluation of the switch design was carried out with an X-band (11.43 GHz) prototype, built to incorporate all the features required for the Ka-band version.« less

Status of experiments at LLNL on high-power X-band microwave generators

NASA Astrophysics Data System (ADS)

Houck, Timothy L.; Westenskow, Glen A.

1994-05-01

The Microwave Source Facility at the Lawrence Livermore National Laboratory (LLNL) is studying the application of induction accelerator technology to high-power microwave generators suitable for linear collider power sources. We report on the results of two experiments, both using the Choppertron's 11.4 GHz modulator and a 5-MeV, 1-kA induction beam. The first experimental configuration has a single traveling-wave output structure designed to produce in excess of 300 MW in a single fundamental waveguide. This output structure consists of 12 individual cells, the first two incorporating de-Q-ing circuits to dampen higher order resonant modes. The second experiment studies the feasibility of enhancing beam to microwave power conversion by accelerating a modulated beam with induction cells. Referred to as the `reacceleration experiment,' this experiment consists of three traveling-wave output structures designed to produce about 125 MW per output and two induction cells located between the outputs. Status of current and planned experiments are presented.

Microwave-assisted hydrothermal synthesis of Ag₂(W(1-x)Mox)O₄ heterostructures: Nucleation of Ag, morphology, and photoluminescence properties.

PubMed

Silva, M D P; Gonçalves, R F; Nogueira, I C; Longo, V M; Mondoni, L; Moron, M G; Santana, Y V; Longo, E

2016-01-15

Ag2W(1-x)MoxO4 (x=0.0 and 0.50) powders were synthesized by the co-precipitation (drop-by-drop) method and processed using a microwave-assisted hydrothermal method. We report the real-time in situ formation and growth of Ag filaments on the Ag2W(1-x)MoxO4 crystals using an accelerated electron beam under high vacuum. Various techniques were used to evaluate the influence of the network-former substitution on the structural and optical properties, including photoluminescence (PL) emission, of these materials. X-ray diffraction results confirmed the phases obtained by the synthesis methods. Raman spectroscopy revealed significant changes in local order-disorder as a function of the network-former substitution. Field-emission scanning electron microscopy was used to determine the shape as well as dimensions of the Ag2W(1-x)MoxO4 heterostructures. The PL spectra showed that the PL-emission intensities of Ag2W(1-x)MoxO4 were greater than those of pure Ag2WO4, probably because of the increase of intermediary energy levels within the band gap of the Ag2W(1-x)MoxO4 heterostructures, as evidenced by the decrease in the band-gap values measured by ultraviolet-visible spectroscopy. Copyright © 2015 Elsevier B.V. All rights reserved.

Solid-State Powered X-band Accelerator

DOE Office of Scientific and Technical Information (OSTI.GOV)

Othman, Mohamed A.K.; Nann, Emilio A.; Dolgashev, Valery A.

2017-03-06

In this report we disseminate the hot test results of an X-band 100-W solid state amplifier chain for linear accelerator (linac) applications. Solid state power amplifiers have become increasingly attractive solutions for achieving high power in radar and maritime applications. Here the performance of solid state amplifiers when driving an RF cavity is investigated. Commercially available, matched and fully-packaged GaN on SiC HEMTs are utilized, comprising a wideband driver stage and two power stages. The amplifier chain has a high poweradded- efficiency and is able to supply up to ~1.2 MV/m field gradient at 9.2 GHz in a simple testmore » cavity, with a peak power exceeding 100 W. These findings set forth the enabling technology for solid-state powered linacs.« less

Spectrally resolved localized states in GaAs 1– xBi x

DOE PAGES

Christian, Theresa M.; Alberi, Kirstin; Beaton, Daniel A.; ...

2017-02-01

In this study, the role of localized states and their influence on the broader band structure remains a crucial question in understanding the band structure evolution in GaAs 1-xBi x. Here in this work, we present clear spectroscopic observations of recombination at several localized states in GaAs 1-xBi x. Sharp and recognizable photoluminescence features appear in multiple samples and redshift as a function of GaBi fraction between x = 0.16% and 0.4% at a linearized rate of 34 meV per % Bi, weaker than the redshift associated with band-to-band recombination. Interpreting these results in terms of radiative recombination between localizedmore » holes and free electrons sheds light on the relative movement of the conduction band minimum and the characteristics of localized bismuth-related trap states in GaAs 1-xBi x alloys.« less

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Talbert, Eric M.; Zarick, Holly F.; Boulesbaa, Abdelaziz

Here in this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I 1-xBrx)3) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I 1-xBr x) 3 from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, wemore » observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I 1-xBrx) 3 samples exhibit a lower Burstein–Moss shift of 0.07–0.08 eV compared to pure MAPbI 3 (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron–phonon coupling and phonon propagation in the lattice.« less

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films

DOE PAGES

Talbert, Eric M.; Zarick, Holly F.; Boulesbaa, Abdelaziz; ...

2017-08-02

Here in this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I 1-xBrx)3) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I 1-xBr x) 3 from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, wemore » observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I 1-xBrx) 3 samples exhibit a lower Burstein–Moss shift of 0.07–0.08 eV compared to pure MAPbI 3 (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron–phonon coupling and phonon propagation in the lattice.« less

Electronic structure of antifluorite Cu2X (X = S, Se, Te) within the modified Becke-Johnson potential plus an on-site Coulomb U.

PubMed

Zhang, Yubo; Wang, Youwei; Xi, Lili; Qiu, Ruihao; Shi, Xun; Zhang, Peihong; Zhang, Wenqing

2014-02-21

The traditional photon absorbers Cu2-xX (X = S, Se, and Te) have regained significant research attention in the search of earth-abundant photovoltaic materials. These moderate- and narrow-gap materials have also been shown to exhibit excellent thermoelectric properties recently. However, semimetallic band structures with inverted band orderings are predicted for antifluorite structure Cu2X using density functional theory with the local density approximation or the generalized gradient approximation. We find that semiconducting band structures and normal band orderings can be obtained using the modified Becke-Johnson potential plus an on-site Coulomb U (the mBJ+U approach), which is consistent with our earlier finding for diamond-like Cu-based multinary semiconductors [Y. Zhang, J. Zhang, W. Gao, T. A. Abtew, Y. Wang, P. Zhang, and W. Zhang, J. Chem. Phys. 139, 184706 (2013)]. The trend of the chemical bonding of Cu2X is analyzed, which shows that the positions of the valence band maximum and conduction band minimum are strongly affected by the inter-site pd and intra-site sp hybridizations, respectively. The calculated gaps of Cu2S and Cu2Se still seem to be underestimated compared with experimental results. We also discuss the effects of different structural phases and Cu disordering and deficiency on the bandgaps of these materials.

Strain-induced topological quantum phase transition in phosphorene oxide

NASA Astrophysics Data System (ADS)

Kang, Seoung-Hun; Park, Jejune; Woo, Sungjong; Kwon, Young-Kyun

Using ab initio density functional theory, we investigate the structural stability and electronic properties of phosphorene oxides (POx) with different oxygen compositions x. A variety of configurations are modeled and optimized geometrically to search for the equilibrium structure for each x value. Our electronic structure calculations on the equilibrium configuration obtained for each x reveal that the band gap tends to increase with the oxygen composition of x < 0.5, and then to decrease with x > 0.5. We further explore the strain effect on the electronic structure of the fully oxidized phosphorene, PO, with x = 1. At a particular strain without spin-orbit coupling (SOC) is observed a band gap closure near the Γ point in the k space. We further find the strain in tandem with SOC induces an interesting band inversion with a reopened very small band gap (5 meV), and thus gives rise to a topological quantum phase transition from a normal insulator to a topological insulator. Such a topological phase transition is confirmed by the wave function analysis and the band topology identified by the Z2 invariant calculation.

Conduction- and Valence-Band Energies in Bulk InAs(1-x)Sb(x) and Type II InAs(1-x) Sb(x)/InAs Strained-Layer Superlattices

DTIC Science & Technology

2013-03-08

tions in the studied SLS structures . The fit of the dependence of the valence- band energy of unstrained InAs1!xSbx on the composition x with a... band . STRUCTURES Bulk InAsSb epilayers on metamorphic buffers and InAsSb/InAs strained-layer superlattices (SLS) were grown on GaSb substrates by solid...meV in InAs and Ev = 0 meV in InSb. For InAsSb with 22.5% Sb grown on GaSb , an unstrained valence- band energy of Ev = !457 meV was obtained. For the

Band-Gap Engineering at a Semiconductor-Crystalline Oxide Interface

DOE PAGES

Jahangir-Moghadam, Mohammadreza; Ahmadi-Majlan, Kamyar; Shen, Xuan; ...

2015-02-09

The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to integrating the functionalities of oxides onto semiconductors is controlling the band alignment at interfaces between the two materials. Here we apply principles of band gap engineering traditionally used at heterojunctions between conventional semiconductors to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZr xTi 1-xO₃ and Ge, in which the band gap of the former is enhanced with Zr content x.more » We present structural and electrical characterization of SrZr xTi 1-xO₃-Ge heterojunctions and demonstrate a type-I band offset can be achieved. These results demonstrate that band gap engineering can be exploited to realize functional semiconductor crystalline oxide heterojunctions.« less

Interacting quasi-band theory for electronic states in compound semiconductor alloys: Wurtzite structure

NASA Astrophysics Data System (ADS)

Kishi, Ayaka; Oda, Masato; Shinozuka, Yuzo

2016-05-01

This paper reports on the electronic states of compound semiconductor alloys of wurtzite structure calculated by the recently proposed interacting quasi-band (IQB) theory combined with empirical sp3 tight-binding models. Solving derived quasi-Hamiltonian 24 × 24 matrix that is characterized by the crystal parameters of the constituents facilitates the calculation of the conduction and valence bands of wurtzite alloys for arbitrary concentrations under a unified scheme. The theory is applied to III-V and II-VI wurtzite alloys: cation-substituted Al1- x Ga x N and Ga1- x In x N and anion-substituted CdS1- x Se x and ZnO1- x S x . The obtained results agree well with the experimental data, and are discussed in terms of mutual mixing between the quasi-localized states (QLS) and quasi-average bands (QAB): the latter bands are approximately given by the virtual crystal approximation (VCA). The changes in the valence and conduction bands, and the origin of the band gap bowing are discussed on the basis of mixing character.

Optical control of hard X-ray polarization by electron injection in a laser wakefield accelerator

PubMed Central

Schnell, Michael; Sävert, Alexander; Uschmann, Ingo; Reuter, Maria; Nicolai, Maria; Kämpfer, Tino; Landgraf, Björn; Jäckel, Oliver; Jansen, Oliver; Pukhov, Alexander; Kaluza, Malte Christoph; Spielmann, Christian

2013-01-01

Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron–betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators. PMID:24026068

The Origin of Cosmic Rays: What can GLAST Say?

NASA Technical Reports Server (NTRS)

Ormes, Jonathan F.; Digel, Seith; Moskalenko, Igor V.; Moiseev, Alexander; Williamson, Roger

2000-01-01

Gamma rays in the band from 30 MeV to 300 GeV, used in combination with direct measurements and with data from radio and X-ray bands, provide a powerful tool for studying the origin of Galactic cosmic rays. Gamma-ray Large Area Space Telescope (GLAST) with its fine 10-20 arcmin angular resolution will be able to map the sites of acceleration of cosmic rays and their interactions with interstellar matter, It will provide information that is necessary to study the acceleration of energetic particles in supernova shocks, their transport in the interstellar medium and penetration into molecular clouds.

First-principle study of effect of variation of `x' on the band alignment in CZTS1-xSex

NASA Astrophysics Data System (ADS)

Ghemud, Vipul; Kshirsagar, Anjali

2018-04-01

The present work concentrates on the electronic structure study of CZTS1-xSex alloy with x ranging from 0 to 1. For the alloy study, we have carried out first-principles calculations employing generalized gradient approximation for structural optimization and further hybrid functional approach to compare the optical band gap with that obtained from the experiments. A systematic increase in the lattice parameters with lowering of band gap from 1.52eV to 1.04eV is seen with increasing Se concentration from 0 to 100%, however the lowering of valence band edge and conduction band edge is not linear with the concentration variation. Our results indicate that the lowering of band gap is a result increased Cu:d and Se:p hybridization with increasing `x'.

Electronic structure of antifluorite Cu{sub 2}X (X = S, Se, Te) within the modified Becke-Johnson potential plus an on-site Coulomb U

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yubo; Wang, Youwei; Xi, Lili

The traditional photon absorbers Cu{sub 2−x}X (X = S, Se, and Te) have regained significant research attention in the search of earth-abundant photovoltaic materials. These moderate- and narrow-gap materials have also been shown to exhibit excellent thermoelectric properties recently. However, semimetallic band structures with inverted band orderings are predicted for antifluorite structure Cu{sub 2}X using density functional theory with the local density approximation or the generalized gradient approximation. We find that semiconducting band structures and normal band orderings can be obtained using the modified Becke-Johnson potential plus an on-site Coulomb U (the mBJ+U approach), which is consistent with our earliermore » finding for diamond-like Cu-based multinary semiconductors [Y. Zhang, J. Zhang, W. Gao, T. A. Abtew, Y. Wang, P. Zhang, and W. Zhang, J. Chem. Phys. 139, 184706 (2013)]. The trend of the chemical bonding of Cu{sub 2}X is analyzed, which shows that the positions of the valence band maximum and conduction band minimum are strongly affected by the inter-site pd and intra-site sp hybridizations, respectively. The calculated gaps of Cu{sub 2}S and Cu{sub 2}Se still seem to be underestimated compared with experimental results. We also discuss the effects of different structural phases and Cu disordering and deficiency on the bandgaps of these materials.« less

(abstract) Transmission Electron Microscopy of Al(sub x)Ga(sub 1-x)N/SiC Multilayer Structures Grown on Sapphire Substrates

NASA Technical Reports Server (NTRS)

Pike, W. T.; George, T.; Khan, M. A.; Kuznia, J. N.

1994-01-01

The potential of wide-band-gap III-V nitrides as ultraviolet sensors and light emitters has prompted an increasing amount of work recently, including the fabrication of the first UV sensors from as-deposited single crystal GaN. We have used high resolution transmission electron microscopy (TEM) to study the microstructure of two novel developments of wide-band-gap III-V nitrides: the growth of ultra-short period GaN/AlN superlattices; and the incorporation of SiC layers into Al(sub x)Ga(sub 1-x)N structures. By varying the relative periods in a GaN/AlN superlattice, the band gap of the composite can be tailored to lie between the elemental values of 365 nm for GaN and 200 nm for AlN. The group IV semiconductor, SiC, has a wide band-gap and has a close lattice match (less than 3 %) to Al(sub x)Ga(sub 1-x)N for growth on the basal plane. Demonstration of epitaxial growth for Al(sub x)Ga(sub 1-x)N/SiC multilayers would introduce a wide band-gap analog to the already existing family of III-V and Si(sub 1-x)Ge(sub x) heteroepitaxial growth systems. Although good quality growth of GaN on SiC substrates has been demonstrated, Al(sub x)Ga(sub 1-x)N/SiC multilayer structures have never been grown and the interfacial structure is unknown.

Theoretical investigation of the structural, electronic, and thermodynamic properties of CdS1-xSex alloys

NASA Astrophysics Data System (ADS)

Long, Debing; Li, Mingkai; Meng, Dongxue; Ahuja, Rajeev; He, Yunbin

2018-03-01

In this work, the structural, electronic, and thermodynamic properties of wurtzite (WZ) and zincblende (ZB) CdS1-xSex alloys are investigated using the density functional theory (DFT) and the cluster expansion method. A special quasirandom structure containing 16 atoms is constructed to calculate the band structures of random alloys. The band gaps of CdS1-xSex alloys are direct and decrease as the Se content increases. The delta self-consistent-field method is applied to correct band gaps that are underestimated by DFT. The band offsets clearly reflect the variation in valence band maxima and conduction band minima, thus providing information useful to the design of relevant quantum well structures. The positive formation enthalpies of both phases imply that CdS1-xSex is an immiscible system and tends to phase separate. The influence of lattice vibrations on the phase diagram is investigated by calculating the phonon density of states. Lattice vibration effects can reduce the critical temperature Tc and increase alloy solid solubilities. This influence is especially significant in the ZB structure. When only chemical interactions are present, the Tc values for WZ- and ZB-CdS1-xSex are 260 K and 249 K, respectively. The lattice vibration enthalpy and entropy lower the Tc to 255 K and 233 K, respectively.

Band crossing in isovalent semiconductor alloys with large size mismatch

NASA Astrophysics Data System (ADS)

Deng, Hui-Xiong; Wei, Su-Huai

2012-02-01

Mixing isovalent compounds AC with BC to form alloys A1-xBxC has been an effective way in band structure engineering to enhance the availability of material properties. In most cases, the mixed isovalent atoms A and B, such as Al and Ga in Al1-xGaxAs or As and Sb in GaAs1-xSbx are similar in their atomic sizes and chemical potentials; therefore, the physical properties of A1-xBxC change smoothly from AC to BC. However, in some cases when the chemical and size differences between the isovalent atoms A and B are large, adding a small amount of B to AC or vice versa can lead to a discontinuous change in the electronic band structure. These large size- and chemicalmismatched (LSCM) systems often show unusual and abrupt changes in the alloys' material properties, which provide great potential in material design for novel device applications. In this report, based on first-principles band-structure calculations we show that for LSCM GaAs1-xNx and GaAs1-xBix alloys at the impurity limit the N (Bi)-induced impurity level is above (below) the conduction-(valence-) band edge of GaAs. These trends reverse at high concentration, i.e., the conduction-band edge of GaAs1-xNx becomes an N-derived state and the valence-band edge of GaAs1-xBix becomes a Bi-derived state, as expected from their band characters. We show that this band crossing phenomenon cannot be described by the popular BAC model but can be naturally explained by a simple band broadening picture.

Zn x Cd1-x S tunable band structure-directing photocatalytic activity and selectivity of visible-light reduction of CO2 into liquid solar fuels

NASA Astrophysics Data System (ADS)

Tang, Lanqin; Kuai, Libang; Li, Yichang; Li, Haijin; Zhou, Yong; Zou, Zhigang

2018-02-01

A series of Zn x Cd1-x S monodispersed nanospheres were successfully synthesized with tunable band structures. As-prepared Zn x Cd1-x S solid solutions show much enhanced photocatalytic efficiency for CO2 photoreduction in aqueous solutions under visible light irradiation, relative to pure CdS analog. Methanol (CH3OH) and acetaldehyde (CH3CHO) are the major products of CO2 photoreduction for the solid solutions with x = 0, 0.2, and 0.5. Interestingly, Zn0.8Cd0.2S photocatalyst with a wide band gap can also additionally generate ethanol (CH3CH2OH) besides CH3OH and CH3CHO. The balance between the band structure-directing redox capacity and light absorption should be considered to influence both product yield and selectivity of CO2 photoreduction. The possible photoreduction mechanism was tentatively proposed.

High gradient RF test results of S-band and C-band cavities for medical linear accelerators

NASA Astrophysics Data System (ADS)

Degiovanni, A.; Bonomi, R.; Garlasché, M.; Verdú-Andrés, S.; Wegner, R.; Amaldi, U.

2018-05-01

TERA Foundation has proposed and designed hadrontherapy facilities based on novel linacs, i.e. high gradient linacs which accelerate either protons or light ions. The overall length of the linac, and therefore its cost, is almost inversely proportional to the average accelerating gradient. With the scope of studying the limiting factors for high gradient operation and to optimize the linac design, TERA, in collaboration with the CLIC Structure Development Group, has conducted a series of high gradient experiments. The main goals were to study the high gradient behavior and to evaluate the maximum gradient reached in 3 and 5.7 GHz structures to direct the design of medical accelerators based on high gradient linacs. This paper summarizes the results of the high power tests of 3.0 and 5.7 GHz single-cell cavities.

Terahertz-driven linear electron acceleration

PubMed Central

Nanni, Emilio A.; Huang, Wenqian R.; Hong, Kyung-Han; Ravi, Koustuban; Fallahi, Arya; Moriena, Gustavo; Dwayne Miller, R. J.; Kärtner, Franz X.

2015-01-01

The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30–50 MeV m−1 gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams. PMID:26439410

Terahertz-driven linear electron acceleration

DOE PAGES

Nanni, Emilio A.; Huang, Wenqian R.; Hong, Kyung-Han; ...

2015-10-06

The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30–50 MeVm -1 gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/protonmore » accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. As a result, these ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.« less

Electronic structure of scandium-doped MgB2

NASA Astrophysics Data System (ADS)

de La Peña, Omar; Agrestini, Stefano

2005-03-01

Recently has been reported the synthesis of a new superconducting alloy based on MgB2, where Mg is partially substituted with Sc. In order to analyze the effect of Sc doping on the structural and superconducting properties of Mg1-xScxB2, we have performed a detailed study of the electronic structure for this new diboride. The calculations have been done using the first-principles LAPW method, within the supercell approach for modeling the doping. In this work we report results for the electronic band structure, Fermi surface, and density of states. The effect of the Sc-d orbitals on the structural and electronic properties of Mg1-xScxB2 is analyzed. Increasing the Sc concentration (x) the σ-band is gradually filled, because Sc have one valence electron more than Mg. Interestingly, the analysis of the band structure shows that even for ScB2 the top of the σ-band remain above the Fermi level, nevertheless the σ-band presents high dispersion and has an important contribution of d states. In this way, in addition to the band filling effect, Sc doping gradually reduces the two-dimensional character of the σ- band in Mg1-xScxB2 as a result of increasing the sp(B)-d(Sc) hybridization. This research was partially supported by Consejo Nacional de Ciencia y Tecnolog'ia (CONACYT, M'exico) under Grant. No. 43830-F

Design of an X -band electron linear accelerator dedicated to decentralized 99Mo/99mTc supply: From beam energy selection to yield estimation

NASA Astrophysics Data System (ADS)

Jang, Jaewoong; Yamamoto, Masashi; Uesaka, Mitsuru

2017-10-01

The most frequently used radionuclide in diagnostic nuclear medicine, 99mTc, is generally obtained by the decay of its parent radionuclide, 99Mo. Recently, concerns have been raised over shortages of 99Mo/99mTc, owing to aging of the research reactors which have been supplying practically all of the global demand for 99Mo in a centralized fashion. In an effort to prevent such 99Mo/99mTc supply disruption and, furthermore, to ameliorate the underlying instability of the centralized 99Mo/99mTc supply chain, we designed an X -band electron linear accelerator which can be distributed over multiple regions, whereby 99Mo/99mTc can be supplied with improved accessibility. The electron beam energy was designed to be 35 MeV, at which an average beam power of 9.1 kW was calculated by the following beam dynamics analysis. Subsequent radioactivity modeling suggests that 11 of the designed electron linear accelerators can realize self-sufficiency of 99Mo/99mTc in Japan.

Ballistic-electron-emission spectroscopy of Al{sub x}Ga{sub 1{minus}x}As/GaAs heterostructures: Conduction-band offsets, transport mechanisms, and band-structure effects

DOE Office of Scientific and Technical Information (OSTI.GOV)

OShea, J.J.; Brazel, E.G.; Rubin, M.E.

1997-07-01

We report an extensive investigation of semiconductor band-structure effects in single-barrier Al{sub x}Ga{sub 1{minus}x}As/GaAs heterostructures using ballistic-electron-emission spectroscopy (BEES). The transport mechanisms in these single-barrier structures were studied systematically as a function of temperature and Al composition over the full compositional range (0{le}x{le}1). The initial ({Gamma}) BEES thresholds for Al{sub x}Ga{sub 1{minus}x}As single barriers with 0{le}x{le}0.42 were extracted using a model which includes the complete transmission probability of the metal-semiconductor interface and the semiconductor heterostructure. Band offsets measured by BEES are in good agreement with previous measurements by other techniques which demonstrates the accuracy of this technique. BEES measurements atmore » 77 K give the same band-offset values as at room temperature. When a reverse bias is applied to the heterostructures, the BEES thresholds shift to lower voltages in good agreement with the expected bias-induced band-bending. In the indirect band-gap regime ({ital x}{gt}0.45), spectra show a weak ballistic-electron-emission microscopy current contribution due to intervalley scattering through Al{sub x}Ga{sub 1{minus}x}As {ital X} valley states. Low-temperature spectra show a marked reduction in this intervalley current component, indicating that intervalley phonon scattering at the GaAs/Al{sub x}Ga{sub 1{minus}x}As interface produces a significant fraction of this{ital X} valley current. A comparison of the BEES thresholds with the expected composition dependence of the Al{sub x}Ga{sub 1{minus}x}As {Gamma}, {ital L}, and {ital X} points yields good agreement over the entire composition range. {copyright} {ital 1997} {ital The American Physical Society}« less

RF Phase Stability and Electron Beam Characterization for the PLEIADES Thomson X-Ray Source

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, W J; Hartemann, F V; Tremaine, A M

2002-10-16

We report on the performance of an S-band RF photocathode electron gun and accelerator for operation with the PLEIADES Thomson x-ray source at LLNL. To produce picosecond, high brightness x-ray pulses, picosecond timing, terahertz bandwidth diagnostics, and RF phase control are required. Planned optical, RF, x-ray and electron beam measurements to characterize the dependence of electron beam parameters and synchronization on RF phase stability are presented.

Hard X-ray Emission and Efficient Particle Acceleration by Supernova Remnants

NASA Astrophysics Data System (ADS)

Vink, Jacco

2009-05-01

I discuss the non-thermal X-ray emission from young supernova remnants. Over the last decade it has become clear from both X-ray and γ-ray observations that young supernovae accelerate particles up to 100 TeV. In soft X-rays the accelerated >10 TeV electrons produce synchrotron radiation, coming from narrow filaments located at the shock fronts. The width of these filaments shows that the magnetic fields are relatively high, thus providing evidence for magnetic field amplification. The synchrotron radiation of several remnants is known to extend into the hard X-ray regime. In particular Cas A, has a spectrum that appears as a power law up to almost 100 TeV. This is very surprising, as a steepening is expected going from the soft to the hard X-ray band. The spectrum is likely a result of many superimposed individual spectra, each steepening at different energies. This implies considerable spatial variation in hard X-rays, an obvious target for Simbol-X. The variations will be important to infer local shock acceleration properties, but also magnetic field fluctuations may cause spatial and temporal variations. Finally, I draw the attention to super bubbles and supernovae as sources of cosmic rays. As such they may be sources of hard X-ray emission. In particular, supernovae exploding inside the dense red supergiants winds of their progenitors ares promising candidates for hard X-ray emission.

Effect of hydrostatic pressure on the structural and electronic properties of Cd{sub 0.75}Cr{sub 0.25}S

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rani, Anita; Kaur, Kulwinder; Kumar, Ranjan

In this paper we present the results obtained from first principle calculations of the effect of hydrostatic pressure on the structural and electronic properties of Cd{sub 1-x}Cr{sub x}S diluted magnetic semiconductor in Zinc Blende (B3) phase at x=0.25. High pressure behavior of Cd{sub 1-x}Cr{sub x}S has been investigated between 0 GPa to 100 GPa The calculations have been performed using Density functional theory as implemented in the Spanish Initiative for Electronic Simulations with Thousands of Atoms code using local density approximation as exchange-correlation (XC) potential. Calculated electronic band structures of Cd{sub 1-x}Cr{sub x}S are discussed in terms of contribution ofmore » Cr 3d{sup 5} 4s{sup 1}, Cd 4d{sup 10} 5s{sup 2}, S 3s{sup 2} 3p{sup 4} orbital’s. Study of band structures shows half-metallic ferromagnetic nature of Cd{sub 0.75}Cr{sub 0.25}S with 100% spin polarization. Under application of external pressure, the valence band and conduction band are shifted upward which leads to modification of electronic structure.« less

Electronic and optical properties of the LiCdX (X = N, P, As and Sb) filled-tetrahedral compounds with the Tran–Blaha modified Becke–Johnson density functional

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bouhemadou, A., E-mail: a_bouhemadou@yahoo.fr; Bin-Omran, S.; Department of Physics, Faculty of Science & Humanitarian Studies, Salman Bin Abdalaziz University, Alkharj 11942

Highlights: • Electronic and optical properties of the LiCdX compounds have been predicted. • Tran–Blaha-modified Becke–Johnson functional significantly improves the band gap. • We predict a direct band gap in all of the considered LiCdX compounds. • Origin of the peaks in the optical spectra is determined. - Abstract: The structural, electronic and optical properties of the LiCdN, LiCdP, LiCdAs and LiCdSb filled-tetrahedral compounds have been explored from first-principles. The calculated structural parameters are consistent with the available experimental results. Since DFT with the common LDA and GGA underestimates the band gap, we use a new developed functional able tomore » accurately describe the electronic structure of semiconductors, namely the Tran–Blaha-modified Becke–Johnson potential. The four investigated compounds demonstrate semiconducting behavior with direct band gap ranging from about 0.32 to 1.65 eV. The charge-carrier effective masses are evaluated at the topmost valence band and at the bottommost conduction band. The evolution of the value and nature of the energy band gap under pressure effect is also investigated. The frequency-dependent complex dielectric function and some macroscopic optical constants are estimated. The microscopic origins of the structures in the optical spectra are determined in terms of the calculated energy band structures.« less

Determination of the energy structure of recombination centers in heavily doped AlxGa1-xN:Si epitaxial layers with x > 0.5

NASA Astrophysics Data System (ADS)

Osinnykh, I. V.; Malin, T. V.; Zhuravlev, K. S.

2018-03-01

The photoluminescence properties of the intensive defect-related emission in heavily doped Al x Ga l-x N:Si layers with x > 0.5 have been investigated by photoluminescence (PL) spectroscopy. The PL band in AlN was attributed to donor-acceptor (DA) transitions. At the lowest Al content, the impurity band merges with the conduction band and DA transitions are replaced by electron-acceptor transitions involving the same acceptor. The energy structure of recombination centers was obtained using the model of configuration coordinates for Al0.67Ga0.33N.

Compaction bands in shale revealed through digital volume correlation of time-resolved X-ray tomography scans

NASA Astrophysics Data System (ADS)

McBeck, J.; Kobchenko, M.; Hall, S.; Tudisco, E.; Cordonnier, B.; Renard, F.

2017-12-01

Previous studies have identified compaction bands primarily within sandstones, and in fewer instances, within other porous rocks and sediments. Using Digital Volume Correlation (DVC) of X-ray microtomography scans, we find evidence of localized zones of high axial contraction that form tabular structures sub-perpendicular to maximum compression, σ1, in Green River shale. To capture in situ strain localization throughout loading, two shale cores were deformed in the HADES triaxial deformation apparatus installed on the X-ray microtomography beamline ID19 at the European Synchrotron Radiation Facility. In these experiments, we increase σ1 in increments of two MPa, with constant confining pressure (20 MPa), until the sample fails in macroscopic shear. After each stress step, a 3D image of the sample inside the rig is acquired at a voxel resolution of 6.5 μm. The evolution of lower density regions within 3D reconstructions of linear attenuation coefficients reveal the development of fractures that fail with some opening. If a fracture produces negligible dilation, it may remain undetected in image segmentation of the reconstructions. We use the DVC software TomoWarp2 to identify undetected fractures and capture the 3D incremental displacement field between each successive pair of microtomography scans acquired in each experiment. The corresponding strain fields reveal localized bands of high axial contraction that host minimal shear strain, and thus match the kinematic definition of compaction bands. The bands develop sub-perpendicular to σ1 in the two samples in which pre-existing bedding laminations were oriented parallel and perpendicular to σ1. As the shales deform plastically toward macroscopic shear failure, the number of bands and axial contraction within the bands increase, while the spacing between the bands decreases. Compaction band development accelerates the rate of overall axial contraction, increasing the mean axial contraction throughout the sample, and strengthens the shale sufficiently to localize shear faults. These results are critical to robust assessment of deformation patterns in shale rocks in contexts such as nuclear waste storage, hydrocarbon recovery and groundwater access.

Band gap bowing in NixMg1−xO

PubMed Central

Niedermeier, Christian A.; Råsander, Mikael; Rhode, Sneha; Kachkanov, Vyacheslav; Zou, Bin; Alford, Neil; Moram, Michelle A.

2016-01-01

Epitaxial transparent oxide NixMg1−xO (0 ≤ x ≤ 1) thin films were grown on MgO(100) substrates by pulsed laser deposition. High-resolution synchrotron X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the thin films are compositionally and structurally homogeneous, forming a completely miscible solid solution. Nevertheless, the composition dependence of the NixMg1−xO optical band gap shows a strong non-parabolic bowing with a discontinuity at dilute NiO concentrations of x < 0.037. Density functional calculations of the NixMg1−xO band structure and the density of states demonstrate that deep Ni 3d levels are introduced into the MgO band gap, which significantly reduce the fundamental gap as confirmed by optical absorption spectra. These states broaden into a Ni 3d-derived conduction band for x > 0.074 and account for the anomalously large band gap narrowing in the NixMg1−xO solid solution system. PMID:27503808

Fermiology of the strongly spin-orbit coupled superconductor Sn(1-x)In(x)Te: implications for topological superconductivity.

PubMed

Sato, T; Tanaka, Y; Nakayama, K; Souma, S; Takahashi, T; Sasaki, S; Ren, Z; Taskin, A A; Segawa, Kouji; Ando, Yoichi

2013-05-17

We have performed angle-resolved photoemission spectroscopy on the strongly spin-orbit coupled low-carrier density superconductor Sn(1-x)In(x)Te (x = 0.045) to elucidate the electronic states relevant to the possible occurrence of topological superconductivity, as recently reported for this compound based on point-contact spectroscopy. The obtained energy-band structure reveals a small holelike Fermi surface centered at the L point of the bulk Brillouin zone, together with a signature of a topological surface state, indicating that this material is a doped topological crystalline insulator characterized by band inversion and mirror symmetry. A comparison of the electronic states with a band-noninverted superconductor possessing a similar Fermi surface structure, Pb(1-x)Tl(x)Te, suggests that the anomalous behavior in the superconducting state of Sn(1-x)In(x)Te is related to the peculiar orbital characteristics of the bulk valence band and/or the presence of a topological surface state.

D Haas-Van Alphen Oscillations in the Diluted Magnetic Semiconductor MERCURY(1-X)IRON(X)SELENIUM.

NASA Astrophysics Data System (ADS)

Miller, Michael Montgomery

de Haas-van Alphen measurements are performed in oriented single crystals of Hg_{ rm 1-x}Fe_{rm x}Se in the range 0.0 <=q x <=q 0.05 for 0.5 < T < 4.2K for the magnetic field range 0.2 T < H < 1.0 T. These data can be interpreted in terms of a closed orbit magnetic breakdown model. The effect of Fe on the conduction band is explored in some detail. It is found that the presence of Fe lowers the Dingle temperature in a non-monotonic fashion, i.e., there is a minimum in the Dingle temperature for x ~ 0.001. This effect cannot be attributed to a gross modification of the band structure. Effective mass measurements are in good agreement with those expected for HgSe. However, the presence of Fe is seen to have a subtle effect on the band structure. The overall symmetry of the band structure may be modified by the addition of Fe. Furthermore, the presence of Fe tends to decrease the inversion asymmetry splitting of the conduction band as evidenced in the low-field beating.

Hollow-Core Photonic Band Gap Fibers for Particle Acceleration

DOE Office of Scientific and Technical Information (OSTI.GOV)

Noble, Robert J.; Spencer, James E.; /SLAC

Photonic band gap (PBG) dielectric fibers with hollow cores are being studied both theoretically and experimentally for use as laser driven accelerator structures. The hollow core functions as both a longitudinal waveguide for the transverse-magnetic (TM) accelerating fields and a channel for the charged particles. The dielectric surrounding the core is permeated by a periodic array of smaller holes to confine the mode, forming a photonic crystal fiber in which modes exist in frequency pass-bands, separated by band gaps. The hollow core acts as a defect which breaks the crystal symmetry, and so-called defect, or trapped modes having frequencies inmore » the band gap will only propagate near the defect. We describe the design of 2-D hollow-core PBG fibers to support TM defect modes with high longitudinal fields and high characteristic impedance. Using as-built dimensions of industrially-made fibers, we perform a simulation analysis of the first prototype PBG fibers specifically designed to support speed-of-light TM modes.« less

X-ray spectra and electronic structure of the Ca3Ga2Ge3О12 compound

NASA Astrophysics Data System (ADS)

Shcherba, I. D.; Kostyk, L. V.; Noga, H.; Bekenov, L. V.; Uskokovich, D.; Jatsyk, B. M.

2017-09-01

The band structure of Ca3Ga2Ge3О12 with the garnet structure has been determined for the first time by X-ray emission and photoelectron spectroscopy. It has been established that the bottom of the valence band is formed by Ge d states, which are not dominant in the chemical bonding. Strong hybridization of oxygen 2s states with 4p states of Ga and Ge revealed by the presence of an extra structure in the X-ray emission spectra has been found. The middle of the valence band has been demonstrated to be occupied by d states of Ga, while Ga and Ge 4рstates with a considerable admixture of oxygen 2p states form the top of the valence band.

Design and application of multimegawatt X -band deflectors for femtosecond electron beam diagnostics

DOE PAGES

Dolgashev, Valery A.; Bowden, Gordon; Ding, Yuantao; ...

2014-10-02

Performance of the x-ray free electron laser Linac Coherent Light Source (LCLS) and the Facility for Advanced Accelerator Experimental Tests (FACET) is determined by the properties of their extremely short electron bunches. Multi-GeV electron bunches in both LCLS and FACET are less than 100 fs long. Optimization of beam properties and understanding of free-electron laser operation require electron beam diagnostics with time resolution of about 10 fs. We designed, built and commissioned a set of high frequency X-band deflectors which can measure the beam longitudinal space charge distribution and slice energy spread to better than 10 fs resolution at fullmore » LCLS energy (14 GeV), and with 70 fs resolution at full FACET energy (20 GeV). Use of high frequency and high gradient in these devices allows them to reach unprecedented performance. We report on the physics motivation, design considerations, operational configuration, cold tests, and typical results of the X-band deflector systems currently in use at SLAC.« less

Fermi arc electronic structure and Chern numbers in the type-II Weyl semimetal candidate Mo x W 1 - x Te 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Belopolski, Ilya; Xu, Su-Yang; Ishida, Yukiaki

2016-08-15

It has recently been proposed that electronic band structures in crystals can give rise to a previously overlooked type of Weyl fermion, which violates Lorentz invariance and, consequently, is forbidden in particle physics. It was further predicted that Mo x W 1 - x Te 2 may realize such a type-II Weyl fermion. Here, we first show theoretically that it is crucial to access the band structure above the Fermi level ε F to show a Weyl semimetal in Mo x W 1 - x Te 2 . Then, we study Mo x W 1 - x Te 2 bymore » pump-probe ARPES and we directly access the band structure > 0.2 eV above ε F in experiment. By comparing our results with ab initio calculations, we conclude that we directly observe the surface state containing the topological Fermi arc. We propose that a future study of Mo x W 1 - x Te 2 by pump-probe ARPES may directly pinpoint the Fermi arc. Our work sets the stage for the experimental discovery of the first type-II Weyl semimetal in Mo x W 1 - x Te 2 .« less

Acoustic band gaps of the woodpile sonic crystal with the simple cubic lattice

NASA Astrophysics Data System (ADS)

Wu, Liang-Yu; Chen, Lien-Wen

2011-02-01

This study theoretically and experimentally investigates the acoustic band gap of a three-dimensional woodpile sonic crystal. Such crystals are built by blocks or rods that are orthogonally stacked together. The adjacent layers are perpendicular to each other. The woodpile structure is embedded in air background. Their band structures and transmission spectra are calculated using the finite element method with a periodic boundary condition. The dependence of the band gap on the width of the stacked rods is discussed. The deaf bands in the band structure are observed by comparing with the calculated transmission spectra. The experimental transmission spectra for the Γ-X and Γ-X' directions are also presented. The calculated results are compared with the experimental results.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kozlov, D. V., E-mail: dvkoz@ipmras.ru; Rumyantsev, V. V.; Morozov, S. V.

A long-wavelength band caused by transitions between states related to the valence band is detected in the photoconductivity spectra of Hg{sub y}Te{sub 1–y}/Cd{sub x}Hg{sub 1–x}Te (CMT) structures with quantum wells. The energy states of mercury vacancies in quantum wells of CMT structures is calculated taking into account a chemical shift. It is shown that the long-wavelength band observed in the photoconductivity spectra of these structures is associated with the ionization of divalent acceptor centers which are such vacancies.

Design and Simulation of Microstrip Hairpin Bandpass Filter with Open Stub and Defected Ground Structure (DGS) at X-Band Frequency

NASA Astrophysics Data System (ADS)

Hariyadi, T.; Mulyasari, S.; Mukhidin

2018-02-01

In this paper we have designed and simulated a Band Pass Filter (BPF) at X-band frequency. This filter is designed for X-band weather radar application with 9500 MHz center frequency and bandwidth -3 dB is 120 MHz. The filter design was performed using a hairpin microstrip combined with an open stub and defected ground structure (DGS). The substrate used is Rogers RT5880 with a dielectric constant of 2.2 and a thickness of 1.575 mm. Based on the simulation results, it is found that the filter works on frequency 9,44 - 9,56 GHz with insertion loss value at pass band is -1,57 dB.

High-Energy X-Ray Imaging of the Pulsar Wind Nebula MSH 15-52: Constraints on Particle Acceleration and Transport

NASA Technical Reports Server (NTRS)

An, Hongjun; Madsen, Kristin K.; Reynolds, Stephen P.; Kaspi, Victoria M.; Harrison, Fiona A.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Fryer, Chris L.; Grefenstette, Brian W.;

High-energy X-ray imaging of the pulsar wind nebula MSH 15–52: constraints on particle acceleration and transport

DOE Office of Scientific and Technical Information (OSTI.GOV)

An, Hongjun; Kaspi, Victoria M.; Madsen, Kristin K.

2014-10-01

We present the first images of the pulsar wind nebula (PWN) MSH 15–52 in the hard X-ray band (≳8 keV), as measured with the Nuclear Spectroscopic Telescope Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the 3-7 keV band is similar to that seen in Chandra high-resolution imaging. However, the spatial extent decreases with energy, which we attribute to synchrotron energy losses as the particles move away from the shock. The hard-band maps show a relative deficit of counts in the northern region toward the RCW 89 thermal remnant, with significant asymmetry. We find thatmore » the integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a spectral break at 6 keV, which may be explained by a break in the synchrotron-emitting electron distribution at ∼200 TeV and/or imperfect cross calibration. We also measure spatially resolved spectra, showing that the spectrum of the PWN softens away from the central pulsar B1509–58, and that there exists a roughly sinusoidal variation of spectral hardness in the azimuthal direction. We discuss the results using particle flow models. We find non-monotonic structure in the variation with distance of spectral hardness within 50'' of the pulsar moving in the jet direction, which may imply particle and magnetic-field compression by magnetic hoop stress as previously suggested for this source. We also present two-dimensional maps of spectral parameters and find an interesting shell-like structure in the N {sub H} map. We discuss possible origins of the shell-like structure and their implications.« less

Theoretical investigation of structural, mechanical and electronic properties of GaAs1-xNx alloys under ambient and high pressure

NASA Astrophysics Data System (ADS)

Li, Jian; Han, Xiuxun; Dong, Chen; Fan, Changzeng

2017-12-01

Using first-principles total energy calculations, we have studied the structural, mechanical and electronic properties of GaAs1-xNx ternary semiconductor alloys with the zinc-blende crystal structure over the whole nitrogen concentration range (with x from 0 to 1) within density functional theory (DFT) framework. To obtain the ideal band gap, we employ the semi-empirical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U). The calculated results illustrate the varying lattice constants and band gap in GaAs1-xNx alloys as functions of the nitrogen concentration x. According to the pressure dependence of the lattice constants and volume, the higher N concentration alloy exhibits the better anti-compressibility. In addition, an increasing band gap is predicted under 20 GPa pressure for GaAs1-xNx alloys.

Hard X-ray Emission and Efficient Particle Acceleration by Supernova Remnants

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vink, Jacco

I discuss the non-thermal X-ray emission from young supernova remnants. Over the last decade it has become clear from both X-ray and {gamma}-ray observations that young supernovae accelerate particles up to 100 TeV. In soft X-rays the accelerated >10 TeV electrons produce synchrotron radiation, coming from narrow filaments located at the shock fronts. The width of these filaments shows that the magnetic fields are relatively high, thus providing evidence for magnetic field amplification.The synchrotron radiation of several remnants is known to extend into the hard X-ray regime. In particular Cas A, has a spectrum that appears as a power lawmore » up to almost 100 TeV. This is very surprising, as a steepening is expected going from the soft to the hard X-ray band. The spectrum is likely a result of many superimposed individual spectra, each steepening at different energies. This implies considerable spatial variation in hard X-rays, an obvious target for Simbol-X. The variations will be important to infer local shock acceleration properties, but also magnetic field fluctuations may cause spatial and temporal variations.Finally, I draw the attention to super bubbles and supernovae as sources of cosmic rays. As such they may be sources of hard X-ray emission. In particular, supernovae exploding inside the dense red supergiants winds of their progenitors ares promising candidates for hard X-ray emission.« less

Performance of a first generation X-band photoelectron rf gun

DOE PAGES

Limborg-Deprey, C.; Adolphsen, C.; McCormick, D.; ...

2016-05-04

Building more compact accelerators to deliver high brightness electron beams for the generation of high flux, highly coherent radiation is a priority for the photon science community. A relatively straightforward reduction in footprint can be achieved by using high-gradient X-band (11.4 GHz) rf technology. To this end, an X-band injector consisting of a 5.5 cell rf gun and a 1-m long linac has been commissioned at SLAC. It delivers an 85 MeV electron beam with peak brightness somewhat better than that achieved in S-band photoinjectors, such as the one developed for the Linac Coherent Light Source (LCLS). The X-band rfmore » gun operates with up to a 200 MV/m peak field on the cathode, and has been used to produce bunches of a few pC to 1.2 nC in charge. Notably, bunch lengths as short as 120 fs rms have been measured for charges of 5 pC (~3×10 7 electrons), and normalized transverse emittances as small as 0.22 mm-mrad have been measured for this same charge level. Bunch lengths as short as 400 (250) fs rms have been achieved for electron bunches of 100 (20) pC with transverse normalized emittances of 0.7 (0.35) mm-mrad. As a result, we report on the performance and the lessons learned from the operation and optimization of this first generation X-band gun.« less

Performance of a first generation X-band photoelectron rf gun

DOE Office of Scientific and Technical Information (OSTI.GOV)

Limborg-Deprey, C.; Adolphsen, C.; McCormick, D.

Building more compact accelerators to deliver high brightness electron beams for the generation of high flux, highly coherent radiation is a priority for the photon science community. A relatively straightforward reduction in footprint can be achieved by using high-gradient X-band (11.4 GHz) rf technology. To this end, an X-band injector consisting of a 5.5 cell rf gun and a 1-m long linac has been commissioned at SLAC. It delivers an 85 MeV electron beam with peak brightness somewhat better than that achieved in S-band photoinjectors, such as the one developed for the Linac Coherent Light Source (LCLS). The X-band rfmore » gun operates with up to a 200 MV/m peak field on the cathode, and has been used to produce bunches of a few pC to 1.2 nC in charge. Notably, bunch lengths as short as 120 fs rms have been measured for charges of 5 pC (~3×10 7 electrons), and normalized transverse emittances as small as 0.22 mm-mrad have been measured for this same charge level. Bunch lengths as short as 400 (250) fs rms have been achieved for electron bunches of 100 (20) pC with transverse normalized emittances of 0.7 (0.35) mm-mrad. As a result, we report on the performance and the lessons learned from the operation and optimization of this first generation X-band gun.« less

High-resolution absorption cross section measurements of carbon monoxide at 20 K between 96.7 and 98.8 nanometers

NASA Technical Reports Server (NTRS)

Stark, G.; Yoshino, K.; Smith, P. L.; Esmond, J. R.; Ito, K.; Stevens, M. H.

1993-01-01

Photoabsorption cross sections for five CO bands, at wavelengths between 96.7 and 98.8 nm, have been measured at high-resolution in a supersonic jet-cooled source at the Photon Factory synchrotron facility. New integrated cross sections are reported for the K-X, L(prime)-X, and L-X bands. Low-temperature spectra of the J-X and W-X bands, which were used in the determination of the absorbing CO column densities, are also presented. The rotational structures of the K-X, L(prime)-X, and L-X bands do not overlap in the low-temperature spectra, allowing for the first unambiguous determination of these band oscillator strengths. We also report revised room temperature measurements of integrated cross sections for the K-X, L(prime)-X, and L-X bands, in which distortions in the measured spectra due to insufficient instrumental resolution have been minimized; the revised room temperature integrated cross sections are consistent with the low-temperature results.

Engineering the electronic band structures of novel cubic structured germanium monochalcogenides for thermoelectric applications

NASA Astrophysics Data System (ADS)

Ul Haq, Bakhtiar; AlFaify, S.; Ahmed, R.; Butt, Faheem K.; Laref, A.; Goumri-Said, Souraya; Tahir, S. A.

2018-05-01

Germanium mono-chalcogenides have received considerable attention for being a promising replacement for the relatively toxic and expensive chalcogenides in renewable and sustainable energy applications. In this paper, we explore the potential of the recently discovered novel cubic structured (π-phase) GeS and GeSe for thermoelectric applications in the framework of density functional theory coupled with Boltzmann transport theory. To examine the modifications in their physical properties, the across composition alloying of π-GeS and π-GeSe (such as π-GeS1-xSex for x =0, 0.25, 0.50, 0.75, and 1) has been performed that has shown important effects on the electronic band structures and effective masses of charge carriers. An increase in Se composition in π-GeS1-xSex has induced a downward shift in their conduction bands, resulting in the narrowing of their energy band gaps. The thermoelectric coefficients of π-GeS1-xSex have been accordingly influenced by the evolution of the electronic band structures and effective masses of charge carriers. π-GeS1-xSex features sufficiently larger values of Seebeck coefficients, power factors and figures of merit (ZTs), which experience further improvement with an increase in temperature, revealing their potential for high-temperature applications. The calculated results show that ZT values equivalent to unity can be achieved for π-GeS1-xSex at appropriate n-type doping levels. Our calculations for the formation enthalpies indicate that a π-GeS1-xSex alloying system is energetically stable and could be synthesized experimentally. These intriguing characteristics make π-GeS1-xSex a promising candidate for futuristic thermoelectric applications in energy harvesting devices.

Role of biaxial strain and microscopic ordering for structural and electronic properties of InxGa1 -xN

NASA Astrophysics Data System (ADS)

Cui, Ying; Lee, Sangheon; Freysoldt, Christoph; Neugebauer, Jörg

2015-08-01

The structural and electronic properties of InxGa1 -xN alloys are studied as a function of c -plane biaxial strain and In ordering by density functional theory with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. A nonlinear variation of the c lattice parameter with In content is observed in biaxial strain and should be taken into account when deducing In content from interplanar distances. From compressive to tensile strain, the character of the top valence-band state changes, leading to a nonlinear variation of the band gap in InxGa1 -xN . Interestingly, the well-known bowing of the InxGa1 -xN band gap is largely removed for alloys grown strictly coherently on GaN, while the actual values for band gaps at x <0.33 are hardly affected by strain. Ordering plays a minor role for lattice constants but may induce changes of the band gap up to 0.15 eV.

Electronic structure and optical properties of iron based chalcogenide FeX2 (X = S, Se, Te) for photovoltaic applications: a first principle study

NASA Astrophysics Data System (ADS)

Ghosh, Anima; Thangavel, R.

2017-11-01

In present work, the electronic structure and optical properties of the FeX2 (X = S, Se, Te) compounds have been evaluated by the density functional theory based on the scalar-relativistic full potential linear augmented plane wave method via Wien2K. From the total energy calculations, it has been found that all the compounds have direct band nature, which determined by iron 3 d states at valance band edge and anion p dominated at conduction band at Γ-point and the fundamental band gap between the valence band and conduction band are estimated 1.40, 1.02 and 0.88 eV respectively with scissor correction for FeS2, FeSe2 and FeTe2 which are close to the experimental values. The optical properties such as dielectric tensor components and the absorption coefficient of these materials are determined in order to investigate their usefulness in photovoltaic applications.

Electromodulation spectroscopy of direct optical transitions in Ge{sub 1−x}Sn{sub x} layers under hydrostatic pressure and built-in strain

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dybała, F.; Żelazna, K.; Maczko, H.

Unstrained Ge{sub 1−x}Sn{sub x} layers of various Sn concentration (1.5%, 3%, 6% Sn) and Ge{sub 0.97}Sn{sub 0.03} layers with built-in compressive (ε = −0.5%) and tensile (ε = 0.3%) strain are grown by molecular beam epitaxy and studied by electromodulation spectroscopy (i.e., contactless electroreflectance and photoreflectance (PR)). In order to obtain unstrained GeSn layers and layers with different built-in in-plane strains, virtual InGaAs substrates of different compositions are grown prior to the deposition of GeSn layers. For unstrained Ge{sub 1−x}Sn{sub x} layers, the pressure coefficient for the direct band gap transition is determined from PR measurements at various hydrostatic pressures to be 12.2 ± 0.2 meV/kbar, whichmore » is very close to the pressure coefficient for the direct band gap transition in Ge (12.9 meV/kbar). This suggests that the hydrostatic deformation potentials typical of Ge can be applied to describe the pressure-induced changes in the electronic band structure of Ge{sub 1−x}Sn{sub x} alloys with low Sn concentrations. The same conclusion is derived for the uniaxial deformation potential, which describes the splitting between heavy-hole (HH) and light-hole (LH) bands as well as the strain-related shift of the spin-orbit (SO) split-off band. It is observed that the HH, LH, and SO related transitions shift due to compressive and tensile strain according to the Bir-Pikus theory. The dispersions of HH, LH, and SO bands are calculated for compressive and tensile strained Ge{sub 0.97}Sn{sub 0.03} with the 8-band kp Hamiltonian including strain effects, and the mixing of HH and LH bands is discussed. In addition, the dispersion of the electronic band structure is calculated for unstrained Ge{sub 1−x}Sn{sub x} layers (3% and 6% Sn) at high hydrostatic pressure with the 8-band kp Hamiltonian, and the pressure-induced changes in the electronic band structure are discussed.« less

Effect of hydrostatic pressure and uniaxial strain on the electronic structure of Pb 1-xSn xTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Geilhufe, Matthias; Nayak, Sanjeev K.; Thomas, Stefan

2015-12-09

The electronic structure of Pb 1–xSn xTe is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations x, Pb 1–xSn xTe is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will be shown that this particular property can be tuned, first, by alloying PbTe and SnTe and, second, by applying hydrostatic pressure or uniaxial strain. Furthermore, the magnitude of strain needed to switch between the regular and inverted band gap canmore » be tuned by the alloy composition. In conclusion, there is a range of potential usage of Pb 1–xSn xTe for spintronic applications.« less

Radio Observations of Elongated Pulsar Wind Nebulae

NASA Astrophysics Data System (ADS)

Ng, Stephen C.-Y.

2015-08-01

The majority of pulsars' rotational energy is carried away by relativistic winds, which are energetic particles accelerated in the magnetosphere. The confinement of the winds by the ambient medium result in synchrotron bubbles with broad-band emission, which are commonly referred to as pulsar wind nebulae (PWNe). Due to long synchrotron cooling time, a radio PWN reflects the integrated history of the system, complementing information obtained from the X-ray and higher energy bands. In addition, radio polarization measurements can offer a powerful probe of the PWN magnetic field structure. Altogether these can reveal the physical conditions and evolutionary history of a system.I report on preliminary results from high-resolution radio observations of PWNe associated with G327.1-1.1, PSRs J1015-5719, B1509-58, and J1549-4848 taken with the Australia Telescope Compact Array (ATCA). Their magnetic field structure and multiwavelength comparison with other observations are discussed.This work is supported by a ECS grant of the Hong Kong Government under HKU 709713P. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.

Commercialization of an S-band standing-wave electron accelerator for industrial applications

NASA Astrophysics Data System (ADS)

Moon, Jin-Hyeok; Kwak, Gyeong-Il; Han, Jae-Ik; Lee, Gyu-Baek; Jeon, Seong-Hwan; Kim, Jae-Young; Hwang, Cheol-Bin; Lee, Gi-Yong; Kim, Young-Man; Park, Sung-Ju

2016-09-01

An electron accelerator system has been developed for use in industrial, as well as possible medical, applications. Based on our experiences achieved during prototype system development and various electron beam acceleration tests, we have built a stable and compact system for sales purposes. We have integrated a self-developed accelerating cavity, an E-gun pulse driver, a radio-frequency (RF) power system, a vacuum system, a cooling system, etc. into a frame with a size of 1800 × 1000 × 1500 mm3. The accelerating structure is a side-coupled standing-wave type operating in the π/2 mode (tuned to~3 GHz). The RF power is provided by using a magnetron driven by a solid-state modulator. The electron gun is a triode type with a dispenser cathode (diameter of 11 mm). The system is capable of delivering a maximum 900-W average electron beam power with tight focusing at the target. Until now, we have performed various electron beam tests and X-ray beam tests after having built the system, have completed the beam assessment for commercializations, and have been preparing full-fledged sales activity. This article reports on our system development processes and on some of our early test results for commercializations.

Real-structure effects: Band gaps of Mg_xZn_{1-x}O, Cd_xZn_{1-x}O, and n-type ZnO from ab-initio calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schleife, A; Bechstedt, F

2012-02-15

Many-body perturbation theory is applied to compute the quasiparticle electronic structures and the optical-absorption spectra (including excitonic effects) for several transparent conducting oxides. We discuss HSE+G{sub 0}W{sub 0} results for band structures, fundamental band gaps, and effective electron masses of MgO, ZnO, CdO, SnO{sub 2}, SnO, In{sub 2}O{sub 3}, and SiO{sub 2}. The Bethe-Salpeter equation is solved to account for excitonic effects in the calculation of the frequency-dependent absorption coefficients. We show that the HSE+G{sub 0}W{sub 0} approach and the solution of the Bethe-Salpeter equation are very well-suited to describe the electronic structure and the optical properties of various transparentmore » conducting oxides in good agreement with experiment.« less

Experimental high gradient testing of a 17.1 GHz photonic band-gap accelerator structure

DOE PAGES

Munroe, Brian J.; Zhang, JieXi; Xu, Haoran; ...

2016-03-29

In this paper, we report the design, fabrication, and high gradient testing of a 17.1 GHz photonic band-gap (PBG) accelerator structure. Photonic band-gap (PBG) structures are promising candidates for electron accelerators capable of high-gradient operation because they have the inherent damping of high order modes required to avoid beam breakup instabilities. The 17.1 GHz PBG structure tested was a single cell structure composed of a triangular array of round copper rods of radius 1.45 mm spaced by 8.05 mm. The test assembly consisted of the test PBG cell located between conventional (pillbox) input and output cells, with input power ofmore » up to 4 MW from a klystron supplied via a TM 01 mode launcher. Breakdown at high gradient was observed by diagnostics including reflected power, downstream and upstream current monitors and visible light emission. The testing procedure was first benchmarked with a conventional disc-loaded waveguide structure, which reached a gradient of 87 MV=m at a breakdown probability of 1.19 × 10 –1 per pulse per meter. The PBG structure was tested with 100 ns pulses at gradient levels of less than 90 MV=m in order to limit the surface temperature rise to 120 K. The PBG structure reached up to 89 MV=m at a breakdown probability of 1.09 × 10 –1 per pulse per meter. These test results show that a PBG structure can simultaneously operate at high gradients and low breakdown probability, while also providing wakefield damping.« less

First principles calculations of electronic structure and magnetic properties of Cr-based magnetic semiconductors Al{sub 1-x}Cr{sub x}X (X=N, P, As, Sb)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saeed, Y., E-mail: yasir_saeed54321@yahoo.co; Shaukat, A., E-mail: schaukat@gmail.co; Nazir, S., E-mail: nazirsafdar@gmail.co

2010-01-15

First principles calculations based on the density functional theory (DFT) within the local spin density approximation are performed to investigate the electronic structure and magnetic properties of Cr-based zinc blende diluted magnetic semiconductors Al{sub 1-x}Cr{sub x}X (X=N, P, As, Sb) for 0<=x<=0.50.The behaviour of magnetic moment of Al{sub 1-x}Cr{sub x}X at each Cr site as well as the change in the band gap value due to spin down electrons has been studied by increasing the concentration of Cr atom and through changing X from N to Sb. Furthermore, the role of p-d hybridization is analyzed in the electronic band structuremore » and exchange splitting of d-dominated bands. The interaction strength is stronger in Al{sub 1-x}Cr{sub x}N and becomes weaker in Al{sub 1-x}Cr{sub x}Sb. The band gap due to the spin down electrons decreases with the increased concentration of Cr in Al{sub 1-x}Cr{sub x}X, and as one moves down along the isoelectronic series in the group V from N to Sb. Our calculations also verify the half-metallic ferromagnetic character in Cr doped AlX. - Graphical abstract: The prototype structures of Cr doped AlX (X=N, P, As, Sb) compounds: (A) zinc blende AlP for x=0, (B) Cr{sub 1}Al{sub 7}P{sub 8} for x=0.125, (C) Cr{sub 1}Al{sub 3}P{sub 4} for x=0.25, (D) Cr{sub 1}Al{sub 1}P{sub 2} for x=0.5.« less

Multiwavelength Study of Quiescent States of Mrk 421 with Unprecedented Hard X-Ray Coverage Provided by NuSTAR in 2013

NASA Technical Reports Server (NTRS)

Balokovic, M.; Paneque, D.; Madejski, G.; Chiang, J.; Furniss, A.; Ajello, M.; Alexander, D. M.; Barret, D.; Blandford, R. D.; Boggs, S. E.;

High-gradient low-β accelerating structure using the first negative spatial harmonic of the fundamental mode

NASA Astrophysics Data System (ADS)

Kutsaev, Sergey V.; Agustsson, Ronald; Boucher, Salime; Fischer, Richard; Murokh, Alex; Mustapha, Brahim; Nassiri, Alireza; Ostroumov, Peter N.; Plastun, Alexander; Savin, Evgeny; Smirnov, Alexander Yu.

2017-12-01

The development of high-gradient accelerating structures for low-β particles is the key for compact hadron linear accelerators. A particular example of such a machine is a hadron therapy linac, which is a promising alternative to cyclic machines, traditionally used for cancer treatment. Currently, the practical utilization of linear accelerators in radiation therapy is limited by the requirement to be under 50 m in length. A usable device for cancer therapy should produce 200-250 MeV protons and/or 400 - 450 MeV /u carbon ions, which sets the requirement of having 35 MV /m average "real-estate gradient" or gradient per unit of actual accelerator length, including different accelerating sections, focusing elements and beam transport lines, and at least 50 MV /m accelerating gradients in the high-energy section of the linac. Such high accelerating gradients for ion linacs have recently become feasible for operations at S-band frequencies. However, the reasonable application of traditional S-band structures is practically limited to β =v /c >0.4 . However, the simulations show that for lower phase velocities, these structures have either high surface fields (>200 MV /m ) or low shunt impedances (<35 M Ω /m ). At the same time, a significant (˜10 % ) reduction in the linac length can be achieved by using the 50 MV /m structures starting from β ˜0.3 . To address this issue, we have designed a novel radio frequency structure where the beam is synchronous with the higher spatial harmonic of the electromagnetic field. In this paper, we discuss the principles of this approach, the related beam dynamics and especially the electromagnetic and thermomechanical designs of this novel structure. Besides the application to ion therapy, the technology described in this paper can be applied to future high gradient normal conducting ion linacs and high energy physics machines, such as a compact hadron collider. This approach preserves linac compactness in settings with limited space availability.

Deep-Ultraviolet Luminescence of Rocksalt-Structured Mg x Zn1-x O (x > 0.5) Films on MgO Substrates

NASA Astrophysics Data System (ADS)

Kaneko, Kentaro; Tsumura, Keiichi; Ishii, Kyohei; Onuma, Takayoshi; Honda, Tohru; Fujita, Shizuo

2018-04-01

Rocksalt-structured Mg x Zn1-x O films with Mg composition x of 0.47, 0.57, and 0.64 were grown on (100)-oriented MgO substrates using mist chemical vapor deposition. Cathodoluminescence measurements showed deep ultraviolet (DUV) emission peaking at 4.88 eV (254 nm), 5.15 eV (241 nm), and 5.21 eV (238 nm), respectively, at 12 K. The peak energies were lower than the band gap energies by ca. 1 eV, suggesting that the deep ultraviolet (DUV) emission may be recognized as near band edge luminescence but is associated with impurities, defects, or band fluctuations. The use of carbon-free precursors in the growth is suggested to eliminate carbon impurities and to improve the optical properties of Mg x Zn1-x O.

Experimental observation of incoherent-coherent crossover and orbital-dependent band renormalization in iron chalcogenide superconductors

DOE PAGES

Liu, Z. K.; Yi, M.; Zhang, Y.; ...

2015-12-22

The level of electronic correlation has been one of the key questions in understanding the nature of superconductivity. Among the iron-based superconductors, the iron chalcogenide family exhibits the strongest electron correlations. To gauge the correlation strength, we performed a systematic angle-resolved photoemission spectroscopy study on the iron chalcogenide series Fe 1+ySe xTe 1-x (0 < x < 0.59), a model system with the simplest structure. Our measurement reveals an incoherent-to-coherent crossover in the electronic structure as the selenium ratio increases and the system evolves from a weakly localized to a more itinerant state. Furthermore, we found that the effective massmore » of bands dominated by the d xy orbital character significantly decreases with increasing selenium ratio, as compared to the d xz/d yz orbital-dominated bands. The orbital-dependent change in the correlation level agrees with theoretical calculations on the band structure renormalization, and may help to understand the onset of superconductivity in Fe 1+ySe xTe 1-x.« less

Electronic structures of U X3 (X =Al , Ga, and In) studied by photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Fujimori, Shin-ichi; Kobata, Masaaki; Takeda, Yukiharu; Okane, Tetsuo; Saitoh, Yuji; Fujimori, Atsushi; Yamagami, Hiroshi; Haga, Yoshinori; Yamamoto, Etsuji; Ōnuki, Yoshichika

2017-09-01

The electronic structures of U X3 (X =Al , Ga , and In ) were studied by photoelectron spectroscopy to understand the relationship between their electronic structures and magnetic properties. The band structures and Fermi surfaces of UAl3 and UGa3 were revealed experimentally by angle-resolved photoelectron spectroscopy (ARPES), and they were compared with the result of band-structure calculations. The topologies of the Fermi surfaces and the band structures of UAl3 and UGa3 were explained reasonably well by the calculation, although bands near the Fermi level (EF) were renormalized owing to the finite electron correlation effect. The topologies of the Fermi surfaces of UAl3 and UGa3 are very similar to each other, except for some minor differences. Such minor differences in their Fermi surface or electron correlation effect might take an essential role in their different magnetic properties. No significant changes were observed between the ARPES spectra of UGa3 in the paramagnetic and antiferromagnetic phases, suggesting that UGa3 is an itinerant weak antiferromagnet. The effect of chemical pressure on the electronic structures of U X3 compounds was also studied by utilizing the smaller lattice constants of UAl3 and UGa3 than that of UIn3. The valence band spectrum of UIn3 is accompanied by a satellitelike structure on the high-binding-energy side. The core-level spectrum of UIn3 is also qualitatively different from those of UAl3 and UGa3. These findings suggest that the U 5 f states in UIn3 are more localized than those in UAl3 and UGa3.

Electronic structure of ZrX2 (X = Se, Te)

NASA Astrophysics Data System (ADS)

Shkvarin, A. S.; Merentsov, A. I.; Shkvarina, E. G.; Yarmoshenko, Yu. M.; Píš, I.; Nappini, S.; Titov, A. N.

2018-03-01

The electronic structure of the ZrX2 (X = Se, Te) compounds has been studied using photoelectron, resonant photoelectron and X-ray absorption spectroscopy, theoretical calculations of the X-ray absorption spectra, and density of electronic states. It was found that the absorption spectra and valence band spectra are influenced by the chalcogen type. The results of the multiplet calculation of the Zr4+ atom show that the change in the splitting in the crystal field, which is described by the 10Dq parameter, is due to the change in the ratio of covalent and ionic contributions to the chemical bond. The resonance band near the Fermi level in the valence band spectra is observed for ZrTe2 in the Zr 3p-4d resonant excitation mode. The extent of photon energy indicates the charge localization on the Zr atom. Similar resonance band for ZrSe2 is absent; it indicates the presence of a gap at the Fermi level.

Variation of crystal structure and optical properties of wurtzite-type oxide semiconductor alloys of β-Cu(Ga,Al)O2

NASA Astrophysics Data System (ADS)

Nagatani, Hiraku; Mizuno, Yuki; Suzuki, Issei; Kita, Masao; Ohashi, Naoki; Omata, Takahisa

2017-06-01

Band-gap engineering of β-CuGaO2 was demonstrated by the alloying of gallium with aluminum, that is, Cu(Ga1-xAlx)O2. The ternary wurtzite β-NaFeO2-type alloys were obtained in the range 0 ≤ x ≤ 0.7, and γ-LiAlO2-type phase appeared in the range 0.7 ≤ x ≤ 1. The energy band gap of wurtzite β-CuGaO2 was controlled in the range between 1.47 and 2.09 eV. A direct band gap for x < 0.6 and indirect band gap for x ≥ 0.6 were proposed based on the structural distortion in the β-NaFeO2-type phase and density functional theory (DFT) calculation of β-CuAlO2. The DFT calculation also indicated that the γ-LiAlO2-type phases appeared in 0.7 ≤ x ≤ 1 are also indirect-gap semiconductors.

Modulating the band structure and sub-bandgap absorption of Co-hyperdoped silicon by co-doping with shallow-level elements

NASA Astrophysics Data System (ADS)

Dong, Xiao; Fang, Xiuxiu; Wang, Yongyong; Song, Xiaohui; Lu, Zhansheng

2018-06-01

Hyperdoped group-III elements can lower the Fermi energy in the band structures of Co-hyperdoped silicon. When the Co-to-X (X = B, Al, Ga) ratio is 2:1, the intermediate band (IB) in the bandgap includes the Fermi energy and is partially filled by electrons, which is in accordance with the requirement of an IB material. The hyperdoped X atoms can cause the blueshift of the sub-bandgap absorption of the compound compared with the material with no shallow-level elements, which is due to the enlargement of the electronic excitation energy of the Co,X-co-doped silicon.

One-dimensional mercury(II) halide coordination polymers of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine ligand: Synthesis, crystal structure, spectroscopic and DFT studies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saghatforoush, Lotfali, E-mail: saghatforoush@gmail.com; Khoshtarkib, Zeinab; Amani, Vahid

2016-01-15

Three new coordination polymers, [Hg(μ-bptz)X{sub 2}]{sub n} (X=Cl (1), Br (2)) and [Hg{sub 2}(μ-bptz)(μ-I){sub 2}I{sub 2}]{sub n} (3) (bptz=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) were synthesized. X-ray structural analysis indicated that compounds 1 and 2 are composed of one-dimensional (1D) linear chains while the compound 3 has 1D stair-stepped structure. The electronic band structure along with density of states (DOS) calculated by the DFT method indicates that compound 1 and 2 are direct band gap semiconductors; however, compound 3 is an indirect semiconductor. The linear optical properties of the compounds are also calculated by DFT method. According to the DFT calculations, the observed emission bandmore » of the compounds in solid state is due to electron transfer from an excited bptz-π* state (CBs) to the top of VBs. {sup 1}H NMR spectra of the compounds indicate that, in solution phase, the compounds don’t decompose completely. Thermal stability of the compounds is studied using TG, DTA methods. - Graphical abstract: Synthesis, crystal structure and emission spectra of [Hg(μ-bptz)X{sub 2}]{sub n} (X=Cl and Br) and [Hg{sub 2}(μ-bptz)(μ-I){sub 2}I{sub 2}]{sub n} are presented. The electronic band structure and linear optical properties of the compounds are calculated by the DFT method. - Highlights: • Three 1D Hg(II) halide coordination polymers with bptz ligand have been prepared. • The structures of the compounds are determined by single crystal XRD. • DFT calculations show that [Hg(μ-bptz)X{sub 2}]{sub n} (X=Cl and Br) have a direct band gap. • DFT calculations show that [Hg{sub 2}(μ-bptz)(μ-I){sub 2}I{sub 2}]{sub n} has an indirect band gap. • The compounds show an intraligand electron transfer emission band in solid state.« less

Charge deformation and orbital hybridization: intrinsic mechanisms on tunable chromaticity of Y3Al5O12:Ce3+ luminescence by doping Gd3+ for warm white LEDs

PubMed Central

Chen, Lei; Chen, Xiuling; Liu, Fayong; Chen, Haohong; Wang, Hui; Zhao, Erlong; Jiang, Yang; Chan, Ting-Shan; Wang, Chia-Hsin; Zhang, Wenhua; Wang, Yu; Chen, Shifu

2015-01-01

The deficiency of Y3Al5O12:Ce (YAG:Ce) luminescence in red component can be compensated by doping Gd3+, thus lead to it being widely used for packaging warm white light-emitting diode devices. This article presents a systematic study on the photoluminescence properties, crystal structures and electronic band structures of (Y1−xGdx)3Al5O12: Ce3+ using powerful experimental techniques of thermally stimulated luminescence, X-ray diffraction, X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and ultraviolet photoelectron spectra (UPS) of the valence band, assisted with theoretical calculations on the band structure, density of states (DOS), and charge deformation density (CDD). A new interpretation from the viewpoint of compression deformation of electron cloud in a rigid structure by combining orbital hybridization with solid-state energy band theory together is put forward to illustrate the intrinsic mechanisms that cause the emission spectral shift, thermal quenching, and luminescence intensity decrease of YAG: Ce upon substitution of Y3+ by Gd3+, which are out of the explanation of the classic configuration coordinate model. The results indicate that in a rigid structure, the charge deformation provides an efficient way to tune chromaticity, but the band gaps and crystal defects must be controlled by comprehensively accounting for luminescence thermal stability and efficiency. PMID:26175141

Increased photoelectron transmission in High-pressure photoelectron spectrometers using "swift acceleration"

NASA Astrophysics Data System (ADS)

Edwards, Mårten O. M.; Karlsson, Patrik G.; Eriksson, Susanna K.; Hahlin, Maria; Siegbahn, Hans; Rensmo, Håkan; Kahk, Juhan M.; Villar-Garcia, Ignacio J.; Payne, David J.; Åhlund, John

2015-06-01

A new operation mode of a HPXPS (high-pressure X-ray photoelectron spectroscopy) analyzer is evaluated on a HPXPS system fitted with an Al Kα X-ray source. A variety of metal foil samples (gold, silver and copper) were measured in different sample gas environments (N2 and H2O), and a front aperture diameter of 0.8 mm. The new design concept is based upon "swiftly" accelerating the photoelectrons to kinetic energies of several keV after they pass the analyzer front aperture. Compared to the standard mode, in which the front section between the two first apertures is field-free, this gives a wider angular collection and a lower tendency for electron losses in collisions with gas molecules within the analyzer. With the swift-acceleration mode we attain, depending on the experimental conditions, up to about 3 times higher peak intensities in vacuum and about 10 to 20 times higher peak intensities in the 6-9 mbar regime, depending on kinetic energy. These experimental findings agree well with simulated transmission functions for the analyzer. The new mode of operation enables faster data acquisition than the standard mode of operation, particularly valuable in a home laboratory environment. Further demonstrations of performance are highlighted by measurements of the valence band structure in dye-sensitized solar cell photoelectrodes under a 2 mbar H2O atmosphere, a molecularly modified surface of interest in photoelectrochemical devices.

Energy band gap and spectroscopic studies in Mn{sub 1-x}Cu{sub x}WO{sub 4} (0 ≤ x ≤ 0.125)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mal, Priyanath; Rambabu, P.; Turpu, G. R.

2016-05-06

A study on the effect of nonmagnetic Cu{sup 2+} substitution at Mn{sup 2+} site on the structural and energy band gap of the MnWO{sub 4} is reported. Convenient solid state reaction route has been adopted for the synthesis of Mn{sub 1-x}Cu{sub x}WO{sub 4}. X-ray diffraction (XRD) pattern showed high crystalline quality of the prepared samples. Raman spectroscopic studies were carried out to understand the structural aspects of the doping. 15 Raman active modes were identified out of 18, predicted for wolframite type monoclinic structure of MnWO{sub 4}. UV-visible diffuse reflectance spectra were recorded and analyzed to get energy band gapmore » of the studied system and are found in the range of 2.5 eV to 2.04 eV with a systematic decrease with the increase in Cu{sup 2+} concentration. Energy band gap values are verified by Density Functional Theory calculations based on projector augmented wave (PAW) method. The calculated values are in good agreement with the experimental data.« less

HOM-Free Linear Accelerating Structure for e+ e- Linear Collider at C-Band

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kubo, Kiyoshi

2003-07-07

HOM-free linear acceleration structure using the choke mode cavity (damped cavity) is now under design for e{sup +}e{sup -} linear collider project at C-band frequency (5712 MHz). Since this structure shows powerful damping effect on most of all HOMs, there is no multibunch problem due to long range wakefields. The structure will be equipped with the microwave absorbers in each cells and also the in-line dummy load in the last few cells. The straightness tolerance for 1.8 m long structure is closer than 30 {micro}m for 25% emittance dilution limit, which can be achieved by standard machining and braising techniques.more » Since it has good vacuum pumping conductance through annular gaps in each cell, instabilities due to the interaction of beam with the residual-gas and ions can be minimized.« less

Probing dissipation mechanisms in BL Lac jets through X-ray polarimetry

NASA Astrophysics Data System (ADS)

Tavecchio, F.; Landoni, M.; Sironi, L.; Coppi, P.

2018-06-01

The dissipation of energy flux in blazar jets plays a key role in the acceleration of relativistic particles. Two possibilities are commonly considered for the dissipation processes, magnetic reconnection - possibly triggered by instabilities in magnetically-dominated jets - , or shocks - for weakly magnetized flows. We consider the polarimetric features expected for the two scenarios analyzing the results of state-of-the-art simulations. For the magnetic reconnection scenario we conclude, using results from global relativistic MHD simulations, that the emission likely occurs in turbulent regions with unstructured magnetic fields, although the simulations do not allow us to draw firm conclusions. On the other hand, with local particle-in-cell simulations we show that, for shocks with a magnetic field geometry suitable for particle acceleration, the self-generated magnetic field at the shock front is predominantly orthogonal to the shock normal and becomes quasi-parallel downstream. Based on this result we develop a simplified model to calculate the frequency-dependent degree of polarization, assuming that high-energy particles are injected at the shock and cool downstream. We apply our results to HBLs, blazars with the maximum of their synchrotron output at UV-soft X-ray energies. While in the optical band the predicted degree of polarization is low, in the X-ray emission it can ideally reach 50%, especially during active/flaring states. The comparison between measurements in the optical and in the X-ray band made during active states (feasible with the planned IXPE satellite) are expected to provide valuable constraints on the dissipation and acceleration processes.

Interacting quasi-band model for electronic states in compound semiconductor alloys: Zincblende structure

NASA Astrophysics Data System (ADS)

Shinozuka, Yuzo; Oda, Masato

2015-09-01

The interacting quasi-band model proposed for electronic states in simple alloys is extended for compound semiconductor alloys with general lattice structures containing several atoms per unit cell. Using a tight-binding model, a variational electronic wave function for quasi-Bloch states yields a non-Hermitian Hamiltonian matrix characterized by matrix elements of constituent crystals and concentration of constituents. Solving secular equations for each k-state yields the alloy’s energy spectrum for any type of randomness and arbitrary concentration. The theory is used to address III-V (II-VI) alloys with a zincblende lattice with crystal band structures well represented by the sp3s* model. Using the resulting 15 × 15 matrix, the concentration dependence of valence and conduction bands is calculated in a unified scheme for typical alloys: Al1-xGaxAs, GaAs1-xPx, and GaSb1-xPx. Results agree well with experiments and are discussed with respect to the concentration dependence, direct-indirect gap transition, and band-gap-bowing origin.

The ASTRO-H X-ray astronomy satellite

NASA Astrophysics Data System (ADS)

Takahashi, Tadayuki; Mitsuda, Kazuhisa; Kelley, Richard; Aharonian, Felix; Akamatsu, Hiroki; Akimoto, Fumie; Allen, Steve; Anabuki, Naohisa; Angelini, Lorella; Arnaud, Keith; Asai, Makoto; Audard, Marc; Awaki, Hisamitsu; Azzarello, Philipp; Baluta, Chris; Bamba, Aya; Bando, Nobutaka; Bautz, Marshall; Bialas, Thomas; Blandford, Roger D.; Boyce, Kevin; Brenneman, Laura; Brown, Gregory; Cackett, Ed; Canavan, Edgar; Chernyakova, Maria; Chiao, Meng; Coppi, Paolo; Costantini, Elisa; de Plaa, Jelle; den Herder, Jan-Willem; DiPirro, Michael; Done, Chris; Dotani, Tadayasu; Doty, John; Ebisawa, Ken; Enoto, Teruaki; Ezoe, Yuichiro; Fabian, Andrew; Ferrigno, Carlo; Foster, Adam; Fujimoto, Ryuichi; Fukazawa, Yasushi; Funk, Stefan; Furuzawa, Akihiro; Galeazzi, Massimiliano; Gallo, Luigi; Gandhi, Poshak; Gilmore, Kirk; Guainazzi, Matteo; Haas, Daniel; Haba, Yoshito; Hamaguchi, Kenji; Harayama, Atsushi; Hatsukade, Isamu; Hayashi, Katsuhiro; Hayashi, Takayuki; Hayashida, Kiyoshi; Hiraga, Junko; Hirose, Kazuyuki; Hornschemeier, Ann; Hoshino, Akio; Hughes, John; Hwang, Una; Iizuka, Ryo; Inoue, Yoshiyuki; Ishibashi, Kazunori; Ishida, Manabu; Ishikawa, Kumi; Ishimura, Kosei; Ishisaki, Yoshitaka; Itoh, Masayuki; Iwata, Naoko; Iyomoto, Naoko; Jewell, Chris; Kaastra, Jelle; Kallman, Timothy; Kamae, Tuneyoshi; Kataoka, Jun; Katsuda, Satoru; Katsuta, Junichiro; Kawaharada, Madoka; Kawai, Nobuyuki; Kawano, Taro; Kawasaki, Shigeo; Khangaluyan, Dmitry; Kilbourne, Caroline; Kimball, Mark; Kimura, Masashi; Kitamoto, Shunji; Kitayama, Tetsu; Kohmura, Takayoshi; Kokubun, Motohide; Konami, Saori; Kosaka, Tatsuro; Koujelev, Alexander; Koyama, Katsuji; Krimm, Hans; Kubota, Aya; Kunieda, Hideyo; LaMassa, Stephanie; Laurent, Philippe; Lebrun, François; Leutenegger, Maurice; Limousin, Olivier; Loewenstein, Michael; Long, Knox; Lumb, David; Madejski, Grzegorz; Maeda, Yoshitomo; Makishima, Kazuo; Markevitch, Maxim; Masters, Candace; Matsumoto, Hironori; Matsushita, Kyoko; McCammon, Dan; McGuinness, Daniel; McNamara, Brian; Miko, Joseph; Miller, Jon; Miller, Eric; Mineshige, Shin; Minesugi, Kenji; Mitsuishi, Ikuyuki; Miyazawa, Takuya; Mizuno, Tsunefumi; Mori, Koji; Mori, Hideyuki; Moroso, Franco; Muench, Theodore; Mukai, Koji; Murakami, Hiroshi; Murakami, Toshio; Mushotzky, Richard; Nagano, Housei; Nagino, Ryo; Nakagawa, Takao; Nakajima, Hiroshi; Nakamori, Takeshi; Nakashima, Shinya; Nakazawa, Kazuhiro; Namba, Yoshiharu; Natsukari, Chikara; Nishioka, Yusuke; Nobukawa, Masayoshi; Noda, Hirofumi; Nomachi, Masaharu; O'Dell, Steve; Odaka, Hirokazu; Ogawa, Hiroyuki; Ogawa, Mina; Ogi, Keiji; Ohashi, Takaya; Ohno, Masanori; Ohta, Masayuki; Okajima, Takashi; Okazaki, Tsuyoshi; Ota, Naomi; Ozaki, Masanobu; Paerels, Frits; Paltani, Stéphane; Parmar, Arvind; Petre, Robert; Pinto, Ciro; Pohl, Martin; Pontius, James; Porter, F. S.; Pottschmidt, Katja; Ramsey, Brian; Reis, Rubens; Reynolds, Christopher; Ricci, Claudio; Russell, Helena; Safi-Harb, Samar; Saito, Shinya; Sakai, Shin-ichiro; Sameshima, Hiroaki; Sato, Kosuke; Sato, Rie; Sato, Goro; Sawada, Makoto; Serlemitsos, Peter; Seta, Hiromi; Shibano, Yasuko; Shida, Maki; Shimada, Takanobu; Shirron, Peter; Simionescu, Aurora; Simmons, Cynthia; Smith, Randall; Sneiderman, Gary; Soong, Yang; Stawarz, Lukasz; Sugawara, Yasuharu; Sugita, Satoshi; Szymkowiak, Andrew; Tajima, Hiroyasu; Takahashi, Hiroaki; Takahashi, Hiromitsu; Takeda, Shin-ichiro; Takei, Yoh; Tamagawa, Toru; Tamura, Keisuke; Tamura, Takayuki; Tanaka, Takaaki; Tanaka, Yasuyuki; Tanaka, Yasuo; Tashiro, Makoto; Tawara, Yuzuru; Terada, Yukikatsu; Terashima, Yuichi; Tombesi, Francesco; Tomida, Hiroshi; Tsuboi, Yoko; Tsujimoto, Masahiro; Tsunemi, Hiroshi; Tsuru, Takeshi; Uchida, Hiroyuki; Uchiyama, Hideki; Uchiyama, Yasunobu; Ueda, Yoshihiro; Ueda, Shutaro; Ueno, Shiro; Uno, Shinichiro; Urry, Meg; Ursino, Eugenio; de Vries, Cor; Wada, Atsushi; Watanabe, Shin; Watanabe, Tomomi; Werner, Norbert; White, Nicholas; Wilkins, Dan; Yamada, Shinya; Yamada, Takahiro; Yamaguchi, Hiroya; Yamaoka, Kazutaka; Yamasaki, Noriko; Yamauchi, Makoto; Yamauchi, Shigeo; Yaqoob, Tahir; Yatsu, Yoichi; Yonetoku, Daisuke; Yoshida, Atsumasa; Yuasa, Takayuki; Zhuravleva, Irina; Zoghbi, Abderahmen; ZuHone, John

2014-07-01

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions developed by the Institute of Space and Astronautical Science (ISAS), with a planned launch in 2015. The ASTRO-H mission is equipped with a suite of sensitive instruments with the highest energy resolution ever achieved at E > 3 keV and a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. The simultaneous broad band pass, coupled with the high spectral resolution of ΔE <= 7 eV of the micro-calorimeter, will enable a wide variety of important science themes to be pursued. ASTRO-H is expected to provide breakthrough results in scientific areas as diverse as the large-scale structure of the Universe and its evolution, the behavior of matter in the gravitational strong field regime, the physical conditions in sites of cosmic-ray acceleration, and the distribution of dark matter in galaxy clusters at different redshifts.

Optical, structural, and chemical properties of CR-39 implanted with 5.2 MeV doubly charged carbon ions

NASA Astrophysics Data System (ADS)

Ali, Dilawar; Butt, M. Z.; Ishtiaq, Mohsin; Waqas Khaliq, M.; Bashir, Farooq

2016-11-01

Poly-allyl-diglycol-carbonate (CR-39) specimens were irradiated with 5.2 MeV doubly charged carbon ions using Pelletron accelerator. Ion dose was varied from 5 × 1013 to 5 × 1015 ions cm-2. Optical, structural, and chemical properties were investigated by UV-vis spectroscopy, x-ray diffractometer, and FTIR/Raman spectroscopy, respectively. It was found that optical absorption increases with increasing ion dose. Absorption edge shifts from UV region to visible region. The measured opacity values of pristine and ion implanted CR-39 range from 0.0519 to 4.7959 mm-1 following an exponential growth (9141%) with the increase in ion dose. The values of direct and indirect band gap energy decrease exponentially with an increase in ion dose by 59% and 71%, respectively. However, average refractive index in the visible region increases from 1.443 to 2.864 with an increase in ion dose, by 98%. A linear relation between band gap energy and crystallite size was observed. Both the number of carbon atoms in conjugation length and the number of carbon atoms per cluster increase linearly with the increase in ion dose. FTIR spectra showed that on C+2 ions irradiation, the intensity of all bands decreases gradually without appearance of any new band, indicating degradation of polymer after irradiation. Raman spectra revealed that the density of -CH2- group decreases on C+2 ions irradiation. However, the structure of CR-39 is completely destroyed on irradiation with ion dose 1 × 1015 and 5 × 1015 ions cm-2.

Synthesis, Structure, and Optical Properties of Antiperovskite-Derived Ba2MQ3X (M = As, Sb; Q = S, Se; X = Cl, Br, I) Chalcohalides.

PubMed

Wang, Ruiqi; Zhang, Xian; He, Jianqiao; Bu, Kejun; Zheng, Chong; Lin, Jianhua; Huang, Fuqiang

2018-02-05

Six isostructural antiperovskite-derived chalcohalides, Ba 2 MQ 3 X (M = As, Sb; Q = S, Se; X = Cl, Br, I), crystallizing in the space group Pnma, have been synthesized by solid-state reactions. The crystal structure features a 3D framework with the [XBa 5 ] 9+ disordered square pyramids as building blocks and [MQ 3 ] 3- units filling the interspace. [XBa 5 ] 9+ disordered square pyramids are edge-sharing along [010], derived from the fusing of the two pyramids in octahedral [XBa 6 ] 11+ . Surprisingly, Ba 2 AsS 3 X (X = Cl, Br, I) show almost the same optical band gap of 2.80 eV, and Ba 2 AsSe 3 X (X = Br, I) also have a similar band gap of 2.28 eV. The optical band gap of Ba 2 SbS 3 I is 2.64 eV. First-principles calculations reveal that the optical absorption is attributed to the transitions between Q np at the valence band maximum (VBM) and M np-Q np at the conduction band minimum (CBM). These compounds also possess interesting photoluminescence properties with splitting emission peaks on excitation at 200 nm.

Structural phase transition, narrow band gap, and room-temperature ferromagnetism in [KNbO{sub 3}]{sub 1−x}[BaNi{sub 1/2}Nb{sub 1/2}O{sub 3−δ}]{sub x} ferroelectrics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhou, Wenliang; Yang, Pingxiong, E-mail: pxyang@ee.ecnu.edu.cn; Chu, Junhao

2014-09-15

Structural phase transition, narrow band gap (E{sub g}), and room-temperature ferromagnetism (RTFM) have been observed in the [KNbO{sub 3}]{sub 1−x}[BaNi{sub 1/2}Nb{sub 1/2}O{sub 3−δ}]{sub x} (KBNNO) ceramics. All the samples have single phase perovskite structure, but exhibit a gradual transition behaviour from the orthorhombic to a cubic structure with the increase of x. Raman spectroscopy analysis not only corroborates this doping-induced change in normal structure but also shows the local crystal symmetry for x ≥ 0.1 compositions to deviate from the idealized cubic perovskite structure. A possible mechanism for the observed specific changes in lattice structure is discussed. Moreover, it ismore » noted that KBNNO with compositions x = 0.1–0.3 have quite narrow E{sub g} of below 1.5 eV, much smaller than the 3.2 eV band gap of parent KNbO{sub 3} (KNO), which is due to the increasing Ni 3d electronic states within the gap of KNO. Furthermore, the KBNNO materials present RTFM near a tetragonal to cubic phase boundary. With increasing x from 0 to 0.3, the magnetism of the samples develops from diamagnetism to ferromagnetism and paramagnetism, originating from the ferromagnetic–antiferromagnetic competition. These results are helpful in the deeper understanding of phase transitions, band gap tunability, and magnetism variations in perovskite oxides and show the potential role, such materials can play, in perovskite solar cells and multiferroic applications.« less

X-ray driven channeling acceleration in crystals and carbon nanotubes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shin, Young-Min; Still, Dean A.; Shiltsev, Vladimir

2013-12-01

Acceleration of particles channeling in a crystal by means of diffracted x-rays via Bormann anomalous transmission was conceived for heavy ions and muons by Tajima and Cavenago [Phys. Rev. Lett. 59, 1440 (1987)], which potentially offers an appreciably high field gradient on the order of GV/cm. The theoretical model of the high gradient acceleration has been studied in two kinds of atomic structure, crystals and carbon nanotubes (CNTs), with analytic calculations and electromagnetic eigenmode simulations. A range of acceleration gradients and cutoffs of the x-ray power (the lowest power limit to overcome the Bremsstrahlung radiation losses) are characterized in termsmore » of the lattice constants, unit cell sizes, and photon energies. The parametric analysis indicates that the required x-ray power can be reduced to an order of megawatt by replacing crystals with CNTs. Eventually, the equivalent dielectric approximation of a multi-wall nanotube shows that 250–810 MeV muons can be synchronously coupled with x-rays of 0.65–1.32 keV in the accelerating structure.« less

Electronic Band Structure Tuning of Highly-Mismatched-Alloys for Energy Conversion Applications

NASA Astrophysics Data System (ADS)

Ting, Min

Highly-mismatched alloys: ZnO1-xTe x and GaN1-xSb x are discussed within the context of finding the suitable material for a cost-effective Si-based tandem solar cell (SBTSC). SBTSC is an attractive concept for breaking through the energy conversion efficiency theoretical limit of a single junction solar cell. Combining with a material of 1.8 eV band gap, SBTSC can theoretically achieve energy conversion efficiency > 45%. ZnO and GaN are wide band gap semiconductors. Alloying Te in ZnO and alloying Sb in GaN result in large band gap reduction to < 2 eV from 3.3 eV and 3.4 eV respectively. The band gap reduction is majorly achieved by the upward shift of valence band (VB). Incorporating Te in ZnO modifies the VB of ZnO through the valence-band anticrossing (VBAC) interaction between localized Te states and ZnO VB delocalized states, which forms a Te-derived VB at 1 eV above the host VB. Similar band structure modification is resulted from alloying Sb in GaN. Zn1-xTex and GaN 1-xSbx thin films are synthesized across the whole composition range by pulsed laser deposition (PLD) and low temperature molecular beam epitaxy (LT-MBE) respectively. The electronic band edges of these alloys are measured by synchrotron X-ray absorption, emission, and the X-ray photoelectron spectroscopies. Modeling the optical absorption coefficient with the band anticrossing (BAC) model revealed that the Te and Sb defect levels to be at 0.99 eV and 1.2 eV above the VB of ZnO and GaN respectively. Electrically, Zn1-xTex is readily n-type conductive and GaN1-xSbx is strongly p-type conductive. A heterojunction device of p-type GaN 0.93Sb0.07 with n-type ZnO0.77Te0.93 upper cell (band gap at 1.8 eV) on Si bottom cell is proposed as a promising SBTSC device.

Field induced gap infrared detector

NASA Technical Reports Server (NTRS)

Elliott, C. Thomas (Inventor)

1990-01-01

A tunable infrared detector which employs a vanishing band gap semimetal material provided with an induced band gap by a magnetic field to allow intrinsic semiconductor type infrared detection capabilities is disclosed. The semimetal material may thus operate as a semiconductor type detector with a wavelength sensitivity corresponding to the induced band gap in a preferred embodiment of a diode structure. Preferred semimetal materials include Hg(1-x)Cd(x)Te, x is less than 0.15, HgCdSe, BiSb, alpha-Sn, HgMgTe, HgMnTe, HgZnTe, HgMnSe, HgMgSe, and HgZnSe. The magnetic field induces a band gap in the semimetal material proportional to the strength of the magnetic field allowing tunable detection cutoff wavelengths. For an applied magnetic field from 5 to 10 tesla, the wavelength detection cutoff will be in the range of 20 to 50 micrometers for Hg(1-x)Cd(x)Te alloys with x about 0.15. A similar approach may also be employed to generate infrared energy in a desired band gap and then operating the structure in a light emitting diode or semiconductor laser type of configuration.

E+ Transition in GaAs1-xNx and GaAs1-xBix Due to Isoelectronic-Impurity-Induced Perturbation of the Conduction Band

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fluegel, B.; Mascarenhas, A.; Ptak, A. J.

2007-01-01

An above-band-gap transition E{sub +} is experimentally observed in the dilute GaAs{sub 1-x}Bi{sub x} alloy. Precise measurements at very low dilutions are made of the above-band-gap transition E{sub +} that is observed in GaAs{sub 1-x}N{sub x}, making it possible to compare the behavior of the different isoelectronic traps Bi and N in the common host GaAs with respect to their perturbation to the host electronic structure. We suggest that the origin of the E{sub +} level observed in GaAs is not the isolated isoelectronic impurity level N{sub x}, as is presumed in the band-anticrossing model, but rather the isoelectronic-impurity-induced perturbationmore » of the conduction band L{sub 6}{sup c}.« less

Design study of high gradient, low impedance accelerating structures for the FERMI free electron laser linac upgrade

NASA Astrophysics Data System (ADS)

Shafqat, N.; Di Mitri, S.; Serpico, C.; Nicastro, S.

2017-09-01

The FERMI free-electron laser (FEL) of Elettra Sincrotrone Trieste, Italy, is a user facility driven by a 1.5 GeV 10-50 Hz S-band radiofrequency linear accelerator (linac), and it is based on an external laser seeding scheme that allows lasing at the shortest fundamental wavelength of 4 nm. An increase of the beam energy to 1.8 GeV at a tolerable breakdown rate, and an improvement of the final beam quality is desired in order to allow either lasing at 4 nm with a higher flux, or lasing at shorter wavelengths. This article presents the impedance analysis of newly designed S-band accelerating structures, for replacement of the existing backward travelling wave structures (BTWS) in the last portion of the FERMI linac. The new structure design promises higher accelerating gradient and lower impedance than those of the existing BTWS. Particle tracking simulations show that, with the linac upgrade, the beam relative energy spread, its linear and nonlinear z-correlation internal to the bunch, and the beam transverse emittances can be made smaller than the ones in the present configuration, with expected advantage to the FEL performance. The repercussion of the upgrade on the linac quadrupole magnets setting, for a pre-determined electron beam optics, is also considered.

DFT study of structural and electronic properties of MoS2(1-x)Se2x alloy (x = 0.25)

NASA Astrophysics Data System (ADS)

Gusakova, Julia; Gusakov, Vasilii; Tay, Beng Kang

2018-04-01

First-principles calculations have been performed to study the structural features of the monolayer MoS2(1-x)Se2x (x = 0.25) alloy and its electronic properties. We studied the effects of the relative positions of Se atoms in a real monolayer alloy. It was demonstrated that the distribution of the Se atoms between the top and bottom chalcogen planes was most energetically favorable. For a more probable distribution of Se atoms, a MoS2(1-x)Se2x (x = 0.25) monolayer alloy is a direct semiconductor with a fundamental band gap equal to 2.35 eV (calculated with the GVJ-2e method). We also evaluated the optical band gap of the alloy at 77 K (1.86 eV) and at room temperature (1.80 eV), which was in good agreement with the experimentally measured band gap of 1.79 eV.

Experimental and theoretical XANES of CdSxSe1-x nanostructures

NASA Astrophysics Data System (ADS)

Yiu, Y. M.; Murphy, M. W.; Liu, L.; Hu, Y.; Sham, T. K.

2014-03-01

The morphology and electronic properties of the CdSxSe1-x nanostructures with varying alloy compositions have been acquired experimentally by X-ray Absorption Near-Edge Structures (XANES) at the Cd, Se and S K-edge and L3,2-edges. The theoretical XANES spectra have been calculated using the density functional approach. It is found that the optical band-gap emission of these CdSxSe1-x nano-ribbons can be tuned to the range between that of pure CdS (2.43 eV) and CdSe (1.74 eV) by changing the S and Se ratio. This gradual shift in (optical and structural) properties from CdS character to CdSe character is also seen in the electronic structures. The densities of states and band structures show that with the addition of Se replacing S in CdS, the band gap shrinks. The K and L3,2 edges of Cd, Se, and S of the XANES structures of both the CdS and CdSe in B4 (wurtzite) and B3 (cubic zinc-blende) structures have been calculated and compared.

A SPATIALLY RESOLVED STUDY OF THE SYNCHROTRON EMISSION AND TITANIUM IN TYCHO’S SUPERNOVA REMNANT USING NuSTAR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lopez, Laura A.; Grefenstette, Brian W.; Harrison, Fiona A.

2015-12-01

We report results from deep observations (∼750 ks) of Tycho's supernova remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over several energy bands to identify the regions producing the hardest X-rays and to search for radioactive decay line emission from {sup 44}Ti. We find that the hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where recent Chandra observations have revealed high emissivity “stripes” associated with particles accelerated to the knee of the cosmic-ray spectrum. We do not find evidence of {sup 44}Ti, and we set limits on its presence and distribution within the SNR.more » These limits correspond to an upper-limit {sup 44}Ti mass of M{sub 44} < 2.4 × 10{sup −4} M{sub ⊙} for a distance of 2.3 kpc. We perform a spatially resolved spectroscopic analysis of 66 regions across Tycho. We map the best-fit rolloff frequency of the hard X-ray spectra, and we compare these results to measurements of the shock expansion and ambient density. We find that the highest energy electrons are accelerated at the lowest densities and in the fastest shocks, with a steep dependence of the rolloff frequency with shock velocity. Such a dependence is predicted by models where the maximum energy of accelerated electrons is limited by the age of the SNR rather than by synchrotron losses, but this scenario requires far lower magnetic field strengths than those derived from observations in Tycho. One way to reconcile these discrepant findings is through shock obliquity effects, and future observational work is necessary to explore the role of obliquity in the particle acceleration process.« less

A spatially resolved study of the synchrotron emission and titanium in Tycho's supernova remnant using NuSTAR

DOE PAGES

Lopez, Laura A.; Grefenstette, Brian W.; Reynolds, Stephen P.; ...

2015-11-30

Here, we report results from deep observations (~750 ks) of Tycho's supernova remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over several energy bands to identify the regions producing the hardest X-rays and to search for radioactive decay line emission from 44Ti. We find that the hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where recent Chandra observations have revealed high emissivity "stripes" associated with particles accelerated to the knee of the cosmic-ray spectrum. We do not find evidence of 44Ti, and we set limits on its presence and distribution within the SNR. Furthermore,more » these limits correspond to an upper-limit 44Ti mass of M 44 < 2.4 × 10 -4 M⊙ for a distance of 2.3 kpc. We perform a spatially resolved spectroscopic analysis of 66 regions across Tycho. We map the best-fit rolloff frequency of the hard X-ray spectra, and we compare these results to measurements of the shock expansion and ambient density. We also find that the highest energy electrons are accelerated at the lowest densities and in the fastest shocks, with a steep dependence of the rolloff frequency with shock velocity. Such a dependence is predicted by models where the maximum energy of accelerated electrons is limited by the age of the SNR rather than by synchrotron losses, but this scenario requires far lower magnetic field strengths than those derived from observations in Tycho. One way to reconcile these discrepant findings is through shock obliquity effects, and future observational work is necessary to explore the role of obliquity in the particle acceleration process.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Linaburg, Matthew R.; McClure, Eric T.; Majher, Jackson D.

The structures of the lead halide perovskites CsPbCl3 and CsPbBr3 have been determined from X-ray powder diffraction data to be orthorhombic with Pnma space group symmetry. Their structures are distorted from the cubic structure of their hybrid analogs, CH3NH3PbX3 (X = Cl, Br), by tilts of the octahedra (Glazer tilt system a–b+a–). Substitution of the smaller Rb+ for Cs+ increases the octahedral tilting distortion and eventually destabilizes the perovskite structure altogether. To understand this behavior, bond valence parameters appropriate for use in chloride and bromide perovskites have been determined for Cs+, Rb+, and Pb2+. As the tolerance factor decreases, themore » band gap increases, by 0.15 eV in Cs1–xRbxPbCl3 and 0.20 eV in Cs1–xRbxPbBr3, upon going from x = 0 to x = 0.6. The band gap shows a linear dependence on tolerance factor, particularly for the Cs1–xRbxPbBr3 system. Comparison with the cubic perovskites CH3NH3PbCl3 and CH3NH3PbBr3 shows that the band gaps of the methylammonium perovskites are anomalously large for APbX3 perovskites with a cubic structure. This comparison suggests that the local symmetry of CH3NH3PbCl3 and CH3NH3PbBr3 deviate significantly from the cubic symmetry of the average structure.« less

First-principles calculation of the structure and electronic properties of Fe-substituted Bi2Ti2O7

NASA Astrophysics Data System (ADS)

Huang, Jin-Dou; Zhang, Zhenyi; Lin, Feng; Dong, Bin

2017-12-01

We performed first-principles calculations to investigate the formation energy, geometry structure, and electronic property of Fe-doped Bi2Ti2O7 systems with different Fe doping content. The calculated formation energies indicate that the substitutional configurations of Fe-doping Bi2Ti2O7 are easy to obtain under O-rich growth condition, but their thermodynamic stability decreases with the increase of Fe content. The calculated spin-resolved density of states and band structures indicate that the introduction of Fe into Bi2Ti2O7 brings high spin polarization. The spin-down impurity levels in Fe x Bi2-x Ti2O7 and spin-up impurity levels in Fe x Bi2Ti2-x O7 systems locate in the bottom of conduction band and narrow the band gap significantly, thus leading to the absorption of visible light. Interestingly, the impurity states in Fe x Bi2-x Ti2O7 are the efficient separation center of photogenerated electron and hole, and less affected by Fe doping content, in comparison, the levels of impurity band in Fe x Bi2Ti2-x O7 systems are largely effected by the Fe doping content, and high Fe doping content is the key factor to improve the separating rate of photogenerated electron and hole.

Light curve and SED modeling of the gamma-ray binary 1FGL J1018.6–5856: Constraints on the orbital geometry and relativistic flow

DOE PAGES

An, Hongjun; Romani, Roger W.

2017-04-04

We present broadband spectral energy distributions and light curves of the gamma-ray binary 1FGL J1018.6–5856 measured in the X-ray and the gamma-ray bands. We find that the orbital modulation in the low-energy gamma-ray band is similar to that in the X-ray band, suggesting a common spectral component. However, above a GeV the orbital light curve changes significantly. We suggest that the GeV band contains significant flux from a pulsar magnetosphere, while the X-ray to TeV light curves are dominated by synchrotron and Compton emission from an intrabinary shock (IBS). We find that a simple one-zone model is inadequate to explainmore » the IBS emission, but that beamed Synchrotron-self Compton radiation from adiabatically accelerated plasma in the shocked pulsar wind can reproduce the complex multiband light curves, including the variable X-ray spike coincident with the gamma-ray maximum. Furthermore, the model requires an inclination of ~50° and an orbital eccentricity of ~0.35, consistent with the limited constraints from existing optical observations. This picture motivates searches for pulsations from the energetic young pulsar powering the wind shock.« less

Light curve and SED modeling of the gamma-ray binary 1FGL J1018.6–5856: Constraints on the orbital geometry and relativistic flow

DOE Office of Scientific and Technical Information (OSTI.GOV)

An, Hongjun; Romani, Roger W.

We present broadband spectral energy distributions and light curves of the gamma-ray binary 1FGL J1018.6–5856 measured in the X-ray and the gamma-ray bands. We find that the orbital modulation in the low-energy gamma-ray band is similar to that in the X-ray band, suggesting a common spectral component. However, above a GeV the orbital light curve changes significantly. We suggest that the GeV band contains significant flux from a pulsar magnetosphere, while the X-ray to TeV light curves are dominated by synchrotron and Compton emission from an intrabinary shock (IBS). We find that a simple one-zone model is inadequate to explainmore » the IBS emission, but that beamed Synchrotron-self Compton radiation from adiabatically accelerated plasma in the shocked pulsar wind can reproduce the complex multiband light curves, including the variable X-ray spike coincident with the gamma-ray maximum. Furthermore, the model requires an inclination of ~50° and an orbital eccentricity of ~0.35, consistent with the limited constraints from existing optical observations. This picture motivates searches for pulsations from the energetic young pulsar powering the wind shock.« less

Light Curve and SED Modeling of the Gamma-Ray Binary 1FGL J1018.6–5856: Constraints on the Orbital Geometry and Relativistic Flow

DOE Office of Scientific and Technical Information (OSTI.GOV)

An, Hongjun; Romani, Roger W., E-mail: hjan@chungbuk.ac.kr

We present broadband spectral energy distributions and light curves of the gamma-ray binary 1FGL J1018.6−5856 measured in the X-ray and the gamma-ray bands. We find that the orbital modulation in the low-energy gamma-ray band is similar to that in the X-ray band, suggesting a common spectral component. However, above a GeV the orbital light curve changes significantly. We suggest that the GeV band contains significant flux from a pulsar magnetosphere, while the X-ray to TeV light curves are dominated by synchrotron and Compton emission from an intrabinary shock (IBS). We find that a simple one-zone model is inadequate to explainmore » the IBS emission, but that beamed Synchrotron-self Compton radiation from adiabatically accelerated plasma in the shocked pulsar wind can reproduce the complex multiband light curves, including the variable X-ray spike coincident with the gamma-ray maximum. The model requires an inclination of ∼50° and an orbital eccentricity of ∼0.35, consistent with the limited constraints from existing optical observations. This picture motivates searches for pulsations from the energetic young pulsar powering the wind shock.« less

Determination of the optical band-gap energy of cubic and hexagonal boron nitride using luminescence excitation spectroscopy

NASA Astrophysics Data System (ADS)

Evans, D. A.; McGlynn, A. G.; Towlson, B. M.; Gunn, M.; Jones, D.; Jenkins, T. E.; Winter, R.; Poolton, N. R. J.

2008-02-01

Using synchrotron-based luminescence excitation spectroscopy in the energy range 4-20 eV at 8 K, the indirect Γ-X optical band-gap transition in cubic boron nitride is determined as 6.36 ± 0.03 eV, and the quasi-direct band-gap energy of hexagonal boron nitride is determined as 5.96 ± 0.04 eV. The composition and structure of the materials are self-consistently established by optically detected x-ray absorption spectroscopy, and both x-ray diffraction and Raman measurements on the same samples give independent confirmation of their chemical and structural purity: together, the results are therefore considered as providing definitive measurements of the optical band-gap energies of the two materials.

Electronic structure modifications and band gap narrowing in Zn0.95V0.05O

NASA Astrophysics Data System (ADS)

Ahad, Abdul; Majid, S. S.; Rahman, F.; Shukla, D. K.; Phase, D. M.

2018-04-01

We present here, structural, optical and electronic structure studies on Zn0.95V0.05O, synthesized using solid state method. Rietveld refinement of x-ray diffraction pattern indicates no considerable change in the lattice of doped ZnO. The band gap of doped sample, as calculated by Kubelka-Munk transformed reflectance spectra, has been found reduced compared to pure ZnO. Considerable changes in absorbance in UV-Vis range is observed in doped sample. V doping induced decrease in band gap is supported by x-ray absorption spectroscopy measurements. It is experimentally confirmed that conduction band edge in Zn0.95V0.05O has shifted towards Fermi level than in pure ZnO.

Systematic research on Ag2X (X = O, S, Se, Te) as visible and near-infrared light driven photocatalysts and effects of their electronic structures

NASA Astrophysics Data System (ADS)

Jiang, Wei; Wu, Zhaomei; Zhu, Yingming; Tian, Wen; Liang, Bin

2018-01-01

Four silver chalcogen compounds, Ag2O, Ag2S, Ag2Se and Ag2Te, can be utilized as visible-light-driven photocatalysts. In this research, the electronic structures of these compounds were analyzed by simulation and experiments to systematically reveal the relationship between photocatalytic performance and energetic structure. All four chalcogenides exhibited interesting photocatalytic activities under ultraviolet, visible and near-infrared light. However, their photocatalytic performances and stability significantly depended on the band gap width, and the valence band and conduct band position, which was determined by their composition. Increasing the X atomic number from O to Te resulted in the upward movement of the valence band top and the conduct band bottom, which resulted in narrower band gaps, a wider absorption spectrum, a weaker photo-oxidization capacity, a higher recombination probability of hole and electron pairs, lower quantum efficiency, and worse stability. Among them, Ag2O has the highest photocatalytic performance and stability due to its widest band gap and lowest position of VB and CB. The combined action of photogenerated holes and different radicals, depending on the different electronic structures, including anion ozone radical, hydroxide radical, and superoxide radical, was observed and understood. The results of experimental observations and simulations of the four silver chalcogen compounds suggested that a proper electronic structure is necessary to obtain a balance between photocatalytic performance and absorbable light region in the development of new photocatalysts.

Measurement of InAsSb bandgap energy and InAs/InAsSb band edge positions using spectroscopic ellipsometry and photoluminescence spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Webster, P. T.; Riordan, N. A.; Liu, S.

2015-12-28

The structural and optical properties of lattice-matched InAs{sub 0.911}Sb{sub 0.089} bulk layers and strain-balanced InAs/InAs{sub 1−x}Sb{sub x} (x ∼ 0.1–0.4) superlattices grown on (100)-oriented GaSb substrates by molecular beam epitaxy are examined using X-ray diffraction, spectroscopic ellipsometry, and temperature dependent photoluminescence spectroscopy. The photoluminescence and ellipsometry measurements determine the ground state bandgap energy and the X-ray diffraction measurements determine the layer thickness and mole fraction of the structures studied. Detailed modeling of the X-ray diffraction data is employed to quantify unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattices. A Kronig-Penney model of the superlattice miniband structure ismore » used to analyze the valence band offset between InAs and InAsSb, and hence the InAsSb band edge positions at each mole fraction. The resulting composition dependence of the bandgap energy and band edge positions of InAsSb are described using the bandgap bowing model; the respective low and room temperature bowing parameters for bulk InAsSb are 938 and 750 meV for the bandgap, 558 and 383 meV for the conduction band, and −380 and −367 meV for the valence band.« less

Inter-band optoelectronic properties in quantum dot structure of low band gap III-V semiconductors

NASA Astrophysics Data System (ADS)

Dey, Anup; Maiti, Biswajit; Chanda Sarkar, Debasree

2014-04-01

A generalized theory is developed to study inter-band optical absorption coefficient (IOAC) and material gain (MG) in quantum dot structures of narrow gap III-V compound semiconductor considering the wave-vector (k→) dependence of the optical transition matrix element. The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted for analysis of the IOAC and MG taking InAs, InSb, Hg1-xCdxTe, and In1-xGaxAsyP1-y lattice matched to InP, as example of III-V compound semiconductors, having varied split-off energy band compared to their bulk band gap energy. It has been found that magnitude of the IOAC for quantum dots increases with increasing incident photon energy and the lines of absorption are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of energy band parameters. The results show a significant deviation to the MG spectrum of narrow-gap materials having band nonparabolicity compared to the parabolic band model approximations. The results reflect the important role of valence band split-off energies in these narrow gap semiconductors.

A time dependent approach to model X-ray and γ-ray light curves of Mrk 421 observed during the flare in February 2010

NASA Astrophysics Data System (ADS)

Singh, K. K.; Sahayanathan, S.; Sinha, A.; Bhatt, N.; Tickoo, A. K.; Yadav, K. K.; Rannot, R. C.; Chandra, P.; Venugopal, K.; Marandi, P.; Kumar, N.; Goyal, H. C.; Goyal, A.; Agarwal, N. K.; Kothari, M.; Chanchalani, K.; Dhar, V. K.; Chouhan, N.; Bhat, C. K.; Koul, M. K.; Koul, R.

2017-07-01

Strong X-ray and γ-ray flares have been detected in February 2010 from the high synchrotron peaked blazar Mrk 421 (z = 0.031). With the motivation of understanding the physics involved in this flaring activity, we study the variability of the source in X-ray and γ-ray energy bands during the period February 10-23, 2010 (MJD 55237-55250). We use near simultaneous X-ray data collected by MAXI, Swift-XRT and γ-ray data collected by Fermi-LAT and TACTIC along with the optical V-band observations by SPOLat Steward Observatory. We observe that the variation in the one day averaged flux from the source during the flare is characterized by fast rise and slow decay. Besides, the TeV γ-ray flux shows a strong correlation with the X-ray flux, suggesting the former to be an outcome of synchrotron self Compton emission process. To model the observed X-ray and γ-ray light curves, we numerically solve the kinetic equation describing the evolution of particle distribution in the emission region. The injection of particle distribution into the emission region, from the putative acceleration region, is assumed to be a time dependent power law. The synchrotron and synchrotron self Compton emission from the evolving particle distribution in the emission region are used to reproduce the X-ray and γ-ray flares successfully. Our study suggests that the flaring activity of Mrk 421 can be an outcome of an efficient acceleration process associated with the increase in underlying non-thermal particle distribution.

Recent Developments in Quantum-Well Infrared Photodetectors

NASA Technical Reports Server (NTRS)

Gunapala, S. D.; Bandara, K. M. S. V.

1995-01-01

Intrinsic infrared (IR) detectors in the long wavelength range (8-20 Am) are based on an optically excited interband transition, which promotes an electron across the band gap (E(sub g)) from the valence band to the conduction band as shown. These photoelectrons can be collected efficiently, thereby producing a photocurrent in the external circuit. Since the incoming photon has to promote an electron from the valence band to the conduction band, the energy of the photon (h(sub upsilon)) must be higher than the E(sub g) of the photosensitive material. Therefore, the spectral response of the detectors can be controlled by controlling the E(sub g) of the photosensitive material. Examples for such materials are Hg(1-x), Cd(x), Te, and Pb(1-x), Sn(x), Te, in which the energy gap can be controlled by varying x. This means detection of very-long-wavelength IR radiation up to 20 microns requires small band gaps down to 62 meV. It is well known that these low band gap materials, characterized by weak bonding and low melting points, are more difficult to grow and process than large-band gap semiconductors such as GaAs. These difficulties motivate the exploration of utilizing the intersub-band transitions in multiquantum well (MQW) structures made of more refractory large-band gap semiconductors. The idea of using MQW structures to detect IR radiation can be explained by using the basic principles of quantum mechanics. The quantum well is equivalent to the well-known particle in a box problem in quantum mechanics, which can be solved by the time independent Schroudiner equation.

Crystallographic and optical properties and band diagrams of CuGaS2 and CuGa5S8 phases in Cu-poor Cu2S-Ga2S3 pseudo-binary system

NASA Astrophysics Data System (ADS)

Maeda, Tsuyoshi; Yu, Ying; Chen, Qing; Ueda, Kenta; Wada, Takahiro

2017-04-01

We synthesized Cu-poor Cu-Ga-S samples such, as CuGaS2 and CuGa5S8 with the composition of (1 - x)Cu2S-(x)Ga2S3 with 0.5 ≤ x ≤ 1.0, by a mechanochemical process and sequential heating. The crystal structure changes from tetragonal chalcopyrite-type CuGaS2 (0.5 ≤ x ≤ 0.55) to tetragonal stannite-type CuGa5S8 (x = 0.8). For samples with 0.60 ≤ x ≤ 0.75, the diffraction peaks were identified to be those of a mixed phase of the chalcopyrite- and stannite-type structures. The band-gap energies of Cu-poor Cu-Ga-S samples increase in a stepwise manner with increasing x. The band-gap energy of CuGa5S8 (x = 0.8) with the tetragonal stannite-type structure is approximately 2.66 eV, which is wider than that of chalcopyrite-type CuGaS2 (2.45 eV). The energy levels of valence band maxima (VBMs) were estimated from the ionization energies measured by photoemission yield spectroscopy (PYS). The energy levels of the VBM and conduction band minimum (CBM) of the Cu-poor Cu-Ga-S samples decrease significantly with increasing x (decreasing Cu/Ga ratio). The energy level of the VBM of CuGaS2 (-5.8 eV) is considerably deeper than those of CuInSe2 (-5.2 eV) and CuInS2 (-5.5 eV). The VBM of stannite-type CuGa5S8 with x = 0.8 (-6.4 eV) is much deeper than that of chalcopyrite-type CuGaS2 (-5.8 eV) and stannite-type CuIn3Se5 (-5.6 eV). In order to understand the band structures of chalcopyrite-type CuGaS2 and stannite-type CuGa5S8, we performed first-principles calculations using the Heyd-Scuseria-Ernzerhof (HSE06), nonlocal screened hybrid density functional method. The theoretical band-gap energy of stannite-type CuGa5S8 (2.2 eV) is wider than that of chalcopyrite-type CuGaS2 (2.0 eV). Both the theoretical and experimental band gaps of stannite-type CuGa5S8 are about 0.2 eV wider than those of chalcopyrite-type CuGaS2.

Optical spectra and band structure of Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals: experiment and theory.

PubMed

Reshak, A H; Parasyuk, O V; Fedorchuk, A O; Kamarudin, H; Auluck, S; Chyský, J

2013-12-05

Theoretical and experimental studies of the Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals are performed. These crystals possess a lot of intrinsic defects which are responsible for their optoelectronic features. The theoretical investigations were performed by means of DFT calculations using different exchange-correlation potentials. The experimental studies were carried out using the modulated VUV ellipsometry for dielectric constants and birefringence studies. The comparison of the structure obtained from X-ray with the theoretically optimized structure is presented. The crucial role of the intrinsic defect states is manifested in the choice of the exchange correlation potential used. The data may be applicable for a large number of the ternary chalcogenides which are sensitive to the presence of the local disordered states near the band edges.

X-ray coherent scattering tomography of textured material (Conference Presentation)

NASA Astrophysics Data System (ADS)

Zhu, Zheyuan; Pang, Shuo

2017-05-01

Small-angle X-ray scattering (SAXS) measures the signature of angular-dependent coherently scattered X-rays, which contains richer information in material composition and structure compared to conventional absorption-based computed tomography. SAXS image reconstruction method of a 2 or 3 dimensional object based on computed tomography, termed as coherent scattering computed tomography (CSCT), enables the detection of spatially-resolved, material-specific isotropic scattering signature inside an extended object, and provides improved contrast for medical diagnosis, security screening, and material characterization applications. However, traditional CSCT methods assumes materials are fine powders or amorphous, and possess isotropic scattering profiles, which is not generally true for all materials. Anisotropic scatters cannot be captured using conventional CSCT method and result in reconstruction errors. To obtain correct information from the sample, we designed new imaging strategy which incorporates extra degree of detector motion into X-ray scattering tomography for the detection of anisotropic scattered photons from a series of two-dimensional intensity measurements. Using a table-top, narrow-band X-ray source and a panel detector, we demonstrate the anisotropic scattering profile captured from an extended object and the reconstruction of a three-dimensional object. For materials possessing a well-organized crystalline structure with certain symmetry, the scatter texture is more predictable. We will also discuss the compressive schemes and implementation of data acquisition to improve the collection efficiency and accelerate the imaging process.

Structural and electronic properties of monolayer group III monochalcogenides

NASA Astrophysics Data System (ADS)

Demirci, S.; Avazlı, N.; Durgun, E.; Cahangirov, S.

2017-03-01

We investigate the structural, mechanical, and electronic properties of the two-dimensional hexagonal structure of group III-VI binary monolayers, M X (M =B , Al, Ga, In and X =O , S, Se, Te) using first-principles calculations based on the density functional theory. The structural optimization calculations and phonon spectrum analysis indicate that all of the 16 possible binary compounds are thermally stable. In-plane stiffness values cover a range depending on the element types and can be as high as that of graphene, while the calculated bending rigidity is found to be an order of magnitude higher than that of graphene. The obtained electronic band structures show that M X monolayers are indirect band-gap semiconductors. The calculated band gaps span a wide optical spectrum from deep ultraviolet to near infrared. The electronic structure of oxides (M O ) is different from the rest because of the high electronegativity of oxygen atoms. The dispersions of the electronic band edges and the nature of bonding between atoms can also be correlated with electronegativities of constituent elements. The unique characteristics of group III-VI binary monolayers can be suitable for high-performance device applications in nanoelectronics and optics.

Local expansion flows of galaxies: quantifying acceleration effect of dark energy

NASA Astrophysics Data System (ADS)

Chernin, A. D.; Teerikorpi, P.

2013-08-01

The nearest expansion flow of galaxies observed around the Local group is studied as an archetypical example of the newly discovered local expansion flows around groups and clusters of galaxies in the nearby Universe. The flow is accelerating due to the antigravity produced by the universal dark energy background. We introduce a new acceleration measure of the flow which is the dimensionless ``acceleration parameter" Q (x) = x - x-2 depending on the normalized distance x only. The parameter is zero at the zero-gravity distance x = 1, and Q(x) ∝ x, when x ≫ 1. At the distance x = 3, the parameter Q = 2.9. Since the expansion flows have a self-similar structure in normalized variables, we expect that the result is valid as well for all the other expansion flows around groups and clusters of galaxies on the spatial scales from ˜ 1 to ˜ 10 Mpc everywhere in the Universe.

Shear banding leads to accelerated aging dynamics in a metallic glass

NASA Astrophysics Data System (ADS)

Küchemann, Stefan; Liu, Chaoyang; Dufresne, Eric M.; Shin, Jeremy; Maaß, Robert

2018-01-01

Traditionally, strain localization in metallic glasses is related to the thickness of the shear defect, which is confined to the nanometer scale. Using site-specific x-ray photon correlation spectroscopy, we reveal significantly accelerated relaxation dynamics around a shear band in a metallic glass at a length scale that is orders of magnitude larger than the defect itself. The relaxation time in the shear-band vicinity is up to ten times smaller compared to the as-cast matrix, and the relaxation dynamics occurs in a characteristic three-stage aging response that manifests itself in the temperature-dependent shape parameter known from classical stretched exponential relaxation dynamics of disordered materials. We demonstrate that the time-dependent correlation functions describing the aging at different temperatures can be captured and collapsed using simple scaling functions. These insights highlight how a ubiquitous nanoscale strain-localization mechanism in metallic glasses leads to a fundamental change of the relaxation dynamics at the mesoscale.

Shear banding leads to accelerated aging dynamics in a metallic glass

DOE Office of Scientific and Technical Information (OSTI.GOV)

Küchemann, Stefan; Liu, Chaoyang; Dufresne, Eric M.

Traditionally, strain localization in metallic glasses is related to the thickness of the shear defect, which is confined to the nanometer scale. In this study, using site-specific x-ray photon correlation spectroscopy (XPCS), we reveal significantly accelerated relaxation dynamics around a shear band in a metallic glass at a length scale that is orders of magnitude larger than the defect itself. The relaxation time in the shear-band vicinity is up to ten-times smaller compared to the as-cast matrix, and the relaxation dynamics occurs in a characteristic three-stage aging response that manifests itself in the temperature-dependent shape parameter known from classical stretchedmore » exponential relaxation dynamics of disordered materials. We demonstrate that the time-dependent correlation functions describing the aging at different temperatures can be captured and collapsed using simple scaling functions. Finally, these insights highlight how an ubiquitous nano-scale strain-localization mechanism in metallic glasses leads to a fundamental change of the relaxation dynamics at the mesoscale.« less

Shear banding leads to accelerated aging dynamics in a metallic glass

DOE PAGES

Küchemann, Stefan; Liu, Chaoyang; Dufresne, Eric M.; ...

2018-01-11

Traditionally, strain localization in metallic glasses is related to the thickness of the shear defect, which is confined to the nanometer scale. In this study, using site-specific x-ray photon correlation spectroscopy (XPCS), we reveal significantly accelerated relaxation dynamics around a shear band in a metallic glass at a length scale that is orders of magnitude larger than the defect itself. The relaxation time in the shear-band vicinity is up to ten-times smaller compared to the as-cast matrix, and the relaxation dynamics occurs in a characteristic three-stage aging response that manifests itself in the temperature-dependent shape parameter known from classical stretchedmore » exponential relaxation dynamics of disordered materials. We demonstrate that the time-dependent correlation functions describing the aging at different temperatures can be captured and collapsed using simple scaling functions. Finally, these insights highlight how an ubiquitous nano-scale strain-localization mechanism in metallic glasses leads to a fundamental change of the relaxation dynamics at the mesoscale.« less

Design study of an S-band RF cavity of a dual-energy electron LINAC for the CIS

NASA Astrophysics Data System (ADS)

Lee, Byeong-No; Park, Hyungdal; Song, Ki-baek; Li, Yonggui; Lee, Byung Cheol; Cha, Sung-su; Lee, Jong-Chul; Shin, Seung-Wook; Chai, Jong-seo

2014-01-01

The design of a resonance frequency (RF) cavity for the dual-energy S-band electron linear accelerator (LINAC) has been carried out for the cargo inspection system (CIS). This Standing-wave-type RF cavity is operated at a frequency under the 2856-MHz resonance frequency and generates electron beams of 9 MeV (high mode) and 6 MeV (low mode). The electrons are accelerated from the initial energy of the electron gun to the target energy (9 or 6 MeV) inside the RF cavity by using the RF power transmitted from a 5.5-MW-class klystron. Then, electron beams with a 1-kW average power (both high mode and low mode) bombard an X-ray target a 2-mm spot size. The proposed accelerating gradient was 13 MV/m, and the designed Q value was about 7100. On going research on 15-MeV non-destructive inspections for military or other applications is presented.

X -band rf driven free electron laser driver with optics linearization

DOE PAGES

Sun, Yipeng; Emma, Paul; Raubenheimer, Tor; ...

2014-11-13

In this paper, a compact hard X-ray free electron lasers (FEL) design is proposed with all X-band rf acceleration and two stage bunch compression. It eliminates the need of a harmonic rf linearization section by employing optics linearization in its first stage bunch compression. Quadrupoles and sextupoles are employed in a bunch compressor one (BC1) design, in such a way that second order longitudinal dispersion of BC1 cancels the second order energy correlation in the electron beam. Start-to-end 6-D simulations are performed with all the collective effects included. Emittance growth in the horizontal plane due to coherent synchrotron radiation ismore » investigated and minimized, to be on a similar level with the successfully operating Linac coherent light source (LCLS). At a FEL radiation wavelength of 0.15 nm, a saturation length of 40 meters can be achieved by employing an undulator with a period of 1.5 cm. Without tapering, a FEL radiation power above 10 GW is achieved with a photon pulse length of 50 fs, which is LCLS-like performance. The overall length of the accelerator plus undulator is around 250 meters which is much shorter than the LCLS length of 1230 meters. That makes it possible to build hard X-ray FEL in a laboratory with limited size.« less

Local Bonding Influence on the Band Edge and Band Gap Formation in Quaternary Chalcopyrites.

PubMed

Miglio, Anna; Heinrich, Christophe P; Tremel, Wolfgang; Hautier, Geoffroy; Zeier, Wolfgang G

2017-09-01

Quaternary chalcopyrites have shown to exhibit tunable band gaps with changing anion composition. Inspired by these observations, the underlying structural and electronic considerations are investigated using a combination of experimentally obtained structural data, molecular orbital considerations, and density functional theory. Within the solid solution Cu 2 ZnGeS 4- x Se x , the anion bond alteration parameter changes, showing larger bond lengths for metal-selenium than for metal-sulfur bonds. The changing bonding interaction directly influences the valence and conduction band edges, which result from antibonding Cu-anion and Ge-anion interactions, respectively. The knowledge of the underlying bonding interactions at the band edges can help design properties of these quaternary chalcopyrites for photovoltaic and thermoelectric applications.

Multifunctional Binary Monolayers Ge xP y: Tunable Band Gap, Ferromagnetism, and Photocatalyst for Water Splitting.

PubMed

Li, Pengfei; Zhang, Wei; Li, Dongdong; Liang, Changhao; Zeng, Xiao Cheng

2018-06-04

The most stable structures of two-dimensional Ge x P y and Ge x As y monolayers with different stoichiometries (e.g., GeP, GeP 2 , and GeP 3 ) are explored systematically through the combination of the particle-swarm optimization technique and density functional theory optimization. For GeP 3 , we show that the newly predicted most stable C2/ m structure is 0.16 eV/atom lower in energy than the state-of-the-art P3̅m1 structure reported previously ( Nano Lett. 2017, 17, 1833). The computed electronic band structures suggest that all the stable and metastable monolayers of Ge x P y are semiconductors with highly tunable band gaps under the biaxial strain, allowing strain engineering of their band gaps within nearly the whole visible-light range. More interestingly, the hole doping can convert the C2/ m GeP 3 monolayer from nonmagnetic to ferromagnetic because of its unique valence band structure. For the GeP 2 monolayer, the predicted most stable Pmc2 1 structure is a (quasi) direct-gap semiconductor that possesses a high electron mobility of ∼800 cm 2 V -1 s -1 along the k a direction, which is much higher than that of MoS 2 (∼200 cm 2 V -1 s -1 ). More importantly, the Pmc2 1 GeP 2 monolayer not only can serve as an n-type channel material in field-effect transistors but also can be an effective catalyst for splitting water.

Prototype Solid State Induction Modulator for SLAC NLC

NASA Astrophysics Data System (ADS)

Cassel, R. L.; DeLamare, J. E.; Nguyen, M. N.; Pappas, G. C.; Cook, E.

2002-08-01

The Next Linear Collider accelerator proposal at SLAC requires a high efficiency, highly reliable, and low cost pulsed power modulator to drive the X band klystrons. The present NLC envisions a solid-state induction modulator design to drive up to 8 klystrons to 500kV for 3muS at 120 PPS with one modulator (>1,000 megawatt pulse, 500kW average). A prototype modulator is presently under construction, which well power 4 each 5045 SLAC klystron to greater than 380 kV for 3muS (>600 megawatt pulse, >300 kW Ave.). The modulator will be capable of driving the 8 each X band klystrons when they become available. The paper covers the design, construction, fabrication and preliminary testing of the prototype modulator.

The optical, ultraviolet, and X-ray structure of the quasar HE 0435–1223

DOE Office of Scientific and Technical Information (OSTI.GOV)

Blackburne, Jeffrey A.; Kochanek, Christopher S.; Chen, Bin

2014-07-10

Microlensing has proved an effective probe of the structure of the innermost regions of quasars and an important test of accretion disk models. We present light curves of the lensed quasar HE 0435–1223 in the R band and in the ultraviolet (UV), and consider them together with X-ray light curves in two energy bands that are presented in a companion paper. Using a Bayesian Monte Carlo method, we constrain the size of the accretion disk in the rest-frame near- and far-UV, and constrain for the first time the size of the X-ray emission regions in two X-ray energy bands. Themore » R-band scale size of the accretion disk is about 10{sup 15.23} cm (∼23r{sub g}), slightly smaller than previous estimates, but larger than would be predicted from the quasar flux. In the UV, the source size is weakly constrained, with a strong prior dependence. The UV to R-band size ratio is consistent with the thin disk model prediction, with large error bars. In soft and hard X-rays, the source size is smaller than ∼10{sup 14.8} cm (∼10r{sub g} ) at 95% confidence. We do not find evidence of structure in the X-ray emission region, as the most likely value for the ratio of the hard X-ray size to the soft X-ray size is unity. Finally, we find that the most likely value for the mean mass of stars in the lens galaxy is ∼0.3 M{sub ☉}, consistent with other studies.« less

Highly mismatched GaN1-x Sb x alloys: synthesis, structure and electronic properties

NASA Astrophysics Data System (ADS)

Yu, K. M.; Sarney, W. L.; Novikov, S. V.; Segercrantz, N.; Ting, M.; Shaw, M.; Svensson, S. P.; Martin, R. W.; Walukiewicz, W.; Foxon, C. T.

2016-08-01

Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaN x Sb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-x Sb x HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-x Sb x HMA over the entire composition range is well described by a modified BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaN x Sb1-x is general and is applicable to any HMA.

The electronic structure of lithium metagallate.

PubMed

Johnson, N W; McLeod, J A; Moewes, A

2011-11-09

Herein we present a study of the electronic structure of lithium metagallate (LiGaO(2)), a material of interest in the field of optoelectronics. We use soft x-ray spectroscopy to probe the electronic structure of both the valence and conduction bands and compare our measurements to ab initio density functional theory calculations. We use several different exchange-correlation functionals, but find that no single theoretical approach used herein accurately quantifies both the band gap and the Ga 3d(10) states in LiGaO(2). We derive a band gap of 5.6 eV, and characterize electron hybridization in both the valence and conduction bands. Our study of the x-ray spectra may prove useful in analysing spectra from more complicated LiGaO(2) heterostructures. © 2011 IOP Publishing Ltd

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys

NASA Technical Reports Server (NTRS)

Yu, Kin Man; Wu, J.; Walukiewicz, W.; Ager, J. W.; Haller, E. E.; Miotkowski, I.; Su, Ching-Hua; Curreri, Peter A. (Technical Monitor)

2001-01-01

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(sub x)V(sub 1-x) alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(sub x)V(sub 1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(sub x)Te(sub 1-x) and ZnSe(sub y)Te(sub 1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(sub y)Te(sub 1-y) alloys in the entire composition range, y between 0 and 1. The samples used in this study are bulk ZnSe(sub y)Te(sub 1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k(sup dot)p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(sub y)Te(sub 1-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys

NASA Technical Reports Server (NTRS)

Yu, Kin Man; Wu, J.; Walukiewicz, W.; Ager, J. W.; Haller, E. E.; Miotkowski, I.; Ramdas, A.; Su, Ching-Hua; Whitaker, Ann F. (Technical Monitor)

2001-01-01

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(x)V(1-x), alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(x),V(1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(x)Te(l-x) and ZnSe(Y)Te(1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(y)Te(1-y) alloys in the entire composition range, 0 less than or equal to y less than or equal to 1. The samples used in this study are bulk ZnSe(y)Te(1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(y)Te(l-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Understanding the optical properties of ZnO1-xSx and ZnO1-xSex alloys

NASA Astrophysics Data System (ADS)

Baldissera, Gustavo; Persson, Clas

2016-01-01

ZnO1-xYx with chalcogen element Y exhibits intriguing optoelectronic properties as the alloying strongly impacts the band-gap energy Eg(x). In this work, we analyze and compare the electronic structures and the dielectric responses of Zn(O,S) and Zn(O,Se) alloys by means of the density functional theory and the partially self-consistent GW approach. We model the crystalline stability from the total energies, and the results indicate that Zn(O,S) is more stable as alloy than Zn(O,Se). We demonstrate also that ion relaxation strongly affects total energies, and that the band-gap bowing depends primarily on local relaxation of the bonds. Moreover, we show that the composition dependent band-gap needs to be analyzed by the band anti-crossing model for small alloying concentration, while the alloying band-bowing model is accurate for strong alloying. We find that the Se-based alloys have a stronger change in the band-gap energy (for instance, ΔEg(0.50) = Eg(ZnO) - Eg(x = 0.50) ≈ 2.2 eV) compared with that of the S-based alloy (ΔEg(0.50) = 1.2 eV), mainly due to a stronger relaxation of the Zn-anion bonds that affects the electronic structure near the band edges. The optical properties of the alloys are discussed in terms of the complex dielectric function ɛ(ω) = ɛ1(ω) + iɛ2(ω) and the absorption coefficient α(ω). While the large band-gap bowing directly impacts the low-energy absorption spectra, the high-frequency dielectric constant ɛ∞ is correlated to the intensity of the dielectric response at energies above 4 eV. Therefore, the dielectric constant is only weakly affected by the non-linear band-gap variation. Despite strong structural relaxation, the high absorption coefficients of the alloys demonstrate that the alloys have well-behaved optoelectronic properties.

Energies of the X- and L-valleys in In{sub 0.53}Ga{sub 0.47}As from electronic structure calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Greene-Diniz, Gabriel; Greer, J. C.; Fischetti, M. V.

2016-02-07

Several theoretical electronic structure methods are applied to study the relative energies of the minima of the X- and L-conduction-band satellite valleys of In{sub x}Ga{sub 1−x}As with x = 0.53. This III-V semiconductor is a contender as a replacement for silicon in high-performance n-type metal-oxide-semiconductor transistors. The energy of the low-lying valleys relative to the conduction-band edge governs the population of channel carriers as the transistor is brought into inversion, hence determining current drive and switching properties at gate voltages above threshold. The calculations indicate that the position of the L- and X-valley minima are ∼1 eV and ∼1.2 eV, respectively, higher in energymore » with respect to the conduction-band minimum at the Γ-point.« less

Katherine E. Weimer Award: X-ray light sources from laser-plasma and laser-electron interaction: development and applications

NASA Astrophysics Data System (ADS)

Albert, Felicie

2017-10-01

Bright sources of x-rays, such as synchrotrons and x-ray free electron lasers (XFEL) are transformational tools for many fields of science. They are used for biology, material science, medicine, or industry. Such sources rely on conventional particle accelerators, where electrons are accelerated to gigaelectronvolts (GeV) energies. The accelerated particles are wiggled in magnetic structures to emit x-ray radiation that is commonly used for molecular crystallography, fluorescence studies, chemical analysis, medical imaging, and many other applications. One of the drawbacks of these machines is their size and cost, because electric field gradients are limited to about 100 V/M in conventional accelerators. Particle acceleration in laser-driven plasmas is an alternative to generate x-rays via betatron emission, Compton scattering, or bremsstrahlung. A plasma can sustain electrical fields many orders of magnitude higher than that in conventional radiofrequency accelerator structures. When short, intense laser pulses are focused into a gas, it produces electron plasma waves in which electrons can be trapped and accelerated to GeV energies. X-ray sources, driven by electrons from laser-wakefield acceleration, have unique properties that are analogous to synchrotron radiation, with a 1000-fold shorter pulse. An important use of x-rays from laser plasma accelerators is in High Energy Density (HED) science, which requires laser and XFEL facilities to create in the laboratory extreme conditions of temperatures and pressures that are usually found in the interiors of stars and planets. To diagnose such extreme states of matter, the development of efficient, versatile and fast (sub-picosecond scale) x-ray probes has become essential. In these experiments, x-ray photons can pass through dense material, and absorption of the x-rays can be directly measured, via spectroscopy or imaging, to inform scientists about the temperature and density of the targets being studied. Performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, supported by the LLNL LDRD program (16ERD024), and by the DOE Office Science Early Career Research Program (SCW1575).

Monolithically integrated tri-axis shock accelerometers with MHz-level high resonant-frequency

NASA Astrophysics Data System (ADS)

Zou, Hongshuo; Wang, Jiachou; Chen, Fang; Bao, Haifei; Jiao, Ding; Zhang, Kun; Song, Zhaohui; Li, Xinxin

2017-07-01

This paper reports a novel monolithically integrated tri-axis high-shock accelerometer with high resonant-frequency for the detection of a broad frequency-band shock signal. For the first time, a resonant-frequency as high as about 1.4 MHz is designed for all the x-, y- and z-axis accelerometers of the integrated tri-axis sensor. In order to achieve a wide frequency-band detection performance, all the three sensing structures are designed into an axially compressed/stretched tiny-beam sensing scheme, where the p  +  -doped tiny-beams are connected into a Wheatstone bridge for piezoresistive output. By using ordinary (1 1 1) silicon wafer (i.e. non-SOI wafer), a single-wafer based fabrication technique is developed to monolithically integrate the three sensing structures for the tri-axis sensor. Testing results under high-shock acceleration show that each of the integrated three-axis accelerometers exhibit about 1.4 MHz resonant-frequency and 0.2-0.4 µV/V/g sensitivity. The achieved high frequencies for all the three sensing units make the tri-axis sensor promising in high fidelity 3D high-shock detection applications.

Relativistic klystron driven compact high gradient accelerator as an injector to an X-ray synchrotron radiation ring

DOEpatents

Yu, David U. L.

1990-01-01

A compact high gradient accelerator driven by a relativistic klystron is utilized to inject high energy electrons into an X-ray synchrotron radiation ring. The high gradients provided by the relativistic klystron enables accelerator structure to be much shorter (typically 3 meters) than conventional injectors. This in turn enables manufacturers which utilize high energy, high intensity X-rays to produce various devices, such as computer chips, to do so on a cost effective basis.

Valence band offsets of Sc x Ga1-x N/AlN and Sc x Ga1-x N/GaN heterojunctions

NASA Astrophysics Data System (ADS)

Tsui, H. C. L.; Goff, L. E.; Palgrave, R. G.; Beere, H. E.; Farrer, I.; Ritchie, D. A.; Moram, M. A.

2016-07-01

The valence band offsets of Sc x Ga1-x N/AlN heterojunctions were measured by x-ray photoelectron spectroscopy (XPS) and were found to increase from 0.42 eV to 0.95 eV as the Sc content x increased from 0 to 0.15. The increase in valence band offset with increasing x is attributed to the corresponding increase in spontaneous polarization of the wurtzite structure. The Sc x Ga1-x N/AlN heterojunction is type I, similar to other III-nitride-based heterojunctions. The data also indicate that a type II staggered heterojunction, which can enhance spatial charge separation, could be formed if Sc x Ga1-x N is grown on GaN.

Optical and magneto-optical effects in Hg{sub 1-x}Cd{sub x}Cr{sub 2}Se{sub 4} (0 ⩽ x ⩽ 1) single crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sukhorukov, Yu. P., E-mail: suhorukov@imp.uran.ru; Telegin, A. V.; Bebenin, N. G.

2015-09-15

The concentration, temperature, and magnetic-field dependences of the magnetoreflection and magnetotransmission of natural light in the infrared spectral range and the Kerr effect in single crystals of ferromagnetic Hg{sub 1-x}Cd{sub x}Cr{sub 2}Se{sub 4} (0 ⩽ x ⩽ 1) spinels have been studied. A relationship of the magneto-optical properties to the electronic band structure of spinels has been established. The most significant changes in the spectra of magnetoreflection, magnetotransmission, and the Kerr effect are shown to be observed for 0.1 < x < 0.25 and are attributable to a rearrangement of the band structure as the composition changes.

Fourier transform infrared spectral evidences for protein conformational changes in immature cataractous human lens capsules accelerated by myopia and/or systemic hypertension

NASA Astrophysics Data System (ADS)

Lin, Shan-Yang; Lee, Shui-Mei; Li, Mei-Jane; Liang, Run-Chu

1997-08-01

The possible changes in protein structures of the cataractous human lens capsules of the immature patients with myopia and/or systemic hypertension have been investigated using Fourier transform infrared (FT-IR) microspectroscopy. Second-derivative and deconvolution methods have been applied to obtain the position of the overlapping components of the amide I band and assign them to different secondary structures. Changes in the protein secondary structure and composition of amide I band were estimated quantitatively from Fourier self-deconvolution and curve fitting algorithms. The results indicate that myopia and/or systemic hypertension were found to significantly modify the protein secondary structure of the cataractous human lens capsules to increase the β-type structure and random coil and decrease the α-helix structure. Myopia-induced conformational change in triple helix structure was more pronounced. In conclusion, myopia and/or systemic hypertension seem to modify the conformation of the protein structures in cataractous human lens capsule to change ionic permeation through lens capsule to accelerate the cataract formation of senile patients.

An Extremely Rich Group Of Starbursts And Agns At A Z=3.1 Proto-Cluster Core

NASA Astrophysics Data System (ADS)

Umehata, Hideki

2017-06-01

The environment where galaxies inhabit is expected to play a critical role in shaping their evolution. Galaxies and nuclei in the dense environment at high redshift (i.e., proto-clusters) provide a good laboratory to investigate the accelerated, most extreme evolution of galaxies at a given epoch. Using ALMA band 3 and band 6, we mapped a 2'x3' region within the node at the junction of the 50 Mpc-scale filamentary three-dimensional structure traced by Lyman-alpha emitters (LAEs) in this field. We obtained 18 robustly detected 1.1mm sources (here after submillimeter galaxies, SMGs) with a signal-to-noise ratio (SNR) >5. We also detected CO(3-2) line from 8 SMGs, which in general shows relatively extended structure. Totally 12 ALMA SMGs have spectroscopic redshifts of z=3.09 and six of them host a X-ray luminous active galactic nuclei (AGN). We also find that multiple z=3.09 ALMA SMGs contribute to two AzTEC sources, supporting that interaction may be responsible for a significant fraction of multiplicity in single-dish sources. Our results suggest that the vigorous star formation activity and the growth of super massive black holes (SMBHs) occurred simultaneously in the densest regions at z 3, which is likely to correspond to the most active historical phase of the massive galaxy population found in the core of the clusters in the present universe.

Compositional bowing of band energies and their deformation potentials in strained InGaAs ternary alloys: A first-principles study

NASA Astrophysics Data System (ADS)

Khomyakov, Petr A.; Luisier, Mathieu; Schenk, Andreas

2015-08-01

Using first-principles calculations, we show that the conduction and valence band energies and their deformation potentials exhibit a non-negligible compositional bowing in strained ternary semiconductor alloys such as InGaAs. The electronic structure of these compounds has been calculated within the framework of local density approximation and hybrid functional approach for large cubic supercells and special quasi-random structures, which represent two kinds of model structures for random alloys. We find that the predicted bowing effect for the band energy deformation potentials is rather insensitive to the choice of the functional and alloy structural model. The direction of bowing is determined by In cations that give a stronger contribution to the formation of the InxGa1-xAs valence band states with x ≳ 0.5, compared to Ga cations.

Silicon micromechanical sensors model of piezoresistivity

NASA Astrophysics Data System (ADS)

Lysko, Jan M.

2001-08-01

Application of the piezo resistivity model to estimate valence and conduction bands shifts induced by the mechanical stress is presented. Parameters of the silicon pressure and acceleration sensor, which are under development in the ITE, Warsaw, were used. Geometrical and technological data were used in calculations of the silicon energy band structure and longitudinal coefficient of the piezo resistivity.(pi) L.

The electronic band structure of Ge1-x Sn x in the full composition range: indirect, direct, and inverted gaps regimes, band offsets, and the Burstein-Moss effect

NASA Astrophysics Data System (ADS)

Polak, M. P.; Scharoch, P.; Kudrawiec, R.

2017-05-01

A comprehensive and detailed study of the composition dependence of lattice constants, band gaps and band offsets has been performed for bulk Ge1-x Sn x alloy in the full composition range using state-of-the-art density functional theory methods. A spectral weight approach to band unfolding has been applied as a means of distinguishing the indirect and direct band gaps from folded supercell band structures. In this way, four characteristic regions of the band gap character have been identified for Ge1-x Sn x alloy: an indirect band gap (x  <  6.5%), a direct band gap (6.5%  <  x  <  25%) and an inverse band gap (x  >  25%) with inverse spin-orbit split-off for 45%  <  x  <  85%. In general, it has been observed that the bowing parameters of band edges (Γ and L-point in conduction band (CBΓ and CB L ), valence band (VB), and spin-orbit (SO) band) are rather large ({{b}\\text{C{{\\text{B}} Γ }}}   =  2.43  ±  0.06 eV, {{b}\\text{C{{\\text{B}}L}}}   =  0.64  ±  0.04 eV, {{b}\\text{VB}}   =  -0.59  ±  0.04 eV, and {{b}\\text{SO}}   =  -0.49  ±  0.05 eV). This indicates that Ge1-x Sn x behaves like a highly mismatched group IV alloy. The composition dependence of lattice constant shows negligible bowing (b a   =  -0.083 Å). Obtained results have been compared with available experimental data. The origin of band gap reduction and large bowing has been analyzed and conclusions have been drawn regarding the relationship between experimental and theoretical results. It is shown that due to the low DOS at the Γ-point, a significant filling of CB by electrons in the direct gap regime may easily take place. Therefore, the Burstein-Moss effect should be considered when comparing experimental data with theoretical predictions as has already been shown for other intrinsic n-type narrow gap semiconductors (e.g. InN).

High gradient RF test results of S-band and C-band cavities for medical linear accelerators

DOE PAGES

Degiovanni, A.; Bonomi, R.; Garlasche, M.; ...

2018-02-09

TERA Foundation has proposed and designed hadrontherapy facilities based on novel linacs, i.e. high gradient linacs which accelerate either protons or light ions. The overall length of the linac, and therefore its cost, is almost inversely proportional to the average accelerating gradient. With the scope of studying the limiting factors for high gradient operation and to optimize the linac design, TERA, in collaboration with the CLIC Structure Development Group, has conducted a series of high gradient experiments. The main goals were to study the high gradient behavior and to evaluate the maximum gradient reached in 3 and 5.7 GHz structuresmore » to direct the design of medical accelerators based on high gradient linacs. Lastly, this paper summarizes the results of the high power tests of 3.0 and 5.7 GHz single-cell cavities.« less

High gradient RF test results of S-band and C-band cavities for medical linear accelerators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Degiovanni, A.; Bonomi, R.; Garlasche, M.

TERA Foundation has proposed and designed hadrontherapy facilities based on novel linacs, i.e. high gradient linacs which accelerate either protons or light ions. The overall length of the linac, and therefore its cost, is almost inversely proportional to the average accelerating gradient. With the scope of studying the limiting factors for high gradient operation and to optimize the linac design, TERA, in collaboration with the CLIC Structure Development Group, has conducted a series of high gradient experiments. The main goals were to study the high gradient behavior and to evaluate the maximum gradient reached in 3 and 5.7 GHz structuresmore » to direct the design of medical accelerators based on high gradient linacs. Lastly, this paper summarizes the results of the high power tests of 3.0 and 5.7 GHz single-cell cavities.« less

Design and Calibration of an RF Actuator for Low-Level RF Systems

NASA Astrophysics Data System (ADS)

Geng, Zheqiao; Hong, Bo

2016-02-01

X-ray free electron laser (FEL) machines like the Linac Coherent Light Source (LCLS) at SLAC require high-quality electron beams to generate X-ray lasers for various experiments. Digital low-level RF (LLRF) systems are widely used to control the high-power RF klystrons to provide a highly stable RF field in accelerator structures for beam acceleration. Feedback and feedforward controllers are implemented in LLRF systems to stabilize or adjust the phase and amplitude of the RF field. To achieve the RF stability and the accuracy of the phase and amplitude adjustment, low-noise and highly linear RF actuators are required. Aiming for the upgrade of the S-band Linac at SLAC, an RF actuator is designed with an I/Qmodulator driven by two digital-to-analog converters (DAC) for the digital LLRF systems. A direct upconversion scheme is selected for RF actuation, and an on-line calibration algorithm is developed to compensate the RF reference leakage and the imbalance errors in the I/Q modulator, which may cause significant phase and amplitude actuation errors. This paper presents the requirements on the RF actuator, the design of the hardware, the calibration algorithm, and the implementation in firmware and software and the test results at LCLS.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Njema, H.; Debbichi, M., E-mail: mourad_fsm@yahoo.fr; Boughzala, K.

Highlights: • The structural and electronic properties of apatites with the general formula Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} (0 ≤ x ≤ 6) have been investigated by means of the density functional theory. • The calculated enthalpy of formation of the compounds increased with the substitution degree. • The decrease in stability is probably due to the disorder induced in the Ca/LaO{sub 6}F polyhedron, following the substitution. - Abstract: The apatite-type compounds Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} with 0 ≤ x ≤ 6 were prepared using a high temperature solid state reaction and weremore » characterized by X-ray diffraction. The crystal structure, chemical bonding, electronic structure and formation energy of all relaxed structures were analyzed by density functional theory (DFT). The calculated results show that the predicted geometry can well reproduce the structural parameters. The incorporation of La{sup 3+} into the fluorapatite (FA) structure induced especially at the level of the S(2) sites a certain disorder which is responsible for the weakening in the stability with x. Excellent agreement were obtained between the calculated and experimental results. Moreover, the band structure indicates that despite the reduction of the band gap with x content all materials remain insulating.« less

Zn(x)Cd(1-x)Se nanomultipods with tunable band gaps: synthesis and first-principles calculations.

PubMed

Wei, Hao; Su, Yanjie; Han, Ziyi; Li, Tongtong; Ren, Xinglong; Yang, Zhi; Wei, Liangming; Cong, Fengsong; Zhang, Yafei

2013-06-14

In this paper, we demonstrate that ZnxCd1-xSe nanomultipods can be synthesized via a facile and nontoxic solution-based method. Interesting aspects of composition, morphology and optical properties were deeply explored. The value of Zn/(Zn+Cd) could be altered across the entire range from 0.08 to 0.86 by varying the ratio of cation precursor contents. The band gap energy could be linearly tuned from 1.88 to 2.48 eV with respect to the value of Zn/(Zn+Cd). The experiment also showed that oleylamine played a dominant role in the formation of multipod structure. A possible growth mechanism was further suggested. First-principles calculations of band gap energy and density of states in the Vienna ab initio simulation package code were performed to verify the experimental variation tendency of the band gap. Computational results indicated that dissimilarities of electronic band structures and orbital constitutions determined the tunable band gap of the as-synthesized nanomultipod, which might be promising for versatile applications in relevant areas of solar cells, biomedicine, sensors, catalysts and so on.

A study on micro-structural and optical parameters of InxSe1-x thin film

NASA Astrophysics Data System (ADS)

Patel, P. B.; Desai, H. N.; Dhimmar, J. M.; Modi, B. P.

2018-04-01

Thin film of Indium Selenide (InSe) has been deposited by thermal evaporation technique onto pre cleaned glass substrate under high vacuum condition. The micro-structural and optical properties of InxSe1-x (x = 0.6, 1-x = 0.4) thin film have been characterized by X-ray diffractrometer (XRD) and UV-Visible spectrophotometer. The XRD spectra showed that InSe thin film has single phase hexagonal structure with preferred orientation along (1 1 0) direction. The micro-structural parameters (crystallite size, lattice strain, dislocation density, domain population) for InSe thin film have been calculated using XRD spectra. The optical parameters (absorption, transmittance, reflectance, energy band gap, Urbach energy) of InSe thin film have been evaluated from absorption spectra. The direct energy band gap and Urbach energy of InSe thin film is found to be 1.90 eV and 235 meV respectively.

Electrical and band structural analyses of Ti1-x Al x O y films grown by atomic layer deposition on p-type GaAs

NASA Astrophysics Data System (ADS)

An, Youngseo; Mahata, Chandreswar; Lee, Changmin; Choi, Sungho; Byun, Young-Chul; Kang, Yu-Seon; Lee, Taeyoon; Kim, Jiyoung; Cho, Mann-Ho; Kim, Hyoungsub

2015-10-01

Amorphous Ti1-x Al x O y films in the Ti-oxide-rich regime (x  <  0.5) were deposited on p-type GaAs via atomic layer deposition with titanium isopropoxide, trimethylaluminum, and H2O precursor chemistry. The electrical properties and energy band alignments were examined for the resulting materials with their underlying substrates, and significant frequency dispersion was observed in the accumulation region of the Ti-oxide-rich Ti1-x Al x O y films. Although a further reduction in the frequency dispersion and leakage current (under gate electron injection) could be somewhat achieved through a greater addition of Al-oxide in the Ti1-x Al x O y film, the simultaneous decrease in the dielectric constant proved problematic in finding an optimal composition for application as a gate dielectric on GaAs. The spectroscopic band alignment measurements of the Ti-oxide-rich Ti1-x Al x O y films indicated that the band gaps had a rather slow increase with the addition of Al-oxide, which was primarily compensated for by an increase in the valance band offset, while a nearly-constant conduction band offset with a negative electron barrier height was maintained.

A Non-thermal Pulsed X-Ray Emission of AR Scorpii

NASA Astrophysics Data System (ADS)

Takata, J.; Hu, C.-P.; Lin, L. C. C.; Tam, P. H. T.; Pal, P. S.; Hui, C. Y.; Kong, A. K. H.; Cheng, K. S.

2018-02-01

We report the analysis result of UV/X-ray emission from AR Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and an M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orbit, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission from the M star surface rather than that from the accretion column on the white dwarf’s (WD) star, which is similar to usual IPs. Additionally, the observed X-ray emission also modulates with the WD’s spin with a pulse fraction of ∼14%. The peak position is aligned in the optical/UV/X-ray band. This supports the hypothesis that the electrons in AR Scorpii are accelerated to a relativistic speed and emit non-thermal photons via the synchrotron radiation. In the X-ray bands, evidence of the power-law spectrum is found in the pulsed component, although the observed emission is dominated by the optically thin thermal plasma emissions with several different temperatures. It is considered that the magnetic dissipation/reconnection process on the M star surface heats up the plasma to a temperature of several keV and also accelerates the electrons to the relativistic speed. The relativistic electrons are trapped in the WD’s closed magnetic field lines by the magnetic mirror effect. In this model, the observed pulsed component is explained by the emissions from the first magnetic mirror point.

Electronic structures of of PuX (X=S, Se, Te)

NASA Astrophysics Data System (ADS)

Maehira, Takahiro; Sakai, Eijiro; Tatetsu, Yasutomi

2013-08-01

We have calculated the energy band structures and the Fermi surfaces of PuS, PuSe, and PuTe by using a self-consistent relativistic linear augmented-plane-wave method with the exchange and correlation potential in the local density approximation. In general, the energy bands near the Fermi level are mainly caused by the hybridization between the Pu 5 f and the monochalcogenide p electrons. The obtained main Fermi surfaces consisted of two hole sheets and one electron sheet, which were constructed from the band having both the Pu 5 f state and the monochalcogenide p state.

The structure and optical properties of Sr{sub 1−x}Ca{sub x}MoO{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hopper, H.A.; Macphee, D.E.; Mclaughlin, A.C., E-mail: a.c.mclaughlin@abdn.ac.uk

2016-10-15

The solid solution Sr{sub 1−x}Ca{sub x}MoO{sub 3} (x=0.00, 0.05, 0.10, 0.13, 0.15 and 0.17) has successfully been synthesised and X-ray Powder diffraction has revealed the occurrence of structural phase transitions, from cubic Pm−3m to tetragonal I4/mcm, and then to orthorhombic Imma as the value of x increased. Discontinuities were observed in the cell parameters and bond lengths and angles at the transition from tetragonal to orthorhombic symmetry as a result of the switching of the octahedral rotation axis at the tetragonal to orthorhombic transition. The increased octahedral tilting could also be linked to the decrease in the band gap frommore » 2.20 eV to 2.10 eV as x increased from 0 to 0.17. - Graphical abstract: Table of Contents Figure Caption: Ultraviolet-visible absorbance spectra for Sr{sub 1−x}Ca{sub x}MoO{sub 3} showing a reduction in band gap upon increasing x as a result of increased octahedral tilting. - Highlights: • The solid solution Sr{sub 1−x}Ca{sub x}MoO{sub 3} has been synthesised. • Structural phase transitions are observed. • Discontinuities were observed in the cell parameters and bond lengths and angles. • Upon increasing x from 0 to 0.17 the band gap reduces from 2.20 eV to 2.10 eV.« less

Band structure engineering for solar energy applications: Zinc oxide(1-x) selenium(x) films and devices

NASA Astrophysics Data System (ADS)

Mayer, Marie Annette

New technologies motivate the development of new semiconducting materials, for which structural, electrical and chemical properties are not well understood. In addition to new materials systems, there are huge opportunities for new applications, especially in solar energy conversion. In this dissertation I explore the role of band structure engineering of semiconducting oxides for solar energy. Due to the abundance and electrochemical stability of oxides, the appropriate modification could make them appealing for applications in both photovoltaics and photoelectrochemical hydrogen production. This dissertation describes the design, synthesis and evaluation of the alloy ZnO1-xSe x for these purposes. I review several methods of band structure engineering including strain, quantum confinement and alloying. A detailed description of the band anticrossing (BAC) model for highly mismatched alloys is provided, including the derivation of the BAC model as well as recent work and potential applications. Thin film ZnOxSe1-x samples are grown by pulsed laser deposition (PLD). I describe in detail the effect of growth conditions (temperature, pressure and laser fluence) on the chemistry, structure and optoelectronic properties of ZnOxSe1-x. The films are grown using different combinations of PLD conditions and characterized with a variety of techniques. Phase pure films with low roughness and high crystallinity were obtained at temperatures below 450¢ªC, pressures less than 10-4 Torr and laser fluences on the order of 1.5 J/cm 2. Electrical conduction was still observed despite heavy concentrations of grain boundaries. The band structure of ZnO1-xSex is then examined in detail. The bulk electron affinity of a ZnO thin film was measured to be 4.5 eV by pinning the Fermi level with native defects. This is explained in the framework of the amphoteric defect model. A shift in the ZnO1-xSe x valence band edge with x is observed using synchrotron x-ray absorption and emission spectroscopy. Measurement of the optical absorption coefficient, α, shows a significant red shift as well as an increase in the low energy density of states with x. Fitting α revealed that the initial Se defect level is located 0.9 eV above the valence band edge and the coupling strength of the interaction is 1.2 eV. Optical reflection data are good agreement with the absorption onset at 2 eV. Taking the derivative of this data reveals experimental observation of the matrix-like band at higher energies. ZnO1-xSex is explicitly evaluated for photoelectrochemical applications. An introduction to semiconductor electrochemistry is followed by flat band, photocurrent, and spectrally resolved photocurrent measurements. The flat band measurements are in excellent agreement with the measurements of the ZnO electron affinity using bulk methods, but show that the conduction band edge of ZnO1-xSex is too low for spontaneous water splitting. Measurements of the incident photon to current conversion efficiency (IPCE) indicated that photons with energies greater than 2 eV excite carriers that do conduct and induce chemical reactions. Tandem ZnO1-xSe x/Si devices are made with a natural Ohmic contact between the p-Si and n-ZnO1-xSex. Electrochemical testing proves that the presence of the tandem photovoltaic provides an overpotential of ˜0.5 V to electrons enabling the reduction of H+ in solution. Finally, the carrier scattering and recombination lifetimes in ZnO 1-xSex are considered. Resistivity, Hall effect and Seebeck coefficient measurements are used to probe the scattering lifetime, while the recombination lifetime is investigated using photoluminescence spectroscopy. Electrochemical photocurrent measurements in light and dark are a function of the product of both lifetimes. Results indicate that significant scattering in the lateral direction does not prohibit the photoelectrochemical device from operating, but defects from high fluence growth are extremely detrimental to the recombination lifetime. A textured or otherwise irregular crystal that does not function well for a device designed for transport in one direction might be perfectly operational when the current flow is perpendicular. The final chapter provides perspective on the future of ZnO1-xSe x in scientific research and obstacles to overcome before industrial applications are possible. Perspective on sustainable hydrogen production is given. The optimist can see a value for nearly all renewable energy technologies in a variety of value-driven applications.

Optical absorption spectra and energy band gap in manganese containing sodium zinc phosphate glasses

NASA Astrophysics Data System (ADS)

Sardarpasha, K. R.; Hanumantharaju, N.; Gowda, V. C. Veeranna

2018-05-01

Optical band gap energy in the system 25Na2O-(75-x)[0.6P2O5-0.4ZnO]-xMnO2 (where x = 0.5,1,5,10 and 20 mol.%) have been studied. The intensity of the absorption band found to increase with increase of MnO2 content. The decrease in the optical band gap energy with increase in MnO2 content in the investigated glasses is attributed to shifting of absorption edge to a longer wavelength region. The obtained results were discussed in view of the structure of phosphate glass network.

Development and application of compact and on-chip electron linear accelerators for dynamic tracking cancer therapy and DNA damage/repair analysis

NASA Astrophysics Data System (ADS)

Uesaka, M.; Demachi, K.; Fujiwara, T.; Dobashi, K.; Fujisawa, H.; Chhatkuli, R. B.; Tsuda, A.; Tanaka, S.; Matsumura, Y.; Otsuki, S.; Kusano, J.; Yamamoto, M.; Nakamura, N.; Tanabe, E.; Koyama, K.; Yoshida, M.; Fujimori, R.; Yasui, A.

2015-06-01

We are developing compact electron linear accelerators (hereafter linac) with high RF (Radio Frequency) frequency (9.3 GHz, wavelength 32.3 mm) of X-band and applying to medicine and non-destructive testing. Especially, potable 950 keV and 3.95 MeV linac X-ray sources have been developed for on-site transmission testing at several industrial plants and civil infrastructures including bridges. 6 MeV linac have been made for pinpoint X-ray dynamic tracking cancer therapy. The length of the accelerating tube is ∼600 mm. The electron beam size at the X-ray target is less than 1 mm and X-ray spot size at the cancer is less than 3 mm. Several hardware and software are under construction for dynamic tracking therapy for moving lung cancer. Moreover, as an ultimate compact linac, we are designing and manufacturing a laser dielectric linac of ∼1 MeV with Yr fiber laser (283 THz, wavelength 1.06 pm). Since the wavelength is 1.06 μm, the length of one accelerating strcture is tens pm and the electron beam size is in sub-micro meter. Since the sizes of cell and nuclear are about 10 and 1 μm, respectively, we plan to use this “On-chip” linac for radiation-induced DNA damage/repair analysis. We are thinking a system where DNA in a nucleus of cell is hit by ∼1 μm electron or X-ray beam and observe its repair by proteins and enzymes in live cells in-situ.

Electronic structures of anatase (TiO2)1-x(TaON)x solid solutions: a first-principles study.

PubMed

Dang, Wenqiang; Chen, Hungru; Umezawa, Naoto; Zhang, Junying

2015-07-21

Sensitizing wide band gap photo-functional materials under visible-light irradiation is an important task for efficient solar energy conversion. Although nitrogen doping into anatase TiO2 has been extensively studied for this purpose, it is hard to increase the nitrogen content in anatase TiO2 because of the aliovalent nitrogen substituted for oxygen, leading to the formation of secondary phases or defects that hamper the migration of photoexcited charge carriers. In this paper, electronic structures of (TiO2)1-x(TaON)x (0 ≤ x ≤ 1) solid solutions, in which the stoichiometry is satisfied with the co-substitution of Ti for Ta along with O for N, are investigated within the anatase crystal structure using first-principles calculations. Our computational results show that the solid solutions have substantially narrower band gaps than TiO2, without introducing any localized energy states in the forbidden gap. In addition, in comparison with the pristine TiO2, the solid solution has a direct band gap when the content of TaON exceeds 0.25, which is advantageous to light absorption. The valence band maximum (VBM) of the solid solutions, which is mainly composed of N 2p states hybridized with O 2p, Ti 3d or Ta 5d orbitals, is higher in energy than that of pristine anatase TiO2 consisting of non-bonding O 2p states. On the other hand, incorporating TaON into TiO2 causes the formation of d-d bonding states through π interactions and substantially lowers the conduction band minimum (CBM) because of the shortened distance between some metal atoms. As a result, the anatase (TiO2)1-x(TaON)x is expected to become a promising visible-light absorber. In addition, some atomic configurations are found to possess exceptionally narrow band gaps.

Structural analysis and characterization of layer perovskite oxynitrides made from Dion Jacobson oxide precursors

NASA Astrophysics Data System (ADS)

Schottenfeld, Joshua A.; Benesi, Alan J.; Stephens, Peter W.; Chen, Gugang; Eklund, Peter C.; Mallouk, Thomas E.

2005-07-01

A three-layer oxynitride Ruddlesden-Popper phase Rb 1+xCa 2Nb 3O 10-xN x· yH 2O ( x=0.7-0.8, y=0.4-0.6) was synthesized by ammonialysis at 800 °C from the Dion-Jacobson phase RbCa 2Nb 3O 10 in the presence of Rb 2CO 3. Incorporation of nitrogen into the layer perovskite structure was confirmed by XPS, combustion analysis, and MAS NMR. The water content was determined by thermal gravimetric analysis and the rubidium content by ICP-MS. A similar layered perovskite interconversion occurred in the two-layer Dion-Jacobson oxide RbLaNb 2O 7 to yield Rb 1+xLaNb 2O 7-xN x· yH 2O ( x=0.7-0.8, y=0.5-1.0). Both compounds were air- and moisture-sensitive, with rapid loss of nitrogen by oxidation and hydrolysis reactions. The structure of the three-layer oxynitride Rb 1.7Ca 2Nb 3O 9.3N 0.7·0.5H 2O was solved in space group P4 /mmm with a=3.887(3) and c=18.65(1) Å, by Rietveld refinement of X-ray powder diffraction data. The two-layer oxynitride structure Rb 1.8LaNb 2O 6.3N 0.7·1.0H 2O was also determined in space group P4 /mmm with a=3.934(2) and c=14.697(2) Å. GSAS refinement of synchrotron X-ray powder diffraction data showed that the water molecules were intercalated between a double layer of Rb+ ions in both the two- and three-layer Ruddlesden-Popper structures. Optical band gaps were measured by diffuse reflectance UV-vis for both materials. An indirect band gap of 2.51 eV and a direct band gap of 2.99 eV were found for the three-layer compound, while an indirect band gap of 2.29 eV and a direct band gap of 2.84 eV were measured for the two-layer compound. Photocatalytic activity tests of the three-layer compound under 380 nm pass filtered light with AgNO 3 as a sacrificial electron acceptor gave a quantum yield of 0.025% for oxygen evolution.

Preliminary experimental investigation of an X-band Cerenkov-type high power microwave oscillator without guiding magnetic field.

PubMed

Guo, Liming; Shu, Ting; Li, Zhiqiang; Ju, Jinchuan; Fang, Xiaoting

2017-02-01

Among high power microwave (HPM) generators without guiding magnetic field, Cerenkov-type oscillator is expected to achieve a relatively high efficiency, which has already been realized in X-band in our previous simulation work. This paper presents the preliminary experimental investigations into an X-band Cerenkov-type HPM oscillator without guiding magnetic field. Based on the previous simulation structure, some modifications regarding diode structure were made. Different cathode structures and materials were tested in the experiments. By using a ring-shaped graphite cathode, microwave of about one hundred megawatt level was generated with a pure center frequency of 9.14 GHz, and an efficiency of about 1.3%. As analyzed in the paper, some practical issues reduce the efficiency in experiments, such as real features of the electron beam, probable breakdown regions on the cathode surface which can damage the diode, and so forth.

Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α-As2Te3-xSex (0 ≤ x ≤ 1.5).

PubMed

Vaney, Jean-Baptiste; Delaizir, Gaëlle; Wiendlocha, Bartlomiej; Tobola, Janusz; Alleno, Eric; Piarristeguy, Andrea; Gonçalves, Antonio Pereira; Gendarme, Christine; Malaman, Bernard; Dauscher, Anne; Candolfi, Christophe; Lenoir, Bertrand

2017-02-20

We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5-523 K) of the solid solution α-As 2 Te 3-x Se x (0 ≤ x ≤ 1.5). All of the polycrystalline compounds α-As 2 Te 3-x Se x crystallize isostructurally in the monoclinic space group C2/m (No. 12, Z = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing x up to x = 1.5 leaves unchanged both the p-type character and semiconducting nature of α-As 2 Te 3 . The electrical resistivity and thermopower gradually increase with x as a result of the progressive increase in the band gap energy. Despite the fact that α-As 2 Te 3 exhibits very low lattice thermal conductivity κ L , the substitution of Se for Te further lowers κ L to 0.35 W m -1 K -1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point-defect scattering.

Redox-dependent substrate-cofactor interactions in the Michaelis-complex of a flavin-dependent oxidoreductase

NASA Astrophysics Data System (ADS)

Werther, Tobias; Wahlefeld, Stefan; Salewski, Johannes; Kuhlmann, Uwe; Zebger, Ingo; Hildebrandt, Peter; Dobbek, Holger

2017-07-01

How an enzyme activates its substrate for turnover is fundamental for catalysis but incompletely understood on a structural level. With redox enzymes one typically analyses structures of enzyme-substrate complexes in the unreactive oxidation state of the cofactor, assuming that the interaction between enzyme and substrate is independent of the cofactors oxidation state. Here, we investigate the Michaelis complex of the flavoenzyme xenobiotic reductase A with the reactive reduced cofactor bound to its substrates by X-ray crystallography and resonance Raman spectroscopy and compare it to the non-reactive oxidized Michaelis complex mimics. We find that substrates bind in different orientations to the oxidized and reduced flavin, in both cases flattening its structure. But only authentic Michaelis complexes display an unexpected rich vibrational band pattern uncovering a strong donor-acceptor complex between reduced flavin and substrate. This interaction likely activates the catalytic ground state of the reduced flavin, accelerating the reaction within a compressed cofactor-substrate complex.

Redox-dependent substrate-cofactor interactions in the Michaelis-complex of a flavin-dependent oxidoreductase

PubMed Central

Werther, Tobias; Wahlefeld, Stefan; Salewski, Johannes; Kuhlmann, Uwe; Zebger, Ingo; Hildebrandt, Peter; Dobbek, Holger

2017-01-01

How an enzyme activates its substrate for turnover is fundamental for catalysis but incompletely understood on a structural level. With redox enzymes one typically analyses structures of enzyme–substrate complexes in the unreactive oxidation state of the cofactor, assuming that the interaction between enzyme and substrate is independent of the cofactors oxidation state. Here, we investigate the Michaelis complex of the flavoenzyme xenobiotic reductase A with the reactive reduced cofactor bound to its substrates by X-ray crystallography and resonance Raman spectroscopy and compare it to the non-reactive oxidized Michaelis complex mimics. We find that substrates bind in different orientations to the oxidized and reduced flavin, in both cases flattening its structure. But only authentic Michaelis complexes display an unexpected rich vibrational band pattern uncovering a strong donor–acceptor complex between reduced flavin and substrate. This interaction likely activates the catalytic ground state of the reduced flavin, accelerating the reaction within a compressed cofactor–substrate complex.

Effect of band filling on anomalous Hall conductivity and magneto-crystalline anisotropy in NiFe epitaxial thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Zhong; Jiang, Hang-Yu; Zhou, Shi-Ming, E-mail: shiming@tongji.edu.cn

2016-01-15

The anomalous Hall effect (AHE) and magneto-crystalline anisotropy (MCA) are investigated in epitaxial Ni{sub x}Fe{sub 1−x} thin films grown on MgO (001) substrates. The scattering independent term b of anomalous Hall conductivity shows obvious correlation with cubic magneto-crystalline anisotropy K{sub 1}. When nickel content x decreasing, both b and K{sub 1} vary continuously from negative to positive, changing sign at about x = 0.85. Ab initio calculations indicate Ni{sub x}Fe{sub 1−x} has more abundant band structures than pure Ni due to the tuning of valence electrons (band fillings), resulting in the increased b and K{sub 1}. This remarkable correlation betweenmore » b and K{sub 1} can be attributed to the effect of band filling near the Fermi surface.« less

X-ray absorption and reflection as probes of the GaN conduction bands: Theory and experiments

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lambrecht, W.R.L.; Rashkeev, S.N.; Segall, B.

1997-04-01

X-ray absorption measurements are a well-known probe of the unoccupied states in a material. The same information can be obtained by using glancing angle X-ray reflectivity. In spite of several existing band structure calculations of the group III nitrides and previous optical studies in UV range, a direct probe of their conduction band densities of states is of interest. The authors performed a joint experimental and theoretical investigation using both of these experimental techniques for wurtzite GaN.

Effects of Doping Ratio of Cobalt and Iron on the Structure and Optical Properties of Bi3.25La0.75Fe(x)Co(1-x)Ti2O12 (X = 0, 0.25, 0.5, 0.75, 1).

PubMed

Song, Myoung Geun; Han, Jun Young; Bark, Chung Wung

2015-10-01

The wide band gap of complex oxides is one of the major obstacles limiting their use in photovoltaic cells. To identify an effective route for tailoring the band gap of complex oxides, this study examined the effects of cobalt and iron doping on lanthanum-modified Bi4Ti3O2-based oxides synthesized using a solid reaction. The structural and optical properties were analyzed by X-ray diffraction and ultraviolet-visible absorption spectroscopy. As a result, the optimal iron to cobalt doping ratio in bismuth titanate powder resulted in an ~1.8 eV decrease in the optical band gap. This new route to reduce the optical bandgap can be adapted to the synthesis of other complex oxides.

DSN 70-meter antenna microwave optics design and performance improvements. Part 1: Design optimization

NASA Technical Reports Server (NTRS)

Bathker, D. A.; Slobin, S. D.

1989-01-01

The design optimizations associated with the microwave and structural upgrade of the DSN 64-m antennas are discussed. Expected area efficiency/gain performances at S- and X-band are given for both the original 64-m systems and the upgraded 70-m systems, and error estimates are developed. The DSN 70-m Upgrade Project specifications, based on predesign estimates, were 1.4-dB gain at S-band and 1.9-dB at X-band, with no degradation to critical receiving system noise temperatures. The measurements show an S-band gain increase of 1.9 dB and an average increase of 2.1 dB at X-band. The Project also delivered small receiving system noise decreases at both frequency bands. The three DSN 70-m antennas, in the initial state of mechanical adjustment as of the end of calendar year 1988, are performing with very high peak microwave area efficiencies at very nearly the engineering design expectations of 76 percent at S-band and 71 percent at X-band.

Electronic structure of β-Ga2O3 single crystals investigated by hard X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Li, Guo-Ling; Zhang, Fabi; Cui, Yi-Tao; Oji, Hiroshi; Son, Jin-Young; Guo, Qixin

2015-07-01

By combination of hard X-ray photoelectron spectroscopy (HAXPES) and first-principles band structure calculations, the electronic states of β-Ga2O3 were investigated to deepen the understanding of bulk information for this compound. The valence band spectra of HAXPES presented the main contribution from Ga 4sp, which are well represented by photoionization cross section weighted partial density of states. The experimental data complemented with the theoretical study yield a realistic picture of the electronic structure for β-Ga2O3.

Synthesis and enhanced electrochemical catalytic performance of monolayer WS2(1-x) Se2x with a tunable band gap.

PubMed

Fu, Qi; Yang, Lei; Wang, Wenhui; Han, Ali; Huang, Jian; Du, Pingwu; Fan, Zhiyong; Zhang, Jingyu; Xiang, Bin

2015-08-26

The first realization of a tunable band-gap in monolayer WS2(1-x) Se2x is demonstrated. The tuning of the bandgap exhibits a strong dependence of S and Se content, as proven by PL spectroscopy. Because of its remarkable electronic structure, monolayer WS2(1-x) Se2x exhibits novel electrochemical catalytic activity and offers long-term electrocatalytic stability for the hydrogen evolution reaction. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode.

PubMed

Blizanac, Berislav B; Arenz, Matthias; Ross, Philip N; Marković, Nenad M

2004-08-18

The electrooxidation of CO has been studied on reconstructed gold single-crystal surfaces by a combination of electrochemical (EC) and infrared reflection absorption spectroscopy (IRAS) measurements. Emphasis is placed on relating the vibrational properties of the CO adlayer to the voltammetric and other macroscopic electrochemical responses, including rotating disk electrode measurements of the catalytic activity. The IRAS data show that the C-O stretching frequencies are strongly dependent on the surface orientation and can be observed in the range 1940-1990 cm(-1) for the 3-fold bridging, 2005-2070 cm(-1) for the 2-fold bridging, and 2115-2140 for the terminal position. The most complex CO spectra are found for the Au(110)-(1 x 2) surface, i.e., a band near 1965 cm(-1), with the second, weaker band shifted positively by about 45 cm(-1) and, finally, a weak band near 2115 cm(-1). While the C-O stretching frequencies for a CO adlayer adsorbed on Au(111)-(1 x 23) show nu(CO) bands at 2029-2069 cm(-1) and at 1944-1986 cm(-1), on the Au(100)-"hex" surface a single CO band is observed at 2004-2029 cm(-1). In the "argon-purged" solution, the terminal nu(CO) band on Au(110)-(1 x 2) and the 3-fold bridging band on the Au(111)-(1 x 23) disappear entirely. The IRAS/EC data show that the kinetics of CO oxidation are structure sensitive; i.e., the onset of CO oxidation increases in the order Au(110)-(1 x 2) > or = Au(100)-"hex" > Au(111)-(1 x 23). Possible explanations for the structure sensitivity are discussed.

Electronic structure of p-type transparent conducting oxide CuAlO2

NASA Astrophysics Data System (ADS)

Mo, Sung-Kwan; Yoon, Joonseok; Liu, Xiaosong; Yang, Wanli; Mun, Bongjin; Ju, Honglyoul

2014-03-01

CuAlO2 is a prototypical p-type transparent conducting oxide. Despite its importance for potential applications and number of studies on its band structure and gap characteristics, experimental study on the momentum-resolved electronic structure has been lacking. We present angle-resolved photoemission data on single crystalline CuAlO2 using synchrotron light source to reveal complete band structure. Complemented by the x-ray absorption and emission spectra, we also study band gap characteristics and compare them with theory.

Interface energetics and atomic structure of epitaxial La{sub 1−x}Sr{sub x}CoO{sub 3} on Nb:SrTiO{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Van Overmeere, Quentin, E-mail: quentin.vanovermeere@uclouvain.be, E-mail: john.d.baniecki@jp.fujitsu.com; Baniecki, John D., E-mail: quentin.vanovermeere@uclouvain.be, E-mail: john.d.baniecki@jp.fujitsu.com; Yamazaki, Takashi

2015-06-15

The energetics at oxide semiconductor/La{sub 1−x}Sr{sub x}CoO{sub 3} heterojunctions, including the respective alignment of the valence and conduction bands, govern charge transfer and have to be determined for the design of future La{sub 1−x}Sr{sub x}CoO{sub 3}-based devices. In this letter, the electronic and atomic structures of epitaxial La{sub 1−x}Sr{sub x}CoO{sub 3} on Nb-doped strontium titanate are revealed by scanning transmission electron microscopy, electron energy loss spectroscopy, and in situ x-ray and ultra violet photoelectron spectroscopies. For LaCoO{sub 3}, a valence band (VB) offset of 2.8 ± 0.1 eV is deduced. The large offset is attributed to the orbital contributions of the Co 3dmore » states to the VB maximum of the LaCoO{sub 3} thin films, with no evidence of interface dipole contributions. The sensitivity of the valence band orbital character to spin state ordering and oxygen vacancies is assessed using density functional theory.« less

Density functional theory calculations for the band gap and formation energy of Pr4-xCaxSi12O3+xN18-x; a highly disordered compound with low symmetry and a large cell size.

PubMed

Hong, Sung Un; Singh, Satendra Pal; Pyo, Myoungho; Park, Woon Bae; Sohn, Kee-Sun

2017-06-28

A novel oxynitride compound, Pr 4-x Ca x Si 12 O 3+x N 18-x , synthesized using a solid-state route has been characterized as a monoclinic structure in the C2 space group using Rietveld refinement on synchrotron powder X-ray diffraction data. The crystal structure of this compound was disordered due to the random distribution of Ca/Pr and N/O ions at various Wyckoff sites. A pragmatic approach for an ab initio calculation based on density function theory (DFT) for this disordered compound has been implemented to calculate an acceptable value of the band gap and formation energy. In general, for the DFT calculation of a disordered compound, a sufficiently large super cell and infinite variety of ensemble configurations is adopted to simulate the random distribution of ions; however, such an approach is time consuming and cost ineffective. Even a single unit cell model gave rise to 43 008 independent configurations as an input model for the DFT calculations. Since it was nearly impossible to calculate the formation energy and the band gap energy for all 43 008 configurations, an elitist non-dominated sorting genetic algorithm (NSGA-II) was employed to find the plausible configurations. In the NSGA-II, all 43 008 configurations were mathematically treated as genomes and the calculated band gap and the formation energy as the objective (fitness) function. Generalized gradient approximation (GGA) was first employed in the preliminary screening using NSGA-II, and thereafter a hybrid functional calculation (HSE06) was executed only for the most plausible GGA-relaxed configurations with lower formation and higher band gap energies. The final band gap energy (3.62 eV) obtained after averaging over the selected configurations, resembles closely the experimental band gap value (4.11 eV).

Analysis of InxGa1-xN/Si p-n heterojunction solar cells and the effects of spontaneous and piezoelectric polarization charges

NASA Astrophysics Data System (ADS)

Zheng, Yangdong; Mihara, Akihiro; Yamamoto, Akio

2013-10-01

The band structures, current-voltage characteristics under solar illumination, and photovoltaic (PV) properties of InxGa1-xN/Si p-n heterojunction solar cells (SCs), as well as the effects of spontaneous and piezoelectric polarization (Psp-PPZ) induced charges are investigated theoretically and numerically. We find that the energy peaks on the conduction and valence bands could exponentially reduce the diffusion currents and photocurrents, thus profoundly affect the PV properties of the SCs. Except for large values, the Psp-PPZ induced interface charges have little influence on the band structures and the PV properties. These results should be useful in analysis and design for multijunction tandem InxGa1-xN/Si SC devices.

Isoelectronic tungsten doping in monolayer MoSe 2 for carrier type modulation

DOE PAGES

Li, Xufan; Lin, Ming -Wei; Basile, Leonardo; ...

2016-07-06

Doping and alloying are effective ways to engineer the band structure and modulate the optoelectronic functionality of monolayer transition metal dichalcogenides (TMDs). In this work, we explore the synthesis and electronic properties of monolayer Mo 1-xW xSe 2 (0 < x < 0.18) alloys with almost 100% alloying degree. The isoelectronic substitutional doping of tungsten for molybdenum in the monolayer MoSe 2 is shown to suppress its intrinsically n-type conduction behavior, with p-type conduction gradually emerging to become dominant with increasing W concentration in the alloys. Atomic resolution Z-contrast electron microscopy show that W is shown to substitute directly formore » Mo without the introduction of noticeable vacancy or interstitial defects, however with randomly-distributed W-rich regions ~2 nm in diameter. Scanning tunneling microscopy/spectroscopy measurements reveal that these W-rich regions exhibit a local band structure with the valence band maximum (VBM) closer to the Fermi level as compared with the Mo-rich regions in the monolayer Mo 1-xW xSe 2 crystal. These localized upshifts of the VBM in the local band structure appear responsible for the overall p-type behavior observed for the monolayer Mo 1-xW xSe 2 crystals. Stacked monolayers of n-type MoSe 2 and p-type Mo 1-xW xSe 2 were demonstrated to form atomically thin, vertically stacked p n homojunctions with gate-tunable characteristics, which appear useful for future optoelectronic applications. Lastly, these results indicate that alloying with isoelectronic dopant atoms appears to be an effective and advantageous alternate strategy to doping or alloying with electron donors or acceptors in two-dimensional TMDs.« less

Laser-free RF-gun as a combined source of THz and ps-sub-ps X-rays

DOE Office of Scientific and Technical Information (OSTI.GOV)

Agustsson, R.; Boucher, S.; Finn, O.

A coherent, mm-sub-mm-wave source driven by a RF electron gun is proposed for wide research applications as well as auxiliary inspection and screening, safe imaging, cancer diagnostics, surface defectoscopy, and enhanced time-domain spectroscopy. It allows generation of high peak and average THz-sub-THz radiation power provided by beam pre-bunching and chirping in the RF gun followed by microbunching in magnetic compressor, and resonant Cherenkov radiation of an essentially flat beam in a robust, ~inch-long, planar, mm-sub-mm gap structure. The proof-of-principle has been successfully demonstrated in Phase I on a 5 MeV beam of L-band thermionic injector of Idaho Accelerator Center. Themore » system can also deliver an intense, ps-sub-ps bursts of low-to-moderate dose of relativistic electrons and X-ray radiation produced by the same beam required for pulsed radiolysis as well as to enhance screening efficiency, throughput and safety.« less

Laser-free RF-gun as a combined source of THz and ps-sub-ps X-rays

DOE PAGES

Agustsson, R.; Boucher, S.; Finn, O.; ...

2015-01-01

A coherent, mm-sub-mm-wave source driven by a RF electron gun is proposed for wide research applications as well as auxiliary inspection and screening, safe imaging, cancer diagnostics, surface defectoscopy, and enhanced time-domain spectroscopy. It allows generation of high peak and average THz-sub-THz radiation power provided by beam pre-bunching and chirping in the RF gun followed by microbunching in magnetic compressor, and resonant Cherenkov radiation of an essentially flat beam in a robust, ~inch-long, planar, mm-sub-mm gap structure. The proof-of-principle has been successfully demonstrated in Phase I on a 5 MeV beam of L-band thermionic injector of Idaho Accelerator Center. Themore » system can also deliver an intense, ps-sub-ps bursts of low-to-moderate dose of relativistic electrons and X-ray radiation produced by the same beam required for pulsed radiolysis as well as to enhance screening efficiency, throughput and safety.« less

Ultrasonic irradiation-assisted synthesis of Bi2S3 nanoparticles in aqueous ionic liquid at ambient condition.

PubMed

de la Parra-Arciniega, Salomé M; Garcia-Gomez, Nora A; Garza-Tovar, Lorena L; García-Gutiérrez, Domingo I; Sánchez, Eduardo M

2017-05-01

In this work, an easy, fast and environmentally friendly method to obtain Bi 2 S 3 nanostructures with sphere-like morphology is introduced. The promising material was successfully synthesized by a sonochemical route in 20% 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtSO 4 ] ionic liquid solution (IL). Morphological studies by electron microscopy (SEM and TEM) show that the use of IL in the synthesis of Bi 2 S 3 favors the formation of nanocrystals non-agglomerated. Micro Raman and energy dispersive X-ray spectroscopy (EDXS) were used to determine the composition and purity of the synthesized material. X-ray powder diffraction (XRD) and selective area electron diffraction (SAED) revealed that ultrasonic radiation accelerated the crystallization of Bi 2 S 3 into orthorhombic bismuthinite structure. The band gap calculated from the diffuse reflectance spectra (DRS) was found to be 1.5eV. Copyright © 2016 Elsevier B.V. All rights reserved.

Effect of Γ-X band mixing on the donor binding energy in a Quantum Wire

NASA Astrophysics Data System (ADS)

Vijaya Shanthi, R.; Jayakumar, K.; Nithiananthi, P.

2015-02-01

To invoke the technological applications of heterostructure semiconductors like Quantum Well (QW), Quantum Well Wire (QWW) and Quantum Dot (QD), it is important to understand the property of impurity energy which is responsible for the peculiar electronic & optical behavior of the Low Dimensional Semiconductor Systems (LDSS). Application of hydrostatic pressure P>35kbar drastically alters the band offsets leading to the crossover of Γ band of the well & X band of the barrier resulting in an indirect transition of the carrier and this effect has been studied experimentally and theoretically in a QW structure. In this paper, we have investigated the effect of Γ-X band mixing due to the application of hydrostatic pressure in a GaAs/AlxGa1-xAs QWW system. The results are presented and discussed for various widths of the wire.

Systematic approach for simultaneously correcting the band-gap and p - d separation errors of common cation III-V or II-VI binaries in density functional theory calculations within a local density approximation

DOE PAGES

Wang, Jianwei; Zhang, Yong; Wang, Lin-Wang

2015-07-31

We propose a systematic approach that can empirically correct three major errors typically found in a density functional theory (DFT) calculation within the local density approximation (LDA) simultaneously for a set of common cation binary semiconductors, such as III-V compounds, (Ga or In)X with X = N,P,As,Sb, and II-VI compounds, (Zn or Cd)X, with X = O,S,Se,Te. By correcting (1) the binary band gaps at high-symmetry points , L, X, (2) the separation of p-and d-orbital-derived valence bands, and (3) conduction band effective masses to experimental values and doing so simultaneously for common cation binaries, the resulting DFT-LDA-based quasi-first-principles methodmore » can be used to predict the electronic structure of complex materials involving multiple binaries with comparable accuracy but much less computational cost than a GW level theory. This approach provides an efficient way to evaluate the electronic structures and other material properties of complex systems, much needed for material discovery and design.« less

Systematic approach for simultaneously correcting the band-gap and p -d separation errors of common cation III-V or II-VI binaries in density functional theory calculations within a local density approximation

NASA Astrophysics Data System (ADS)

Wang, Jianwei; Zhang, Yong; Wang, Lin-Wang

2015-07-01

We propose a systematic approach that can empirically correct three major errors typically found in a density functional theory (DFT) calculation within the local density approximation (LDA) simultaneously for a set of common cation binary semiconductors, such as III-V compounds, (Ga or In)X with X =N ,P ,As ,Sb , and II-VI compounds, (Zn or Cd)X , with X =O ,S ,Se ,Te . By correcting (1) the binary band gaps at high-symmetry points Γ , L , X , (2) the separation of p -and d -orbital-derived valence bands, and (3) conduction band effective masses to experimental values and doing so simultaneously for common cation binaries, the resulting DFT-LDA-based quasi-first-principles method can be used to predict the electronic structure of complex materials involving multiple binaries with comparable accuracy but much less computational cost than a GW level theory. This approach provides an efficient way to evaluate the electronic structures and other material properties of complex systems, much needed for material discovery and design.

High-energy Emission from Nonrelativistic Radiative Shocks: Application to Gamma-Ray Novae

NASA Astrophysics Data System (ADS)

Vurm, Indrek; Metzger, Brian D.

2018-01-01

The observation of GeV gamma-rays from novae by Fermi/LAT demonstrates that the nonrelativistic radiative shocks in these systems can accelerate particles to energies of at least ∼10 GeV. The low-energy extension of the same nonthermal particle distribution inevitably gives rise to emission in the hard X-ray band. Above ≳ 10 {keV}, this radiation can escape the system without significant absorption/attenuation, and can potentially be detected by NuSTAR. We present theoretical models for hard X-ray and gamma-ray emission from radiative shocks in both leptonic and hadronic scenarios, accounting for the rapid evolution of the downstream properties due to the fast cooling of thermal plasma. We find that due to strong Coulomb losses, only a fraction of {10}-4{--}{10}-3 of the gamma-ray luminosity is radiated in the NuSTAR band; nevertheless, this emission could be detectable simultaneously with the LAT emission in bright gamma-ray novae with a ∼50 ks exposure. The spectral slope in hard X-rays is α ≈ 0 for typical nova parameters, thus serving as a testable prediction of the model. Our work demonstrates how combined hard X-ray and gamma-ray observations can be used to constrain properties of the nova outflow (velocity, density, and mass outflow rate) and particle acceleration at the shock. A very low X-ray to gamma-ray luminosity ratio ({L}{{X}}/{L}γ ≲ 5× {10}-4) would disfavor leptonic models for the gamma-ray emission. Our model can also be applied to other astrophysical environments with radiative shocks, including SNe IIn and colliding winds in massive star binaries.

Understanding of sub-band gap absorption of femtosecond-laser sulfur hyperdoped silicon using synchrotron-based techniques

PubMed Central

Limaye, Mukta V.; Chen, S. C.; Lee, C. Y.; Chen, L. Y.; Singh, Shashi B.; Shao, Y. C.; Wang, Y. F.; Hsieh, S. H.; Hsueh, H. C.; Chiou, J. W.; Chen, C. H.; Jang, L. Y.; Cheng, C. L.; Pong, W. F.; Hu, Y. F.

2015-01-01

The correlation between sub-band gap absorption and the chemical states and electronic and atomic structures of S-hyperdoped Si have been extensively studied, using synchrotron-based x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge spectroscopy (XANES), extended x-ray absorption fine structure (EXAFS), valence-band photoemission spectroscopy (VB-PES) and first-principles calculation. S 2p XPS spectra reveal that the S-hyperdoped Si with the greatest (~87%) sub-band gap absorption contains the highest concentration of S2− (monosulfide) species. Annealing S-hyperdoped Si reduces the sub-band gap absorptance and the concentration of S2− species, but significantly increases the concentration of larger S clusters [polysulfides (Sn2−, n > 2)]. The Si K-edge XANES spectra show that S hyperdoping in Si increases (decreased) the occupied (unoccupied) electronic density of states at/above the conduction-band-minimum. VB-PES spectra evidently reveal that the S-dopants not only form an impurity band deep within the band gap, giving rise to the sub-band gap absorption, but also cause the insulator-to-metal transition in S-hyperdoped Si samples. Based on the experimental results and the calculations by density functional theory, the chemical state of the S species and the formation of the S-dopant states in the band gap of Si are critical in determining the sub-band gap absorptance of hyperdoped Si samples. PMID:26098075

Theoretical band structure of the superconducting antiperovskite oxide Sr3-xSnO

NASA Astrophysics Data System (ADS)

Ikeda, Atsutoshi; Fukumoto, Toshiyuki; Oudah, Mohamed; Hausmann, Jan Niklas; Yonezawa, Shingo; Kobayashi, Shingo; Sato, Masatoshi; Tassel, Cédric; Takeiri, Fumitaka; Takatsu, Hiroshi; Kageyama, Hiroshi; Maeno, Yoshiteru

2018-05-01

In order to investigate the position of the strontium deficiency in superconductive Sr3-xSnO, we synthesized and measured X-ray-diffraction patterns of Sr3-xSnO (x ∼ 0.5). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on Sr3-xSnO (x = 0, 0.5) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of Sr3-xSnO in case of x ∼ 0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr3SnO, where the mixing of the valence p and conduction d orbitals are considered to be small.

Mechanical, Anisotropic, and Electronic Properties of XN (X = C, Si, Ge): Theoretical Investigations.

PubMed

Ma, Zhenyang; Liu, Xuhong; Yu, Xinhai; Shi, Chunlei; Wang, Dayun

2017-08-08

The structural, mechanical, elastic anisotropic, and electronic properties of Pbca -XN (X = C, Si, Ge) are investigated in this work using the Perdew-Burke-Ernzerhof (PBE) functional, Perdew-Burke-Ernzerhof for solids (PBEsol) functional, and Ceperly and Alder, parameterized by Perdew and Zunger (CA-PZ) functional in the framework of density functional theory. The achieved results for the lattice parameters and band gap of Pbca -CN with the PBE functional in this research are in good accordance with other theoretical results. The band structures of Pbca -XN (X = C, Si, Ge) show that Pbca -SiN and Pbca -GeN are both direct band gap semiconductor materials with a band gap of 3.39 eV and 2.22 eV, respectively. Pbca -XN (X = C, Si, Ge) exhibits varying degrees of mechanical anisotropic properties with respect to the Poisson's ratio, bulk modulus, shear modulus, Young's modulus, and universal anisotropic index. The (001) plane and (010) plane of Pbca -CN/SiN/GeN both exhibit greater elastic anisotropy in the bulk modulus and Young's modulus than the (100) plane.

Mechanical, Anisotropic, and Electronic Properties of XN (X = C, Si, Ge): Theoretical Investigations

PubMed Central

Ma, Zhenyang; Liu, Xuhong; Yu, Xinhai; Shi, Chunlei; Wang, Dayun

2017-01-01

The structural, mechanical, elastic anisotropic, and electronic properties of Pbca-XN (X = C, Si, Ge) are investigated in this work using the Perdew–Burke–Ernzerhof (PBE) functional, Perdew–Burke–Ernzerhof for solids (PBEsol) functional, and Ceperly and Alder, parameterized by Perdew and Zunger (CA–PZ) functional in the framework of density functional theory. The achieved results for the lattice parameters and band gap of Pbca-CN with the PBE functional in this research are in good accordance with other theoretical results. The band structures of Pbca-XN (X = C, Si, Ge) show that Pbca-SiN and Pbca-GeN are both direct band gap semiconductor materials with a band gap of 3.39 eV and 2.22 eV, respectively. Pbca-XN (X = C, Si, Ge) exhibits varying degrees of mechanical anisotropic properties with respect to the Poisson’s ratio, bulk modulus, shear modulus, Young’s modulus, and universal anisotropic index. The (001) plane and (010) plane of Pbca-CN/SiN/GeN both exhibit greater elastic anisotropy in the bulk modulus and Young’s modulus than the (100) plane. PMID:28786960

The Pressure Dependence of Structural, Electronic, Mechanical, Vibrational, and Thermodynamic Properties of Palladium-Based Heusler Alloys

NASA Astrophysics Data System (ADS)

Çoban, Cansu

2017-08-01

The pressure dependent behaviour of the structural, electronic, mechanical, vibrational, and thermodynamic properties of Pd2TiX (X=Ga, In) Heusler alloys was investigated by ab initio calculations. The lattice constant, the bulk modulus and its first pressure derivative, the electronic band structure and the density of states (DOS), mechanical properties such as elastic constants, anisotropy factor, Young's modulus, etc., the phonon dispersion curves and phonon DOS, entropy, heat capacity, and free energy were obtained under pressure. It was determined that the calculated lattice parameters are in good agreement with the literature, the elastic constants obey the stability criterion, and the phonon dispersion curves have no negative frequency which shows that the compounds are stable. The band structures at 0, 50, and 70 GPa showed valence instability at the L point which explains the superconductivity in Pd2TiX (X=Ga, In).

Electrically coupling complex oxides to semiconductors: A route to novel material functionalities

DOE PAGES

Ngai, J. H.; Ahmadi-Majlan, K.; Moghadam, J.; ...

2017-01-12

Complex oxides and semiconductors exhibit distinct yet complementary properties owing to their respective ionic and covalent natures. By electrically coupling complex oxides to traditional semiconductors within epitaxial heterostructures, enhanced or novel functionalities beyond those of the constituent materials can potentially be realized. Essential to electrically coupling complex oxides to semiconductors is control of the physical structure of the epitaxially grown oxide, as well as the electronic structure of the interface. In this paper, we discuss how composition of the perovskite A- and B-site cations can be manipulated to control the physical and electronic structure of semiconductor—complex oxide heterostructures. Two prototypicalmore » heterostructures, Ba 1-xSr xTiO 3/Ge and SrZr xTi 1-xO 3/Ge, will be discussed. In the case of Ba 1-xSr xTiO 3/Ge, we discuss how strain can be engineered through A-site composition to enable the re-orientable ferroelectric polarization of the former to be coupled to carriers in the semiconductor. In the case of SrZr xTi 1-xO 3/Ge we discuss how B-site composition can be exploited to control the band offset at the interface. Finally, analogous to heterojunctions between compound semiconducting materials, control of band offsets, i.e., band-gap engineering, provides a pathway to electrically couple complex oxides to semiconductors to realize a host of functionalities.« less

Electrically coupling complex oxides to semiconductors: A route to novel material functionalities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ngai, J. H.; Ahmadi-Majlan, K.; Moghadam, J.

Complex oxides and semiconductors exhibit distinct yet complementary properties owing to their respective ionic and covalent natures. By electrically coupling complex oxides to traditional semiconductors within epitaxial heterostructures, enhanced or novel functionalities beyond those of the constituent materials can potentially be realized. Essential to electrically coupling complex oxides to semiconductors is control of the physical structure of the epitaxially grown oxide, as well as the electronic structure of the interface. In this paper, we discuss how composition of the perovskite A- and B-site cations can be manipulated to control the physical and electronic structure of semiconductor—complex oxide heterostructures. Two prototypicalmore » heterostructures, Ba 1-xSr xTiO 3/Ge and SrZr xTi 1-xO 3/Ge, will be discussed. In the case of Ba 1-xSr xTiO 3/Ge, we discuss how strain can be engineered through A-site composition to enable the re-orientable ferroelectric polarization of the former to be coupled to carriers in the semiconductor. In the case of SrZr xTi 1-xO 3/Ge we discuss how B-site composition can be exploited to control the band offset at the interface. Finally, analogous to heterojunctions between compound semiconducting materials, control of band offsets, i.e., band-gap engineering, provides a pathway to electrically couple complex oxides to semiconductors to realize a host of functionalities.« less

AB INITIO STUDY OF STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF MgxCd1-xX (X = S, Se, Te) ALLOYS

NASA Astrophysics Data System (ADS)

Noor, N. A.; Shaukat, A.

2012-12-01

This study describes structural, electronic and optical properties of MgxCd1-xX (X = S, Se, Te) alloys in the complete range 0≤x ≤1 of composition x in the zinc-blende (ZB) phase with the help of full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method within density functional theory (DFT). In order to calculate total energy, generalized gradient approximation (Wu-Cohen GGA) has been applied, which is based on optimization energy. For electronic structure calculations, the corresponding potential is being optimized by Engel-Vosko GGA formalism. Our calculations reveal the nonlinear variation of lattice constant and bulk modulus with different concentration for the end binary and their ternary alloys, which slightly deviates from Vegard's law. The calculated band structures show a direct band gap for all three alloys with increasing order in the complete range of the compositional parameter x. In addition, we have discussed the disorder parameter (gap bowing) and concluded that the total band gap bowing is substantially influenced by the chemical (electronegativity) contribution. The calculated density of states (DOS) of these alloys is discussed in terms of contribution from various s-, p- and d-states of the constituent atoms and charge density distributions plots are analyzed. Optical properties have been presented in the form of the complex dielectric function ɛ(ω), refractive index n(ω) and extinction coefficient k(ω) as function of the incident photon energy, and the results have been compared with existing experimental data and other theoretical calculations.

Control of Ge1-x-ySixSny layer lattice constant for energy band alignment in Ge1-xSnx/Ge1-x-ySixSny heterostructures

NASA Astrophysics Data System (ADS)

Fukuda, Masahiro; Watanabe, Kazuhiro; Sakashita, Mitsuo; Kurosawa, Masashi; Nakatsuka, Osamu; Zaima, Shigeaki

2017-10-01

The energy band alignment of Ge1-xSnx/Ge1-x-ySixSny heterostructures was investigated, and control of the valence band offset at the Ge1-xSnx/Ge1-x-ySixSny heterointerface was achieved by controlling the Si and Sn contents in the Ge1-x-ySixSny layer. The valence band offset in the Ge0.902Sn0.098/Ge0.41Si0.50Sn0.09 heterostructure was evaluated to be as high as 330 meV, and its conduction band offset was estimated to be 150 meV by considering the energy bandgap calculated from the theoretical prediction. In addition, the formation of the strain-relaxed Ge1-x-ySixSny layer was examined and the crystalline structure was characterized. The epitaxial growth of a strain-relaxed Ge0.64Si0.21Sn0.15 layer with the degree of strain relaxation of 55% was examined using a virtual Ge substrate. Moreover, enhancement of the strain relaxation was demonstrated by post-deposition annealing, where a degree of strain relaxation of 70% was achieved after annealing at 400 °C. These results indicate the possibility for enhancing the indirect-direct crossover with a strained and high-Sn-content Ge1-xSnx layer on a strain-relaxed Ge1-x-ySixSny layer, realizing preferable carrier confinement by type-I energy band alignment with high conduction and valence band offsets.

Rectifying behavior in the GaN/graded-AlxGa1‑xN/GaN double heterojunction structure

NASA Astrophysics Data System (ADS)

Wang, Caiwei; Jiang, Yang; Ma, Ziguang; Zuo, Peng; Yan, Shen; Die, Junhui; Wang, Lu; Jia, Haiqiang; Wang, Wenxin; Chen, Hong

2018-05-01

Rectifying characteristics induced by the polarization fields are achieved in the GaN/graded-AlxGa1‑xN/GaN double heterojunction structure (DHS). By grading AlxGa1‑xN from x  =  0.4(0.3) to 0.1, the DHS displays a better conductivity for smaller reverse bias than for forward bias voltages (reverse rectifying behavior) which is opposite to p–n junction rectifying characteristics. The mechanism of reverse rectifying behavior is illustrated via calculating the energy band structures of the samples. The band gap narrowing caused by decreasing Al composition could compensate the for the band tilt due to the polarization effect in AlxGa1‑xN barriers, thus lowering the barrier height for electron transport from top to bottom. The reverse rectifying behavior could be enhanced by increasing the Al content and the thickness of the multi-layer graded AlxGa1‑xN barriers. This work gives a better understanding of the mechanism of carrier transport in a DHS and makes it possible to realize novel GaN-based heterojunction transistors.

Experimental and theoretical study of the electronic structure of single-crystal BaBiO3

NASA Astrophysics Data System (ADS)

Balandeh, Shadi; Green, Robert J.; Foyevtsova, Kateryna; Chi, Shun; Foyevtsov, Oleksandr; Li, Fengmiao; Sawatzky, George A.

2017-10-01

High quality single crystals of BaBiO3 were grown by congruent melting technique and characterized with x-ray diffraction, x-ray photoemission, and transport property studies. The perovskite oxide BaBiO3 is a negative charge transfer gap high Tc oxide parent superconducting compound exhibiting self-doping of holes into the oxygen 2 p band. We study the low energy scale valence and conduction bands in detail from both a theoretical perspective as well as through x ray, absorption/emission, and photoelectron spectroscopies. X-ray spectroscopy verifies the results of density functional theory (DFT) regarding the overall band structure featuring strong O 2 p character of the empty antibonding combination of the hybridized Bi 6 s and O 2 p states. From the analysis of the core level line shapes we conclude that the dominant O 2 p -Bi 6 s hybridization energy scale determines the low energy scale electronic structure. This analysis provides further insight into the importance of self-doped oxygen 2 p states in this high Tc family of oxides.

Measurements of Local Strain Variation in Si(1-x)Ge(x)/Si Heterostructures

NASA Technical Reports Server (NTRS)

Bell, L. D.; Kaiser, W. J.; Manion, S. J.; Milliken, S. J.; Pike, W. T.; Fathauer, R. W.

1995-01-01

The energy splitting of the conduction-band minimum of Si(1-x), Ge(x), due to strain has been directly measured by the application of ballistic-electron-emission microscope (BEEM) spectroscopy to Ag/Si(1-x), Ge(x) structures. Experimental values for this conduction-band splitting agree well with calculations. For Au/Si(1-x), Ge(x), however, heterogeneity in the strain of the Si(1-x), Ge(x) layer is introduced by deposition of the Au. This variation is attributed to species interdiffusion, which produces a rough Si(1-x)Ge(x) surface. Preliminary modeling indicates that the observed roughness is consistent with the strain variation measured by BEEM.

Structural, electronic, magnetic and optical properties of semiconductor Zn1-xMoxTe compound

NASA Astrophysics Data System (ADS)

Feng, Zhong-Ying; Zhang, Jian-Min

2018-03-01

The structural, electronic, magnetic and optical properties of the Zn1-xMoxTe (x = 0.00, 0.25, 0.50, 0.75, 1.00) have been investigated by the spin-polarized first-principles calculations. The Zn0.50Mo0.50Te has tetragonal structure while the Zn1-xMoxTe (x = 0.00, 0.25, 0.75, 1.00) crystallize in cubic structures. For Zn1-xMoxTe (x = 0.25, 0.50, 0.75, 1.00) alloys, the lattice constant and the volume are found larger than those of pure ZnTe alloy. The Zn1-xMoxTe (x = 0.25, 0.50, 0.75, 1.00) is magnetic and the Mo element is found dominant in the bands crossing the Fermi level in the spin-up channel. The Zn0.75Mo0.25Te and MoTe have half-metallic (HM) behavior. In spin-down channel of the Zn0.75Mo0.25Te, the Zn atom mainly contributed to the conduction band minimum (CBM), while the valence band maximum (VBM) appears mainly due to contribution of Te element. A positive spin splitting and crystal field splitting of d-states of Mo atom has been observed for Zn0.75Mo0.25Te alloy. The maximum values of the absorption coefficients αMAX(ω) of the Zn0.50Mo0.50Te alloy along a or b axes are smaller than the absorption coefficient along c axis. The first absorption peak appearing in the energy range of 0.000-1.000 eV for Zn1-xMoxTe (x = 0.25, 0.50, 0.75 or 1.00) alloys is the new peak which is not observed in ZnTe.

Structural and electronic properties of Sr{sub x}Ba{sub 1-x}SnO{sub 3} from first principles calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moreira, E.; Henriques, J.M.; Azevedo, D.L.

2012-03-15

Neutron diffraction data for Sr{sub x}Ba{sub 1-x}SnO{sub 3} (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) solid solutions were used as inputs to obtain optimized geometries and electronic properties using the density functional theory (DFT) formalism considering both the local density and generalized gradient approximations, LDA and GGA, respectively. The crystal structures and SnO{sub 6} octahedra tilting angles found after total energy minimization agree well with experiment, specially for the GGA data. Elastic constants were also obtained and compared with theoretical and experimental results for cubic BaSnO{sub 3}. While the alloys with cubic unit cell have an indirect band gap, tetragonalmore » and orthorhombic alloys exhibit direct band gaps (exception made to x=1.0). The Kohn-Sham minimum electronic band gap oscillates from 1.52 eV (cubic x=0.0, LDA) to 2.61 eV (orthorhombic x=1.0, LDA), and from 0.74 eV (cubic BaSnO{sub 3}, GGA) to 1.97 eV (orthorhombic SrSnO{sub 3}, GGA). Parabolic interpolation of bands has allowed us to estimate the effective masses for charge carriers, which are shown to be anisotropic and larger for holes. - Graphical Abstract: Highlights: Black-Right-Pointing-Pointer DFT calculations were performed on Sr{sub x}Ba{sub 1-x}SnO{sub 3} solid solutions. Black-Right-Pointing-Pointer Calculated crystal structures agree well with experiment. Black-Right-Pointing-Pointer Alloys have direct or indirect gaps depending on the Sr molar fraction. Black-Right-Pointing-Pointer The Kohn-Sham gap variation from x=0.0 to x=1.0 is close to the experimental value. Black-Right-Pointing-Pointer Carrier effective masses are very anisotropic, specially for holes.« less

Two Step Acceleration Process of Electrons in the Outer Van Allen Radiation Belt by Time Domain Electric Field Bursts and Large Amplitude Chorus Waves

NASA Astrophysics Data System (ADS)

Agapitov, O. V.; Mozer, F.; Artemyev, A.; Krasnoselskikh, V.; Lejosne, S.

2014-12-01

A huge number of different non-linear structures (double layers, electron holes, non-linear whistlers, etc) have been observed by the electric field experiment on the Van Allen Probes in conjunction with relativistic electron acceleration in the Earth's outer radiation belt. These structures, found as short duration (~0.1 msec) quasi-periodic bursts of electric field in the high time resolution electric field waveform, have been called Time Domain Structures (TDS). They can quite effectively interact with radiation belt electrons. Due to the trapping of electrons into these non-linear structures, they are accelerated up to ~10 keV and their pitch angles are changed, especially for low energies (˜1 keV). Large amplitude electric field perturbations cause non-linear resonant trapping of electrons into the effective potential of the TDS and these electrons are then accelerated in the non-homogeneous magnetic field. These locally accelerated electrons create the "seed population" of several keV electrons that can be accelerated by coherent, large amplitude, upper band whistler waves to MeV energies in this two step acceleration process. All the elements of this chain acceleration mechanism have been observed by the Van Allen Probes.

Electronic structure and thermoelectric properties of half-Heusler compounds with eight electron valence count—KScX (X = C and Ge)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ciftci, Yasemin O.; Mahanti, Subhendra D.

Electronic band structure and structural properties of two representative half-Heusler (HH) compounds with 8 electron valence count (VC), KScC and KScGe, have been studied using first principles methods within density functional theory and generalized gradient approximation. These systems differ from the well studied class of HH compounds like ZrNiSn and ZrCoSb which have VC = 18 because of the absence of d electrons of the transition metal atoms Ni and Co. Electronic transport properties such as Seebeck coefficient (S), electrical conductivity (σ), electronic thermal conductivity (κ{sub e}) (the latter two scaled by electronic relaxation time), and the power factor (S{sup 2}σ) havemore » been calculated using semi-classical Boltzmann transport theory within constant relaxation time approximation. Both the compounds are direct band gap semiconductors with band extrema at the X point. Their electronic structures show a mixture of heavy and light bands near the valance band maximum and highly anisotropic conduction and valence bands near the band extrema, desirable features of good thermoelectric. Optimal p- or n-type doping concentrations have been estimated based on thermopower and maximum power factors. The optimum room temperature values of S are ∼1.5 times larger than that of the best room temperature thermoelectric Bi{sub 2}Te{sub 3}. We also discuss the impact of the band structure on deviations from Weidemann-Franz law as one tunes the chemical potential across the band gap.« less

Tunable electronic structure in stained two dimensional van der Waals g-C2N/XSe2 (X = Mo, W) heterostructures

NASA Astrophysics Data System (ADS)

Zheng, Z. D.; Wang, X. C.; Mi, W. B.

2017-10-01

The electronic structure of the strained g-C2N/XSe2 (X=Mo, W) van der Waals heterostructures are investigated by first-principles calculations. The g-C2N/MoSe2 heterostructure is an indirect band gap semiconductor at a strain from 0% to 8%, where its band gap is 0.66, 0.61, 0.73, 0.60 and 0.33 eV. At K point, the spin splitting is 186, 181, 39, 13 and 9 meV, respectively. For g-C2N/WSe2 heterostructures, the band gap is 0.32, 0.37, 0.42, 0.45 and 0.36 eV, and the conduction band minimum is shifted from Г-M region to K-Г region as the strain increases from 0% to 8%. Its spin splitting monotonically decreases as a strain raises to 8%, which is 445, 424, 261, 111 and 96 meV, respectively. Moreover, at a strain less than 4%, the conduction band mainly comes from g-C2N, but it comes from XSe2 (X=Mo, W) above 6%. Our results show that the g-C2N/XSe2 heterostructures have tunable electronic structures, which makes it a potential candidate for novel electronic devices.

Experimental visualization of covalent bonds and structural disorder in a gallium zinc oxynitride photocatalyst (Ga(1-x)Znx)(N(1-x)Ox): origin of visible light absorption.

PubMed

Yashima, Masatomo; Yamada, Hiroki; Maeda, Kazuhiko; Domen, Kazunari

2010-04-14

We present the experimental visualization of covalent bonding, positional disorders and split anion sites in visible-light responsive photocatalyst (Ga(0.885)Zn(0.115))(N(0.885)O(0.115)). ZnO alloying into GaN reduces the band gap, leading to the visible-light response. DFT calculations indicated no significant difference in band gap between structural models with and without split sites.

Design of Semiconducting Tetrahedral Mn 1-xZn xO Alloys and Their Application to Solar Water Splitting

DOE PAGES

Peng, Haowei; Ndione, Paul F.; Ginley, David S.; ...

2015-03-18

Transition metal oxides play important roles as contact and electrode materials, but their use as active layers in solar energy conversion requires achieving semiconducting properties akin to those of conventional semiconductors like Si or GaAs. In particular, efficient bipolar carrier transport is a challenge in these materials. Based on the prediction that a tetrahedral polymorph of MnO should have such desirable semiconducting properties, and the possibility to overcome thermodynamic solubility limits by nonequilibrium thin-film growth, we exploit both structure-property and composition-structure relationships to design and realize novel wurtzite-structure Mn 1₋xZn xO alloys. At Zn compositions above x≈0.3, thin films ofmore » these alloys assume the tetrahedral wurtzite structure instead of the octahedral rocksalt structure of MnO, thereby enabling semiconductor properties that are unique among transition metal oxides, i.e., a band gap within the visible spectrum, a band-transport mechanism for both electron and hole carriers, electron doping, and a band lineup suitable for solar hydrogen generation. In conclusion, a proof of principle is provided by initial photo-electrocatalytic device measurements, corroborating, in particular, the predicted favorable hole-transport properties of these alloys.« less

Diamond field emitter array cathodes and possibilities of employing additive manufacturing for dielectric laser accelerating structures

NASA Astrophysics Data System (ADS)

Simakov, Evgenya I.; Andrews, Heather L.; Herman, Matthew J.; Hubbard, Kevin M.; Weis, Eric

2017-03-01

Demonstration of a stand-alone practical dielectric laser accelerator (DLA) requires innovation in two major critical components: high-current ultra-low-emittance cathodes and efficient laser accelerator structures. LANL develops two technologies that in our opinion are applicable to the novel DLA architectures: diamond field emitter array (DFEA) cathodes and additive manufacturing of photonic band-gap (PBG) structures. This paper discusses the results of testing of DFEA cathodes in the field-emission regime and the possibilities for their operation in the photoemission regime, and compares their emission characteristics to the specific needs of DLAs. We also describe recent advances in additive manufacturing of dielectric woodpile structures using a Nanoscribe direct laser-writing device capable of maskless lithography and additive manufacturing, and the development of novel infrared dielectric materials compatible with additive manufacturing.

Design and simulation of RF MEMS SPST shunt and SPDT shunt-shunt switches for X-band and Ku-band applications

NASA Astrophysics Data System (ADS)

Lenka, Manas K.; Sharma, Amit; Sharma, Jaibir; DasGupta, Amitava

2012-10-01

This paper describes the design and simulation of RF MEMS SPST shunt and SPDT shunt-shunt switches with modified coplanar waveguide (CPW) configuration for X-band and Ku-band applications exhibiting high isolation and low insertion loss. By modifying the basic CPW structure for a six-strip membrane having length 720 μm, the resonant frequency can be reduced from 33.5 GHz to 13.5 GHz with isolation as high as -30 dB(-63 dB at resonant frequency) in Ku-band. Similar results are also found in case SPST and SPDT switches with other membrane types.

The Physics and Applications of High Brightness Electron Beams

NASA Astrophysics Data System (ADS)

Palumbo, Luigi; Rosenzweig, J.; Serafini, Luca

2007-09-01

Plenary sessions. RF deflector based sub-Ps beam diagnostics: application to FEL and advanced accelerators / D. Alesini. Production of fermtosecond pulses and micron beam spots for high brightness electron beam applications / S.G. Anderson ... [et al.]. Wakefields of sub-picosecond electron bunches / K.L.F. Bane. Diamond secondary emitter / I. Ben-Zvi ... [et al.]. Parametric optimization for an X-ray free electron laser with a laser wiggler / R. Bonifacio, N. Piovella and M.M. Cola. Needle cathodes for high-brightness beams / C.H. Boulware ... [et al.]. Non linear evolution of short pulses in FEL cascaded undulators and the FEL harmonic cascade / L. Giannessi and P. Musumeci. High brightness laser induced multi-meV electron/proton sources / D. Giulietti ... [et al.]. Emittance limitation of a conditioned beam in a strong focusing FEL undulator / Z. Huang, G. Stupakov and S. Reiche. Scaled models: space-charge dominated electron storage rings / R.A. Kishek ... [et al.]. High brightness beam applications: energy recovered linacs / G.A. Krafft. Maximizing brightness in photoinjectors / C. Limborg-Deprey and H. Tomizawa. Ultracold electron sources / O.J. Luiten ... [et al.]. Scaling laws of structure-based optical accelerators / A. Mizrahi, V. Karagodsky and L. Schächter. High brightness beams-applications to free-electron lasers / S. Reiche. Conception of photo-injectors for the CTF3 experiment / R. Roux. Superconducting RF photoinjectors: an overview / J. Sekutowicz. Status and perspectives of photo injector developments for high brightness beams / F. Stephan. Results from the UCLA/FNLP underdense plasma lens experiment / M.C. Thompson ... [et al.]. Medical application of multi-beam compton scattering monochromatic tunable hard X-ray source / M. Uesaka ... [et al.]. Design of a 2 kA, 30 fs RF-photoinjector for waterbag compression / S.B. Van Der Geer, O.J. Luiten and M.J. De Loos. Proposal for a high-brightness pulsed electron source / M. Zolotorev ... [et al.]. -- Working Group 1. Summary of working group 1 on electron sources / M. Ferrario and G. Gatti. Design and RF measurements of an X-band accelerating structure for the SPARC project / D. Alesini ... [et al.]. Mitigation of RF gun breakdown by removal of tuning rods in high field regions / A.M. Cook... [et al.]. Measurements of quantum efficiency of Mg films produced by pulsed laser ablation deposition for application to bright electron sources / G. Gatti ... [et al.]. The S-band 1.6 cell RF gun correlated energy spread dependence on Pi and 0 mode relative amplitude / F. Schmerge ... [et al.]. RF gun photo-emission model for metal cathodes including time dependent emission / J.F. Schmerge ... [et al.]. Superconducting photocathodes / J. Smedley ... [et al.]. -- Working Group 2. Summary of Working Group 2: diagnostics and beam manipulation / G. Travish. Observation of coherent edge radiation emitted by a 100 Femtosecond compressed electron beam / G. Andonian, M, Dunning, E. Hemsing, J. B. Rosenzweig ... [et al.]. PARMELA simulations for PITZ: first machine studies and interpretation of measurements / M. Boscolo ... [et al.]. The LCLS single-shot relative bunch length monitor system / M.P. Dunning ... [et al.]. Beam shaping and permanent magnet quadrupole focusing with applications to the plasma wakefield accelerator / R.J. England ... [et al.]. Commissioning of the SPARC movable emittance meter and its first operation at PITZ / D. Filippetto... [et al.]. Experimental testing of dynamically optimized photoelectron beams / J.B. Rosenzweig ... [et al.]. Synchronization between the laser and electron beam in a photocathode RF gun / A. Sakumi ... [et al.]. Method of bunch radiation photochronography with 10 Femtosecond and less resolution / A. Tron and I. Merinov -- Working Group 3. New challenges in theory and modeling-summary for working group 3. L. Giannessi. Resonant modes in a 1.6 cells RF gun / M. Ferrario and C. Ronsivalle. Emittance degradation due to wake fields in a high brightness photoinjector / M. Ferrario, V. Fusco, M. Migliorati and L. Palumbo. Simulations of coherent synchroton radiation effects in electron machines / M. Migliorati, A, Schiavi and G. Dattoli. QFEL: A numerical code for multi-dimensional simulation of free electron lasers in the quantum regime / A. Schiavi ... [et al.]. First simulations results on laser pulse jitter and microbunching instability at Saprxino / M. Boscolo ... [et al.]. -- Working Group 4. Working group 4 summary: applications of high brightness beams to advanced accelerators and light sources / M. Uesaka and A. Rossi. Study of transverse effects in the production of X-rays with free-electron laser based on an optical ondulator / A. Bacci ... [et al.]. Channeling projects at LNF: from crystal undulators to capillary waveguides / S.B. Dabagov ... [et al.]. Mono-Energetic electron generation and plasma diagnosis experiments in a laser plasma cathode / K. Kinoshita ... [et al.]. A high-density electron beam and quad-scan measurements at Pleiades Thompson X-ray source / J.K. Lim ... [et al.]. Laser pulse circulation system for compact monochromatic tunable hard X-ray source / H. Ogino ... [et al.]. Limits on production of narrow band photons from inverse compton scattering / J. Rosenzweig and O. Williams. Preliminary results from the UCLA/SLAC ultra-high gradient Cerenkov wakefield accelerator experiment / M.C. Thompson ... [et al.]. Status of the polarized nonlinear inverse compton scattering experiment at UCLA / O. Williams... [et al.]. Coupling laser power into a slab-symmetric accelerator structure / R.B. Yoder and J.B. Rosenzweig.

Effect of chlorination on the TlBr band edges for improved room temperature radiation detectors: Effect of chlorination on the TlBr band edges for radiation detectors

DOE PAGES

Varley, J. B.; Conway, A. M.; Voss, L. F.; ...

2015-02-09

Thallium bromide (TlBr) crystals subjected to hydrochloric acid (HCl) chemical treatments have been shown to advantageously affect device performance and longevity in TlBr-based room temperature radiation detectors, yet the exact mechanisms of the improvements remain poorly understood. Here in this paper, we investigate the influence of several HCl chemical treatments on device-grade TlBr and describe the changes in the composition and electronic structure of the surface. Composition analysis and depth profiles obtained from secondary ion mass spectrometry (SIMS) identify the extent to which each HCl etch condition affects the detector surface region and forms of a graded TlBr/TlBr 1-xCL xmore » surface heterojunction. Using a combination of X-ray photoemission spectroscopy (XPS) and hybrid density functional calculations, we are able to determine the valence band offsets, band gaps, and conduction band offsets as a function of Cl content over the entire composition range of TIBr 1-xC1 X. This study establishes a strong correlation between device process conditions, surface chemistry, and electronic structure with the goal of further optimizing the long-term stability and radiation response of TlBr-based detectors.« less

Diverse carrier mobility of monolayer BNC x : a combined density functional theory and Boltzmann transport theory study.

PubMed

Wu, Tao; Deng, Kaiming; Deng, Weiqiao; Lu, Ruifeng

2017-10-19

BNC x monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNC x (x  =  1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC 2 -1 is a material with very small band gap (0.02 eV) among all the structures while other BNC x monolayers are semiconductors with band gap ranging from 0.51 eV to 1.32 eV. The carrier mobility of BNC x varies considerably from tens to millions of cm 2 V -1 s -1 . For BNC 2 -1, the hole mobility and electron mobility along both x and y directions can reach 10 5 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNC x monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC 4 , its hole mobility along the y direction and electron mobility along the x direction unexpectedly reach 10 6 orders of magnitude, even higher than that of graphene. Our findings suggest that BNC x layered materials with the proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices.

Diverse carrier mobility of monolayer BNC x : a combined density functional theory and Boltzmann transport theory study

NASA Astrophysics Data System (ADS)

Wu, Tao; Deng, Kaiming; Deng, Weiqiao; Lu, Ruifeng

2017-11-01

BNC x monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNC x (x  =  1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC2-1 is a material with very small band gap (0.02 eV) among all the structures while other BNC x monolayers are semiconductors with band gap ranging from 0.51 eV to 1.32 eV. The carrier mobility of BNC x varies considerably from tens to millions of cm2 V-1 s-1. For BNC2-1, the hole mobility and electron mobility along both x and y directions can reach 105 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNC x monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC4, its hole mobility along the y direction and electron mobility along the x direction unexpectedly reach 106 orders of magnitude, even higher than that of graphene. Our findings suggest that BNC x layered materials with the proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices.

Stable monolayer honeycomb-like structures of RuX2 (X =S,Se)

NASA Astrophysics Data System (ADS)

Ersan, Fatih; Cahangirov, Seymur; Gökoǧlu, Gökhan; Rubio, Angel; Aktürk, Ethem

2016-10-01

Recent studies show that several metal oxides and dichalcogenides (M X2) , which exist in nature, can be stable in two-dimensional (2D) form and each year several new M X2 structures are explored. The unstable structures in H (hexagonal) or T (octahedral) forms can be stabilized through Peierls distortion. In this paper, we propose new 2D forms of RuS2 and RuSe2 materials. We investigate in detail the stability, electronic, magnetic, optical, and thermodynamic properties of 2D Ru X2 (X =S,Se) structures from first principles. While their H and T structures are unstable, the distorted T structures (T'-Ru X2) are stable and have a nonmagnetic semiconducting ground state. The molecular dynamic simulations also confirm that T'-Ru X2 systems are stable even at 500 K without any structural deformation. T'-RuS2 and T'-RuSe2 have indirect band gaps with 0.745 eV (1.694 eV with HSE) and 0.798 eV (1.675 eV with HSE) gap values, respectively. We also examine their bilayer and trilayer forms and find direct and smaller band gaps. We find that AA stacking is more favorable than the AB configuration. The new 2D materials obtained can be good candidates with striking properties for applications in semiconductor electronic, optoelectronic devices, and sensor technology.

Effect of Nb Content on Mechanical Behavior and Structural Properties of W/(Zr55Cu30Al10Ni5)100- x Nb x Composite

NASA Astrophysics Data System (ADS)

Mahmoodan, Morteza; Gholamipour, Reza; Mirdamadi, Shamseddin; Nategh, Said

2017-05-01

In the present study, (Zr55Cu30Al10Ni5)100- x Nb( x=0,1,2,3) bulk metallic glass matrix/tungsten wire composites were fabricated by infiltration process. Structural studies were investigated by scanning electron microscopy and X-ray diffraction method. Also, mechanical behaviors of the materials were analyzed using quasi-static compressive tests. Results indicated that the best mechanical properties i.e., 2105 MPa compressive ultimate strength and 28 pct plastic strain before failure, were achieved in the composite sample with X = 2. It was also found that adding Nb to the matrix modified interface structure in W fiber/(Zr55Cu30Al10Ni5)98Nb2 since the stable diffusion band formation acts as a functionally graded layer. Finally, the observation of multiple shear bands formation in the matrix could confirm the excellent plastic deformation behavior of the composite.

First principles examination of electronic structure and optical features of 4H-GaN1-xPx polytype alloys

NASA Astrophysics Data System (ADS)

Laref, A.; Hussain, Z.; Laref, S.; Yang, J. T.; Xiong, Y. C.; Luo, S. J.

2018-04-01

By using first-principles calculations, we compute the electronic band structures and typical aspects of the optical spectra of hexagonally structured GaN1-xPx alloys. Although a type III-V semiconductor, GaP commonly possesses a zinc-blende structure with an indirect band gap; as such, it may additionally form hexagonal polytypes under specific growth conditions. The electronic structures and optical properties are calculated by combining a non-nitride III-V semiconductor and a nitride III-V semiconductor, as GaP and GaN crystallizing in a 4H polytype, with the N composition ranging between x = 0-1. For all studied materials, the energy gap is found to be direct. The optical properties of the hexagonal materials may illustrate the strong polarization dependence owing to the crystalline anisotropy. This investigation for GaN1-xPx alloys is anticipated to supply paramount information for applications in the visible/ultraviolet spectral regions. At a specific concentration, x, these alloys would be exclusively appealing candidates for solar-cell applications.

Estimating tropical forest structure using LIDAR AND X-BAND INSAR

NASA Astrophysics Data System (ADS)

Palace, M. W.; Treuhaft, R. N.; Keller, M. M.; Sullivan, F.; Roberto dos Santos, J.; Goncalves, F. G.; Shimbo, J.; Neumann, M.; Madsen, S. N.; Hensley, S.

2013-12-01

Tropical forests are considered the most structurally complex of all forests and are experiencing rapid change due to anthropogenic and climatic factors. The high carbon stocks and fluxes make understanding tropical forests highly important to both regional and global studies involving ecosystems and climate. Large and remote areas in the tropics are prime targets for the use of remotely sensed data. Radar and lidar have previously been used to estimate forest structure, with an emphasis on biomass. These two remote sensing methods have the potential to yield much more information about forest structure, specifically through the use of X-band radar and waveform lidar data. We examined forest structure using both field-based and remotely sensed data in the Tapajos National Forest, Para, Brazil. We measured multiple structural parameters for about 70 plots in the field within a 25 x 15 km area that have TanDEM-X single-pass horizontally and vertically polarized radar interferometric data. High resolution airborne lidar were collected over a 22 sq km portion of the same area, within which 33 plots were co-located. Preliminary analyses suggest that X-band interferometric coherence decreases by about a factor of 2 (from 0.95 to 0.45) with increasing field-measured vertical extent (average heights of 7-25 m) and biomass (10-430 Mg/ha) for a vertical wavelength of 39 m, further suggesting, as has been observed at C-band, that interferometric synthetic aperture radar (InSAR) is substantially more sensitive to forest structure/biomass than SAR. Unlike InSAR coherence versus biomass, SAR power at X-band versus biomass shows no trend. Moreover, airborne lidar coherence at the same vertical wavenumbers as InSAR is also shown to decrease as a function of biomass, as well. Although the lidar coherence decrease is about 15% more than the InSAR, implying that lidar penetrates more than InSAR, these preliminary results suggest that X-band InSAR may be useful for structure and biomass estimation over large spatial scales not attainable with airborne lidar. In this study, we employed a set of less commonly used lidar metrics that we consider analogous to field-based measurements, such as the number of canopy maxima, measures of canopy vegetation distribution diversity and evenness (entropy), and estimates of gap fraction. We incorporated these metrics, as well as lidar coherence metrics pulled from discrete Fourier transforms of pseudowaveforms, and hypothetical stand characteristics of best-fit synthetic vegetation profiles into multiple regression analysis of forest biometric properties. Among simple and complex measures of forest structure, ranging from tree density, diameter at breast height, and various canopy geometry parameters, we found strong relationships with lidar canopy vegetation profile parameters. We suggest that the sole use of lidar height is limited in understanding biomass in a forest with little variation across the landscape and that there are many parameters that may be gleaned by lidar data that inform on forest biometric properties.

Chemical doping in pnictides superconductors: The case of Ca(Fe1-xXx) 2As2 , X = Co, Ni, Pt

NASA Astrophysics Data System (ADS)

Continenza, Alessandra; Profeta, Gianni

2018-04-01

We present a comparative and detailed study of transition metal doping in CaFe2As2. Comparing with several experimental results and carefully analyzing how the states at the Fermi level are affected by doping we show that: i) simulation of real doping and considering induces structural relaxations are crucial to correctly address the physical mechanisms induced by transition metal substitutions; ii) different dopant concentration induces changes on the band structure that can not be described within a simple rigid-band picture; iii) careful comparison with the available ARPES results shows that the main effects on band filling and symmetry can be caught within DFT.

Multi-wave band SMM-VLA observations of an M2 flare and an associated coronal mass ejection

NASA Technical Reports Server (NTRS)

Willson, Robert F.; Lang, Kenneth R.; Schmelz, Joan T.; Gonzalez, Raymond D.; Smith, Kermit L.

1991-01-01

Results are presented of observations of an M2 flare and an associated coronal mass ejection CME by instruments on the SMM as well as by the VLA and other ground-based observatories on September 30, 1988. The multiwave band data show a gradual slowly changing event which lasted several hours. The microwave burst emission was found to originate in compact moderately circularly polarized sources located near the sites of bright H-alpha and soft X-ray emission. These data are combined with estimates of an electron temperature of 1.5 x 10 to the 7th K and an emission measure of about 2.0 x 10 to the 49th/cu cm obtained from Ca XIX and Fe XXV spectra to show that the microwave emission can be attributed to thermal gyrosynchrotron radiation in regions where the magnetic field strength is 425-650 G. The CME acceleration at low altitudes is measured on the basis of ground- and space-based coronagraphs.

Optical and structural properties of amorphous Se x Te100- x aligned nanorods

NASA Astrophysics Data System (ADS)

Al-Agel, Faisal A.

2013-12-01

In the present work, we report studies on optical and structural phenomenon in as-deposited thin films composed of aligned nanorods of amorphous Se x Te100- x ( x = 3, 6, 9, and 12). In structural studies, field emission scanning electron microscopic (FESEM) images suggest that these thin films contain high yield of aligned nanorods. These nanorods show a completely amorphous nature, which is verified by X-ray diffraction patterns of these thin films. Optical studies include the measurement of spectral dependence of absorption, reflection, and transmission of these thin films, respectively. On the basis of optical absorption data, a direct optical band gap is observed. This observation of a direct optical band gap in these nanorods is interesting as chalcogenides normally show an indirect band gap, and due to this reason, these materials could not become very popular for semiconducting devices. Therefore, this is an important report and will open up new directions for the application of these materials in semiconducting devices. The value of this optical band gap is found to decrease with the increase in selenium (Se) concentration. The reflection and absorption data are employed to estimate the values of optical constants (extinction coefficient ( k) and refractive index ( n)). From the spectral dependence of these optical constants, it is found that the values of refractive index ( n) increase, whereas the values of extinction coefficient ( k) decrease with the increase in photon energy. The real and imaginary parts of dielectric constants calculated with the values of extinction coefficient ( k) and refractive index ( n), are found to vary with photon energy and dopant concentration.

Electrical, Optical and Structural Studies of INAS/INGASB VLWIR Superlattices

DTIC Science & Technology

2013-01-01

period measured by x-ray diffraction and the optical band gap energy determined by the photoresponse spectra. Sample InAs (Å) GaSb (Å) In (%) IF (Å...8x8 EFA. 22 Temperature-dependent lattice constants, band gap energies , and other physical data for InAs and GaSb are taken from Vurgaftman et al...gallium antimonide to achieve energy band gaps less than 50 meV with a superlattice period on the order of 68 Å. Similar to the work reported on

Two-dimensional wide-band-gap II-V semiconductors with a dilated graphene-like structure

NASA Astrophysics Data System (ADS)

Zhang, Xue-Jing; Liu, Bang-Gui

2016-12-01

Since the advent of graphene, two-dimensional (2D) materials have become very attractive and there is growing interest in exploring new 2D materials beyond graphene. Here, through density-functional theory (DFT) calculations, we predict 2D wide-band-gap II-V semiconductor materials of M3X2 (M = Zn, Cd and X = N, P, As) with a dilated graphene-like honeycomb structure. In this structure the group-V X atoms form two X-atomic planes symmetrically astride the centering group-IIB M atomic plane. Our DFT calculation shows that 2D Zn3N2, Zn3P2 and Zn3As2 have direct band gaps of 2.87, 3.81 and 3.55 eV, respectively, and 2D Cd3N2, Cd3P2 and Cd3As2 exhibit indirect band gaps of 2.74, 3.51 and 3.29 eV, respectively. Each of the six 2D materials is shown to have effective carrier (either hole or electron) masses down to 0.03m 0-0.05m 0. The structural stability and feasibility of experimental realization of these 2D materials has been shown in terms of DFT phonon spectra and total energy comparison with related existing bulk materials. On the experimental side, there already are many similar two-coordinate structures of Zn and other transition metals in various organic materials. Therefore, these 2D semiconductors can enrich the family of 2D electronic materials and may have promising potential for achieving novel transistors and optoelectronic devices.

Structural and optical properties of co-precipitated copper doped zinc oxide

NASA Astrophysics Data System (ADS)

Pandey, Devendra K.; Modi, Anchit; Pandey, Padmini; Gaur, N. K.

2018-05-01

We have synthesized pure and copper doped zinc oxide Zn1-xO:Cux (x = 0, 0.03) powder by wet chemical co-precipitation method followed by sintering of the co-precipitated amorphous phase powder at 450°C for 4 hours. The experiment is performed to recognize the effect of nominal doping of transition metal over the structural, morphological and optical properties. The structural parameters are observed by using Rietveld refinement of X-ray diffraction data which clearly represents that Cu ion is perfectly incorporated at the Zn site with minimal distortions within the lattice. The crystallite size is estimated by Debye-Scherrer and Hall-Williamson formulation. The particle morphology and size is determined with scanning electron microscopic (SEM) technique. The band gap and optical measurements are carried out with UV-visible absorption and photoluminescence (PL) spectroscopic technique, respectively. Enhanced PL spectral response is observed for ZnO:Cu along with non-radiative transitions from conduction band to valence band. The energy levels near the conduction band that are commonly involved in the optoelectronic transitions in the UV-region are traced by using absorption and luminescence spectral graphs.

Doping-dependent correlation effects in (Sr1-xLax) 3Ir2O7

NASA Astrophysics Data System (ADS)

Affeldt, Gregory; Hogan, Tom; Denlinger, Jonathan D.; Vishwanath, Ashvin; Wilson, Stephen D.; Lanzara, Alessandra

2018-03-01

We have measured the signatures of electronic energy scales and their doping evolution in the band structure of (Sr1-xLax) 3Ir2O7 using angle-resolved photoemission spectroscopy. While band splittings and positions corresponding to the bilayer splitting and spin-orbit coupling undergo only small changes, the Mott gap and effective mass of both the lower Hubbard band and conduction band exhibit strong variations with doping. These changes correspond to similar observations in the cuprate superconductors, and are likely connected to the changing effective Coulomb interaction upon addition of itinerant carriers.

Structural and optical manipulation of colloidal Ge1-xSnx nanocrystals with experimentally synthesized sizes: Atomistic tight-binding theory

NASA Astrophysics Data System (ADS)

Sukkabot, Worasak

2017-02-01

Nontoxic, maintainable and cost-effective group IV semiconductors are gorgeous for an expansive range of electronic and optoelectronic applications, even though the presence of the indirect band gap obstructs the optical performance. However, band structures can be modified from indirect to direct band gaps by constructing the nanostructures or by alloying with tin (Sn) material. In the study presented here, I investigate the impact of ion-centred types, Sn compositions and dimensions on the electronic structures and optical properties in Ge1-xSnx diamond cubic nanocrystals of the experimentally synthesized Sn contents and diameters using the atomistic tight-binding theory (TB) in the conjunction with the configuration interaction description (CI). The analysis of the mechanism suggests that the physical properties are mainly sensitive with ion-centred types (anion (a) and cation (c)), Sn compositions and dimensions of Ge1-xSnx diamond cubic nanocrystals. The reduction of optical band gaps is reported with the increasing diameters and Sn alloying contents. The visible spectral range is obtained allowing for the applications in bio imaging and chemical sensing. The optical band gaps based on tight-binding calculations are in close agreement with the experimental data for Ge1-xSnx nanocrystals with diameter of 2.1 nm, while for Ge1-xSnx nanocrystals with diameter of 2.7 nm there is a discrepancy of 0.4 eV with experimental results and first-principles calculations. An improvement in the luminescence properties of such Ge1-xSnx nanocrystals becomes possible in the presence of the Sn contents. The electron-hole coulomb interaction is reduced with the increasing Sn components, while the electron-hole exchange interaction is increased with the increasing Sn contents. In addition, I have to point out an astonishing phenomenon, stokes shift and fine structure splitting, with the aim for the realization of the entangled source. The stokes shift and fine structure splitting are enhanced with the increasing Sn contents and decreasing diameters as can be elucidated by the trend of ground electron-hole wave function overlaps. Ge1-xSnx nanocrystal with Sn-free content and large size is the best candidate to be a source of entangled photon pairs. Finally, the combinations of direct band gap character and broad tunable visible spectra advise the promise for use in optoelectronic devices as well as solar cells.

Electromagnetic properties of absorber fabric coated with BaFe12O19/MWCNTs/PANi nanocomposite in X and Ku bands frequency

NASA Astrophysics Data System (ADS)

Afzali, Arezoo; Mottaghitalab, Vahid; Seyyed Afghahi, Seyyed Salman; Jafarian, Mojtaba; Atassi, Yomen

2017-11-01

Current investigation focuses on the electromagnetic properties of nonwoven fabric coated with BaFe12O19 (BHF) /MWCNTs/PANi nanocomposite in X and Ku bands. The BHF/MWCNTs and BHF/MWCNTs/PANi nanocomposites are prepared using the sol gel and in-situ polymerization methods respectively. The absorbent fabric was prepared based on applying a 40 wt% of BHF/MWCNTs/PANi nanocomposite in silicon resin on nonwoven fabric via roller coating technique The X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and vector network analysis (VNA) are used to peruse microstructural, magnetic and electromagnetic features of the composite and absorber fabric respectively. The microscopic images of the fabric coated with magnetic nanocomposite shows a homogenous layer of nanoparticles on the fabric surface. The maximum reflection loss of binary nano-composite BHF/MWCNTs was measured about -28.50 dB at 11.72 GHz with 1.7 GHz bandwidth (RL < -10 dB) in X band. Moreover in Ku band, the maximum reflection loss is -29.66 dB at 15.78 GHz with 3.2 GHz bandwidths. Also the ternary nanocomposite BHF/MWCNTs/PANi exhibits a broad band absorber over a wide range of X band with a maximum reflection loss of -36.2 dB at 10.2 GHz with 1.5 GHz bandwidth and in the Ku band has arrived a maximum reflection loss of -37.65 dB at 12.84 GHz with 2.43 GHz bandwidth. This result reflects the synergistic effect of the different components with different loss mechanisms. As it is observed due to the presence of PANi in the structure of nanocomposite, the amount of absorption has increased extraordinarily. The absorber fabric exhibits a maximum reflection loss of -24.2 dB at 11.6 GHz with 4 GHz bandwidth in X band. However, in Ku band, the absorber fabric has had the maximum absorption in 16.88 GHz that is about -24.34 dB with 6 GHz bandwidth. Therefore, results indicate that the fabric samples coated represents appreciable maximum absorption value of more than 99% in X and Ku bands which can be attributed to presence of carbon and polyaniline structure in composite material.

Structural investigation of phosphate - bismuth glasses with vanadium

NASA Astrophysics Data System (ADS)

Stǎnescu, R.; Vedeanu, N.; Cozar, I. B.; Mǎgdaş, A.

2013-11-01

The xV2O5(1-dx)[0.5P2O5ṡ0.5Bi2O3] glass system with 0 ≤ x ≤ 50 mol% is investigated by IR and Raman spectroscopy. Both P2O5 and Bi2O3 oxides are known as network formers, but Bi2O3 is an unconventional one. At low content of vanadium oxide (x ≤ 5 mol%), both IR and Raman spectra are dominated by vibration bands characteristics to structural groups of phosphate and bismuthate lattices. Due to the network modifier role, vanadium oxide acts mainly on the Bi2O3 network allowing the phosphate groups to impose their characteristics absorption bands in spectra. These bands are strongly reduced for x ≥ 20 mol% due to the phosphate network depolymerization and the appearance of new vibrations characteristic to P-O-V, Bi-O-V and V-O-V groups showing the network former role of V2O5.

First-principles study of structural, electronic, and optical properties of surface defects in GaAs(001) - β2(2x4)

NASA Astrophysics Data System (ADS)

Bacuyag, Dhonny; Escaño, Mary Clare Sison; David, Melanie; Tani, Masahiko

2018-06-01

We performed first-principles calculations based on density functional theory (DFT) to investigate the role of point defects in the structural, electronic, and optical properties of the GaAs(001)- β2(2x4). In terms of structural properties, AsGa is the most stable defect structure, consistent with experiments. With respect to the electronic structure, band structures revealed the existence of sub-band and midgap states for all defects. The induced sub-bands and midgap states originated from the redistributions of charges towards these defects and neighboring atoms. The presence of these point defects introduced deep energy levels characteristic of EB3 (0.97 eV), EL4 (0.52 eV), and EL2 (0.82 eV) for AsGa, GaAs, GaV, respectively. The optical properties are found to be strongly related to these induced gap states. The calculated onset values in the absorption spectra, corresponding to the energy gaps, confirmed the absorption below the known bulk band gap of 1.43 eV. These support the possible two-step photoabsorption mediated by midgap states as observed in experiments.

Broadband Observations and Modeling of the Shell-Type Supernova Remnant G347.3-0.5

NASA Technical Reports Server (NTRS)

Ellison, Donald C.; Slane, Patrick O.; Gaensler, Bryan M.

2002-01-01

The supernova remnant G347.3-0.5 emits a featureless power law in X-rays, thought to indicate shock acceleration of electrons to high energies. We here produce a broadband spectrum of the bright northwest limb of this source by combining radio observations from the Australia Telescope Compact Array (ATCA), X-ray observations from the Advanced Satellite for Cosmology and Astrophysics (ASCA), and TeV gamma-ray observations from the CANGAROO imaging Cerenkov telescope. We assume that this emission is produced by an electron population generated by diffusive shock acceleration at the remnant forward shock. The nonlinear aspects of the particle acceleration force a connection between the widely different wavelength bands and between the electrons and the unseen ions, presumably accelerated simultaneously with the electrons. This allows us to infer the relativistic proton spectrum and estimate ambient parameters such as the supernova explosion energy, magnetic field, matter density in the emission region, and efficiency of the shock acceleration process. We find convincing evidence that the shock acceleration is efficient, placing greater than 25% of the shock kinetic energy flux into relativistic ions. Despite this high efficiency, the maximum electron and proton energies, while depending somewhat on assumptions for the compression of the magnetic field in the shock, are well below the observed 'knee' at 10(exp 15) eV in the Galactic cosmic-ray spectrum.

Anomalous structural disorder and distortion in metal-to-insulator-transition Ti{sub 2}O{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hwang, In-Hui; Jin, Zhenlan; Park, Chang-In

2016-01-07

Mott proposed that impurity bands in corundum-symmetry Ti{sub 2}O{sub 3} at high temperatures caused a collapse in the bandgap. However, the origin of the impurity bands has not yet been clarified. We examine the local structural properties of metal-to-insulator-transition Ti{sub 2}O{sub 3} using in-situ x-ray absorption fine structure (XAFS) measurements at the Ti K edge in the temperature range from 288 to 739 K. The Ti{sub 2}O{sub 3} powder is synthesized by using a chemical reaction method. X-ray diffraction (XRD) measurements from Ti{sub 2}O{sub 3} with a Rietveld refinement demonstrate a single-phased R-3c symmetry without additional distortion. Extended-XAFS combined with XRDmore » reveals a zigzag patterned Ti position and an anomalous structural disorder in Ti-Ti pairs, accompanied by a bond length expansion of the Ti-Ti pairs along the c-axis for T > 450 K. The local structural distortion and disorder of the Ti atoms would induce impurity levels in the band gap between the Ti 3d a{sub 1g} and e{sub g}{sup π} bands, resulting in a collapse of the band gap for T > 450 K.« less

Studies on the S-band bunching system with the Hybrid Bunching-accelerating Structure

NASA Astrophysics Data System (ADS)

Pei, Shi-Lun; Gao, Bin

2018-04-01

Generally, a standard bunching system is composed of a standing-wave (SW) pre-buncher (PB), a traveling-wave (TW) buncher (B) and a standard accelerating structure. In the industrial area, the bunching system is usually simplified by eliminating the PB and integrating the B and the standard accelerating structure together to form a β-varied accelerating structure. The beam capturing efficiency for this kind of simplified system is often worse than that for the standard one. The hybrid buncher (HB) has been proved to be a successful attempt to reduce the cost but preserve the beam quality as much as possible. Here we propose to exclusively simplify the standard bunching system by integrating the PB, the B and the standard accelerating structure together to form a Hybrid Bunching-accelerating Structure (HBaS). Compared to the standard bunching system, the one based on the HBaS is more compact, and the cost is lowered to the largest extent. With almost the same beam transportation efficiency (∼70%) from the electron gun to the linac exit, the peak-to-peak (p-to-p) beam energy spread and the 1 σ emittance of the linac with the HBaS are ∼20% and ∼60% bigger than those of the linac based on the split PB/B/standard accelerating structure system. Nonetheless, the proposed HBaS can be widely applied in the industrial linacs to greatly increase the beam capturing efficiency without fairly increasing the construction cost.

High power experimental studies of hybrid photonic band gap accelerator structures

DOE PAGES

Zhang, JieXi; Munroe, Brian J.; Xu, Haoran; ...

2016-08-31

This paper reports the first high power tests of hybrid photonic band gap (PBG) accelerator structures. Three hybrid PBG (HPBG) structures were designed, built and tested at 17.14 GHz. Each structure had a triangular lattice array with 60 inner sapphire rods and 24 outer copper rods sandwiched between copper disks. The dielectric PBG band gap map allows the unique feature of overmoded operation in a TM 02 mode, with suppression of both lower order modes, such as the TM 11 mode, as well as higher order modes. The use of sapphire rods, which have negligible dielectric loss, required inclusion ofmore » the dielectric birefringence in the design. The three structures were designed to sequentially reduce the peak surface electric field. Simulations showed relatively high surface fields at the triple point as well as in any gaps between components in the clamped assembly. The third structure used sapphire rods with small pin extensions at each end and obtained the highest gradient of 19 MV/m, corresponding to a surface electric field of 78 MV/m, with a breakdown probability of 5×10 –1 per pulse per meter for a 100-ns input power pulse. Operation at a gradient above 20 MV/m led to runaway breakdowns with extensive light emission and eventual damage. For all three structures, multipactor light emission was observed at gradients well below the breakdown threshold. As a result, this research indicated that multipactor triggered at the triple point limited the operational gradient of the hybrid structure.« less

Cementum structure in Beluga whale teeth

DOE PAGES

Stock, S. R.; Finney, L. A.; Telser, A.; ...

2016-11-09

We report that a large fraction of the volume of Beluga whale (Delphinapterus leucas) teeth consists of cementum, a mineralized tissue which grows throughout the life of the animal and to which the periodontal ligaments attach. Annular growth bands or growth layer groups (GLGs) form within Beluga cementum, and this study investigates GLG structure using X-ray fluorescence mapping and X-ray diffraction mapping with microbeams of synchrotron radiation. The Ca and Zn fluorescent intensities and carbonated hydroxyapatite (cAp) diffracted intensities rise and fall together and match the light-dark bands visible in transmitted light micrographs. Within the bands of maximum Ca andmore » Zn intensity, the ratio of Zn to Ca is slightly higher than in the minima bands. Further, the GLG cAp, Ca and Zn modulation is preserved throughout the cementum for durations >25 year.« less

Potential Fluctuations and Localization Effects in CZTS Single Crystals, as Revealed by Optical Spectroscopy

NASA Astrophysics Data System (ADS)

Bleuse, Joël; Ducroquet, Frédérique; Mariette, Henri

2018-03-01

Reports on Cu_2 ZnSn(S_x Se_{1-x} )_4 (CZTSSe) solar cell devices all show an open-circuit voltage lower than expected, especially when compared to CuIn_x Ga_{1-x} (S,Se)_2 devices, which reduces their power efficiency and delays their development. A high concentration of intrinsic defects in CZTSSe, and their stabilization through neutral complex formation, which induces some local fluctuations, are at the origin of local energy shifts in the conduction and valence band edges. The implied band tail in Cu_2 ZnSnS_4 is studied in this work by combining three types of optical spectroscopy data: emission spectra compared to photoluminescence excitation spectroscopy, emission spectra as a function of excitation power, and time-resolved photoluminescence spectra. All these data converge to show that both the bandgap and the band tail of localized states just below are dependent on the degree of order/disorder in the Cu/Zn cation sublattice of the quaternary structure: in the more ordered structures, the bandgap increases by about 50 meV, and the energy range of the band tail is decreased from about 110 to 70 meV.

Stability and Elastic, Electronic, and Thermodynamic Properties of Fe2TiSi1- x Sn x Compounds

NASA Astrophysics Data System (ADS)

Jong, Ju-Yong; Yan, Jihong; Zhu, Jingchuan; Kim, Chol-Jin

2017-10-01

We have systematically studied the structural, phase, and mechanical stability and elastic, electronic, and thermodynamic properties of Fe2TiSi1- x Sn x ( x = 0, 0.25, 0.5, 0.75, 1) compounds using first-principles calculations. The structural and phase stability and elastic properties of Fe2TiSi1- x Sn x ( x = 0, 0.25, 0.5, 0.75, 1) indicated that all of the compounds are thermodynamically and mechanically stable. The shear modulus, bulk modulus, Young's modulus, Poisson's ratio, electronic band structure, density of states, Debye temperature, and Grüneisen parameter of all the substituted compounds were studied. The results show that Sn substitution in Fe2TiSi enhances its stability and mechanical and thermoelectric properties. The Fe2TiSi1- x Sn x compounds have narrow bandgap from 0.144 eV and 0.472 eV for Sn substitution from 0 to 1. The calculated band structure and density of states (DOS) of Fe2TiSi1- x Sn x show that the thermoelectric properties can be improved at substituent concentration x of 0.75. The lattice thermal conductivity was significantly decreased in the Sn-substituted compounds, and all the results indicate that Fe2TiSi0.25Sn0.75 could be a new candidate high-performance thermoelectric material.

Luminescent zero-dimensional organic metal halide hybrids with near-unity quantum efficiency.

PubMed

Zhou, Chenkun; Lin, Haoran; Tian, Yu; Yuan, Zhao; Clark, Ronald; Chen, Banghao; van de Burgt, Lambertus J; Wang, Jamie C; Zhou, Yan; Hanson, Kenneth; Meisner, Quinton J; Neu, Jennifer; Besara, Tiglet; Siegrist, Theo; Lambers, Eric; Djurovich, Peter; Ma, Biwu

2018-01-21

Single crystalline zero-dimensional (0D) organic-inorganic hybrid materials with perfect host-guest structures have been developed as a new generation of highly efficient light emitters. Here we report a series of lead-free organic metal halide hybrids with a 0D structure, (C 4 N 2 H 14 X) 4 SnX 6 (X = Br, I) and (C 9 NH 20 ) 2 SbX 5 (X = Cl), in which the individual metal halide octahedra (SnX 6 4- ) and quadrangular pyramids (SbX 5 2- ) are completely isolated from each other and surrounded by the organic ligands C 4 N 2 H 14 X + and C 9 NH 20 + , respectively. The isolation of the photoactive metal halide species by the wide band gap organic ligands leads to no interaction or electronic band formation between the metal halide species, allowing the bulk materials to exhibit the intrinsic properties of the individual metal halide species. These 0D organic metal halide hybrids can also be considered as perfect host-guest systems, with the metal halide species periodically doped in the wide band gap matrix. Highly luminescent, strongly Stokes shifted broadband emissions with photoluminescence quantum efficiencies (PLQEs) of close to unity were realized, as a result of excited state structural reorganization of the individual metal halide species. Our discovery of highly luminescent single crystalline 0D organic-inorganic hybrid materials as perfect host-guest systems opens up a new paradigm in functional materials design.

Optical, x-ray and microwave diagnostics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tudisco, S.; Mascali, D.; Altana, C.

2013-07-26

Laser-driven ion acceleration is a new approach for the particles acceleration, which allows obtaining ion beams with unique properties, such as short burst duration, large particle number, small size source size, low transverse emittance. Currently, two main acceleration mechanisms have been identified and investigated: target normal sheath acceleration (TNSA) and radiation pressure acceleration (RPA). Electrons dynamics and energies are strongly coupled to these acceleration mechanisms and they can be investigated with optical and X-ray techniques. The main aim of these studies are the identification of few physical observables that can be directly correlated to the proton emission obtained (in termsmore » of reproducibility and intensity) in operations with different target material and structure and laser-target interaction parameters.« less

Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy.

PubMed

Wei, Wei; Qin, Zhixin; Fan, Shunfei; Li, Zhiwei; Shi, Kai; Zhu, Qinsheng; Zhang, Guoyi

2012-10-10

A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV.

Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy

PubMed Central

2012-01-01

A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV. PMID:23046910

Effect of Halide Composition on the Photochemical Stability of Perovskite Photovoltaic Materials.

PubMed

Misra, Ravi K; Ciammaruchi, Laura; Aharon, Sigalit; Mogilyansky, Dmitry; Etgar, Lioz; Visoly-Fisher, Iris; Katz, Eugene A

2016-09-22

The photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I 1-x Br x ) 3 (MA=methylammonium) compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. We found that MAPbBr 3 films exhibited no degradation, while MAPbI 3 and mixed halide MAPb(I 1-x Br x ) 3 films decomposed yielding crystallization of inorganic PbI 2 accompanied by degradation of the perovskite solar light absorption, with faster absorption degradation in mixed halide films. The crystal coherence length was found to correlate with the stability of the films. We postulate that the introduction of Br into the mixed halide solid solution stressed its structure and induced more structural defects and/or grain boundaries compared to pure halide perovskites, which might be responsible for the accelerated degradation. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Pulsed laser deposited BexZn1-xO1-ySy quaternary alloy films: structure, composition, and band gap bowing

NASA Astrophysics Data System (ADS)

Zhang, Wuzhong; Xu, Maji; Zhang, Mi; Cheng, Hailing; Li, Mingkai; Zhang, Qingfeng; Lu, Yinmei; Chen, Jingwen; Chen, Changqing; He, Yunbin

2018-03-01

In this work, c-axis preferentially oriented BexZn1-xO1-ySy (BeZnOS) quaternary alloy films were prepared successfully on c-plane sapphire by pulsed laser deposition for the first time. By appropriate adjustment of O2 pressure during the deposition, the grown films exhibited a single-phase hexagonal structure and good crystalline quality. The solid solubility of S in BexZn1-xO1-ySy quaternary alloy was significantly expanded (y ≤ 0.17 or y ≥ 0.35) as a result of simultaneous substitution of cation Zn2+ by smaller Be2+ and anion O2- by bigger S2-. Besides, due to the introduction of BeO with a wide band gap, BeZnOS quaternary films exhibited wider band gaps than the ternary ZnOS films with similar S contents. As the O2 pressure increased from 0.05 Pa to 6 Pa, the band gap of BeZnOS displayed an interesting bowing behavior. The variation range of the band gap was between 3.55 eV and 3.10 eV. The BeZnOS films with a wide band gap show potential applications in fabricating optoelectronic devices such as UV-detectors.

Highly strained InxGa(1-x)As-InyAl(1-y)As (x>0.8,y<0.3) layers for short wavelength QWIP and QCL structures grown by MBE

NASA Astrophysics Data System (ADS)

Missous, M.; Mitchell, C.; Sly, J.; Lai, K. T.; Gupta, R.; Haywood, S. K.

2004-01-01

Highly strained quantum cascade laser (QCL) and quantum well infrared photodetector (QWIPs) structures based on InxGa(1-x)As-InyAl(1-y)As (x>0.8,y<0.3) layers have been grown by molecular beam epitaxy. Conditions of exact stoichiometric growth were used at a temperature of ∼420°C to produce structures that are suitable for both emission and detection in the 2- 5 μm mid-infrared regime. High structural integrity, as assessed by double crystal X-ray diffraction, room temperature photoluminescence and electrical characteristics were observed. Strong room temperature intersubband absorption in highly tensile strained and strain-compensated In 0.84Ga 0.16As/AlAs/In 0.52Al 0.48As double barrier quantum wells grown on InP substrates is demonstrated. Γ- Γ intersubband transitions have been observed across a wide range of the mid-infrared spectrum (2- 7 μm) in three structures of differing In 0.84Ga 0.16As well width (30, 45, and 80 Å). We demonstrate short-wavelength IR, intersubband operation in both detection and emission for application in QC and QWIP structures. By pushing the InGaAs-InAlAs system to its ultimate limit, we have obtained the highest band offsets that are theoretically possible in this system both for the Γ- Γ bands and the Γ-X bands, thereby opening up the way for both high power and high efficiency coupled with short-wavelength operation at room temperature. The versatility of this material system and technique in covering a wide range of the infrared spectrum is thus demonstrated.

Influence of calcium on transport properties, band spectrum and superconductivity of YBa2Cu3O(y) and YBa(1.5)La(0.5)Cu3O(y)

NASA Technical Reports Server (NTRS)

Gasumyants, V. E.; Vladimirskaya, E. V.; Patrina, I. B.

1995-01-01

The comparative investigation of transport phenomena in Y(1-x)Ca(x)Ba2Cu3O(y) (0 is less than x is less than 0.25; 6.96 is greater than y is greater than 6.87 and 6.73 is less than x is less than 6.53); Y(1-x)Ca(x)Ba(1.5)La(0.5)Cu3O(y) (0 is less than x is less than 0.5; 7.12 is greater than y is greater than 6.96) and YBa(2-x)La(x)Cu3O(y) (0 is less than x is less than 0.5; 6.95 is less than y is less than 7.21) systems have been carried out. The temperature dependencies of resistivity and thermopower have been measured. It was found that the S(T) dependencies take some additional features with Ca content increase. The results obtained have been analyzed on the basis of the phenomenological theory of electron transport in the case of the narrow conductive band. The main parameters of the band spectrum (the band filling with electrons degree and the total effective band width) have been determined. The dependencies of these from contents of substituting elements are discussed. Analyzing the results obtained simultaneously with the tendencies in oxygen content and critical temperature change we have confirmed the conclusion that the oxygen sublattice disordering has a determinant effect on band structure parameters and superconductive properties of YBa2Cu3O(y). The results obtained suggest that Ca gives rise to some peculiarities in band spectrum of this compound.

Density Measurements in Air by Optically Exciting the Cordes Bands of I2

NASA Technical Reports Server (NTRS)

Balla, R. Jeffrey; Exton, Reginald J.

2000-01-01

We describe an optical method based on laser-induced fluorescence for obtaining instantaneous measurements of density along a line in low-density air seeded with I2. The Cordes bands of I2 (D(sup 1)sigma(sup +, sub u)) left arrow X(sup 1)sigma(sup +, sub g)) are excited with a tunable ArF excimer laser. air densities in the range (0.1-6.5) x 10(exp 17) cm(exp -3) are measured over 295-583 K using the density-dependent emission ratio of two emission bands of I2; the 340 nm bands and the diffuse-structured McLennan bands near 320 nm.

The effect of Y3+ substitution on the structural, optical band-gap, and magnetic properties of cobalt ferrite nanoparticles.

PubMed

Alves, T E P; Pessoni, H V S; Franco, A

2017-06-28

In this study we investigated the structural, optical band-gap, and magnetic properties of CoY x Fe 2-x O 4 (0 ≤ x ≤ 0.04) nanoparticles (NPs) synthesized using a combustion reaction method without the need for subsequent heat treatment or the calcing process. The particle size measured from X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) images confirms the nanostructural character in the range of 16-36 nm. The optical band-gap (E g ) values increase with the Y 3+ ion (x) concentration being 3.30 and 3.58 eV for x = 0 and x = 0.04, respectively. The presence of yttrium in the cobalt ferrite (Y-doped cobalt ferrite) structure affects the magnetic properties. For instance, the saturation magnetization, M s and remanent magnetization, M r , decrease from 69 emu g -1 to 33 and 28 to 12 emu g -1 for x = 0 and x = 0.04, respectively. On the other hand the coercivity, H c , increases from 1100 to 1900 Oe for x = 0 and x = 0.04 at room temperature. Also we found that M s , M r , and H c decreased with increasing temperature up to 773 K. The cubic magnetocrystalline constant, K 1 , determined by using the "law of approach" (LA) to saturation decreases with Y 3+ ion concentration and temperature. K 1 values for x = 0 (x = 0.04) were 3.3 × 10 6 erg cm -3 (2.0 × 10 6 erg cm -3 ) and 0.4 × 10 6 erg cm -3 (0.3 × 10 6 erg cm -3 ) at 300 K and 773 K, respectively. The results were discussed in terms of inter-particle interactions induced by thermal fluctuations, and Co 2+ ion distribution over tetrahedral A-sites and octahedral B-sites of the spinel structure due to Y 3+ ion substitution.

Defect engineering of complex semiconductor alloys: Cu2-2xMxO1-yXy

NASA Astrophysics Data System (ADS)

Lany, Stephan; Stevanovic, Vladan

2013-03-01

The electrical properties of semiconductors are generally controlled via doping, i.e., the incorporation of dilute concentrations of aliovalent impurity atoms, whereas the band structure properties (gap, effective masses, optical properties) are manipulated by alloying, i.e., the incorporation of much larger amounts of isovalent elements. Theoretical approaches usually address either doping or alloying, but rarely both problems at the same time. By combining defect supercell calculations, GW quasi-particle energy calculation, and thermodynamic modeling, we study the range of electrical and band structure properties accessible by alloying aliovalent cations (M = Mg, Zn, Cd) and isovalent anions (X = S, Se) in Cu2O. In order to extend dilute defect models to higher concentrations, we take into account the association/dissociation of defect pairs and complexes, as well as the composition dependence of the band gap and the band edge energies. Considering a composition window for the Cu2-2xMxO1-yXy alloys of 0 <= (x,y) <= 0.2, we predict a wide range of possible band gaps from 1.7 to 2.6 eV, and net doping concentrations between p = 1019 cm-3 and n = 1017cm-3, notably achieving type conversion from p- to n-type at Zn or Cd compositions around x = 0.1. This work is supported as part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.

Optical characterization of type-I to type-II band alignment transition in GaAs/Al x Ga1-x As quantum rings grown by droplet epitaxy

NASA Astrophysics Data System (ADS)

Su, Linlin; Wang, Ying; Guo, Qinglin; Li, Xiaowei; Wang, Shufang; Fu, Guangsheng; Mazur, Yuriy I.; E Ware, Morgan; Salamo, Gregory J.; Liang, Baolai; Huffaker, Diana L.

2017-08-01

Optical properties of GaAs/Al x Ga1-x As quantum rings (QRs) grown on GaAs (1 0 0) by droplet epitaxy have been investigated as a function of the Al-composition in the Al x Ga1-x As barrier. A transition from type-I to type-II band alignment is observed for the QRs via photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. While x  ⩽  0.45, the QR PL spectra show a blue-shift and an increasing intensity with increasing Al-composition, revealing the enhancement of quantum confinement in the QRs with type-I band alignment. While x  ⩾  0.60, the characteristic large blue-shift with excitation intensity and the much longer lifetime indicate the realization of a type-II band alignment. Due to the height fluctuation of QR structures grown by droplet epitaxy mode, it is not the large blue-shift of emission energy, but the long lifetime that becomes the more important feature to identify the type-II band alignment.

High-power klystrons

NASA Astrophysics Data System (ADS)

Siambis, John G.; True, Richard B.; Symons, R. S.

1994-05-01

Novel emerging applications in advanced linear collider accelerators, ionospheric and atmospheric sensing and modification and a wide spectrum of industrial processing applications, have resulted in microwave tube requirements that call for further development of high power klystrons in the range from S-band to X-band. In the present paper we review recent progress in high power klystron development and discuss some of the issues and scaling laws for successful design. We also discuss recent progress in electron guns with potential grading electrodes for high voltage with short and long pulse operation via computer simulations obtained from the code DEMEOS, as well as preliminary experimental results. We present designs for high power beam collectors.

Tunneling calculations for GaAs-Al(x)Ga(1-x) as graded band-gap sawtooth superlattices. Thesis

NASA Technical Reports Server (NTRS)

Forrest, Kathrine A.; Meijer, Paul H. E.

1991-01-01

Quantum mechanical tunneling calculations for sawtooth (linearly graded band-gap) and step-barrier AlGaAs superlattices were performed by means of a transfer matrix method, within the effective mass approximation. The transmission coefficient and tunneling current versus applied voltage were computed for several representative structures. Particular consideration was given to effective mass variations. The tunneling properties of step and sawtooth superlattices show some qualitative similarities. Both structures exhibit resonant tunneling, however, because they deform differently under applied fields, the J-V curves differ.

Titanium-silicon oxide film structures for polarization-modulated infrared reflection absorption spectroscopy

PubMed Central

Dunlop, Iain E.; Zorn, Stefan; Richter, Gunther; Srot, Vesna; Kelsch, Marion; van Aken, Peter A.; Skoda, Maximilian; Gerlach, Alexander; Spatz, Joachim P.; Schreiber, Frank

2010-01-01

We present a titanium-silicon oxide film structure that permits polarization modulated infrared reflection absorption spectroscopy on silicon oxide surfaces. The structure consists of a ~6 nm sputtered silicon oxide film on a ~200 nm sputtered titanium film. Characterization using conventional and scanning transmission electron microscopy, electron energy loss spectroscopy, X-ray photoelectron spectroscopy and X-ray reflectometry is presented. We demonstrate the use of this structure to investigate a selectively protein-resistant self-assembled monolayer (SAM) consisting of silane-anchored, biotin-terminated poly(ethylene glycol) (PEG). PEG-associated IR bands were observed. Measurements of protein-characteristic band intensities showed that this SAM adsorbed streptavidin whereas it repelled bovine serum albumin, as had been expected from its structure. PMID:20418963

Spin splitting in band structures of BiTeX (X=Cl, Br, I) monolayers

NASA Astrophysics Data System (ADS)

Hvazdouski, D. C.; Baranava, M. S.; Stempitsky, V. R.

2018-04-01

In systems with breaking of inversion symmetry a perpendicular electric field arises that interacts with the conduction electrons. It may give rise to electron state splitting even without influence of external magnetic field due to the spin-orbital interaction (SOI). Such a removal of the spin degeneracy is called the Rashba effect. Nanostructure with the Rashba effect can be part of a spin transistor. Spin degeneracy can be realized in a channel from a material of this type without additive of magnetic ions. Lack of additive increases the charge carrier mobility and reliability of the device. Ab initio simulations of BiTeX (X=Cl, Br, I) monolayers have been carried out using VASP wherein implemented DFT method. The study of this structures is of interest because such sort of structures can be used their as spin-orbitronics materials. The crystal parameters of BiTeCl, BiTeBr, BiTeI have been determined by the ionic relaxation and static calculations. It is necessary to note that splitting of energy bands occurs in case of SOI included. The values of the Rashba coefficient aR (in the range from 6.25 to 10.00 eV·Å) have high magnitudes for spintronics materials. Band structure of monolayers structures have ideal Rashba electron gas, i.e. there no other energy states near to Fermi level except Rashba states.

Electronic structure and optical properties of defect chalcopyrite HgGa2Se4

NASA Astrophysics Data System (ADS)

Gabrelian, B. V.; Lavrentyev, A. A.; Vu, Tuan V.; Parasyuk, O. V.; Khyzhun, O. Y.

2018-01-01

We report on studies from an experimental and theoretical viewpoint of the electronic structure of mercury digallium selenide, HgGa2Se4, a very promising optoelectronic material. In particular, the method of X-ray photoelectron spectroscopy (XPS) was used to evaluate binding energies of the constituent element core electrons and the shape of the valence band for pristine and Ar+-ion bombarded surfaces of HgGa2Se4 single crystal. First principles band-structure calculations were performed in the present work using the augmented plane wave + local orbitals (APW+lo). These calculations indicate that the Se 4p states are the main contributors at the top and in the upper portion of the valence band with slightly smaller contributions of the Ga 4p states in the upper portion of the band as well. Further, the central portion of the valence band is determined mainly by contributions of the Ga 4s states, and the Hg 5d states are the principal contributors to the bottom of the valence band. These theoretical data are in fair agreement when matching on a common energy scale of the X-ray emission bands giving information on the energy distribution of the Se 4p and Ga 4p states and the XPS valence-band spectrum of the HgGa2Se4 crystal. The principal optical constants are elucidated from the DFT calculations.

A Chemical Understanding of the Band Convergence in Thermoelectric CoSb 3 Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hanus, Riley; Guo, Xingyu; Tang, Yinglu

2017-01-13

N-Type skutterudites, such as Yb xCo 4Sb 12, have recently been shown to exhibit high valley degeneracy with possible band convergence, explaining the excellent thermoelectric efficiency of these materials. Using a combined theoretical and experimental approach involving temperature-dependent synchrotron diffraction, molecular orbital diagrams, and computational studies, the chemical nature of critical features in the band structure is highlighted. We identify how n-type doping on the filler site induces structural changes that are observed in both the diffraction data and computational results. Additionally, we show how chemical n-type doping slightly alters the electronic band structure, moving the high-valley degeneracy secondary conductionmore » band closer to the primary conduction band and thus inducing band convergence.« less

Development of new S-band SLED for PAL-XFEL Linac

NASA Astrophysics Data System (ADS)

Joo, Youngdo; Park, Yongjung; Heo, Hoon; Heo, Jinyul; Park, Sung-Soo; Kim, Sang-Hee; Kim, Kwang-Hoon; Kang, Heung-Sik; Lee, Heung-Soo; Noh, Sungju; Oh, Kyoungmin

2017-01-01

In order to achieve beam acceleration to the beam energy of 10 GeV at the end of its 716 m-long linear accelerator (Linac), the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) is going to operate the Stanford Linear Accelerator Energy Doubler (SLED) at the maximum klystron output peak power of 80 MW, with a pulse length of 4 μs, and at a repetition rate of 60 Hz. The original SLED that had been used in Pohang Light Source-II (PLS-II) can no longer sustain such a high-power operation because excessive radiation caused by RF breakdown has been frequently detected even at the lower klystron peak power during the PLS-II operation. Therefore, a new SLED is designed by modifying both the 3-dB power hybrid and the waveguide-cavity coupling structure of the original SLED where the excessive radiation has been mainly detected. The finite-difference time-domain (FDTD) simulation in the CST Microwave Studio shows that the new SLED has a peak electric field and a surface current lower than those of the original SLED at the same level of the RF input peak power, which would secure stable high-power operation. All of the 42 SLEDs in the PAL-XFEL Linac are newly fabricated and installed. During the RF conditioning of the PAL-XFEL Linac, no significant vacuum and radiation issue was found in the new SLEDs. Finally, the accelerated electron beam energy of 10 GeV obtained at the end of the PAL-XFEL Linac verified that the RF performance of the new SLED is stable.

Measuring the band structures of periodic beams using the wave superposition method

NASA Astrophysics Data System (ADS)

Junyi, L.; Ruffini, V.; Balint, D.

2016-11-01

Phononic crystals and elastic metamaterials are artificially engineered periodic structures that have several interesting properties, such as negative effective stiffness in certain frequency ranges. An interesting property of phononic crystals and elastic metamaterials is the presence of band gaps, which are bands of frequencies where elastic waves cannot propagate. The presence of band gaps gives this class of materials the potential to be used as vibration isolators. In many studies, the band structures were used to evaluate the band gaps. The presence of band gaps in a finite structure is commonly validated by measuring the frequency response as there are no direct methods of measuring the band structures. In this study, an experiment was conducted to determine the band structure of one dimension phononic crystals with two wave modes, such as a bi-material beam, using the frequency response at only 6 points to validate the wave superposition method (WSM) introduced in a previous study. A bi-material beam and an aluminium beam with varying geometry were studied. The experiment was performed by hanging the beams freely, exciting one end of the beams, and measuring the acceleration at consecutive unit cells. The measured transfer function of the beams agrees with the analytical solutions but minor discrepancies. The band structure was then determined using WSM and the band structure of one set of the waves was found to agree well with the analytical solutions. The measurements taken for the other set of waves, which are the evanescent waves in the bi-material beams, were inaccurate and noisy. The transfer functions at additional points of one of the beams were calculated from the measured band structure using WSM. The calculated transfer function agrees with the measured results except at the frequencies where the band structure was inaccurate. Lastly, a study of the potential sources of errors was also conducted using finite element modelling and the errors in the dispersion curve measured from the experiments were deduced to be a result of a combination of measurement noise, the different placement of the accelerometer with finite mass, and the torsional mode.

Ionic-Liquid-Assisted Microwave Synthesis of Solid Solutions of Sr 1–xBa xSnO 3 Perovskite for Photocatalytic Applications

DOE PAGES

Alammar, Tarek; Slowing, Igor I.; Anderegg, Jim; ...

2017-06-06

Nanocrystalline Sr 1–xBa xSnO 3 (x = 0, 0.2, 0.4, 0.8, 1) perovskite photocatalysts were prepared by microwave synthesis in an ionic liquid (IL) and subsequent heat-treatment. The influence of the Sr/Ba substitution on the structure, crystallization, morphology, and photocatalytic efficiency was investigated and the samples were fully characterized. On the basis of X-ray diffraction results, as the Ba content in the SrSnO 3 lattice increases, a symmetry increase was observed from the orthorhombic perovskite structure for SrSnO 3 to the cubic BaSnO 3 structure. The analysis of the sample morphology by SEM reveals that the Sr 1–xBa xSnO 3more » samples favor the formation of nanorods (500 nm–5 μm in diameter and several micrometers long). The photophysical properties were examined by UV/Vis diffuse reflectance spectroscopy. The band gap decreases from 3.85 to 3.19 eV with increasing Ba 2+ content. Furthermore, the photocatalytic properties were evaluated for the hydroxylation of terephthalic acid (TA). The order of the activities for TA hydroxylation was Sr 0.8Ba 0.2SnO 3 > SrSnO 3 > BaSnO 3 > Sr 0.6Ba 0.4SnO 3 > Sr 0.2Ba 0.8SnO 3. Here, the highest photocatalytic activity was observed for Sr 0.8Ba 0.2SnO 3, and this can be attributed to the synergistic impacts of the modification of the crystal structure and morphology, the relatively large surface area associated with the small crystallite size, and the suitable band gap and band-edge position.« less

Ab-initio study of electronic structure and magnetic properties of half-metallic Fe{sub 2}Mn{sub 1−x}V{sub x}Si{sub 0.5}Al{sub 0.5} alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Go, Anna, E-mail: annago@alpha.uwb.edu.pl

2014-11-15

Ab-initio electronic structure calculations are carried out for quinternary Fe{sub 2}Mn{sub 1−x}V{sub x}Si{sub 0.5}Al{sub 0.5} alloys. When x=0 the alloy is half-metallic ferromagnet, with magnetic moment following the Slater–Pauling rule. Replacement of Mn by V, changes its electronic and magnetic structure. V-doped alloys exhibit half-metallic behavior for x≤0.25. However, even for higher V concentrations, electronic spin polarization is still very high, what makes the alloys interesting for spintronic applications. - Graphical abstract: Densities of states of Fe{sub 2}MnSi{sub 0.5}Al{sub 0.5} and magnetic moments of Fe{sub 2}Mn{sub 1−x}V{sub x}Si{sub 0.5}Al{sub 0.5}. - Highlights: • Fe{sub 2}MnSi{sub 0.5}Al{sub 0.5} is a half-metallicmore » ferromagnet with a minority band gap of 0.49 eV. • Half-metallic band gap is very stable against the change of the lattice parameter. • Half-metallic band gap is obtained for Fe{sub 2}Mn{sub 1−x}V{sub x}Si{sub 0.5}Al{sub 0.5} for x≤0.25. • Electronic spin polarization is very high and equal to at least 95% for x≤0.625. • The main carrier of magnetism of the compound is manganese.« less

R&D status of linear collider technology at KEK

NASA Astrophysics Data System (ADS)

Urakawa, Junji

1992-02-01

This paper gives an outline of the Japan Linear Collider (JLC) project, especially JLC-I. The status of the various R&D works is particularly presented for the following topics: (1) electron and positron sources, (2) S-band injector linacs, (3) damping rings, (4) high power klystrons and accelerating structures, (5) the final focus system. Finally, the status of the construction and design studies for the Accelerator Test Facility (ATF) is summarized.

Structural studies and band gap tuning of Cr doped ZnO nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Srinet, Gunjan, E-mail: gunjansrinet@gmail.com; Kumar, Ravindra, E-mail: gunjansrinet@gmail.com; Sajal, Vivek, E-mail: gunjansrinet@gmail.com

2014-04-24

Structural and optical properties of Cr doped ZnO nanoparticles prepared by the thermal decomposition method are presented. X-ray diffraction studies confirmed the substitution of Cr on Zn sites without changing the wurtzite structure of ZnO. Modified form of W-H equations was used to calculate various physical parameters and their variation with Cr doping is discussed. Significant red shift was observed in band gap, i.e., a band gap tuning is achieved by Cr doping which could eventually be useful for optoelectronic applications.

Spin-polarized structural, elastic, electronic and magnetic properties of half-metallic ferromagnetism in V-doped ZnSe

NASA Astrophysics Data System (ADS)

Monir, M. El Amine.; Baltache, H.; Murtaza, G.; Khenata, R.; Ahmed, Waleed K.; Bouhemadou, A.; Omran, S. Bin; Seddik, T.

2015-01-01

Based on first principles spin-polarized density functional theory, the structural, elastic electronic and magnetic properties of Zn1-xVxSe (for x=0.25, 0.50, 0.75) in zinc blende structure have been studied. The investigation was done using the full-potential augmented plane wave method as implemented in WIEN2k code. The exchange-correlation potential was treated with the generalized gradient approximation PBE-GGA for the structural and elastic properties. Moreover, the PBE-GGA+U approximation (where U is the Hubbard correlation terms) is employed to treat the "d" electrons properly. A comparative study between the band structures, electronic structures, total and partial densities of states and local moments calculated within both GGA and GGA+U schemes is presented. The analysis of spin-polarized band structure and density of states shows the half-metallic ferromagnetic character and are also used to determine s(p)-d exchange constants N0α (conduction band) and N0β (valence band) due to Se(4p)-V(3d) hybridization. It has been clearly evidence that the magnetic moment of V is reduced from its free space change value of 3 μB and the minor atomic magnetic moment on Zn and Se are generated.

Tunable optical properties of ZnCdTe2-xSex(x =0.625) chalcopyrite for photovoltaics; a mBJLDA approach

NASA Astrophysics Data System (ADS)

Kashyap, Manish K.; Paudyal, D.; Harmon, B. N.

In the present study, we have performed ab-initio simulations of sp-element defect in ZnCdTe2-xSex (x =0.625) chalcopyrite to check the tuning of band gap as compared to the pristine case. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke-Johnson (mBJ) potential as coupled with Local Density Approximation (LDA) for these calculations. The calculated energy band structures show a direct band gap at the point in the brillouin zone for the pristine as well as the defected case and the band gap decreases with inclusion of sp-disorder. The imaginary dielectric function predicts the optical band gap of pristine ZnCdTe2 very close to the experimental value and the results are in reasonable agreement without applying any scissor operator. With inclusion of sp-element defect, the optical spectra is tuned to optimal region, suitable for photovoltaics. It is apparent that mBJ functional is well suited for calculating electronic structure of pristine as well as defected ZnCdTe2chalcopyrite. MKK acknowledges financial support from UGC, India in the form of RAMAN Post-doctoral fellowship. This work at Ames Laboratory was supported by the DOE, Office of Basic Energy Sciences, Materials Sciences Division under contract No. DE-AC02-07CH11358.

Design of a low emittance and high repetition rate S-band photoinjector

NASA Astrophysics Data System (ADS)

Han, Jang-Hui

2014-09-01

As an electron beam injector of X-ray free-electron lasers (FELs), photoinjectors have been developed for the past few decades. Such an injector starting with a photocathode RF gun provides high brightness beams and therefore it is being adopted as an injector of X-ray FELs. In this paper we show how to improve photoinjector performance in terms of emittance and repetition rates by means of injector components optimization, especially with the gun. Transverse emittance at the end of an injector is reduced by optimizing the gun design, gun solenoid position, and accelerating section position. The repetition rate of an injector mainly depends on the gun. It is discussed that a repetition rate of 1 kHz at a normal-conducting S-band photoinjector is feasible by adopting a coaxial RF coupler and improving cooling-water channels surrounding the gun.

Modelling Hard Gamma-Ray Emission from Supernova Remnants

NASA Technical Reports Server (NTRS)

Baring, Matthew

2000-01-01

The observation by the CANGAROO experiment of TeV emission from SN 1006, in conjunction with several instances of non-thermal X-ray emission from supernova remnants, has led to inferences of super-TeV electrons in these extended sources. While this is sufficient to propel the theoretical community in their modelling of particle acceleration and associated radiation, the anticipated emergence in the next decade of a number of new experiments probing the TeV and sub-TeV bands provides further substantial motivation for modellers. In particular, the quest for obtaining unambiguous gamma-ray signatures of cosmic ray ion acceleration defines a "Holy Grail" for observers and theorists alike. This review summarizes theoretical developments in the prediction of MeV-TeV gamma-rays from supernova remnants over the last five years, focusing on how global properties of models can impact, and be impacted by, hard gamma-ray observational programs, thereby probing the supernova remnant environment. Properties of central consideration include the maximum energy of accelerated particles, the density of the unshocked interstellar medium, the ambient magnetic field, and the relativistic electron-to-proton ratio. Criteria for determining good candidate remnants for observability in the TeV band are identified.

Structural characteristics of Mg-doped (1-x)(K0.5Na0.5)NbO3-xLiSbO3 lead-free ceramics as revealed by Raman spectroscopy

NASA Astrophysics Data System (ADS)

Zhu, W. L.; Zhu, J. L.; Meng, Y.; Wang, M. S.; Zhu, B.; Zhu, X. H.; Zhu, J. G.; Xiao, D. Q.; Pezzotti, G.

2011-12-01

This paper presents a Raman spectroscopic study of compositional-change-induced structure variation and of the related mechanism of Mg doping in LiSbO3 (LS)-modified (K0.5Na0.5)NbO3 (KNN) ceramics. With increasing LS content from 0 to 0.06, a discontinuous shift towards higher wavenumbers was found for the band position of the A1g(v1) stretching mode of KNN, accompanied by a clearly nonlinear broadening of this band and a decrease in its intensity. Such morphological changes in the Raman spectrum result from two factors: (i) changes in polarizability/binding strength of the O-Nb-O vibration upon incorporation of Li ions in the KNN perovskitic structure and (ii) a polymorphic phase transition (PPT) from orthorhombic to tetragonal (O → T) phase at x > 0.04. Upon increasing the amount, w, of Mg dopant incorporated into the (1-x)KNN-xLS ceramic structure, the intensity of the Raman bands are enhanced, while the peak position and the full width at half maximum of the A1g(v1) mode was found to experience a clear dependence on both w and x. Raman characterization revealed that the mechanism of Mg doping is strongly correlated with the concentration of Li in the perovskite structure: Mg2+ ions will preferentially replace Li+ ions for low Mg doping while replace K/Na ions for higher doping of Mg. The PPT O → T was also found to be altered by the introduction of Mg and the critical value of LS concentration, xO-T, for incipient O → T transition in the KNN-xLS-wMT system was strongly dependent on Mg content, with xO → T being roughly equal to 0.04 + 2w, for the case of dilute Mg alloying.

Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag2Ga2SiS6 compound

NASA Astrophysics Data System (ADS)

Piasecki, M.; Myronchuk, G. L.; Parasyuk, O. V.; Khyzhun, O. Y.; Fedorchuk, A. O.; Pavlyuk, V. V.; Kozer, V. R.; Sachanyuk, V. P.; El-Naggar, A. M.; Albassam, A. A.; Jedryka, J.; Kityk, I. V.

2017-02-01

For the first time phase equilibria and phase diagram of the AgGaS2-SiS2 system were successfully explored by differential thermal and X-ray phase analysis methods. Crystal structure of low-temperature (LT) modification of Ag2Ga2SiS6 (LT- Ag2Ga2SiS6) was studied by X-ray powder method and it belongs to tetragonal space group I-42d, with unit cell parameters a=5.7164(4) Å, c=9.8023(7) Å, V=320.32(7) Å3. Additional details regarding the crystal structure exploration are available at the web page Fachinformationszentrum Karlsruhe. X-ray photoelectron core-level and valence-band spectra were measured for pristine LT- Ag2Ga2SiS6 crystal surface. In addition, the X-ray photoelectron valence-band spectrum of LT-Ag2Ga2SiS6 was matched on a common energy scale with the X-ray emission S Kβ1,3 and Ga Kβ2 bands, which give information on the energy distribution of the S 3p and Ga 4p states, respectively. The presented X-ray spectroscopy results indicate that the valence S p and Ga p atomic states contribute mainly to the upper and central parts of the valence band of LT-Ag2Ga2SiS6, respectively, with a less significant contribution also to other valence-band regions. Band gap energy was estimated by measuring the quantum energy in the spectral range of the fundamental absorption. We have found that energy gap Eg is equal to 2.35 eV at 300 K. LT-Ag2Ga2SiS6 is a photosensitive material and reveals two spectral maxima on the curve of spectral photoconductivity spectra at λmax1 =590 nm and λmax2 =860 nm. Additionally, linear electro-optical effect of LT-Ag2Ga2SiS6 for the wavelengths of a cw He-Ne laser at 1150 nm was explored.

Electronic and atomic structures of Ti{sub 1-x}Al{sub x}N thin films related to their damage behavior

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tuilier, M.-H.; Pac, M.-J.; Girleanu, M.

2008-04-15

Ti and Al K-edge x-ray absorption spectroscopy is used to investigate the electronic structure of Ti{sub 1-x}Al{sub x}N thin films deposited by reactive magnetron sputtering. The experimental near edge spectra of TiN and AlN are interpreted in the light of unoccupied density of state band structure calculations. The comparison of the structural parameters derived from x-ray absorption fine structure and x-ray diffraction reveals segregation between Al-rich and Ti-rich domains within the Ti{sub 1-x}Al{sub x}N films. Whereas x-ray diffraction probes only the crystallized domains, the structural information derived from extended x-ray absorption fine structure analysis turns on both crystalline and grainmore » boundaries. The results are discussed by considering the damage behavior of the films depending on the composition.« less

Optoelectronic Properties of X-Doped (X = O, S, Te) Photovoltaic CSe with Puckered Structure.

PubMed

Zhang, Qiang; Xin, Tianyuan; Lu, Xiaoke; Wang, Yuexia

2018-03-16

We exploited novel two-dimensional (2D) carbon selenide (CSe) with a structure analogous to phosphorene, and probed its electronics and optoelectronics. Calculating phonon spectra using the density functional perturbation theory (DFPT) method indicated that 2D CSe possesses dynamic stability, which made it possible to tune and equip CSe with outstanding properties by way of X-doping (X = O, S, Te), i.e., X substituting Se atoms. Then systematic investigation on the structural, electronic, and optical properties of pristine and X-doped monolayer CSe was carried out using the density functional theory (DFT) method. It was found that the bonding feature of C-X is intimately associated with the electronegativity and radius of the doping atoms, which leads to diverse electronic and optical properties for doping different group VI elements. All the systems possess direct gaps, except for O-doping. Substituting O for Se atoms in monolayer CSe brings about a transition from a direct Γ-Γ band gap to an indirect Γ-Y band gap. Moreover, the value of the band gap decreases with increased doping concentration and radius of doping atoms. A red shift in absorption spectra occurs toward the visible range of radiation after doping, and the red-shift phenomenon becomes more obvious with increased radius and concentration of doping atoms. The results can be useful for filtering doping atoms according to their radius or electronegativity in order to tailor optical spectra efficiently.

Optoelectronic Properties of X-Doped (X = O, S, Te) Photovoltaic CSe with Puckered Structure

PubMed Central

Zhang, Qiang; Xin, Tianyuan; Lu, Xiaoke; Wang, Yuexia

2018-01-01

We exploited novel two-dimensional (2D) carbon selenide (CSe) with a structure analogous to phosphorene, and probed its electronics and optoelectronics. Calculating phonon spectra using the density functional perturbation theory (DFPT) method indicated that 2D CSe possesses dynamic stability, which made it possible to tune and equip CSe with outstanding properties by way of X-doping (X = O, S, Te), i.e., X substituting Se atoms. Then systematic investigation on the structural, electronic, and optical properties of pristine and X-doped monolayer CSe was carried out using the density functional theory (DFT) method. It was found that the bonding feature of C-X is intimately associated with the electronegativity and radius of the doping atoms, which leads to diverse electronic and optical properties for doping different group VI elements. All the systems possess direct gaps, except for O-doping. Substituting O for Se atoms in monolayer CSe brings about a transition from a direct Γ-Γ band gap to an indirect Γ-Y band gap. Moreover, the value of the band gap decreases with increased doping concentration and radius of doping atoms. A red shift in absorption spectra occurs toward the visible range of radiation after doping, and the red-shift phenomenon becomes more obvious with increased radius and concentration of doping atoms. The results can be useful for filtering doping atoms according to their radius or electronegativity in order to tailor optical spectra efficiently. PMID:29547504

Space Radar Image of Safsaf Oasis, Egypt

NASA Technical Reports Server (NTRS)

1994-01-01

This three-frequency space radar image of south-central Egypt demonstrates the unique capability of imaging radar to penetrate thin sand cover in arid regions to reveal hidden details below the surface. Nearly all of the structures seen in this image are invisible to the naked eye and to conventional optical satellite sensors. Features appear in various colors because the three separate radar wavelengths are able to penetrate the sand to different depths. Areas that appear red or orange are places that can be seen only by the longest wavelength, L-band, and they are the deepest of the buried structures. Field studies in this area indicate L-band can penetrate as much as 2 meters (6.5 feet) of very dry sand to image buried rock structures. Ancient drainage channels at the bottom of the image are filled with sand more than 2 meters (6.5 feet) thick and therefore appear dark because the radar waves cannot penetrate them. The fractured orange areas at the top of the image and the blue circular structures in the center of the image are granitic areas that may contain mineral ore deposits. Scientists are using the penetrating capabilities of radar imaging in desert areas in studies of structural geology, mineral exploration, ancient climates, water resources and archaeology. This image is 51.9 kilometers by 30.2 kilometers (32.2 miles by 18.7 miles) and is centered at 22.7 degrees north latitude, 29.3degrees east longitude. North is toward the upper right. The colors are assigned to different radar frequencies and polarizations as follows: red is L-band, horizontally transmitted and received; green is C-band, horizontally transmitted and received; and blue is X-band, vertically transmitted and received. The image was acquired by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) on April 16, 1994, on board the space shuttle Endeavour. SIR-C/X-SAR, a joint mission of the German, Italian and United States space agencies, is part of NASA's Mission to Planet Earth program.

Analysis of Rotationally Resolved Spectra to Non-Degenerate (a''_1) Upper-State Vibronic Levels in the tilde{A} ^2E''-tilde{X}^2A^'_2 Electronic Transition of NO_3

NASA Astrophysics Data System (ADS)

Roudjane, Mourad; Codd, Terrance Joseph; Chen, Ming-Wei; Tran, Henry; Melnik, Dmitry G.; Miller, Terry A.; Stanton, John F.

2015-06-01

The vibronic structure of the tilde{A}-tilde{X} electronic spectrum of NO_3 has been observed using both room-temperature and jet-cooled samples. A recent analysis of this structure is consistent with the Jahn-Teller effect (JTE) in the e^' ν_3 vibrational mode (N-O stretch) being quite strong while the JTE in the e^' ν_4 mode (O-N-O) bend) is rather weak. Electronic structure calculations qualitatively predict these results but the calculated magnitude of the JTE is quantitatively inconsistent with the spectral analysis. Rotationally resolved spectra have been obtained for over a dozen vibronic bands of the tilde{A}-tilde{X} electronic transition in NO_3. An analysis of these spectra should provide considerably more experimental information about the JTE in the tilde{A} state of NO_3 as the rotational structure should be quite sensitive to the geometric distortion of the molecule due to the JTE. This talk will focus upon the parallel bands, which terminate on tilde{A} state levels of a''_1 vibronic symmetry, which were the subject of a preliminary analysis reported at this meeting in 2014. We have now recorded the rotational structure of over a half-dozen parallel bands and have completed analysis on the 3^1_0 and 3^1_0 4^1_0 transitions with several other bands being reasonably well understood. Two general conclusions emerge from this work. (i) All the spectral bands show evidence of perturbations which can reasonably be assumed to result from interactions of the observed tilde{A} state levels with high vibrational levels of the tilde{X} state. The perturbations range from severe in some bands to quite modest in others. (ii) Analyses of observed spectra, insofar as the perturbations permit, have all been performed with an oblate symmetric top model including only additional spin-rotation effects. This result is, of course, consistent with an effective, undistorted geometry for NO_3 of D3h symmetry on the rotational timescale.

Tailoring the electronic structure of β-Ga2O3 by non-metal doping from hybrid density functional theory calculations.

PubMed

Guo, Weiyan; Guo, Yating; Dong, Hao; Zhou, Xin

2015-02-28

A systematic study using density functional theory has been performed for β-Ga2O3 doped with non-metal elements X (X = C, N, F, Si, P, S, Cl, Se, Br, and I) to evaluate the effect of doping on the band edges and photocatalytic activity of β-Ga2O3. The utilization of a more reliable hybrid density functional, as prescribed by Heyd, Scuseria and Ernzerhof, is found to be effective in predicting the band gap of β-Ga2O3 (4.5 eV), in agreement with the experimental result (4.59 eV). Based on the relaxed structures of X-doped systems, the defect formation energies and the plots of density of states have been calculated to analyze the band edges, the band gap states and the preferred doping sites. Our results show that the doping is energetically favored under Ga-rich growth conditions with respect to O-rich growth conditions. It is easier to replace the threefold coordinated O atom with non-metal elements compared to the fourfold coordinated O atom. X-doped systems (X = C, Si, P) show no change in the band gap, with the presence of discrete midgap states, which have adverse effect on the photocatalytic properties. The photocatalytic redox ability can be improved to a certain extent by doping with N, S, Cl, Se, Br, and I. The band alignments for Se-doped and I-doped β-Ga2O3 are well positioned for the feasibility of both photo-oxidation and photo-reduction of water, which are promising photocatalysts for water splitting in the visible region.

Density-functional studies of tungsten trioxide, tungsten bronzes, and related systems

NASA Astrophysics Data System (ADS)

Ingham, B.; Hendy, S. C.; Chong, S. V.; Tallon, J. L.

2005-08-01

Tungsten trioxide adopts a variety of structures which can be intercalated with charged species to alter the electronic properties, thus forming “tungsten bronzes.” Similar effects are observed upon removing oxygen from WO3 . We present a computational study of cubic and hexagonal alkali bronzes and examine the effects on cell size and band structure as the size of the intercalated ion is increased. With the exception of hydrogen (which is predicted to be unstable as an intercalate), the behavior of the bronzes are relatively consistent. NaWO3 is the most stable of the cubic systems, although in the hexagonal system the larger ions are more stable. The band structures are identical, with the intercalated atom donating its single electron to the tungsten 5d valence band. A study of fractional doping in the NaxWO3 system (0⩽x⩽1) showed a linear variation in cell parameter and a systematic shift in the Fermi level into the conduction band. In the oxygen-deficient WO3-x system the Fermi level undergoes a sudden jump into the conduction band at around x=0.2 . Lastly, three compounds of a layered WO4•α,ω -diaminoalkane hybrid series were studied and found to be insulating, with features in the band structure similar to those of the parent WO3 compound that relate well to experimental UV-visible spectroscopy results.

Shear bands and anisotropy of the mechanical properties of an MA2-1pch magnesium alloy after equal-channel angular pressing

NASA Astrophysics Data System (ADS)

Serebryany, V. N.; Khar'kova, M. A.; D'yakonov, G. S.; Kopylov, V. I.; Dobatkin, S. V.

2017-10-01

Effect of structure and texture on the anisotropy of the mechanical properties of the MA2-1pch magnesium alloy subjected to equal-channel angular pressing and subsequent annealing has been studied in two mutually perpendicular planes Y and X (along and across the pressing direction). The anisotropy of the mechanical properties is shown to be due to various orientations of shear bands and various types of texture inside the bands and outside them in planes X and Y.

Multicolor emission from intermediate band semiconductor ZnO 1-xSe x

DOE Office of Scientific and Technical Information (OSTI.GOV)

Welna, M.; Baranowski, M.; Linhart, W. M.

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E - and upper E + valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emissionmore » is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.« less

Multicolor emission from intermediate band semiconductor ZnO 1-xSe x

DOE PAGES

Welna, M.; Baranowski, M.; Linhart, W. M.; ...

2017-03-13

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E - and upper E + valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emissionmore » is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.« less

Multi-band implications of external-IC flares

NASA Astrophysics Data System (ADS)

Richter, Stephan; Spanier, Felix

2015-02-01

Very fast variability on scales of minutes is regularly observed in Blazars. The assumption that these flares are emerging from the dominant emission zone of the very high energy (VHE) radiation within the jet challenges current acceleration and radiation models. In this work we use a spatially resolved and time dependent synchrotron-self-Compton (SSC) model that includes the full time dependence of Fermi-I acceleration. We use the (apparent) orphan γ -ray flare of Mrk501 during MJD 54952 and test various flare scenarios against the observed data. We find that a rapidly variable external radiation field can reproduce the high energy lightcurve best. However, the effect of the strong inverse Compton (IC) cooling on other bands and the X-ray observations are constraining the parameters to rather extreme ranges. Then again other scenarios would require parameters even more extreme or stronger physical constraints on the rise and decay of the source of the variability which might be in contradiction with constraints derived from the size of the black hole's ergosphere.

Production and evolution of A-centers in n-type Si1-xGex

NASA Astrophysics Data System (ADS)

Sgourou, E. N.; Andrianakis, A.; Londos, C. A.; Chroneos, A.

2013-03-01

The vacancy-oxygen pair (VO or A-center) in n-type Si1-xGex crystals (x = 0, 0.025, 0.055) has been studied using infrared (IR) spectroscopy. It is determined that the VO production is suppressed in the case of n-type Si1-xGex as compared to Si. It is observed that the annealing temperature of the VO defect in Si1-xGex is substantially lower as compared to Si. The decay of the VO (830 cm-1) band, in the course of 20 min isochronal anneals, shows two stages: The onset of the first stage is at ˜180 °C and the decrease of the VO signal is accompanied in the spectra by the increase of the intensity of two bands at ˜834 and 839 cm-1. These bands appear in the spectra immediately after irradiation and were previously correlated with (VO-Ge) structures. The onset of the second stage occurs at ˜250 °C were the 830 cm-1 band of VO and the above two bands of (VO-Ge) decrease together in the spectra accompanied by the simultaneous growth of the 885 cm-1 band of the VO2 defect. Interestingly, the percentage of the VO pairs that are converted to VO2 defects is larger in the Si1-xGex samples with intermediate Ge content (x = 0.025) as compared with Si (x = 0) and with the high Ge content samples (x = 0.055). The results are discussed in view of the association of VO pairs with Ge.

Structural and Optical Properties of Cd 1- x Se x Thin Films Deposited by Electron Beam Evaporation Technique

NASA Astrophysics Data System (ADS)

Tripathi, Ravishankar Nath; Verma, Aneet Kumar; Rahul, Vishwakarma, S. R.

2011-10-01

Cadmium selenide (CdSe) thin films deposited by means of electron beam evaporation technique under high vacuum ˜10 -5 torr on ultrasonically cleaned glass substrate. Using stating materials of various compositions of cadmium and selenium using formula Cd 1- x Se x where x is orbitory constant having value 0.20≤ x ≤0.40 here we take less value of x for the creation of anion vacancy in thin films. In present work the structural properties have been studies using XRD technique and found that starting materials and thin films both are polycrystalline in nature having hexagonal structure. Here we study the effect of composition ratio Cd/Se in starting material and its prepared thin films on its grain size and lattice parameter. From the analysis of X-Ray diffractogram found that lattice parameter and grain size both are decreases with increasing Cd/Se ratio in thin films as well as in starting material the preferred orientation in thin films along (100) plane. The surface morphology was studied using SEM characterization and found that films are smooth and homogeneous. The films have been analysed for optical band gap and absorbed a direct band gap.

Optical, Structural, and Thermal Properties of Cerium-Doped Zinc Borophosphate Glasses.

PubMed

Choi, Su-Yeon; Ryu, Bong-Ki

2015-11-01

In this study, we verify the relationship between the optical properties and structure of cerium-doped zinc borophosphate glasses that have concurrence of non-bridging oxygen (NBO) and bridging oxygen (BO), Ce3+ and Ce4+, and BO3 structure and BO4 structure. We prepared cerium-doped zinc borophosphate glass with various compositions, given by xCeO2-(100-x)[50ZnO-10B2O3 -40P2O5] (x = 1 mol% to 6 mol%), and analyzed their optical band energy, glass transition temperature, crystallization temperature, density, and molar volume. Some of the techniques used for analysis were Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In the investigated glasses, the optical band gap energy decreased from 3.28 eV to 1.73 eV. From these results, we can deduce the changes when transitions occur from BO to NBO, from Ce3+ to Ce4+, and from the BO3 structure to the BO4 structure with increasing CeO2 content using FT-IR and XPS analysis. We also verified the changes in structural and physical properties from quantitative properties such as glass transition temperature, crystallization temperature, density, and molar volume.

Growth and applications of GeSn-related group-IV semiconductor materials.

PubMed

Zaima, Shigeaki; Nakatsuka, Osamu; Taoka, Noriyuki; Kurosawa, Masashi; Takeuchi, Wakana; Sakashita, Mitsuo

2015-08-01

We review the technology of Ge 1- x Sn x -related group-IV semiconductor materials for developing Si-based nanoelectronics. Ge 1- x Sn x -related materials provide novel engineering of the crystal growth, strain structure, and energy band alignment for realising various applications not only in electronics, but also in optoelectronics. We introduce our recent achievements in the crystal growth of Ge 1- x Sn x -related material thin films and the studies of the electronic properties of thin films, metals/Ge 1- x Sn x , and insulators/Ge 1- x Sn x interfaces. We also review recent studies related to the crystal growth, energy band engineering, and device applications of Ge 1- x Sn x -related materials, as well as the reported performances of electronic devices using Ge 1- x Sn x related materials.

First-principles study on half-metallic ferromagnetic properties of Zn1- x V x Se ternary alloys

NASA Astrophysics Data System (ADS)

Khatta, Swati; Tripathi, S. K.; Prakash, Satya

2017-09-01

The spin-polarised density functional theory along with self-consistent plane-wave pseudopotential is used to investigate the half-metallic ferromagnetic properties of ternary alloys Zn1- x V x Se. The generalized gradient approximation is used for exchange-correlation potential. The equilibrium lattice constants, bulk modulus, and its derivatives are calculated. The calculated spin-polarised energy-band structures reveal that these alloys are half-metallic for x = 0.375 and 0.50 and nearly half-metallic for other values of x. The estimated direct and indirect bandgaps may be useful for the magneto-optical absorption experiments. It is found that there is strong Zn 4s, Se 4p, and V 3d orbital hybridization in the conduction bands of both the spins, while Se 4p and V 3d orbital hybridization predominates in the valence bands of both the spins. The s, p-d, and p-d orbital hybridization reduces the local magnetic moment of V atoms and small local magnetic moments are produced on Zn and Se atoms which get coupled with V atoms in ferromagnetic and antiferromagnetic phases, respectively. The conduction and valence-band-edge splittings and exchange constants predict the ferromagnetism in these alloys. The conduction band-impurity (s and p-d) exchange interaction is more significant for ferromagnetism in these alloys than the valence band-impurity (p-d) exchange interaction.

Structural and electronic properties of GaN nanowires with embedded In{sub x}Ga{sub 1−x}N nanodisks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kioseoglou, J., E-mail: sifisl@auth.gr; Pavloudis, Th.; Kehagias, Th.

2015-07-21

In the present study, the effects of various types of strain and indium concentration on the total energy and optoelectronic properties of GaN nanowires (NWs) with embedded In{sub x}Ga{sub 1−x}N nanodisks (NDs) are examined. In particular, the bi-axial, hydrostatic, and uniaxial strain states of the embedded In{sub x}Ga{sub 1−x}N NDs are investigated for multiple In concentrations. Density functional theory is employed to calculate the band structure of the NWs. The theoretical analysis finds that the supercell-size-dependent characteristics calculated for our 972-atom NW models are very close to the infinite supercell-size limit. It is established that the embedded In{sub x}Ga{sub 1−x}Nmore » NDs do not induce deep states in the band gap of the NWs. A bowing parameter of 1.82 eV is derived from our analysis in the quadratic Vegard's formula for the band gaps at the various In concentrations of the investigated In{sub x}Ga{sub 1−x}N NDs in GaN NW structures. It is concluded that up to ∼10% of In, the hydrostatic strain state is competitive with the bi-axial due to the radial absorption of the strain on the surfaces. Above this value, the dominant strain state is the bi-axial one. Thus, hydrostatic and bi-axial strain components coexist in the embedded NDs, and they are of different physical origin. The bi-axial strain comes from growth on lattice mismatched substrates, while the hydrostatic strain originates from the lateral relaxation of the surfaces.« less

Thermodynamical and thermoelectric properties of boron doped YPd{sub 3} and YRh{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dwivedi, Shalini; Sharma, Yamini, E-mail: sharma.yamini62@gmail.com; Sharma, Ramesh

2016-05-23

The structural, electronic, thermal, and optical properties of borides of cubic non-magnetic YX{sub 3} (X=Rh, Pd) compounds and their borides which crystallize in the AuCu{sub 3} structure have been studied using the density functional theory (DFT). The flat bands in the vicinity of E{sub F} which are associated with superconductivity appear in YPd{sub 3} and YRh{sub 3} band structures. However, the B s-states enhance the flat band only in YRh{sub 3}B. The optical properties clearly show that boron insertion modifies the absorption and transmittance. The YX{sub 3} alloys and their borides exhibit valuable changes in the thermopower and ZT. Itmore » is observed that the properties of the Y-X intermetallics change significantly for the Y-Rh and Y-Pd alloys and the presence of single boron atom modifies the properties to a great extent.« less

Electronic Structures of Purple Bronze KMo6O17 Studied by X-Ray Photoemission Spectra

NASA Astrophysics Data System (ADS)

Qin, Xiaokui; Wei, Junyin; Shi, Jing; Tian, Mingliang; Chen, Hong; Tian, Decheng

X-ray photoemission spectroscopy study has been performed for the purple bronze KMo6O17. The structures of conduction band and valence band are analogous to the results of ultraviolet photoemission spectra and are also consistent with the model of Travaglini et al., but the gap between conduction and valence band is insignificant. The shape of asymmetric and broadening line of O-1s is due to unresolved contributions from the many inequivalent oxygen sites in this crystal structure. Mo 3d core-level spectrum reveals that there are two kinds of valence states of Molybdenum (Mo+5 and Mo+6). The calculated average valence state is about +5.6, which is consistent with the expectation value from the composition of this material. The tail of Mo-3d spectrum toward higher binding energy is the consequence of the excitation of electron-hole pairs with singularity index of 0.21.

Verification of band offsets and electron effective masses in GaAsN/GaAs quantum wells: Spectroscopic experiment versus 10-band k·p modeling

NASA Astrophysics Data System (ADS)

Ryczko, K.; Sek, G.; Sitarek, P.; Mika, A.; Misiewicz, J.; Langer, F.; Höfling, S.; Forchel, A.; Kamp, M.

2013-06-01

Optical transitions in GaAs1-xNx/GaAs quantum wells (QWs) have been probed by two complementary techniques, modulation spectroscopy in a form of photoreflectance and surface photovoltage spectroscopy. Transition energies in QWs of various widths and N contents have been compared with the results of band structure calculations based on the 10-band k.p Hamiltonian. Due to the observation of higher order transitions in the measured spectra, the band gap discontinuities at the GaAsN/GaAs interface and the electron effective masses could be determined, both treated as semi-free parameters to get the best matching between the theoretical and experimental energies. We have obtained the chemical conduction band offset values of 86% for x = 1.2% and 83% for x = 2.2%, respectively. For these determined band offsets, the electron effective masses equal to about 0.09 mo in QWs with 1.2% N and 0.15 mo for the case of larger N content of 2.2%.

The optical spectrum of ternary alloy BBi1-xAsx

NASA Astrophysics Data System (ADS)

Yalcin, Battal G.; Aslan, M.; Ozcan, M. H.; Rahnamaye Aliabad, H. A.

2016-06-01

Among the III-V semiconductors, boron BBi and BAs as well as their alloys have attracted both scientific and technological interest in recent years. We present a calculation of the structural, electronic and optical properties of ternary alloy BBi1-xAsx by means of the WIEN2k software package. The exchange-correlation potential is treated by the generalized gradient approximation (GGA) within the schema of Wu and Cohen. Also, we have used the modified Becke-Johnson (mBJ) formalism to improve the band gap results. All the calculations have been performed after geometry optimization. In this study, we have investigated structural properties such as the lattice constant (a0), bulk modulus (B0) and its pressure derivative (B‧), and calculated the electronic band structures of the studied materials. Accurate calculation of linear optical properties, such as real (ɛ 1) and imaginary (ɛ 2) dielectric functions, reflectivity (R), electron energy loss spectrum, absorption coefficient (α), refractive index (n) and sum rule (Neff) are investigated. Our obtained results for studied binary compounds, BBi and BAs, fairly coincide with other theoretical calculations and experimental measurements. According to the best of our knowledge, no experimental or theoretical data are presently available for the studied ternary alloy BBi1-xAsx (0 < x < 1). The role of electronic band structure calculation with regards to the linear optical properties of BBi1-xAsx is discussed. The effect of the spin-orbit interaction (SOI) is also investigated and found to be quite small.

Electronic structure and optical properties of noncentrosymmetric LiGaSe2: Experimental measurements and DFT band structure calculations

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.

2017-04-01

We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.

Substitution of Li for Cu in Cu2ZnSnS4: Toward Wide Band Gap Absorbers with Low Cation Disorder for Thin Film Solar Cells.

PubMed

Lafond, A; Guillot-Deudon, C; Vidal, J; Paris, M; La, C; Jobic, S

2017-03-06

The substitution of lithium for copper in Cu 2 ZnSnS 4 (CZTS) has been experimentally and theoretically investigated. Formally, the (Cu 1-x Li x )ZnSnS 4 system exhibits two well-defined solid solutions. Indeed, single crystal structural analyses demonstrate that the low (x < 0.4) and high (x > 0.6) lithium-content compounds adopt the kesterite structure and the wurtz-kesterite structure, respectively. For x between 0.4 and 0.6, the two aforementioned structure types coexist. Moreover, 119 Sn NMR analyses carried out on a (Cu 0.7 Li 0.3 ) 2 ZnSnS 4 sample clearly indicate that lithium replaces copper preferentially on two of the three available 2-fold crystallographic sites commonly occupied by Cu and Zn in disordered kesterite. Furthermore, the observed individual lines in the NMR spectrum suggest that the propensity of Cu and Zn atoms to be randomly distributed over the 2c and 2d crystallographic sites is lowered when lithium is partially substituted for copper. Additionally, the first-principles calculations provide insights into the arrangement of Li atoms as a function of the Cu/Zn disorder and its effect on the structural (lattice parameters) and optical properties of CZTS (band gap evolution). Those calculations agree with the experimental observations and account for the evolutions of the unit cell parameters as well as for the increase of band gap when the Li-content increases. The calculation of the formation enthalpy of point defect unambiguously indicates that Li modifies the Cu/Zn disorder in a manner similar to the change of Cu/Zn disorder induced by Ag alloying. Overall, it was found that Li alloying is a versatile way of tuning the optoelectronic properties of CZTS making it a good candidate as wide band gap materials for the top cells of tandem solar cells.

Ballistic-Electron-Emission-Microscopy of Strained Si(sub 1-x)Ge(sub x) Layers

NASA Technical Reports Server (NTRS)

Bell, L. D.; Milliken, A. M.; Manion, S. J.; Kaiser, W. J.; Fathauer, R. W.; Pike, W. T.

1994-01-01

Ballistic-electron-emission microscopy (BEEM) has been used to investigate the effects of strain on Si(sub 1-x)Ge(sub x) alloys. Lifting of the degeneracy of the conduction-band minimum of Si(sub 1-x)Ge(sub x), due to lattice deformation has been directly measured by application of BEEM spectroscopy to Ag/Si structures. Experimental values for this conduction-band splitting agree well with calculations. In addition, an unexpected heterogeneity in the strain of the Si(sub 1-x)Ge(sub x) layer is introduced by deposition of Au. This effect, not observed with Ag, is attributed to species interdiffusion and has important implications for metal-semiconductor devices based oil pseudomorphic Si(sub 1-x)Ge(sub x)/Si material systems.

Carbon Dioxide Electroreduction into Syngas Boosted by a Partially Delocalized Charge in Molybdenum Sulfide Selenide Alloy Monolayers.

PubMed

Xu, Jiaqi; Li, Xiaodong; Liu, Wei; Sun, Yongfu; Ju, Zhengyu; Yao, Tao; Wang, Chengming; Ju, Huanxin; Zhu, Junfa; Wei, Shiqiang; Xie, Yi

2017-07-24

Structural parameters of ternary transition-metal dichalcogenide (TMD) alloy usually obey Vegard law well, while interestingly it often exhibits boosted electrocatalytic performances relative to its two pristine binary TMDs. To unveil the underlying reasons, we propose an ideal model of ternary TMDs alloy monolayer. As a prototype, MoSeS alloy monolayers are successfully synthesized, in which X-ray absorption fine structure spectroscopy manifests their shortened Mo-S and lengthened Mo-Se bonds, helping to tailor the d-band electronic structure of Mo atoms. Density functional theory calculations illustrate an increased density of states near their conduction band edge, which ensures faster electron transfer confirmed by their lower work function and smaller charge-transfer resistance. Energy calculations show the off-center charge around Mo atoms not only benefits for stabilizing COOH* intermediate confirmed by its most negative formation energy, but also facilitates the rate-limiting CO desorption step verified by CO temperature programmed desorption and electro-stripping tests. As a result, MoSeS alloy monolayers attain the highest 45.2 % Faradaic efficiency for CO production, much larger than that of MoS 2 monolayers (16.6 %) and MoSe 2 monolayers (30.5 %) at -1.15 V vs. RHE. This work discloses how the partially delocalized charge in ternary TMDs alloys accelerates electrocatalytic performances at atomic level, opening new horizons for manipulating CO 2 electroreduction properties. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

An unsupervised two-stage clustering approach for forest structure classification based on X-band InSAR data - A case study in complex temperate forest stands

NASA Astrophysics Data System (ADS)

Abdullahi, Sahra; Schardt, Mathias; Pretzsch, Hans

2017-05-01

Forest structure at stand level plays a key role for sustainable forest management, since the biodiversity, productivity, growth and stability of the forest can be positively influenced by managing its structural diversity. In contrast to field-based measurements, remote sensing techniques offer a cost-efficient opportunity to collect area-wide information about forest stand structure with high spatial and temporal resolution. Especially Interferometric Synthetic Aperture Radar (InSAR), which facilitates worldwide acquisition of 3d information independent from weather conditions and illumination, is convenient to capture forest stand structure. This study purposes an unsupervised two-stage clustering approach for forest structure classification based on height information derived from interferometric X-band SAR data which was performed in complex temperate forest stands of Traunstein forest (South Germany). In particular, a four dimensional input data set composed of first-order height statistics was non-linearly projected on a two-dimensional Self-Organizing Map, spatially ordered according to similarity (based on the Euclidean distance) in the first stage and classified using the k-means algorithm in the second stage. The study demonstrated that X-band InSAR data exhibits considerable capabilities for forest structure classification. Moreover, the unsupervised classification approach achieved meaningful and reasonable results by means of comparison to aerial imagery and LiDAR data.

Electronic Structure and Thermoelectric Properties of Transition Metal Monosilicides

NASA Astrophysics Data System (ADS)

Pshenay-Severin, D. A.; Ivanov, Yu. V.; Burkov, A. T.; Novikov, S. V.; Zaitsev, V. K.; Reith, H.

2018-06-01

We present theoretical and experimental results on electronic structure and thermoelectric properties of cobalt monosilicide (CoSi) and of Co1- x M x Si diluted alloys (M = Fe and Ni) at temperatures from 2 K to 800 K. CoSi crystallizes into a non-centrosymmetric cubic B20 structure, which suggests the possibility of a topologically non-trivial electronic structure. We show that the electronic structure of CoSi exhibits linear band crossings in close vicinity to Fermi energy, confirming the possibility of non-trivial topology. The proximity of the linear-dispersion bands to Fermi energy implies their important contribution to the electronic transport. Calculation of thermopower of CoSi, using ab initio band structure and the constant relaxation time approximation, is carried out. It reveals that many body corrections to the electronic spectrum are important in order to obtain qualitative agreement of theoretical and experimental temperature dependences of thermopower. Phonon dispersion and lattice thermal conductivity are calculated. The phonons give a major contribution to the thermal conductivity of the compound below room temperature.

Electronic structure of germanium selenide investigated using ultra-violet photo-electron spectroscopy

NASA Astrophysics Data System (ADS)

Mishra, P.; Lohani, H.; Kundu, A. K.; Patel, R.; Solanki, G. K.; Menon, Krishnakumar S. R.; Sekhar, B. R.

2015-07-01

The valence band electronic structure of GeSe single crystals has been investigated using angle resolved photoemission spectroscopy (ARPES) and x-ray photoelectron spectroscopy. The experimentally observed bands from ARPES, match qualitatively with our LDA-based band structure calculations along the Γ-Z, Γ-Y and Γ-T symmetry directions. The valence band maximum occurs nearly midway along the Γ-Z direction, at a binding energy of -0.5 eV, substantiating the indirect band gap of GeSe. Non-dispersive features associated with surface states and indirect transitions have been observed. The difference in hybridization of Se and Ge 4p orbitals leads to the variation of dispersion along the three symmetry directions. The predominance of the Se 4pz orbitals, evidenced from theoretical calculations, may be the cause for highly dispersive bands along the Γ-T direction. Detailed electronic structure analysis reveals the significance of the cation-anion 4p orbitals hybridization in the valence band dispersion of IV-VI semiconductors. This is the first comprehensive report of the electronic structure of a GeSe single crystal using ARPES in conjugation with theoretical band structure analysis.

Tailoring optical properties of TiO2-Cr co-sputtered films using swift heavy ions

NASA Astrophysics Data System (ADS)

Gupta, Ratnesh; Sen, Sagar; Phase, D. M.; Avasthi, D. K.; Gupta, Ajay

2018-05-01

Effect of 100 MeV Au7+ ion irradiation on structure and optical properties of Cr-doped TiO2 films has been studied using X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy, UV-Visible spectroscopy, X-ray reflectivity, and atomic force microscopy. X-ray reflectivity measurement implied that film thickness reduces as a function of ion fluence while surface roughness increases. The variation in surface roughness is well correlated with AFM results. Ion irradiation decreases the band gap energy of the film. Swift heavy ion irradiation enhances the oxygen vacancies in the film, and the extra electrons in the vacancies act as donor-like states. In valence band spectrum, there is a shift in the Ti3d peak towards lower energies and the shift is equivalent to the band gap energy obtained from UV spectrum. Evidence for band bending is also provided by the corresponding Ti XPS peak which exhibits a shift towards lower energy due to the downward band bending. X-ray absorption studies on O Kand Cr L3,2 edges clearly indicate that swift heavy ion irradiation induces formation of Cr-clusters in TiO2 matrix.

Electronic band structure and optical gain of GaN{sub x}Bi{sub y}As{sub 1−x−y}/GaAs pyramidal quantum dots

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Zhi-Gang; State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083; Bose, Sumanta

The electronic band structure and optical gain of GaN{sub x}Bi{sub y}As{sub 1−x−y}/GaAs pyramidal quantum dots (QDs) are investigated using the 16-band k ⋅ p model with constant strain. The optical gain is calculated taking both homogeneous and inhomogeneous broadenings into consideration. The effective band gap falls as we increase the composition of nitrogen (N) and bismuth (Bi) and with an appropriate choice of composition we can tune the emission wavelength to span within 1.3 μm–1.55 μm, for device application in fiber technology. The extent of this red shift is more profound in QDs compared with bulk material due to quantum confinement. Othermore » factors affecting the emission characteristics include virtual crystal, strain profile, band anticrossing (BAC), and valence band anticrossing (VBAC). The strain profile has a profound impact on the electronic structure, specially the valence band of QDs, which can be determined using the composition distribution of wave functions. All these factors eventually affect the optical gain spectrum. With an increase in QD size, we observe a red shift in the emission energy and emergence of secondary peaks owing to transitions or greater energy compared with the fundamental transition.« less

Multi-beam linear accelerator EVT

NASA Astrophysics Data System (ADS)

Teryaev, Vladimir E.; Kazakov, Sergey Yu.; Hirshfield, Jay L.

2016-09-01

A novel electron multi-beam accelerator is presented. The accelerator, short-named EVT (Electron Voltage Transformer) belongs to the class of two-beam accelerators. It combines an RF generator and essentially an accelerator within the same vacuum envelope. Drive beam-lets and an accelerated beam are modulated in RF modulators and then bunches pass into an accelerating structure, comprising uncoupled with each other and inductive tuned cavities, where the energy transfer from the drive beams to the accelerated beam occurs. A phasing of bunches is solved by choice correspond distances between gaps of the adjacent cavities. Preliminary results of numerical simulations and the initial specification of EVT operating in S-band, with a 60 kV gun and generating a 2.7 A, 1.1 MV beam at its output is presented. A relatively high efficiency of 67% and high design average power suggest that EVT can find its use in industrial applications.

Multi-beam linear accelerator EVT

DOE PAGES

Teryaev, Vladimir E.; Kazakov, Sergey Yu.; Hirshfield, Jay L.

2016-03-29

A novel electron multi-beam accelerator is presented. The accelerator, short-named EVT (Electron Voltage Transformer) belongs to the class of two-beam accelerators. It combines an RF generator and essentially an accelerator within the same vacuum envelope. Drive beam-lets and an accelerated beam are modulated in RF modulators and then bunches pass into an accelerating structure, comprising uncoupled with each other and inductive tuned cavities, where the energy transfer from the drive beams to the accelerated beam occurs. A phasing of bunches is solved by choice correspond distances between gaps of the adjacent cavities. Preliminary results of numerical simulations and the initialmore » specification of EVT operating in S-band, with a 60 kV gun and generating a 2.7 A, 1.1 MV beam at its output is presented. Furthermore, a relatively high efficiency of 67% and high design average power suggest that EVT can find its use in industrial applications.« less

The simultaneous low state spectral energy distribution of 1ES 2344+514 from radio to very high energies

DOE PAGES

Aleksić, J.; Antonelli, L. A.; Antoranz, P.; ...

2013-07-31

Here, BL Lacertae objects are variable at all energy bands on time scales down to minutes. To construct and interpret their spectral energy distribution (SED), simultaneous broad-band observations are mandatory. Up to now, the number of objects studied during such campaigns is very limited and biased towards high flux states. Furthermore, we present the results of a dedicated multi-wavelength study of the high-frequency peaked BL Lacertae (HBL) object and known TeV emitter 1ES 2344+514 by means of a pre-organised campaign. The observations were conducted during simultaneous visibility windows of MAGIC and AGILE in late 2008. The measurements were complemented bymore » Metsähovi, RATAN-600, KVA+Tuorla, Swift and VLBA pointings. Additional coverage was provided by the ongoing long-term F-GAMMA and MOJAVE programs, the OVRO 40-m and CrAO telescopes as well as the Fermi satellite. The obtained SEDs are modelled using a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. As a result, 1ES 2344+514 was found at very low flux states in both X-rays and very high energy gamma rays. Variability was detected in the low frequency radio and X-ray bands only, where for the latter a small flare was observed. The X-ray flare was possibly caused by shock acceleration characterised by similar cooling and acceleration time scales. MOJAVE VLBA monitoring reveals a static jet whose components are stable over time scales of eleven years, contrary to previous findings. There appears to be no significant correlation between the 15 GHz and R-band monitoring light curves. The observations presented here constitute the first multi-wavelength campaign on 1ES 2344+514 from radio to VHE energies and one of the few simultaneous SEDs during low activity states. The quasi-simultaneous Fermi-LAT data poses some challenges for SED modelling, but in general the SEDs are described well by both applied models. The resulting parameters are typical for TeV emitting HBLs. Consequently it remains unclear whether a so-called quiescent state was found in this campaign.« less

Influence of Bi-related impurity states on the bandgap and spin-orbit splitting energy of dilute III-V-Bi alloys: InP1-xBix, InAs1-xBix, InSb1-xBix and GaSb1-xBix

NASA Astrophysics Data System (ADS)

Samajdar, D. P.; Dhar, S.

2016-01-01

Valence Band Anticrossing (VBAC) Model is used to calculate the changes in band structure of Bi containing alloys such as InP1-xBix, InAs1-xBix, InSb1-xBix and GaSb1-xBix due to the incorporation of dilute concentrations of bismuth. The coupling parameter CBi which gives the magnitude of interaction of Bi impurity states with the LH, HH and SO sub bands in VBAC depends on the increase in the HH/LH related energy level EHH/LH+, location of the Bi related impurity level EBi and valence band offset ΔEVBM between the endpoint compounds in the corresponding III-V-Bi. The reduction in band gap as well as the enhancement of the spin-orbit splitting energy is well explained using this model and the calculated results are compared with the results of Virtual Crystal Approximation (VCA) and Density Functional Theory (DFT) calculations, as well as with the available experimental data and are found to have good agreement. The incorporation of Bi mainly perturbs the valence band due to the interaction of the Bi impurity states with the HH, LH and SO bands. The lowering of the conduction band minimum (CBM) due to VCA is added with the upward movement of the HH/LH bands to get the total reduction in band gap for the bismides. The valence band shifts of 31.9, 32.5, 20.8 and 12.4 meV/at%Bi for InP1-xBix, InAs1-xBix, InSb1-xBix and GaSb1-xBix respectively constitute 65, 76, 59 and 31% of the total band gap reduction and the rest is the contribution of the conduction band shift. The spin-orbit splitting energy also shows significant increase with the maximum change in InPBi and the minimum in InSbBi. The same is true for Ga containing bismides if we make a comparison with the available values for GaAsBi and GaPBi with that of GaSbBi. It has also been observed that the increase in splitting energy is greater in case of the bismides such as InAsBi, InPBi and GaAsBi than the bismides such as InSbBi and GaSbBi with the parent substrates having higher values of splitting energy. This may be due to the proximity of the Bi related impurity level EBi with the SO bands of InAs, InP and GaAs.

Multi-shell spherical GaAs /AlxGa1-x As quantum dot shells-size distribution as a mechanism to generate intermediate band energy levels

NASA Astrophysics Data System (ADS)

Rodríguez-Magdaleno, K. A.; Pérez-Álvarez, R.; Martínez-Orozco, J. C.; Pernas-Salomón, R.

2017-04-01

In this work the generation of an intermediate band of energy levels from multi-shell spherical GaAs /AlxGa1-x As quantum dot shells-size distribution is reported. Within the effective mass approximation the electronic structure of a GaAs spherical quantum-dot surrounded by one, two and three shells is studied in detail using a numerically stable transfer matrix method. We found that a shells-size distribution characterized by continuously wider GaAs domains is a suitable mechanism to generate the intermediate band whose width is also dependent on the Aluminium concentration x. Our results suggest that this effective mechanism can be used for the design of wider intermediate band than reported in other quantum systems with possible solar cells enhanced performance.

High-spin structures in the 139Pr nucleus

NASA Astrophysics Data System (ADS)

Yeoh, E. Y.; Zhu, S. J.; Wang, J. G.; Xiao, Z. G.; Zhang, M.; Yan, W. H.; Wang, R. S.; Xu, Q.; Wu, X. G.; He, C. Y.; Li, G. S.; Zheng, Y.; Li, C. B.; Cao, X. P.; Hu, S. P.; Yao, S. H.; Yu, B. B.

2012-06-01

Background: 139Pr is located in a transitional region of neutron number close to the N=82 shell. The study of its high-spin states and collective bands is important for systematically understanding the nuclear structural characteristics in this region.Purpose: To investigate the high-spin levels and to search for oblate bands in 139Pr.Methods: The high-spin states of 139Pr have been studied via the reaction 124Sn(19F,4n) at a beam energy of 80 MeV. The experiment was carried out at the HI-13 Tandem Accelerator at the China Institute of Atomic Energy (CIAE). The data analysis was done by using the γ-γ coincidence method.Results: The level scheme of 139Pr has been expanded with spin up to 45/2ℏ. A total of 39 new levels and 45 new transitions are identified. Four collective band structures at high-spin states have been newly established. From systematic analysis, one of the bands is proposed as a double decoupled band; two bands are proposed as oblate bands with γ˜-60∘; another band is suggested as an oblate-triaxial band with γ˜-90∘. The other characteristics for these bands are discussed.Conclusions: A new level scheme in 139Pr has been established and the collective bands at high spin have been identified. The result shows that the strong oblate shape-driving effect is caused by neutrons at the high-spin states in 139Pr.

W-band GaAs camel-cathode Gunn devices produced by MBE

NASA Astrophysics Data System (ADS)

Beall, R. B.; Battersby, S. J.; Grecian, P. J.; Jones, S.; Smith, G.

1989-06-01

The dc and microwave performance of a novel second-harmonic W-band GaAs Gunn device incorporating a camel barrier are reported. Comparison with conventional Gunn devices shows significant improvement in power output and dc to RF conversion efficiency for the new structure. The frequency at which the maximum power is produced is lower for the camel cathode Gunn device, an observation attributed to a reduction in the length of the acceleration zone.

Synthesis, X-ray structure, magnetic resonance, and DFT analysis of a soluble copper(II) phthalocyanine lacking C-H bonds.

PubMed

Moons, Hans; Łapok, Łukasz; Loas, Andrei; Van Doorslaer, Sabine; Gorun, Sergiu M

2010-10-04

The synthesis, crystal structure, and electronic properties of perfluoro-isopropyl-substituted perfluorophthalocyanine bearing a copper atom in the central cavity (F(64)PcCu) are reported. While most halogenated phthalocyanines do not exhibit long-term order sufficient to form large single crystals, this is not the case for F(64)PcCu. Its crystal structure was determined by X-ray analysis and linked to the electronic properties determined by electron paramagnetic resonance (EPR). The findings are corroborated by density functional theory (DFT) computations, which agree well with the experiment. X-band continuous-wave EPR spectra of undiluted F(64)PcCu powder, indicate the existence of isolated metal centers. The electron-withdrawing effect of the perfluoroalkyl (R(f)) groups significantly enhances the complexes solubility in organic solvents like alcohols, including via their axial coordination. This coordination is confirmed by X-band (1)H HYSCORE experiments and is also seen in the solid state via the X-ray structure. Detailed X-band CW-EPR, X-band Davies and Mims ENDOR, and W-band electron spin-echo-detected EPR studies of F(64)PcCu in ethanol allow the determination of the principal g values and the hyperfine couplings of the metal, nitrogen, and fluorine nuclei. Comparison of the g and metal hyperfine values of F(64)PcCu and other PcCu complexes in different matrices reveals a dominant effect of the matrix on these EPR parameters, while variations in the ring substituents have only a secondary effect. The relatively strong axial coordination occurs despite the diminished covalency of the C-N bonds and potentially weakening Jahn-Teller effects. Surprisingly, natural abundance (13)C HYSCORE signals could be observed for a frozen ethanol solution of F(64)PcCu. The (13)C nuclei contributing to the HYSCORE spectra could be identified as the pyrrole carbons by means of DFT. Finally, (19)F ENDOR and easily observable paramagnetic NMR were found to relate well to the DFT computations, revealing negligible isotropic hyperfine (Fermi contact) contributions. The single-site isolation in solution and solid state and the relatively strong coordination of axial ligands, both attributed to the introduction of R(f) groups, are features important for materials and catalyst design.

Magnetic and structural studies of trivalent Co-substituted Cd-Mn ferrites

NASA Astrophysics Data System (ADS)

Amer, M. A.; Meaz, T. M.; El-Kestawy, M.; Ghoneim, A. I.

2016-05-01

Series of polycrystalline Cd0.4Mn0.6CoxFe2-xO4 ferrites, 0≤x≤1, were prepared by solid state reaction method. The samples were characterized by inductive coupling plasma, X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectra and vibrating sample magnetometry. This study proved that all samples have single-phase cubic spinel structure. The true lattice constant, saturation magnetization, magnetic moment and trend of grain size and IR band νA showed decrease against x, whereas the trend of crystallite size, threshold frequency, Debye temperature, IR bands ν1 and ν2 and force constants F1 and F2, coercivity, anisotropy constant and residual magnetization showed increase. The IR analysis proved existence of Fe2+, Co2+, Fe4+, Co4+ and/or Mn4+ ions amongst the crystal sublattices. The characteristic bands ν1 and ν2 and force constants F1 and F2 showed decrease versus the tetrahedral- and octahedral-site bond length, respectively. The strain, specific surface area, refractive index, velocity, jump rate and remnant magnetization proved dependence on Co3+ ion content x.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Varley, J. B.; Conway, A. M.; Voss, L. F.

Thallium bromide (TlBr) crystals subjected to hydrochloric acid (HCl) chemical treatments have been shown to advantageously affect device performance and longevity in TlBr-based room temperature radiation detectors, yet the exact mechanisms of the improvements remain poorly understood. Here in this paper, we investigate the influence of several HCl chemical treatments on device-grade TlBr and describe the changes in the composition and electronic structure of the surface. Composition analysis and depth profiles obtained from secondary ion mass spectrometry (SIMS) identify the extent to which each HCl etch condition affects the detector surface region and forms of a graded TlBr/TlBr 1-xCL xmore » surface heterojunction. Using a combination of X-ray photoemission spectroscopy (XPS) and hybrid density functional calculations, we are able to determine the valence band offsets, band gaps, and conduction band offsets as a function of Cl content over the entire composition range of TIBr 1-xC1 X. This study establishes a strong correlation between device process conditions, surface chemistry, and electronic structure with the goal of further optimizing the long-term stability and radiation response of TlBr-based detectors.« less

Computational screening of organic polymer dielectrics for novel accelerator technologies

DOE PAGES

Pilania, Ghanshyam; Weis, Eric; Walker, Ethan M.; ...

2018-06-18

The use of infrared lasers to power accelerating dielectric structures is a developing area of research. Within this technology, the choice of the dielectric material forming the accelerating structures, such as the photonic band gap (PBG) structures, is dictated by a range of interrelated factors including their dielectric and optical properties, amenability to photo-polymerization, thermochemical stability and other target performance metrics of the particle accelerator. In this direction, electronic structure theory aided computational screening and design of dielectric materials can play a key role in identifying potential candidate materials with the targeted functionalities to guide experimental synthetic efforts. In anmore » attempt to systematically understand the role of chemistry in controlling the electronic structure and dielectric properties of organic polymeric materials, here we employ empirical screening and density functional theory (DFT) computations, as a part of our multi-step hierarchal screening strategy. Our DFT based analysis focused on the bandgap, dielectric permittivity, and frequency-dependent dielectric losses due to lattice absorption as key properties to down-select promising polymer motifs. In addition to the specific application of dielectric laser acceleration, the general methodology presented here is deemed to be valuable in the design of new insulators with an attractive combination of dielectric properties.« less

Thermal, optical and structural properties of Dy3+ doped sodium aluminophosphate glasses

NASA Astrophysics Data System (ADS)

Kaur, Manpreet; Singh, Anupinder; Thakur, Vanita; Singh, Lakhwant

2016-03-01

Trivalent Dysprosium doped sodium aluminophosphate glasses with composition 50P2O5-10Al2O3-(20-x)Na2O-20CaO-xDy2O3 (x varying from 0 to 5 mol%) were prepared by melt quench technique. The density of the prepared samples was measured using Archimedes principle and various physical properties like molar volume, rare earth ion concentration, polaron radius, inter nuclear distance and field strength were calculated using different formulae. The differential scanning calorimetry (DSC) was carried out to study the thermal stability of prepared glasses. The UV Visible absorption spectra of the dysprosium doped glasses were found to be comprised of ten absorption bands which correspond to transitions from ground state 6H15/2 to various excited states. The indirect optical band gap energy of the samples was calculated by Tauc's plot and the optical energy was found to be attenuated with Dy3+ ions. The photoluminescence spectrum revealed that Dy3+ doped aluminophosphate glasses have strong emission bands in the visible region. A blue emission band centred at 486 nm, a bright yellow band centred at 575 nm and a weak red band centred at 668 nm were observed in the emission spectrum due to excitation at 352 nm wavelength. Both FTIR and Raman spectra assert slight structural changes induced in the host glass network with Dy3+ ions.

Structure, morphology and Raman and optical spectroscopic analysis of In1-xCuxP thin films grown by MOCVD technique for solar cell applications

NASA Astrophysics Data System (ADS)

Alshahrie, Ahmed; Juodkazis, S.; Al-Ghamdi, A. A.; Hafez, M.; Bronstein, L. M.

2017-10-01

Nanocrystalline In1-xCuxP thin films (0 ≤ x ≤ 0.5) have been deposited on quartz substrates by a Metal-Organic Chemical Vapor Deposition (MOCVD) technique. The effect of the copper ion content on the structural crystal lattice, morphology and optical behavior of the InP thin films was assessed using X-ray diffraction, scanning electron microscopy, atomic force microscopy, Raman spectroscopy and spectrophotometry. All films exhibited a crystalline cubic zinc blende structure, inferring the solubility of the Cu atoms in the InP crystal structure. The XRD patterns demonstrated that the inclusion of Cu atoms into the InP films forced the nanoparticles in the films to grow along the (1 1 1) direction. The AFM topography showed that the Cu ions reduce the surface roughness of deposited films. The Raman spectra of the deposited films contain the first and second order anti-stoke ΓTO, ΓLO, ΧLO + ΧTO, 2ΓTO, and ΓLO + ΓTO bands which are characteristic of the InP crystalline structure. The intensities of these bands decreased with increasing the content of the Cu atoms in the InP crystals implying the creation of a stacking fault density in the InP crystal structure. The In1-xCuxP thin films have shown high optical transparency of 90%. An increase of the optical band gap from 1.38 eV to 1.6 eV was assigned to the increase of the amount of Cu ions in the InP films. The In0.5Cu0.5P thin film exhibited remarkable optical conductivity with very low dissipation factor which makes it a promising buffer window for solar energy applications.

Efficiency of Synchrotron Radiation from Rotation-powered Pulsars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kisaka, Shota; Tanaka, Shuta J., E-mail: kisaka@phys.aoyama.ac.jp, E-mail: sjtanaka@center.konan-u.ac.jp

2017-03-01

Synchrotron radiation is widely considered to be the origin of the pulsed non-thermal emissions from rotation-powered pulsars in optical and X-ray bands. In this paper, we study the synchrotron radiation emitted by the created electron and positron pairs in the pulsar magnetosphere to constrain the energy conversion efficiency from the Poynting flux to the particle energy flux. We model two pair creation processes, two-photon collision, which efficiently works in young γ -ray pulsars (≲10{sup 6} year), and magnetic pair creation, which is the dominant process to supply pairs in old pulsars (≳10{sup 6} year). Using the analytical model, we derivemore » the maximum synchrotron luminosity as a function of the energy conversion efficiency. From the comparison with observations, we find that the energy conversion efficiency to the accelerated particles should be an order of unity in the magnetosphere, even though we make a number of the optimistic assumptions to enlarge the synchrotron luminosity. In order to explain the luminosity of the non-thermal X-ray/optical emission from pulsars with low spin-down luminosity L {sub sd} ≲ 10{sup 34} erg s{sup −1}, non-dipole magnetic field components should be dominant at the emission region. For the γ -ray pulsars with L {sub sd} ≲ 10{sup 35} erg s{sup −1}, observed γ -ray to X-ray and optical flux ratios are much higher than the flux ratio between curvature and the synchrotron radiations. We discuss some possibilities such as the coexistence of multiple accelerators in the magnetosphere as suggested from the recent numerical simulation results. The obtained maximum luminosity would be useful to select observational targets in X-ray and optical bands.« less

Fermi-LAT Discovery of Extended Gamma-Ray Emission in the Direction of Supernova Remnant W51C

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abdo, A.A.; /Naval Research Lab, Wash., D.C. /Federal City Coll.; Ackermann, M.

The discovery of bright gamma-ray emission coincident with supernova remnant (SNR) W51C is reported using the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. W51C is a middle-aged remnant ({approx}10{sup 4} yr) with intense radio synchrotron emission in its shell and known to be interacting with a molecular cloud. The gamma-ray emission is spatially extended, broadly consistent with the radio and X-ray extent of SNR W51C. The energy spectrum in the 0.2-50 GeV band exhibits steepening toward high energies. The luminosity is greater than 1 x 10{sup 36} erg s{sup -1} given the distance constraint of D >more » 5.5 kpc, which makes this object one of the most luminous gamma-ray sources in our Galaxy. The observed gamma-rays can be explained reasonably by a combination of efficient acceleration of nuclear cosmic rays at supernova shocks and shock-cloud interactions. The decay of neutral p mesons produced in hadronic collisions provides a plausible explanation for the gamma-ray emission. The product of the average gas density and the total energy content of the accelerated protons amounts to {bar n}{sub H} W{sub p} {approx_equal} 5 x 10{sup 51} (D/6 kpc){sup 2} erg cm{sup -3}. Electron density constraints from the radio and X-ray bands render it difficult to explain the LAT signal as due to inverse Compton scattering. The Fermi LAT source coincident with SNR W51C sheds new light on the origin of Galactic cosmic rays.« less

Possibilities for LWIR detectors using MBE-grown Si(/Si(1-x)Ge(x) structures

NASA Technical Reports Server (NTRS)

Hauenstein, Robert J.; Miles, Richard H.; Young, Mary H.

1990-01-01

Traditionally, long wavelength infrared (LWIR) detection in Si-based structures has involved either extrinsic Si or Si/metal Schottky barrier devices. Molecular beam epitaxially (MBE) grown Si and Si/Si(1-x)Ge(x) heterostructures offer new possibilities for LWIR detection, including sensors based on intersubband transitions as well as improved conventional devices. The improvement in doping profile control of MBE in comparison with conventional chemical vapor deposited (CVD) Si films has resulted in the successful growth of extrinsic Si:Ga, blocked impurity-band conduction detectors. These structures exhibit a highly abrupt step change in dopant profile between detecting and blocking layers which is extremely difficult or impossible to achieve through conventional epitaxial growth techniques. Through alloying Si with Ge, Schottky barrier infrared detectors are possible, with barrier height values between those involving pure Si or Ge semiconducting materials alone. For both n-type and p-type structures, strain effects can split the band edges, thereby splitting the Schottky threshold and altering the spectral response. Measurements of photoresponse of n-type Au/Si(1-x)Ge(x) Schottky barriers demonstrate this effect. For intersubband multiquntum well (MQW) LWIR detection, Si(1-x)Ge(x)/Si detectors grown on Si substrates promise comparable absorption coefficients to that of the Ga(Al)As system while in addition offering the fundamental advantage of response to normally incident light as well as the practical advantage of Si-compatibility. Researchers grew Si(1-x)Ge(x)/Si MQW structures aimed at sensitivity to IR in the 8 to 12 micron region and longer, guided by recent theoretical work. Preliminary measurements of n- and p-type Si(1-x)Ge(x)/Si MQW structures are given.

A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals.

PubMed

Jovic, Vedran; Rettie, Alexander J E; Singh, Vijay R; Zhou, Jianshi; Lamoureux, Bethany; Buddie Mullins, C; Bluhm, Hendrik; Laverock, Jude; Smith, Kevin E

2016-11-23

Doped BiVO 4 is a promising photoelectrochemical water splitting anode, whose activity is hampered by poor charge transport. Here we use a set of X-ray spectroscopic methods to probe the origin and nature of localized electron states in W:BiVO 4 . Furthermore, using the polarized nature of the X-rays, we probe variations in the electronic structure along the crystal axes. In this manner, we reveal aspects of the electronic structure related to electron localization and observations consistent with conductivity anisotropy between the ab-plane and c-axis. We verify that tungsten substitutes as W 6+ for V 5+ in BiVO 4 . This is shown to result in the presence of inter-band gap states related to electrons at V 4+ sites of e symmetry. The energetic position of the states in the band gap suggest that they are highly localized and may act as recombination centres. Polarization dependent X-ray absorption spectra reveal anisotropy in the electronic structure between the ab-plane and c-axis. Results show the superior hybridization between V 3d and O 2p states, higher V wavefunction overlap and broader conduction bands in the ab-plane than in the c-axis. These insights into the electronic structure are discussed in the context of existing experimental and theoretical reports regarding charge transport in BiVO 4 .

Theoretical Study of the Transverse Dielectric Constant of Superlattices and Their Alloys

NASA Astrophysics Data System (ADS)

Kahen, Keith Brian

The optical properties of III-V binary and ternary compounds and GaAs-Al(,x)Ga(,1-x)As superlattices are determined by calculating the real and imaginary parts of the transverse dielectric constant, (epsilon)((omega)) = (epsilon)(,1)((omega)) + i(epsilon)(,2)((omega)). Emphasis is given to determining the influence of different material and superlattice (layer thickness and Al composi- tion) parameters on the values of the index of refraction. (eta)((omega)) and absorption coefficient, (alpha)((omega)). In order to calculate the optical properties of a material, it is necessary to compute its electronic band structure. We accomplish this by introducing a partition band structure approach based on a combination of the (')k(.)(')p and nonlocal pseudopotential techniques. In this approach the bulk Brillouin zone is partitioned into the (GAMMA), X, and L regions by performing (')k(.)(')p expansions about these three symmetry points. The results for (eta)((omega)) and (alpha)((omega)) of bulk III-V compounds com- pare well with other one-electron band structure models, and our calculations show that for small frequencies, the index of refraction is determined mainly by the contributions of the outer regions of the Brillouin zone. The effects of alloy scattering are incorporated into the model using a perturbative CPA approach which only includes the influence of compositional disorder. The results for the disorder-induced, (GAMMA) point, energy -gap bowings are shown to be nearly comparable to those calculated using more sophisticated CPA approaches. Further - more, the calculated absorption coefficient of Al(,x)Ga(,1 -x)As is found to be in good agreement with the experimental data. The model is extended to heterostructures by using the envelope-function approximation. Valence-band mixing and (GAMMA)-region exciton effects are also included in the model. Our results show that the anisotropy and structure dependence of the refractive index of superlattices result mainly from the contribution of the (GAMMA) region, while the contributions of the outer regions of the zone are rather insensitive to the superlattice structure. The superlattice index of refraction values is determined to attain maxima at the various (GAMMA)-region, quantized, transition energies, where for certain structures the difference between the refractive indices of the superlattice and its corresponding Al(,x)Ga(,1-x)As alloy can be as large as 2%. (Abstract shortened with permission of author.).

Four-Way Ka-Band Power Combiner

NASA Technical Reports Server (NTRS)

Perez, Raul; Li, Samuel

2007-01-01

A waveguide structure for combining the outputs of four amplifiers operating at 35 GHz (Ka band) is based on a similar prior structure used in the X band. The structure is designed to function with low combining loss and low total reflected power at a center frequency of 35 GHz with a 160 MHz bandwidth. The structure (see figure) comprises mainly a junction of five rectangular waveguides in a radial waveguide. The outputs of the four amplifiers can be coupled in through any four of the five waveguide ports. Provided that these four signals are properly phased, they combine and come out through the fifth waveguide port.

Temperature-induced band shift in bulk γ-InSe by angle-resolved photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Xu, Huanfeng; Wang, Wei; Zhao, Yafei; Zhang, Xiaoqian; Feng, Yue; Tu, Jian; Gu, Chenyi; Sun, Yizhe; Liu, Chang; Nie, Yuefeng; Edmond Turcu, Ion C.; Xu, Yongbing; He, Liang

2018-05-01

Indium selenide (InSe) has recently become popular research topics because of its unique layered crystal structure, direct band gap and high electron mobilities. In this work, we have acquired the electronic structure of bulk γ-InSe at various temperatures using angle-resolved photoemission spectroscopy (ARPES). We have also found that as the temperature decreases, the valence bands of γ-InSe exhibit a monotonic shift to lower binding energies. This band shift is attributed to the change of lattice parameters and has been validated by variable temperature X-ray diffraction measurements and theoretical calculations.

Synthesis and optical properties of Pr and Ti doped BiFeO{sub 3} ceramics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Singh, Vikash, E-mail: vikash.singh@abes.ac.in; Applied Science and Humanities, ABES EC, Ghaziabad; Sharma, Subhash

2016-05-23

Bi{sub 1-x}Pr{sub x}Fe{sub 1-x}Ti{sub x}O{sub 3} ceramics with x = 0.00, 0.10 and 0.20 were synthesized by solid state reaction method. Rietveld fitting of diffraction data reveals structural transition from rhombohedral phase (R{sub 3C}) for x ≤ 0.10 to orthorhombic phase (P{sub nma}) for x = 0.20. FTIR spectra exhibit broad absorption bands, which may be due to the overlapping of Fe-O and Bi-O vibrations in these ceramics. UV-visible spectroscopy results show strong absorption of light in the spectral range of 400-600 nm, indicating optical band gap in the visible region for these samples.

Photonic band structures solved by a plane-wave-based transfer-matrix method.

PubMed

Li, Zhi-Yuan; Lin, Lan-Lan

2003-04-01

Transfer-matrix methods adopting a plane-wave basis have been routinely used to calculate the scattering of electromagnetic waves by general multilayer gratings and photonic crystal slabs. In this paper we show that this technique, when combined with Bloch's theorem, can be extended to solve the photonic band structure for 2D and 3D photonic crystal structures. Three different eigensolution schemes to solve the traditional band diagrams along high-symmetry lines in the first Brillouin zone of the crystal are discussed. Optimal rules for the Fourier expansion over the dielectric function and electromagnetic fields with discontinuities occurring at the boundary of different material domains have been employed to accelerate the convergence of numerical computation. Application of this method to an important class of 3D layer-by-layer photonic crystals reveals the superior convergency of this different approach over the conventional plane-wave expansion method.

Interaction of an ultrarelativistic electron bunch train with a W-band accelerating structure: High power and high gradient

DOE PAGES

Wang, D.; Antipov, S.; Jing, C.; ...

2016-02-05

Electron beam interaction with high frequency structures (beyond microwave regime) has a great impact on future high energy frontier machines. We report on the generation of multimegawatt pulsed rf power at 91 GHz in a planar metallic accelerating structure driven by an ultrarelativistic electron bunch train. This slow-wave wakefield device can also be used for high gradient acceleration of electrons with a stable rf phase and amplitude which are controlled by manipulation of the bunch train. To achieve precise control of the rf pulse properties, a two-beam wakefield interferometry method was developed in which the rf pulse, due to themore » interference of the wakefields from the two bunches, was measured as a function of bunch separation. As a result, measurements of the energy change of a trailing electron bunch as a function of the bunch separation confirmed the interferometry method.« less

Solar Hard X-ray Observations with NuSTAR

NASA Astrophysics Data System (ADS)

Marsh, Andrew; Smith, D. M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Harrison, F. A.; Grefenstette, B. W.; Stern, D.

2012-05-01

High-sensitivity imaging of coronal hard X-rays allows detection of freshly accelerated nonthermal electrons at the acceleration site. A few such observations have been made with Yohkoh and RHESSI, but a leap in sensitivity could help pin down the time, place, and manner of reconnection. Around the time of this meeting, the Nuclear Spectroscopic Telescope ARray (NuSTAR), a NASA Small Explorer for high energy astrophysics that uses grazing-incidence optics to focus X-rays up to 80 keV, will be launched. Three weeks will be dedicated to solar observing during the baseline two-year mission. NuSTAR will be 200 times more sensitive than RHESSI in the hard X-ray band. This will allow the following new observations, among others: 1) Extrapolation of the micro/nanoflare distribution by two orders of magnitude down in flux; 2) Search for hard X-rays from network nanoflares (soft X-ray bright points) and evaluation of their role in coronal heating; 3) Discovery of hard X-ray bremsstrahlung from the electron beams driving type III radio bursts, and measurement of their electron spectrum; 4) Hard X-ray studies of polar soft X-ray jets and impulsive solar energetic particle events at the edge of coronal holes; 5) Study of coronal bremsstrahlung from particles accelerated by coronal mass ejections as they are first launched; 6) Study of particles at the coronal reconnection site when flare footpoints and loops are occulted; 7) Search for weak high-temperature coronal plasmas in active regions that are not flaring; and 8) Search for hypothetical axion particles created in the solar core via the hard X-ray signal from their conversion to X-rays in the coronal magnetic field. NuSTAR will also serve as a pathfinder for a future dedicated space mission with enhanced capabilities, such as a satellite version of the FOXSI sounding rocket.

Solar Hard X-ray Observations with NuSTAR

NASA Astrophysics Data System (ADS)

Smith, David M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Stern, D.; Grefenstette, B. W.; Harrison, F. A.

2011-05-01

High-sensitivity imaging of coronal hard X-rays allows detection of freshly accelerated nonthermal electrons at the acceleration site. A few such observations have been made with Yohkoh and RHESSI, but a leap in sensitivity could help pin down the time, place, and manner of reconnection. In 2012, the Nuclear Spectroscopic Telescope Array (NuSTAR), a NASA Small Explorer for high energy astrophysics that uses grazing-incidence optics to focus X-rays up to 80 keV, will be launched. NuSTAR is capable of solar pointing, and three weeks will be dedicated to solar observing during the baseline two-year mission. NuSTAR will be 200 times more sensitive than RHESSI in the hard X-ray band. This will allow the following new observations, among others: 1) Extrapolation of the micro/nanoflare distribution by two orders of magnitude down in flux 2) Search for hard X-rays from network nanoflares (soft X-ray bright points) and evaluation of their role in coronal heating 3) Discovery of hard X-ray bremsstrahlung from the electron beams driving type III radio bursts, and measurement of their electron spectrum 4) Hard X-ray studies of polar soft X-ray jets and impulsive solar energetic particle events at the edge of coronal holes, and comparison of these events with observations of 3He and other particles in interplanetary space 5) Study of coronal bremsstrahlung from particles accelerated by coronal mass ejections as they are first launched 6) Study of particles at the coronal reconnection site when flare footpoints are occulted; and 7) Search for hypothetical axion particles created in the solar core via the hard X-ray signal from their conversion to X-rays in the coronal magnetic field. NuSTAR will also serve as a pathfinder for a future dedicated space mission with enhanced capabilities, such as a satellite version of the FOXSI sounding rocket.

Tropical-Forest Structure and Biomass Dynamics from TanDEM-X Radar Interferometry

Treesearch

Robert Treuhaft; Yang Lei; Fabio Gonçalves; Michael Keller; João Santos; Maxim Neumann; André Almeida

2017-01-01

Changes in tropical-forest structure and aboveground biomass (AGB) contribute directly to atmospheric changes in CO2, which, in turn, bear on global climate. This paper demonstrates the capability of radar-interferometric phase-height time series at X-band (wavelength = 3 cm) to monitor changes in vertical structure and AGB, with sub-hectare and monthly spatial and...

An algorithm for the design and tuning of RF accelerating structures with variable cell lengths

NASA Astrophysics Data System (ADS)

Lal, Shankar; Pant, K. K.

2018-05-01

An algorithm is proposed for the design of a π mode standing wave buncher structure with variable cell lengths. It employs a two-parameter, multi-step approach for the design of the structure with desired resonant frequency and field flatness. The algorithm, along with analytical scaling laws for the design of the RF power coupling slot, makes it possible to accurately design the structure employing a freely available electromagnetic code like SUPERFISH. To compensate for machining errors, a tuning method has been devised to achieve desired RF parameters for the structure, which has been qualified by the successful tuning of a 7-cell buncher to π mode frequency of 2856 MHz with field flatness <3% and RF coupling coefficient close to unity. The proposed design algorithm and tuning method have demonstrated the feasibility of developing an S-band accelerating structure for desired RF parameters with a relatively relaxed machining tolerance of ∼ 25 μm. This paper discusses the algorithm for the design and tuning of an RF accelerating structure with variable cell lengths.

Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag{sub 2}Ga{sub 2}SiS{sub 6} compound

DOE Office of Scientific and Technical Information (OSTI.GOV)

Piasecki, M., E-mail: m.piasecki@ajd.czest.pl; Myronchuk, G.L.; Parasyuk, O.V.

For the first time phase equilibria and phase diagram of the AgGaS{sub 2}–SiS{sub 2} system were successfully explored by differential thermal and X-ray phase analysis methods. Crystal structure of low-temperature (LT) modification of Ag{sub 2}Ga{sub 2}SiS{sub 6} (LРў- Ag{sub 2}Ga{sub 2}SiS{sub 6}) was studied by X-ray powder method and it belongs to tetragonal space group I-42d, with unit cell parameters a=5.7164(4) Å, c=9.8023(7) Å, V=320.32(7) Å{sup 3}. Additional details regarding the crystal structure exploration are available at the web page Fachinformationszentrum Karlsruhe. X-ray photoelectron core-level and valence-band spectra were measured for pristine LРў- Ag{sub 2}Ga{sub 2}SiS{sub 6} crystal surface. Inmore » addition, the X-ray photoelectron valence-band spectrum of LРў-Ag{sub 2}Ga{sub 2}SiS{sub 6} was matched on a common energy scale with the X-ray emission S Kβ{sub 1,3} and Ga Kβ{sub 2} bands, which give information on the energy distribution of the S 3p and Ga 4p states, respectively. The presented X-ray spectroscopy results indicate that the valence S p and Ga p atomic states contribute mainly to the upper and central parts of the valence band of LРў-Ag{sub 2}Ga{sub 2}SiS{sub 6}, respectively, with a less significant contribution also to other valence-band regions. Band gap energy was estimated by measuring the quantum energy in the spectral range of the fundamental absorption. We have found that energy gap Eg is equal to 2.35 eV at 300 K. LT-Ag{sub 2}Ga{sub 2}SiS{sub 6} is a photosensitive material and reveals two spectral maxima on the curve of spectral photoconductivity spectra at λ{sub max1} =590 nm and λ{sub max2} =860 nm. Additionally, linear electro-optical effect of LT-Ag{sub 2}Ga{sub 2}SiS{sub 6} for the wavelengths of a cw He-Ne laser at 1150 nm was explored. - Graphical abstract: Manuscript present the technology of growth and investigation of properties a new quaternary compound Ag{sub 2}Ga{sub 2}SiS{sub 6} including the detailed study of the phase equilibria in the AgGaS{sub 2}–SiS{sub 2} system to determine the physico-chemical conditions of its formation. We examined crystal structure of the compound by X-ray powder method, its electronic structure by XPS method. Finally we have found these crystals like promising material for nonlinear optical, electrooptical and piezoelectric applications. - Highlights: • Phase diagram of the AgGaS{sub 2}–SiS{sub 2} system was successfully described at first time. • Crystal Ag{sub 2}Ga{sub 2}SiS{sub 6} was grown and its structure was determined. • Electronic structure and chemical bonding of the Ag{sub 2}Ga{sub 2}SiS{sub 6} crystal were examined by. • X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) methods. • Optical and photoelectrical properties were investigated.« less

Accelerators for Discovery Science and Security applications

NASA Astrophysics Data System (ADS)

Todd, A. M. M.; Bluem, H. P.; Jarvis, J. D.; Park, J. H.; Rathke, J. W.; Schultheiss, T. J.

2015-05-01

Several Advanced Energy Systems (AES) accelerator projects that span applications in Discovery Science and Security are described. The design and performance of the IR and THz free electron laser (FEL) at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin that is now an operating user facility for physical chemistry research in molecular and cluster spectroscopy as well as surface science, is highlighted. The device was designed to meet challenging specifications, including a final energy adjustable in the range of 15-50 MeV, low longitudinal emittance (<50 keV-psec) and transverse emittance (<20 π mm-mrad), at more than 200 pC bunch charge with a micropulse repetition rate of 1 GHz and a macropulse length of up to 15 μs. Secondly, we will describe an ongoing effort to develop an ultrafast electron diffraction (UED) source that is scheduled for completion in 2015 with prototype testing taking place at the Brookhaven National Laboratory (BNL) Accelerator Test Facility (ATF). This tabletop X-band system will find application in time-resolved chemical imaging and as a resource for drug-cell interaction analysis. A third active area at AES is accelerators for security applications where we will cover some top-level aspects of THz and X-ray systems that are under development and in testing for stand-off and portal detection.

Enhanced carrier mobility and direct tunneling probability of biaxially strained Ge{sub 1−x}Sn{sub x} alloys for field-effect transistors applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Lei; Liang, Renrong, E-mail: liangrr@tsinghua.edu.cn, E-mail: junxu@tsinghua.edu.cn; Wang, Jing

The carrier transport and tunneling capabilities of biaxially strained Ge{sub 1−x}Sn{sub x} alloys with (001), (110), and (111) orientations were comprehensively investigated and compared. The electron band structures of biaxially strained Ge{sub 1−x}Sn{sub x} alloys were calculated by the nonlocal empirical pseudopotential method and the modified virtual crystal approximation was adopted in the calculation. The electron and hole effective masses at the band edges were extracted using a parabolic line fit. It is shown that the applied biaxial strain and the high Sn composition are both helpful for the reduction of carrier effective masses, which leads to the enhanced carriermore » mobility and the boosted direct band-to-band-tunneling probability. Furthermore, the strain induced valance band splitting reduces the hole interband scattering, and the splitting also results in the significantly enhanced direct tunneling rate along the out-of-plane direction compared with that along the in-plane direction. The biaxially strained (111) Ge{sub 1−x}Sn{sub x} alloys exhibit the smallest band gaps compared with (001) and (110) orientations, leading to the highest in-plane and out-of-plane direct tunneling probabilities. The small effective masses on (110) and (111) planes in some strained conditions also contribute to the enhanced carrier mobility and tunneling probability. Therefore, the biaxially strained (110) and (111) Ge{sub 1−x}Sn{sub x} alloys have the potential to outperform the corresponding (001) Ge{sub 1−x}Sn{sub x} devices. It is important to optimize the applied biaxial strain, the Sn composition, and the substrate orientation for the design of high performance Ge{sub 1−x}Sn{sub x} field-effect transistors.« less

Abnormal broadening of the optical transitions in (Ga,As)N/GaAs quantum wells

NASA Astrophysics Data System (ADS)

Turcotte, S.; Beaudry, J.-N.; Masut, R. A.; Desjardins, P.; Bentoumi, G.; Leonelli, R.

2012-01-01

We have measured the near band-gap absorption of structurally well characterized GaAs1-xNx quantum wells grown on GaAs(001) with x<0.014. The spectra were reproduced by a model that includes electron-hole correlations. We find that the width of the excitonic and band-to-band optical transitions are more than twice larger than what is found in conventional III-V alloy heterostructures. This confirms the presence of strong nitrogen-configuration induced band-gap fluctuations reported previously by Bentoumi [Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.70.035315 70, 035315 (2004)] for bulk dilute GaAsN alloys.

Venus spherical harmonic gravity model to degree and order 60

NASA Technical Reports Server (NTRS)

Konopliv, Alex S.; Sjogren, William L.

1994-01-01

The Magellan and Pioneer Venus Orbiter radiometric tracking data sets have been combined to produce a 60th degree and order spherical harmonic gravity field. The Magellan data include the high-precision X-band gravity tracking from September 1992 to May 1993 and post-aerobraking data up to January 5, 1994. Gravity models are presented from the application of Kaula's power rule for Venus and an alternative a priori method using surface accelerations. Results are given as vertical gravity acceleration at the reference surface, geoid, vertical Bouguer, and vertical isostatic maps with errors for the vertical gravity and geoid maps included. Correlation of the gravity with topography for the different models is also discussed.

Spectral tailoring of nanoscale EUV and soft x-ray multilayer optics

NASA Astrophysics Data System (ADS)

Huang, Qiushi; Medvedev, Viacheslav; van de Kruijs, Robbert; Yakshin, Andrey; Louis, Eric; Bijkerk, Fred

2017-03-01

Extreme ultraviolet and soft X-ray (XUV) multilayer optics have experienced significant development over the past few years, particularly on controlling the spectral characteristics of light for advanced applications like EUV photolithography, space observation, and accelerator- or lab-based XUV experiments. Both planar and three dimensional multilayer structures have been developed to tailor the spectral response in a wide wavelength range. For the planar multilayer optics, different layered schemes are explored. Stacks of periodic multilayers and capping layers are demonstrated to achieve multi-channel reflection or suppression of the reflective properties. Aperiodic multilayer structures enable broadband reflection both in angles and wavelengths, with the possibility of polarization control. The broad wavelength band multilayer is also used to shape attosecond pulses for the study of ultrafast phenomena. Narrowband multilayer monochromators are delivered to bridge the resolution gap between crystals and regular multilayers. High spectral purity multilayers with innovated anti-reflection structures are shown to select spectrally clean XUV radiation from broadband X-ray sources, especially the plasma sources for EUV lithography. Significant progress is also made in the three dimensional multilayer optics, i.e., combining micro- and nanostructures with multilayers, in order to provide new freedom to tune the spectral response. Several kinds of multilayer gratings, including multilayer coated gratings, sliced multilayer gratings, and lamellar multilayer gratings are being pursued for high resolution and high efficiency XUV spectrometers/monochromators, with their advantages and disadvantages, respectively. Multilayer diffraction optics are also developed for spectral purity enhancement. New structures like gratings, zone plates, and pyramids that obtain full suppression of the unwanted radiation and high XUV reflectance are reviewed. Based on the present achievement of the spectral tailoring multilayer optics, the remaining challenges and opportunities for future researches are discussed.

Electronic structure and optical properties of CdSxSe1-x solid solution nanostructures from X-ray absorption near edge structure, X-ray excited optical luminescence, and density functional theory investigations

NASA Astrophysics Data System (ADS)

Murphy, M. W.; Yiu, Y. M.; Ward, M. J.; Liu, L.; Hu, Y.; Zapien, J. A.; Liu, Yingkai; Sham, T. K.

2014-11-01

The electronic structure and optical properties of a series of iso-electronic and iso-structural CdSxSe1-x solid solution nanostructures have been investigated using X-ray absorption near edge structure, extended X-ray absorption fine structure, and X-ray excited optical luminescence at various absorption edges of Cd, S, and Se. It is found that the system exhibits compositions, with variable local structure in-between that of CdS and CdSe accompanied by tunable optical band gap between that of CdS and CdSe. Theoretical calculation using density functional theory has been carried out to elucidate the observations. It is also found that luminescence induced by X-ray excitation shows new optical channels not observed previously with laser excitation. The implications of these observations are discussed.

Self-shielded electron linear accelerators designed for radiation technologies

NASA Astrophysics Data System (ADS)

Belugin, V. M.; Rozanov, N. E.; Pirozhenko, V. M.

2009-09-01

This paper describes self-shielded high-intensity electron linear accelerators designed for radiation technologies. The specific property of the accelerators is that they do not apply an external magnetic field; acceleration and focusing of electron beams are performed by radio-frequency fields in the accelerating structures. The main characteristics of the accelerators are high current and beam power, but also reliable operation and a long service life. To obtain these characteristics, a number of problems have been solved, including a particular optimization of the accelerator components and the application of a variety of specific means. The paper describes features of the electron beam dynamics, accelerating structure, and radio-frequency power supply. Several compact self-shielded accelerators for radiation sterilization and x-ray cargo inspection have been created. The introduced methods made it possible to obtain a high intensity of the electron beam and good performance of the accelerators.

Back-bombardment compensation in microwave thermionic electron guns

NASA Astrophysics Data System (ADS)

Kowalczyk, Jeremy M. D.; Madey, John M. J.

2014-12-01

The development of capable, reliable, and cost-effective compact electron beam sources remains a long-standing objective of the efforts to develop the accelerator systems needed for on-site research and industrial applications ranging from electron beam welding to high performance x-ray and gamma ray light sources for element-resolved microanalysis and national security. The need in these applications for simplicity, reliability, and low cost has emphasized solutions compatible with the use of the long established and commercially available pulsed microwave rf sources and L-, S- or X-band linear accelerators. Thermionic microwave electron guns have proven to be one successful approach to the development of the electron sources for these systems providing high macropulse average current beams with picosecond pulse lengths and good emittance out to macropulse lengths of 4-5 microseconds. But longer macropulse lengths are now needed for use in inverse-Compton x-ray sources and other emerging applications. We describe in this paper our approach to extending the usable macropulse current and pulse length of these guns through the use of thermal diffusion to compensate for the increase in cathode surface temperature due to back-bombardment.

Determination of a natural valence-band offset - The case of HgTe and CdTe

NASA Technical Reports Server (NTRS)

Shih, C. K.; Spicer, W. E.

1987-01-01

A method to determine a natural valence-band offset (NVBO), i.e., the change in the valence-band maximum energy which is intrinsic to the bulk band structures of semiconductors is proposed. The HgTe-CdTe system is used as an example in which it is found that the valence-band maximum of HgTe lies 0.35 + or - 0.06 eV above that of CdTe. The NVBO of 0.35 eV is in good agreement with the X-ray photoemission spectroscopy measurement of the heterojunction offset. The procedure to determine the NVBO between semiconductors, and its implication on the heterojunction band lineup and the electronic structures of semiconductor alloys, are discussed.

Characterization of electronic structure of periodically strained graphene

DOE PAGES

Aslani, Marjan; Garner, C. Michael; Kumar, Suhas; ...

2015-11-03

We induced periodic biaxial tensile strain in polycrystalline graphene by wrapping it over a substrate with repeating pillar-like structures with a periodicity of 600 nm. Using Raman spectroscopy, we determined to have introduced biaxial strains in graphene in the range of 0.4% to 0.7%. Its band structure was characterized using photoemission from valance bands, shifts in the secondary electron emission, and x-ray absorption from the carbon 1s levels to the unoccupied graphene conduction bands. It was observed that relative to unstrained graphene, strained graphene had a higher work function and higher density of states in the valence and conduction bands.more » Furthermore, we measured the conductivity of the strained and unstrained graphene in response to a gate voltage and correlated the changes in their behavior to the changes in the electronic structure. From these sets of data, we propose a simple band diagram representing graphene with periodic biaxial strain.« less

Metal insulator transition in nickel substituted FeSi

NASA Astrophysics Data System (ADS)

Krishnan, M.; Mishra, Ashish; Singh, Durgesh; Venkatesh, R.; Gangrade, Mohan; Ganesan, V.

2018-04-01

Resistivity of Fe1-xNixSi has been reported. Metal Insulator transition (MIT) is observed in Nickel (Ni) substituted FeSi for x in the range from 2 to 4 percentage. Two Band Model has been employed in order to calculate activation energy and to predict how band structure renormalized with substitution of nickel in FeSi. At sufficient level of nickel concentration an impurity band forms around Fermi level and contributes to the conduction heavily at low temperatures. Concentration around x = 0.04, displays metallic property below ˜ 70 K and is quantitatively similar to systems like Fe1-xTxSi (T = Co, Mn). Metallic component thus derived from Ni substituted FeSi seems to have an unconventional temperature dependence that may be attributed to the onset of departures from Fermi liquid picture.

Room Temperature Thin Film Ba(x)Sr(1-x)TiO3 Ku-Band Coupled MicrostripPhase Shifters: Effects of Film Thickness, Doping, Annealing and Substrate Choice

NASA Technical Reports Server (NTRS)

VanKeuls, F. W.; Mueller, C. H.; Miranda, F. A.; Romanofsky, R. R.; Canedy, C. L.; Aggarwal, S.; Venkatesan, T.; Ramesh, R.; Horwitz, S.; Chang, W.

1999-01-01

We report on measurements taken on over twenty Ku-band coupled microstrip phase shifters (CMPS) using thin ferroelectric films of Ba(x)Sr(1-x)TiO3. This CMPS design is a recent innovation designed to take advantage of the high tunability and tolerate the high dielectric constant of ferroelectric films at Ku- and K-band frequencies. These devices are envisioned as a component in low-cost steerable beam phased area antennas, Comparisons are made between devices with differing film thickness, annealed vs unannealed, Mn-doped vs. undoped, and also substrates of LaAlO3 and MgO. A comparison between the CMPS structure and a CPW phase shifter was also made oil the same ferroelectric film.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Grayson, Katie J.; Faries, Kaitlyn M.; Huang, Xia

Photosynthesis uses a limited range of the solar spectrum, so enhancing spectral coverage could improve the efficiency of light capture. Here, we show that a hybrid reaction centre (RC)/yellow fluorescent protein (YFP) complex accelerates photosynthetic growth in the bacterium Rhodobacter sphaeroides. The structure of the RC/YFP-light-harvesting 1 (LH1) complex shows the position of YFP attachment to the RC-H subunit, on the cytoplasmic side of the RC complex. Fluorescence lifetime microscopy of whole cells and ultrafast transient absorption spectroscopy of purified RC/YFP complexes show that the YFP–RC intermolecular distance and spectral overlap between the emission of YFP and the visible-region (Qmore » X) absorption bands of the RC allow energy transfer via a Fo¨rster mechanism, with an efficiency of 40±10%. Finally, this proof-of-principle study demonstrates the feasibility of increasing spectral coverage for harvesting light using non-native genetically-encoded light-absorbers, thereby augmenting energy transfer and trapping in photosynthesis.« less

Rare-earth gate oxides for GaAs MOSFET application

NASA Astrophysics Data System (ADS)

Kwon, Kwang-Ho; Yang, Jun-Kyu; Park, Hyung-Ho; Kim, Jongdae; Roh, Tae Moon

2006-08-01

Rare-earth oxide films for gate dielectric on n-GaAs have been investigated. The oxide films were e-beam evaporated on S-passivated GaAs, considering interfacial chemical bonding state and energy band structure. Rare-earth oxides such as Gd 2O 3, (Gd xLa 1- x) 2O 3, and Gd-silicate were employed due to high resistivity and no chemical reaction with GaAs. Structural and bonding properties were characterized by X-ray photoemission, absorption, and diffraction. The electrical characteristics of metal-oxide-semiconductor (MOS) diodes were correlated with material properties and energy band structures to guarantee the feasibility for MOS field effect transistor (FET) application. Gd 2O 3 films were grown epitaxially on S-passivated GaAs (0 0 1) at 400 °C. The passivation induced a lowering of crystallization temperature with an epitaxial relationship of Gd 2O 3 (4 4 0) and GaAs (0 0 1). A better lattice matching relation between Gd 2O 3 and GaAs substrate was accomplished by the substitution of Gd with La, which has larger ionic radius. The in-plane relationship of (Gd xLa 1- x) 2O 3 (4 4 0) with GaAs (0 0 1) was found and the epitaxial films showed an improved crystalline quality. Amorphous Gd-silicate film was synthesized by the incorporation of SiO 2 into Gd 2O 3. These amorphous Gd-silicate films excluded defect traps or current flow path due to grain boundaries and showed a relatively larger energy band gap dependent on the contents of SiO 2. Energy band parameters such as Δ EC, Δ EV, and Eg were effectively controlled by the film composition.

Structural, thermal and optical absorption features of heavy metal oxides doped tellurite rich glasses

NASA Astrophysics Data System (ADS)

Kaky, Kawa M.; Lakshminarayana, G.; Baki, S. O.; Kityk, I. V.; Taufiq-Yap, Y. H.; Mahdi, M. A.

In order to improve tellurite glass stability to be applicable for optical fiber amplifier applications, glasses with the composition of (70 - x)TeO2. (10)ZnO. (10)WO3. (5)Na2O. (5)TiO2. (x)Bi2O3 (x = 1, 2, 3, 4, and 5 mol%) have been produced and characterized using the related methods. Structural properties were investigated using X-ray diffraction (XRD) which confirms the non-crystalline structure and scanning electron microscopy (SEM) micrographs also confirm the XRD results. The energy dispersive X-ray (EDX) analysis profiles show that all the mentioned elements are present in the prepared glasses. Following the IR spectra, all the tellurium bonds such as stretching vibrations of TeO4 tbp and TeO3/TeO3+1 unit are revealed. Raman spectra confirm the presence of different functional groups, actually, it shows bands mainly in four spectral regions: R1 (65-150) cm-1, R2 (280-550) cm-1, R3 (880-950) cm-1 and R4 (916-926) cm-1 and the identified bands are assigned to respective molecular groups. The thermal study was carried out using Differential scanning calorimetry (DSC) which indicates good thermal stability of the synthesized glasses with increasing Bi concentration. From the optical absorption spectra, we evaluated cut-off edge wavelengths and found increasing cutoff wavelength with an increase in Bi2O3 concentration. In the UV-Visible region, optical band gap energy and allowed transitions were investigated using three methods; direct, indirect, and absorption spectrum fitting (ASF), and band gaps from indirect and ASF were matched.

BL-Lacs in X-Ray Outburst

NASA Technical Reports Server (NTRS)

Remillard, Ronald A.; Urry, C. Megan; Aharonian, Felix; Pian, Elena; Sambruna, Rita; Coppi, Paolo

2000-01-01

We conducted a multifrequency campaign for the TeV blazar Markarian 421 in 1998 April. The campaign started from a pronounced high-amplitude flare recorded by BeppoSAX and Whipple; the Advanced Satellite for Cosmology and Astrophysics (ASCA) observation started three days later. In the X-ray data, we detected multiple flares, occurring on timescales of about one day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, the Extreme Ultraviolet Explorer, and the Rossi X-Ray Timing Explorer indicates that the variability amplitudes in the low-energy synchrotron component are larger at higher photon energies. In TeV and gamma-rays, large intraday variations-which were correlated with the X-ray flux-were observed when results from three Cerenkov telescopes were combined. The rms variability of TeV and gamma-rays was similar to that observed in hard X-rays, above ten keV. The X-ray light curve reveals flares that are almost symmetric for most cases, implying that the dominant timescale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of seven days indicates that the characteristic timescale is approx. 0.5 days. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are real, these imply that the particle acceleration and X-ray cooling timescales are similar.

Electron Beam Production and Characterization for the PLEIADES Thomson X-ray Source

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, W J; Hartemann, F V; Tremaine, A M

2002-10-14

We report on the performance of an S-band RF photocathode electron gun and accelerator for operation with the PLEIADES Thomson x-ray source at LLNL. Simulations of beam production, transport, and focus are presented. It is shown that a 1 ps, 500 pC electron bunch with a normalized emittance of less than 5 {pi}mm-mrad can be delivered to the interaction point. Initial electron measurements are presented. Calculations of expected x-ray flux are also performed, demonstrating an expected peak spectral brightness of 10{sup 20} photons/s/mm{sup 2}/mrad{sup 2}/0.1% bandwidth. Effects of RF phase jitter are also presented, and planned phase measurements and controlmore » methods are discussed.« less

An all-perovskite p-n junction based on transparent conducting p -La 1-x Sr x CrO 3 epitaxial layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Du, Yingge; Li, Chen; Zhang, Kelvin H. L.

2017-08-07

Transparent, conducting p -La 1-x Sr x CrO 3 epitaxial layers were deposited on Nb-doped SrTiO 3(001) by oxygen-assisted molecular beam epitaxy to form structurally coherent p-n junctions. X-ray photoelectron spectroscopy reveals a type II or “staggered” band alignment, with valence and conduction band offsets of 2.0 eV and 0.9 eV, respectively. Diodes fabricated from these heterojunctions exhibit rectifying behavior, and the I-V characteristics are different from those for traditional semiconductor p-n junctions. A rather large ideality factor is ascribed to the complex nature of the interface.

Anomalous Hall conductivity and electronic structures of Si-substituted Mn2CoAl epitaxial films

NASA Astrophysics Data System (ADS)

Arima, K.; Kuroda, F.; Yamada, S.; Fukushima, T.; Oguchi, T.; Hamaya, K.

2018-02-01

We study anomalous Hall conductivity (σAHC) and electronic band structures of Si-substituted Mn2CoAl (Mn2CoAl1 -xSix ). First-principles calculations reveal that the electronic band structure is like a spin-gapless system even after substituting a quaternary element of Si for Al up to x =0.2 in Mn2CoAl1 -xSix . This means that the Si substitution enables the Fermi-level shift without largely changing the electronic structures in Mn2CoAl . By using molecular beam epitaxy techniques, Mn2CoAl1 -xSix epitaxial films can be grown, leading to the systematic control of x (0 ⩽x ⩽0.3 ). In addition to the electrical conductivity, the values of σAHC for the Mn2CoAl1 -xSix films are similar to those in Mn2CoAl films shown in previous reports. We note that a very small σAHC of ˜1.1 S/cm is obtained for x = 0.225, and the sign of σAHC is changed from positive to negative at around x = 0.25. We discuss the origin of the sign reversal of σAHC as a consequence of the Fermi-level shift in Mn2CoAl . Considering the presence of the structural disorder in the Mn2CoAl1 -xSix films, we can conclude that the small value and sign reversal of σAHC are not related to the characteristics of spin-gapless semiconductors.

Surface and bulk electronic structures of unintentionally and Mg-doped In0.7Ga0.3N epilayer by hard X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Imura, Masataka; Tsuda, Shunsuke; Takeda, Hiroyuki; Nagata, Takahiro; Banal, Ryan G.; Yoshikawa, Hideki; Yang, AnLi; Yamashita, Yoshiyuki; Kobayashi, Keisuke; Koide, Yasuo; Yamaguchi, Tomohiro; Kaneko, Masamitsu; Uematsu, Nao; Wang, Ke; Araki, Tsutomu; Nanishi, Yasushi

2018-03-01

The surface and bulk electronic structures of In0.7Ga0.3N epilayers are investigated by angle-resolved hard X-ray photoelectron spectroscopy (HX-PES) combined with soft X-PES. The unintentionally and Mg-doped In0.7Ga0.3N (u-In0.7Ga0.3N and In0.7Ga0.3N:Mg, respectively) epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. Here three samples with different Mg concentrations ([Mg] = 0, 7 × 1019, and 4 × 1020 cm-3) are chosen for comparison. It is found that a large downward energy band bending exists in all samples due to the formation of a surface electron accumulation (SEA) layer. For u-In0.7Ga0.3N epilayer, band bending as large as 0.8 ± 0.05 eV occurs from bulk to surface. Judged from the valence band spectral edge and numerical analysis of energy band with a surface quantum well, the valence band maximum (VBM) with respect to Fermi energy (EF) level in the bulk is determined to be 1.22 ± 0.05 eV. In contrast, for In0.7Ga0.3N:Mg epilayers, the band bending increases and the VBM only in the bulk tends to shift toward the EF level owing to the Mg acceptor doping. Hence, the energy band is considered to exhibit a downward bending structure due to the coexistence of the n+ SEA layer and Mg-doped p layer formed in the bulk. When [Mg] changes from 7 × 1019 to 4 × 1020 cm-3, the peak split occurs in HX-PES spectra under the bulk sensitive condition. This result indicates that the energy band forms an anomalous downward bending structure with a singular point due to the generation of a thin depleted region at the n+ p interface. For In0.7Ga0.3N:Mg epilayers, the VBM in the bulk is assumed to be slightly lower than EF level within 0.1 eV.

X-Ray Photoelectron Spectroscopic Characterization of Iron Oxide Nanoparticles

NASA Astrophysics Data System (ADS)

Radu, T.; Iacovita, C.; Benea, D.; Turcu, R.

2017-05-01

We report X-ray photoelectron spectroscopy (XPS) results on iron oxide magnetic nanoparticle (Fe3O4) synthesized using solvothermal reduction in the presence of polyethylene glycol. The magnetite obtained was employed as precursor for the synthesis of γ-Fe2O3 (by oxygen dissociation) which in turn was transformed into α-Fe2O3. We confirmed the magnetite, maghemite and hematite structure by Fourier Transformed Spectroscopy (FTIR) and X-ray diffraction (XRD). The analysis of the XPS core level and valence band (VB) photoemission spectra for all investigated samples is discussed in terms of the degree of iron oxidation. This is of fundamental importance to better understand the electronic structure of the obtained iron oxide nanoparticles in order to control and improve their quality for specific biomedical applications. Moreover, theoretical band structure calculations are performed for magnetite and the separate contributions of Fe in tetragonal and octahedral environment are shown.

ARPES studies on metal-insulator-transition in NiS2-xSex

NASA Astrophysics Data System (ADS)

Han, Garam; Kim, Y. K.; Kyung, W. S.; Kim, Chul; Koh, Y. Y.; Lee, K. D.; Kim, C.

2012-02-01

Understanding Metal insulator transition (MIT) is one of the most challenging issues in condensed matter physics. NiS2-xSex (NSS) is a well known system for band width controlled MIT studies while most of High-Tc superconductors (HTSCs) are described within band filling MIT picture. Cubic pyrite NiS2 is known as a charge-transfer (CT) insulator and easily forms a solid solution with NiSe2, which is a good metal even though it is isostrucural and isoelectronic to NiS2. MIT is induced by Se alloying and is observed at a low temperature for x=0.5. The important merit is that there is no structure transition which often accompanies MIT. In spite of the importance of the system, even the experimental band dispersion is not known so far along with many controversies. For this reason, we performed angle resolved photoemission spectroscopy on high quality single crystals and successfully obtained Fermi surface maps of x=0.5, x=0.7 and x=0.8 systems (the metallic side). By doping dependent systematic studies on NSS and comparison with LDA calculation, we try to explain the relationship between band width and the MIT.

Structure and optical band gaps of (Ba,Sr)SnO{sub 3} films grown by molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schumann, Timo; Raghavan, Santosh; Ahadi, Kaveh

2016-09-15

Epitaxial growth of (Ba{sub x}Sr{sub 1−x})SnO{sub 3} films with 0 ≤ x ≤ 1 using molecular beam epitaxy is reported. It is shown that SrSnO{sub 3} films can be grown coherently strained on closely lattice and symmetry matched PrScO{sub 3} substrates. The evolution of the optical band gap as a function of composition is determined by spectroscopic ellipsometry. The direct band gap monotonously decreases with x from to 4.46 eV (x = 0) to 3.36 eV (x = 1). A large Burnstein-Moss shift is observed with La-doping of BaSnO{sub 3} films. The shift corresponds approximately to the increase in Fermi level and is consistent with the low conduction band mass.

Computational study of electronic, optical and thermoelectric properties of X3PbO (X = Ca, Sr, Ba) anti-perovskites

NASA Astrophysics Data System (ADS)

Hassan, M.; Arshad, I.; Mahmood, Q.

2017-11-01

We report the structural, electronic, optical and thermoelectric (TE) properties of X3PbO (X = Ca, Sr, Ba) anti-perovskites as a function of X cations belonging to the group IIA. The computations are done by using the most recently introduced modified Becke-Johnson potential. It has been observed that the cubic lattice constant increases as the cations change from Ca to Ba, consequently, the bulk modulus reduces. The bottom of conduction band shows strong hybridization between Pb-6p, O-2p and X-s states, in contrast, valence band maxima are mainly manufactured by Pb-6p states. The anti-perovskites exhibit narrow direct band gap that show an inverse relation to the static real dielectric constants that verifies Penn’s model. In addition, the X cations induced tuning of the absorption edge in the visible and the ultraviolet energy suggest optical device applications. The computed TE parameters have been found sensitive to the X cations and have been demonstrated to be best suited for the TE devices operating at high temperatures.

Electronic properties and bonding in Zr Hx thin films investigated by valence-band x-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Magnuson, Martin; Schmidt, Susann; Hultman, Lars; Högberg, Hans

2017-11-01

The electronic structure and chemical bonding in reactively magnetron sputtered Zr Hx (x =0.15 , 0.30, 1.16) thin films with oxygen content as low as 0.2 at.% are investigated by 4d valence band, shallow 4p core-level, and 3d core-level x-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity toward the H 1s-Zr 4d hybridization region at ˜6 eV below the Fermi level. For low hydrogen content (x =0.15 , 0.30), the films consist of a superposition of hexagonal closest-packed metal (α phase) and understoichiometric δ -Zr Hx (Ca F2 -type structure) phases, while for x =1.16 , the films form single-phase Zr Hx that largely resembles that of stoichiometric δ -Zr H2 phase. We show that the cubic δ -Zr Hx phase is metastable as thin film up to x =1.16 , while for higher H contents the structure is predicted to be tetragonally distorted. For the investigated Zr H1.16 film, we find chemical shifts of 0.68 and 0.51 eV toward higher binding energies for the Zr 4 p3 /2 and 3 d5 /2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as a function of hydrogen content is discussed in relation to the charge transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.

High Power Factor and Enhanced Thermoelectric Performance of SnTe-AgInTe2: Synergistic Effect of Resonance Level and Valence Band Convergence.

PubMed

Banik, Ananya; Shenoy, U Sandhya; Saha, Sujoy; Waghmare, Umesh V; Biswas, Kanishka

2016-10-05

Understanding the basis of electronic transport and developing ideas to improve thermoelectric power factor are essential for production of efficient thermoelectric materials. Here, we report a significantly large thermoelectric power factor of ∼31.4 μW/cm·K 2 at 856 K in Ag and In co-doped SnTe (i.e., SnAg x In x Te 1+2x ). This is the highest power factor so far reported for SnTe-based material, which arises from the synergistic effects of Ag and In on the electronic structure and the improved electrical transport properties of SnTe. In and Ag play different but complementary roles in modifying the valence band structure of SnTe. In-doping introduces resonance levels inside the valence bands, leading to a significant improvement in the Seebeck coefficient at room temperature. On the other hand, Ag-doping reduces the energy separation between light- and heavy-hole valence bands by widening the principal band gap, which also results in an improved Seebeck coefficient. Additionally, Ag-doping in SnTe enhances the p-type carrier mobility. Co-doping of In and Ag in SnTe yields synergistically enhanced Seebeck coefficient and power factor over a broad temperature range because of the synergy of the introduction of resonance states and convergence of valence bands, which have been confirmed by first-principles density functional theory-based electronic structure calculations. As a consequence, we have achieved an improved thermoelectric figure of merit, zT ≈ 1, in SnAg 0.025 In 0.025 Te 1.05 at 856 K.

Bi2MoxW1-xO6 solid solutions with tunable band structure and enhanced visible-light photocatalytic activities

NASA Astrophysics Data System (ADS)

Li, Wenqi; Ding, Xingeng; Wu, Huating; Yang, Hui

2018-07-01

Semiconductor photocatalysis is an effective green way to combat water pollution. For the first time, this study reports a novel method to develop Bi2MoxW1-xO6 solid solution with microsphere structure through anion-exchange method. All Bi2MoxW1-xO6 samples exhibit an Aurivillius-type crystal structure without any secondary phase, confirming that in complete solid solutions as the value of x increases, the band gap energy of Bi2MoxW1-xO6 solid solutions decreases, while the optical absorption edge moves to longer wavelength. The Raman spectra research shows an increase in orthorhombic distortion with progressive replacement of W sites in Bi2WO6 with Mo6+ ions. Compared to Bi2MoO6 and Bi2WO6 samples, Bi2Mo0.4W0.6O6 sample displayed best photocatalytic activity and cycling stability for degradation of RhB dye. The enhanced photocatalytic activity of Bi2Mo0.4W0.6O6 sample can be synergetically linked to hierarchical hollow structure, enhanced light absorbance, and high carrier-separation efficiency. Additionally, the hollow Bi2MoxW1-xO6 microspheres formation can be attributed to the Kirkendall effect.

Generation of X-rays by electrons recycling through thin internal targets of cyclic accelerators

NASA Astrophysics Data System (ADS)

Kaplin, V.; Kuznetsov, S.; Uglov, S.

2018-05-01

The use of thin (< 10‑3 radiation length) internal targets in cyclic accelerators leads to multiple passes (recycling effect) of electrons through them. The multiplicity of electron passes (M) is determined by the electron energy, accelerator parameters, the thickness, structure and material of a target and leads to an increase in the effective target thickness and the efficiency of radiation generation. The increase of M leads to the increase in the emittance of electron beams which can change the characteristics of radiation processes. The experimental results obtained using the Tomsk synchrotron and betatron showed the possibility of increasing the yield and brightness of coherent X-rays generated by the electrons passing (recycling) through thin crystals and periodic multilayers placed into the chambers of accelerators, when the recycling effect did not influence on the spectral and angular characteristics of generated X-rays.

Two-stage Electron Acceleration by 3D Collisionless Guide-field Magnetic Reconnection

NASA Astrophysics Data System (ADS)

Buechner, J.; Munoz, P.

2017-12-01

We discuss a two-stage process of electron acceleration near X-lines of 3D collisionless guide-field magnetic reconnection. Non-relativistic electrons are first pre-accelerated by magnetic-field-aligned (parallel) electric fields. At the nonlinear stage of 3D guide-field magnetic reconnection electric and magnetic fields become filamentary structured due to streaming instabilities. This causes an additional curvature-driven electron acceleration in the guide-field direction. The resulting spectrum of the accelerated electrons follows a power law.

Chemical Modification of Graphene Oxide by Nitrogenation: An X-ray Absorption and Emission Spectroscopy Study

DOE PAGES

Chuang, Cheng-Hao; Ray, Sekhar C.; Mazumder, Debarati; ...

2017-02-10

Nitrogen-doped graphene oxides (GO:N x) were synthesized by a partial reduction of graphene oxide (GO) using urea [CO(NH 2) 2 ]. Their electronic/bonding structures were investigated using X-ray absorption near-edge structure (XANES), valence-band photoemission spectroscopy (VB-PES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). During GO:N x synthesis, different nitrogen-bonding species, such as pyrrolic/graphitic-nitrogen, were formed by replacing of oxygen-containing functional groups. At lower N-content (2.7 at%), pyrrolic-N, owing to surface and subsurface diffusion of C, N and NH is deduced from various X-ray spectroscopies. In contrast, at higher N-content (5.0 at%) graphitic nitrogen was formed in whichmore » each N-atom trigonally bonds to three distinct sp 2 -hybridized carbons with substitution of the N-atoms for C atoms in the graphite layer. Upon nitrogen substitution, the total density of state close to Fermi level is increased to raise the valence-band maximum, as revealed by VB-PES spectra, indicating an electron donation from nitrogen, molecular bonding C/N/O coordination or/and lattice structure reorganization in GO:N x . The well-ordered chemical environments induced by nitrogen dopant are revealed by XANES and RIXS measurements.« less

K-Band Si/SiGe HBT MMIC Amplifiers Using Lumped Passive Components with a Micromachined Structure

NASA Technical Reports Server (NTRS)

Lu, Liang-Hung; Rieh, Jae-Sung; Bhattacharya, Pallab; Katechi, Linda P. B.; Croke, E. T.; Ponchak, George E.; Alterovitz, Samuel A.

2000-01-01

Using Si/SiGe heterojunction bipolar transistors with a maximum oscillation frequency of 52 GHz and a novel structure for passive components, a two-stage K-band lumped-element amplifier has been designed and fabricated on high-resistivity Si substrates. The chip size including biasing and RF chokes is 0.92 x 0.67 sq mm.

Annealing Effect on (FAPbI3)1−x(MAPbBr3)x Perovskite Films in Inverted-Type Perovskite Solar Cells

PubMed Central

Chen, Lung-Chien; Wu, Jia-Ren; Tseng, Zhong-Liang; Chen, Cheng-Chiang; Chang, Sheng Hsiung; Huang, Jun-Kai; Lee, King-Lien; Cheng, Hsin-Ming

2016-01-01

This study determines the effects of annealing treatment on the structure and the optical and electronic behaviors of the mixed (FAPbI3)1−x(MAPbBr3)x perovskite system. The experimental results reveal that (FAPbI3)1−x(MAPbBr3)x (x ~ 0.2) is an effective light-absorbing material for use in inverted planar perovskite solar cells owing to its large absorbance and tunable band gap. Therefore, good band-matching between the (FAPbI3)1−x(MAPbBr3)x and C60 in photovoltaic devices can be controlled by annealing at various temperatures. Accordingly, an inverted mixed perovskite solar cell with a record efficiency of 12.0% under AM1.5G irradiation is realized. PMID:28773874

Theoretical Study of the Transverse Dielectric Constant of Superlattices and Their Alloys. Ph.D Thesis

NASA Technical Reports Server (NTRS)

Kahen, K. B.

1986-01-01

The optical properties of III to V binary and ternary compounds and GaAs-Al(x)Ga(1-x)As superlattices are determined by calculating the real and imaginary parts of the transverse dielectric constant. Emphasis is given to determining the influence of different material and superlattice parameters on the values of the index of refraction and absorption coefficient. In order to calculate the optical properties of a material, it is necessary to compute its electronic band structure. This was accomplished by introducing a partition band structure approach based on a combination of the vector k x vector p and nonlocal pseudopotential techniques. The advantages of this approach are that it is accurate, computationally fast, analytical, and flexible. These last two properties enable incorporation of additional effects into the model, such as disorder scattering, which occurs for alloy materials and excitons. Furthermore, the model is easily extended to more complex structures, for example multiple quantum wells and superlattices. The results for the transverse dielectric constant and absorption coefficient of bulk III to V compounds compare well with other one-electron band structure models and the calculations show that for small frequencies, the index of refraction is determined mainly by the contibution of the outer regions of the Brillouin zone.

Automated phase mapping with AgileFD and its application to light absorber discovery in the V–Mn–Nb oxide system

DOE PAGES

Suram, Santosh K.; Xue, Yexiang; Bai, Junwen; ...

2016-11-21

Rapid construction of phase diagrams is a central tenet of combinatorial materials science with accelerated materials discovery efforts often hampered by challenges in interpreting combinatorial X-ray diffraction data sets, which we address by developing AgileFD, an artificial intelligence algorithm that enables rapid phase mapping from a combinatorial library of X-ray diffraction patterns. AgileFD models alloying-based peak shifting through a novel expansion of convolutional nonnegative matrix factorization, which not only improves the identification of constituent phases but also maps their concentration and lattice parameter as a function of composition. By incorporating Gibbs’ phase rule into the algorithm, physically meaningful phase mapsmore » are obtained with unsupervised operation, and more refined solutions are attained by injecting expert knowledge of the system. The algorithm is demonstrated through investigation of the V–Mn–Nb oxide system where decomposition of eight oxide phases, including two with substantial alloying, provides the first phase map for this pseudoternary system. This phase map enables interpretation of high-throughput band gap data, leading to the discovery of new solar light absorbers and the alloying-based tuning of the direct-allowed band gap energy of MnV 2O 6. Lastly, the open-source family of AgileFD algorithms can be implemented into a broad range of high throughput workflows to accelerate materials discovery.« less

Automated phase mapping with AgileFD and its application to light absorber discovery in the V–Mn–Nb oxide system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suram, Santosh K.; Xue, Yexiang; Bai, Junwen

Rapid construction of phase diagrams is a central tenet of combinatorial materials science with accelerated materials discovery efforts often hampered by challenges in interpreting combinatorial X-ray diffraction data sets, which we address by developing AgileFD, an artificial intelligence algorithm that enables rapid phase mapping from a combinatorial library of X-ray diffraction patterns. AgileFD models alloying-based peak shifting through a novel expansion of convolutional nonnegative matrix factorization, which not only improves the identification of constituent phases but also maps their concentration and lattice parameter as a function of composition. By incorporating Gibbs’ phase rule into the algorithm, physically meaningful phase mapsmore » are obtained with unsupervised operation, and more refined solutions are attained by injecting expert knowledge of the system. The algorithm is demonstrated through investigation of the V–Mn–Nb oxide system where decomposition of eight oxide phases, including two with substantial alloying, provides the first phase map for this pseudoternary system. This phase map enables interpretation of high-throughput band gap data, leading to the discovery of new solar light absorbers and the alloying-based tuning of the direct-allowed band gap energy of MnV 2O 6. Lastly, the open-source family of AgileFD algorithms can be implemented into a broad range of high throughput workflows to accelerate materials discovery.« less

Band structure calculations of CuAlO2, CuGaO2, CuInO2, and CuCrO2 by screened exchange

NASA Astrophysics Data System (ADS)

Gillen, Roland; Robertson, John

2011-07-01

We report density functional theory band structure calculations on the transparent conducting oxides CuAlO2, CuGaO2, CuInO2, and CuCrO2. The use of the hybrid functional screened-exchange local density approximation (sX-LDA) leads to considerably improved electronic properties compared to standard LDA and generalized gradient approximation (GGA) approaches. We show that the resulting electronic band gaps compare well with experimental values and previous quasiparticle calculations, and show the correct trends with respect to the atomic number of the cation (Al, Ga, In). The resulting energetic depths of Cu d and O p levels and the valence-band widths are considerable improvements compared to LDA and GGA and are in good agreement with available x-ray photoelectron spectroscopy data. Lastly, we show the calculated imaginary part of the dielectric function for all four systems.

Strong Correlation and Topological States in Orbital-Active Dirac Materials

NASA Astrophysics Data System (ADS)

Xu, Shenglong; Wu, Congjun

Two dimensional Dirac materials, starting with graphene, have drawn tremendous research interests in the past decade. Instead of focusing on the pz orbital as in graphene, we go a step further and study its two orbitals counterpart, namely the px and py orbitals on a honeycomb lattice. The model applies to both optical lattices and several solid state systems including organic material, fluoridated tin film, BiX/SBX (X=H.F.CI.Br). In the band structure, besides the well known Dirac points in the graphene band structure, the orbital degrees of freedom give rise to flat bands as well as quadratic band touching points. These new features provide an even wider playground for searching exotic states of matter. With help of mean field theory and functional renormalization group (FRG) method, we explore the effects of interaction on the system and investigate the consequential interesting states such as ferromagnetism, Wigner crystallization, quantum anomalous Hall states and f-wave superconductivity.

Electronic Structure of HgBa2CaCu2O(6+delta) Epitaxial films measured by x-ray Photoemission

NASA Technical Reports Server (NTRS)

Vasquez, R. P.; Rupp, M.; Gupta, A.; Tsuei, C. C.

1995-01-01

The electronic structure and chemical states of HgBa2CaCu20(sub 6 + delta), epitaxial films have been studied with x-ray photelectron spectroscopy. Signals from the superconducting phase dominate all the core-level spectra, and a clear Fermi edge is observed in the valence-band region. The Ba, Ca, Cu, and O core levels are similar to those of Tl2Ba2CaCu208(+)O(sub 6 + delta), but distinct differences are observed in the valence bands which are consistent with differences in the calculated densities of states.

Modelling Hard Gamma-Ray Emission from Supernova Remnants

NASA Technical Reports Server (NTRS)

Baring, Matthew G.

1999-01-01

The observation by the CANGAROO (Collaboration of Australia and Nippon Gamma Ray Observatory at Outback) experiment of TeV emission from SN 1006, in conjunction with several instances of non-thermal X-ray emission from supernova remnants, has led to inferences of super-TeV electrons in these extended sources. While this is sufficient to propel the theoretical community in their modelling of particle acceleration and associated radiation, the anticipated emergence in the next decade of a number of new experiments probing the TeV and sub-TeV bands provides further substantial motivation for modellers. In particular, the quest for obtaining unambiguous gamma-ray signatures of cosmic ray ion acceleration defines a "Holy Grail" for observers and theorists alike. This review summarizes theoretical developments in the prediction of MeV-TeV gamma-rays from supernova remnants over the last five years, focusing on how global properties of models can impact, and be impacted by, hard gamma-ray observational programs, thereby probing the supernova remnant environment. Properties of central consideration include the maximum energy of accelerated particles, the density of the unshocked interstellar medium, the ambient magnetic field, and the relativistic electron-to-proton ratio. Criteria for determining good candidate remnants for observability in the TeV band are identified.

120 MeV Ni Ion beam induced modifications in poly (ethylene terephthalate) used in commercial bottled water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kumar, Vijay; Sonkawade, R. G.; Ali, Yasir

2012-06-05

We report the effects of heavy ion irradiation on the optical, structural, and chemical properties of polyethylene terephthalate (PET) film used in commercial bottled water. PET bottles were exposed with 120 MeV Ni ions at fluences varying from 3 x 10{sup 10} to 3 x 10{sup 12} ion/cm{sup 2}. The modifications so induced were analyzed by using UV-Vis, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. Substantial decrease in optical band gap is observed with the increase in ion fluence. In the FTIR spectra, most of bands are decreased due the degradation of the molecular structure. XRD measurements showmore » the decrease in peak intensity, which reflects the loss of crystallinity after irradiation.« less

Surface modification effects of fluorine-doped tin dioxide by oxygen plasma ion implantation

NASA Astrophysics Data System (ADS)

Tang, Peng; Liu, Cai; Zhang, Jingquan; Wu, Lili; Li, Wei; Feng, Lianghuan; Zeng, Guanggen; Wang, Wenwu

2018-04-01

SnO2:F (FTO), as a kind of transparent conductive oxide (TCO), exhibits excellent transmittance and conductivity and is widely used as transparency electrodes in solar cells. It's very important to modifying the surface of FTO for it plays a critical role in CdTe solar cells. In this study, modifying effects of oxygen plasma on FTO was investigated systematically. Oxygen plasma treatment on FTO surface with ion accelerating voltage ranged from 0.4 kV to 1.6 kV has been processed. The O proportion of surface was increased after ion implantation. The Fermi level of surface measurement by XPS valance band spectra was lowered as the ion accelerating voltage increased to 1.2 kV and then raised as accelerating voltage was elevated to 1.6 kV. The work function measured by Kelvin probe force microscopy increased after ion implanting, and it was consistent with the variation of Fermi level. The change of energy band structure of FTO surface mainly originated from the surface composition variation. As FTO conduction was primarily due to oxyanion hole, the carrier was electron and its concentration was reduced while O proportion was elevated at the surface of FTO, as a result, the Fermi level lowered and the work function was enlarged. It was proved that oxygen plasma treatment is an effective method to modulate the energy band structure of the surface as well as other properties of FTO, which provides much more space for interface and surface modification and then photoelectric device performance promotion.

Effect of Ni doping on structural and optical properties of Zn{sub 1−x}Ni{sub x}O nanopowder synthesized via low cost sono-chemical method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Singh, Budhendra, E-mail: bksingh@ua.pt; Kaushal, Ajay, E-mail: ajay.kaushal@ua.pt; Bdikin, Igor

2015-10-15

Highlights: • Pure and Ni doped ZnO nanopowders were synthesized by low cost sonochemical method. • The optical properties of Zn{sub 1−x}Ni{sub x}O nanopowders can be tuned by varying Ni content. • The results reveal the solubility limit of Ni into ZnO matrix as below 8%. - Abstract: Zn{sub 1−x}Ni{sub x}O nanopowders with different Ni contents of x = 0.0, 0.04 and 0.08 were synthesized via cost effective sonochemical reaction method. X-ray diffraction (XRD) pattern reveals pure wurtzite phase of prepared nanostructures with no additional impurity peaks. The morphology and dimensions of nanoparticles were investigated using scanning electron microscope (SEM).more » A sharp and strong peak for first order optical mode for wurtzite zinc oxide (ZnO) structure was observed at ∼438 cm{sup −1} in Raman spectra. The calculated optical band gap (E{sub g}) from UV–vis transmission data was found to decrease with increase in Ni content. The observed red shift in E{sub g} with increasing Ni content in ZnO nanopowders were in agreement with band gap behaviours found in their photoluminescence (PL) spectra. The synthesised ZnO nanopowders with controlled band gap on Ni doping reveals their potential for use in various electronic and optical device applications. The results were discussed in detail.« less

Extending the ICRF to Higher Radio Frequencies

NASA Technical Reports Server (NTRS)

Jacobs, C. S.; Jones, D. L.; Lanyi, G. E.; Lowe, S. T.; Naudet, C. J.; Resch, G. M.; Steppe, J. A.; Zhang, L. D.; Ulvestad, J. S.; Taylor, G. B.

2002-01-01

The ICRF forms the basis for all astrometry including use as the inertial coordinate system for navigating deep space missions. This frame was defined using S/X-band observations over the past 20+ years. In January 2002, the VLBA approved our proposal for observing time to extend the ICRF to K-band (24 GHz) and Q-band (43 GHz). The first step will be observations at K- and Q-bands on a subset of ICRF sources. Eventually, K- and Q-band multi-epoch observations will be used to estimate positions, flux density and source structure for a large fraction of the current S/X-band ICRF source list. This work will benefit the radio astronomy community by extending the VLBA calibrator list at these bands. In the longer term, we would also like to extend the ICRF to Ka-band (32 GHz). A celestial reference frame will be needed at this frequency to support deep space navigation. A navigation demonstration is being considered for NASA's Mars 2005 mission. The initial K- and Q-band work will serve to identify candidate sources at Ka-band for use with that mission.

Spatial and temporal characteristics of flare energy release determined from X-ray and radio imaging observations

NASA Technical Reports Server (NTRS)

Hernandez, A. M.; Machado, M. E.; Vilmer, N.; Trottet, G.

1986-01-01

Using the Hard X-ray Imaging Spectrometer (HXIS) from the Solar Maximum Mission Satellite, the morphological aspects and temporal evolution of three major flares which occurred on June 29, 1980 are studied. One of these events, observed at 10:40 UT, is analyzed in particular detail, including Hard X-ray Burst Spectrometer (HXRBS) data and metric wavelength data from the Nancay radioheliograph. The flares occurred during the interaction of two distinct magnetic structures. There is an early onset phase during which there is a weak level of particle acceleration, perhaps accompanied by strong heating within the magnetic interaction region. The impulsive phase of high power energy release is associated with a major interaction between the two structures and accompanied by strong acceleration and heating.

Photocatalytic hydrogen production from water-methanol mixtures using N-doped Sr2Nb2O7 under visible light irradiation: effects of catalyst structure.

PubMed

Ji, Sang Min; Borse, Pramod H; Kim, Hyun Gyu; Hwang, Dong Won; Jang, Jum Suk; Bae, Sang Won; Lee, Jae Sung

2005-03-21

Nitrogen-doped perovskite type materials, Sr2Nb2O7-xNx (0, 1.5 < x < 2.8), have been studied as visible light-active photocatalysts for hydrogen production from methanol-water mixtures. Nitrogen doping in Sr2Nb2O7 red-shifted the light absorption edge into the visible light range and induced visible light photocatalytic activity. There existed an optimum amount of nitrogen doping that showed the maximum rate of hydrogen production. Among the potential variables that might cause this activity variation, the crystal structure appeared to be the most important. Thus, as the extent of N-doping increased, the original orthorhombic structure of the layered perovskite was transformed into an unlayered cubic oxynitride structure. The most active catalytic phase was an intermediate phase still maintaining the original layered perovskite structure, but with a part of its oxygen replaced by nitrogen and oxygen vacancy to adjust the charge difference between oxygen and doped nitrogen. These experimental observations were explained by density functional theory calculations. Thus, in Sr2Nb2O7-xNx, N2p orbital was the main contributor to the top of the valence band, causing band gap narrowing while the bottom of conduction band due to Nb 4d orbital remained almost unchanged.

Cd{sub 0.9375}Mn{sub 0.0625}S diluted magnetic semiconductor: A DFT study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rani, Anita; Kaur, Kulwinder; Kumar, Ranjan, E-mail: ranianita64@gmail.com

We studied the spin polarized electronic band structures and magnetic properties of the diluted magnetic semiconductor Cd{sub 1-x}Mn{sub x}S in Zinc Blende phase (B3) with 0.0625 Mn by using ab initio method. The calculations were performed by using Density Functional Theory as implemented in the Spanish Initiative for Electronic Simulations with Thousands of Atoms code using local density approximation (LDA). Calculated electronic band structures and magnetic properties of Cd{sub 1-x}Mn{sub x}S are discussed in terms of contribution of Mn 3d{sup 5} 4s{sup 2}, Cd 4d{sup 10} 5s{sup 2}, S 3s{sup 2} 3p{sup 4} orbitals. The total magnetic moment is foundmore » to be 5.00 µb for Cd{sub 1−x}Mn{sub x}S at x=0.0625. This value indicate that Mn atom adds no hole carrier to the perfect CdS crystal. We found that Mn doped systems are ferromagnetic. Calculated results are in good agreement with previous studies.« less

Study on structural and optical properties of α-(AlxCr1-x)2O3 (0 ≤ x ≤ 1) solid solutions

NASA Astrophysics Data System (ADS)

Jangir, Ravindra; Kumar, Dharmendra; Srihari, Velaga; Ganguli, Tapas

2018-04-01

We report on structural and optical properties for ternary α-(AlxCr1-x)2O3 (0 ≤ x ≤ 1) solid solutions synthesized by using solid sate reaction method. Single R-3c phase was obtained for the Aluminum composition of 0 ≤ x ≤ 1. Due to difference in the ionic radia of Al3+ and Cr3+, in plane lattice parameter showed deviation from the vegard's law. Optical absorption spectra for the solid solutions showed a blue shift of ˜ 0.5 eV in the optical gap. It has also been observed that Cr 3d level shifted towards the O 2p level in the valance band which indicates the enhancement of hybridization in the d and p levels, which is related to the delocalization of hole states, responsible for p-type conduction in wide band gap semiconductors. The results suggests that ternary α-(AlxCr1-x)2O3 (0 ≤ x ≤ 1) can be useful in the field of UV transparent electronics and UV photodetectors.

Synthesis of Nanocrystalline SnOx (x = 1–2) Thin Film Using a Chemical Bath Deposition Method with Improved Deposition Time, Temperature and pH

PubMed Central

Ebrahimiasl, Saeideh; Yunus, Wan Md. Zin Wan; Kassim, Anuar; Zainal, Zulkarnain

2011-01-01

Nanocrystalline SnOx (x = 1–2) thin films were prepared on glass substrates by a simple chemical bath deposition method. Triethanolamine was used as complexing agent to decrease time and temperature of deposition and shift the pH of the solution to the noncorrosive region. The films were characterized for composition, surface morphology, structure and optical properties. X-ray diffraction analysis confirms that SnOx thin films consist of a polycrystalline structure with an average grain size of 36 nm. Atomic force microscopy studies show a uniform grain distribution without pinholes. The elemental composition was evaluated by energy dispersive X-ray spectroscopy. The average O/Sn atomic percentage ratio is 1.72. Band gap energy and optical transition were determined from optical absorbance data. The film was found to exhibit direct and indirect transitions in the visible spectrum with band gap values of about 3.9 and 3.7 eV, respectively. The optical transmittance in the visible region is 82%. The SnOx nanocrystals exhibit an ultraviolet emission band centered at 392 nm in the vicinity of the band edge, which is attributed to the well-known exciton transition in SnOx. Photosensitivity was detected in the positive region under illumination with white light. PMID:22163690

Design and theoretical calculation of novel GeSn fully-depleted n-tunneling FET with quantum confinement model for suppression on GIDL effect

NASA Astrophysics Data System (ADS)

Liu, Xiangyu; Hu, Huiyong; Wang, Meng; Miao, Yuanhao; Han, Genquan; Wang, Bin

2018-06-01

In this paper, a novel fully-depleted Ge1-xSnx n-Tunneling FET (FD Ge1-xSnx nTFET) with field plate is investigated theoretically based on the experiment previously published. The energy band structures of Ge1-xSnx are calculated by EMP and the band-to-band tunneling (BTBT) parameters of Ge1-xSnx are calculated by Kane's model. The electrical characteristics of FD Ge1-xSnx nTFET and FD Ge1-xSnx nTFET with field plate (FD-FP Ge1-xSnx nTFET) having various Sn compositions are investigated and simulated with quantum confinement model. The results indicated that the GIDL effect is serious in FD Ge1-xSnx nTFET. By employing the field plate structure, the GIDL effect of FD-FP Ge1-xSnx nTFET is suppressed and the off-state current Ioff is decreased more than 2 orders of magnitude having Sn compositions from 0 to 0.06 compared with FD Ge1-xSnx nTFET. The impact of the difference of work function between field plate metal and channel Φfps is also studied. With the optimized Φfps = 0.0 eV, the on-state current Ion = 4.6 × 10-5 A/μm, the off-state current Ioff = 1.6 × 10-13 A/μm and the maximum on/off ration Ion/Ioff = 2.9 × 108 are achieved.

Band-gap engineering of La1-x Nd x AlO3 (x = 0, 0.25, 0.50, 0.75, 1) perovskite using density functional theory: A modified Becke Johnson potential study

NASA Astrophysics Data System (ADS)

Sandeep; D, P. Rai; A, Shankar; M, P. Ghimire; Anup Pradhan, Sakhya; T, P. Sinha; R, Khenata; S, Bin Omran; R, K. Thapa

2016-06-01

The structural, electronic, and magnetic properties of the Nd-doped Rare earth aluminate, La1-x Nd x AlO3 (x = 0% to 100%) alloys are studied using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory. The effects of the Nd substitution in LaAlO3 are studied using the supercell calculations. The computed electronic structure with the modified Becke-Johnson (mBJ) potential based approximation indicates that the La1-x Nd x AlO3 alloys may possess half-metallic (HM) behaviors when doped with Nd of a finite density of states at the Fermi level (E F). The direct and indirect band gaps are studied each as a function of x which is the concentration of Nd-doped LaAlO3. The calculated magnetic moments in the La1-x Nd x AlO3 alloys are found to arise mainly from the Nd-4f state. A probable half-metallic nature is suggested for each of these systems with supportive integral magnetic moments and highly spin-polarized electronic structures in these doped systems at E F. The observed decrease of the band gap with the increase in the concentration of Nd doping in LaAlO3 is a suitable technique for harnessing useful spintronic and magnetic devices. Project supported by the DST-SERB, Dy (Grant No. SERB/3586/2013-14), the UGCBSR, FRPS (Grant No. F.30-52/2014), the UGC (New Delhi, India) Inspire Fellowship DST (India), and the Deanship of Scientific Research at King Saud University (Grant No. RPG-VPP-088). M P Ghimire thanks the Alexander von Humboldt Foundation, Germany for the financial support.

Application of Skylab EREP photographs to study of the modern episode of accelerated erosion in southern Arizona

NASA Technical Reports Server (NTRS)

Morrison, R. B. (Principal Investigator)

1974-01-01

The author has identified the following significant results. Indexing and analysis of the SL 2, SL 3, and SL 4 photos of the project area has shown that S-190A coverage with less than 30% clouds totals about 123,000 sq km. The 70-mm unenlarged color, color-infrared, B/W red, and B/W green bands from S-190A are of good to excellent quality; the B/W IR bands from SL 2 are excessively grainy and have very low resolution; those from SL 3 are better but nevertheless have low resolution. The 5-inch unenlarged color transparencies from S-190B are generally of excellent photographic quality. However, where cloud cover is extensive, commonly the S-190A and S-190B color and color-IR photos are correctly exposed for the clouds but considerably underexposed for the ground. The 4X enlargements of all bands of S-190A photos taken by SL 2 are much fuzzier than they should be; evidently the enlarger was not focused properly. The 4X enlargements from SL 3 are much superior.

New quaternary thallium indium germanium selenide TlInGe2Se6: Crystal and electronic structure

NASA Astrophysics Data System (ADS)

Khyzhun, O. Y.; Parasyuk, O. V.; Tsisar, O. V.; Piskach, L. V.; Myronchuk, G. L.; Levytskyy, V. O.; Babizhetskyy, V. S.

2017-10-01

Crystal structure of a novel quaternary thallium indium germanium selenide TlInGe2Se6 was investigated by means of powder X-ray diffraction method. It was determined that the compound crystallizes in the trigonal space group R3 with the unit cell parameters a = 10.1798(2) Å, c = 9.2872(3) Å. The relationship with similar structures was discussed. The as-synthesized TlInGe2Se6 ingot was tested with X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). In particular, the XPS valence-band and core-level spectra were recorded for initial and Ar+ ion-bombarded surfaces of the sample under consideration. The XPS data allow for statement that the TlInGe2Se6 surface is rigid with respect to Ar+ ion-bombardment. Particularly, Ar+ ion-bombardment (3.0 keV, 5 min duration, ion current density fixed at 14 μA/cm2) did not cause substantial modifications of stoichiometry in topmost surface layers. Furthermore, comparison on a common energy scale of the XES Se Kβ2 and Ge Kβ2 bands and the XPS valence-band spectrum reveals that the principal contributions of the Se 4p and Ge 4p states occur in the upper and central portions of the valence band of TlInGe2Se6, respectively, with also their substantial contributions in other portions of the band. The bandgap energy of TlInGe2Se6 at the level of αg=103 cm-1 is equal to 2.38 eV at room temperature.

Assessment of band gaps for alkaline-earth chalcogenides using improved Tran Blaha-modified Becke Johnson potential

NASA Astrophysics Data System (ADS)

Yedukondalu, N.; Kunduru, Lavanya; Roshan, S. C. Rakesh; Sainath, M.

2018-04-01

Assessment of band gaps for nine alkaline-earth chalcogenides namely MX (M = Ca, Sr, Ba and X = S, Se Te) compounds are reported using Tran Blaha-modified Becke Johnson (TB-mBJ) potential and its new parameterization. From the computed electronic band structures at the equilibrium lattice constants, these materials are found to be indirect band gap semiconductors at ambient conditions. The calculated band gaps are improved using TB-mBJ and its new parameterization when compared to local density approximation (LDA) and Becke Johnson potentials. We also observe that TB-mBJ new parameterization for semiconductors below 7 eV reproduces the experimental trends very well for the small band gap semiconducting alkaline-earth chalcogenides. The calculated band profiles look similar for MX compounds (electronic band structures are provided for BaS for representation purpose) using LDA and new parameterization of TB-mBJ potentials.

Thermodynamical study of boron doped CeX{sub 3} (X=Pd, Rh)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sharma, Ramesh; Dwivedi, Shalini; Sharma, Yamini, E-mail: sharma.yamini62@gmail.com

2016-05-06

The structural, electronic, thermal, and optical properties of cubic non magnetic CeX{sub 3}(X=Pd, Rh) compounds which crystallize in the Au{sub 3}Cu structure have been studied using the projected augmented wave (PAW) method within the density functional theory (DFT) with generalized gradient approximation (GGA) for exchange correlation potential. In this paper we have calculated the band structure which are interpreted using the density of states. The optical properties such as extinction coefficients clearly illustrate the changes in CeX{sub 3} due to intercalation of boron. Lattice instability is observed in CePd{sub 3}B from the calculated dynamical properties.

Ultra-short wavelength x-ray system

DOEpatents

Umstadter, Donald [Ann Arbor, MI; He, Fei [Ann Arbor, MI; Lau, Yue-Ying [Potomac, MD

2008-01-22

A method and apparatus to generate a beam of coherent light including x-rays or XUV by colliding a high-intensity laser pulse with an electron beam that is accelerated by a synchronized laser pulse. Applications include x-ray and EUV lithography, protein structural analysis, plasma diagnostics, x-ray diffraction, crack analysis, non-destructive testing, surface science and ultrafast science.

Structural and optical properties of Sb65Se35-xGex thin films

NASA Astrophysics Data System (ADS)

Saleh, S. A.; Al-Hajry, A.; Ali, H. M.

2011-07-01

Sb65Se35-xGex (x=0-20 at.%) thin films, prepared by the electron beam evaporation technique on ultrasonically cleaned glass substrates at 300 K, were investigated. The amorphous structure of the thin films was confirmed by x-ray diffraction analysis. The structure was deduced from the Raman spectra measured for all germanium contents in the Sb-Se-Ge matrix. The absorption coefficient (α) of the films was determined by optical transmission measurements. The compositional dependence of the optical band gap is discussed in light of topological and chemical ordered network models.

Bandgap Engineering of Lead-Free Double Perovskite Cs2 AgBiBr6 through Trivalent Metal Alloying.

PubMed

Du, Ke-Zhao; Meng, Weiwei; Wang, Xiaoming; Yan, Yanfa; Mitzi, David B

2017-07-03

The double perovskite family, A 2 M I M III X 6 , is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH 3 NH 3 PbI 3 . Given the generally large indirect band gap within most known double perovskites, band-gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs 2 AgBiBr 6 as host, band-gap engineering through alloying of In III /Sb III has been demonstrated in the current work. Cs 2 Ag(Bi 1-x M x )Br 6 (M=In, Sb) accommodates up to 75 % In III with increased band gap, and up to 37.5 % Sb III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs 2 Ag(Bi 0.625 Sb 0.375 )Br 6 . Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three-metal systems are also assessed. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Core x-ray spectra in semiconductors and the Mahan-Nozieres-De Dominicis model

NASA Astrophysics Data System (ADS)

Livins, Peteris

1998-10-01

The Mahan-Nozières-De Dominicis (MND) model of core x-ray spectra is examined for semiconductors. Due to the finite band gap, the Anderson orthogonality does not occur, and thus spectra near the band edge can be calculated without the shakeup contribution. For semiconductors, and not only for metals, we investigate whether the remaining many-particle dynamic exchange effect of the MND model, or so-called replacement, can significantly alter x-ray spectral shapes near the band edge from those obtained from a straightforward final-state rule. For both emission and absorption, in the absence of shakeup, an exact formulation suitable for materials with band structure is discussed. A numerical model for a semiconductor with a 1-eV band gap demonstrates the band-edge modifications, and shows a 50% effect at the band edge, indicating that this dynamic exchange effect can be significant and should be considered in any specific emission or absorption calculation for a semiconductor. Although the ineffectiveness of the orthogonality theorem in semiconductors is emphasized, a suppression near the band edge also remains a possibility. Included is a discussion on the breakdown of the final-state rule. In addition, connection is made to the determinantal approach of Ohtaka and Tanabe.

Influence of Fe ions on structural, optical and thermal properties of SnO{sub 2} nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahmed, Ateeq, E-mail: ateeqamu124@gmail.com; Tripathi, P.; Khan, Wasi

2016-05-23

In the present work, Fe doped SnO{sub 2} nanoparticles with the composition Sn{sub 1-x}Fe{sub x}O{sub 2} (x = 0, 0.02, 0.04 and 0.06) have been successfully synthesized using sol-gel auto combustion technique. The samples are characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDAX), Ultraviolet (UV-Visible) absorption spectroscopy and thermal gravimetric analysis (TGA). The XRD study shows that all the samples have been found in tetragonal rutile structure without any extra phase and average crystallite size which lies in the range of 6-17 nm. The EDAX spectrum confirmed the doping of Fe ion into tin oxidemore » nanomaterial. The optical band gap of doped SnO{sub 2} is found to decrease with increasing Fe ion concentration, which is due to the formation of donor energy levels in the actual band gap of SnO{sub 2}.« less

Synthesis and Luminescence Properties of Blue Na(Sr0.97-xCa(x))PO4:0.03Eu2+ Phosphors for White Light Emitting Diode Applications.

PubMed

Hakeem, D A; Park, K

2015-07-01

The crystal structure and luminescence properties of Na(Sr0.97-xCax)PO4:0.03Eu2+ (0 < x < 1.0) phosphors were studied, depending on the Ca2+ concentration. All the Na(Sr0.97-xCax)PO4:0.03Eu2+ phosphors had a hexagonal crystal structure. The excitation spectra of the prepared phosphors showed a broad band ranging from 250 to 420 nm, which arises due to the 4f-5d transitions of Eu2+ ions. Upon the excitation of 334 nm wavelength, the emission spectra showed a broad blue band ranging from 400 to 700 nm peaking at 450 nm. Among the prepared phosphors, the Na(Sr0.72Ca0.25)PO4:0.03Eu2+ showed the strongest emission intensity and could be applied as a blue emitting phosphor for UV-based w-LEDs.

A Model for AR Scorpii: Emission from Relativistic Electrons Trapped by Closed Magnetic Field Lines of Magnetic White Dwarfs

NASA Astrophysics Data System (ADS)

Takata, J.; Yang, H.; Cheng, K. S.

2017-12-01

AR Scorpii is an intermediate polar binary system composed of a magnetic white dwarf (WD) and an M-type star and shows nonthermal, pulsed, and highly linearly polarized emission. The radio/optical emission modulates with the WD’s spin and shows the double-peak structure in the light curves. In this paper, we discuss a possible scenario for the radiation mechanism of AR Scorpii. The magnetic interaction on the surface of the companion star produces an outflow from the companion star, the heating of the companion star surface, and the acceleration of electrons to a relativistic energy. The accelerated electrons, whose typical Lorentz factor is ∼50–100, from the companion star move along the magnetic field lines toward the WD surface. The electrons injected with the pitch angle of \\sin {θ }p,0> 0.05 are subject to the magnetic mirror effect and are trapped in the closed magnetic field line region. We find that the emission from the first magnetic mirror points mainly contributes to the observed pulsed emission and the formation of the double-peak structure in the light curve. For the inclined rotator, the pulse peak in the calculated light curve shifts the position in the spin phase, and a Fourier analysis exhibits a beat frequency feature, which are consistent with the optical/UV observations. The pulse profile also evolves with the orbital phase owing to the effect of the viewing geometry. The model also interprets the global features of the observed spectral energy distribution in radio to X-ray energy bands. We also discuss the curvature radiation and the inverse-Compton scattering process in the outer gap accelerator of the WD in AR Scorpii and the possibility of the detection by future high-energy missions.

First principles study on structural, electronic and optical properties of Ga1-xBxP ternary alloys (x = 0, 0.25, 0.5, 0.75 and 1)

NASA Astrophysics Data System (ADS)

Hoat, D. M.; Rivas Silva, J. F.; Méndez Blas, A.

2018-07-01

The structural, electronic and optical properties of GaP, BP binary compounds and their ternary alloys Ga1-xBxP (x = 0.25, 0.5 and 0.75) have been studied by full-potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory (DFT) as implemented in WIEN2k package. Local density approximation (LDA) and generalized gradient approximation (GGA) as proposed by Perdew-Burke-Ernzerhof (PBE), Wu-Cohen (WC) and PBE for solid (PBESol) were used for treatment of exchange-correlation effect in calculations. Additionally, the Tran-Blaha modified Becke-Johnson (mBJ) potential was also employed for electronic and optical calculations due to that it gives very accurate band gap of solids. As B concentration increases, the lattice constant reduces and the energy band gap firstly decreases for small composition x and then it shows increasing trend until pure BP. Our results show that the indirect-direct band gap transition can be reached from x = 0.33. The linear optical properties, such as reflectivity, absorption coefficient, refractive index and optical conductivity of binary compounds and ternary alloys were derived from their calculated complex dielectric function in wide energy range up to 30 eV, and the alloying effect on these properties was also analyzed in detail.

First-principle calculations of structural, electronic, optical, elastic and thermal properties of MgXAs2 (X=Si, Ge) compounds

NASA Astrophysics Data System (ADS)

Cheddadi, S.; Boubendira, K.; Meradji, H.; Ghemid, S.; Hassan, F. El Haj; Lakel, S.; Khenata, R.

2017-12-01

First-principle calculations on the structural, electronic, optical, elastic and thermal properties of the chalcopyrite MgXAs2 (X=Si, Ge) have been performed within the density functional theory (DFT) using the full-potential linearized augmented plane wave (FP-LAPW) method. The obtained equilibrium structural parameters are in good agreement with the available experimental data and theoretical results. The calculated band structures reveal a direct energy band gap for the interested compounds. The predicted band gaps using the modified Becke-Johnson (mBJ) exchange approximation are in fairly good agreement with the experimental data. The optical constants such as the dielectric function, refractive index, and the extinction coefficient are calculated and analysed. The independent elastic parameters namely, C_{11}, C_{12}, C_{13}, C_{33}, C_{44} and C_{66 } are evaluated. The effects of temperature and pressure on some macroscopic properties of MgSiAs2 and MgGeAs2 are predicted using the quasiharmonic Debye model in which the lattice vibrations are taken into account.

Structure-Dependent Optical Properties of Self-Organized Bi2Se3 Nanostructures: From Nanocrystals to Nanoflakes.

PubMed

Yang, Shang-Dong; Yang, Liao; Zheng, Yu-Xiang; Zhou, Wen-Jie; Gao, Meng-Yu; Wang, Song-You; Zhang, Rong-Jun; Chen, Liang-Yao

2017-08-30

Bismuth selenide (Bi 2 Se 3 ), with a wide bulk band gap and single massless Dirac cone at the surface, is a promising three-dimensional topological insulator. Bi 2 Se 3 possesses gapless surface states and an insulator-like bulk band gap as a new type of quantum matter. Different Bi 2 Se 3 nanostructures were prepared using electron beam evaporation with high production efficiency. Structural investigations by energy-dispersive X-ray analysis, scanning electron microscopy, and X-ray diffraction revealed the sample stoichiometries and the structural transition mechanism from nanocrystals to nanoflakes. The optical properties systematically probed and analyzed by spectroscopic ellipsometry showed strong dependence on the nanostructures and were also predicted to have structure-modifiable technological prospects. The optical parameters, plasma frequencies, scattering rates of the free electrons, and optical band gaps were related to the topological properties of the Bi 2 Se 3 nanostructures via light-matter interactions, offering new opportunities and approaches for studies on topological insulators and spintronics. The high-quality Bi 2 Se 3 nanostructures provide advantages in exploring novel physics and exploiting prospective applications.

Overview of Mono-Energetic Gamma-Ray Sources and Applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hartemann, Fred; /LLNL, Livermore; Albert, Felicie

2012-06-25

Recent progress in accelerator physics and laser technology have enabled the development of a new class of tunable gamma-ray light sources based on Compton scattering between a high-brightness, relativistic electron beam and a high intensity laser pulse produced via chirped-pulse amplification (CPA). A precision, tunable Mono-Energetic Gamma-ray (MEGa-ray) source driven by a compact, high-gradient X-band linac is currently under development and construction at LLNL. High-brightness, relativistic electron bunches produced by an X-band linac designed in collaboration with SLAC NAL will interact with a Joule-class, 10 ps, diode-pumped CPA laser pulse to generate tunable {gamma}-rays in the 0.5-2.5 MeV photon energymore » range via Compton scattering. This MEGaray source will be used to excite nuclear resonance fluorescence in various isotopes. Applications include homeland security, stockpile science and surveillance, nuclear fuel assay, and waste imaging and assay. The source design, key parameters, and current status are presented, along with important applications, including nuclear resonance fluorescence.« less

Structural investigation of new vanadium-bismuth-phosphate glasses by IR and ESR spectroscopy

NASA Astrophysics Data System (ADS)

Vedeanu, N.; Cozar, O.; Stanescu, R.; Cozar, I. B.; Ardelean, I.

2013-07-01

IR spectra changes of the xV2O5(1 - x)[0.8P2O5ṡ0.2Bi2O3] glass system with 0 ⩽ x ⩽ 50 mol% show that vanadium oxide acts as a network modifier at low concentration (x ⩽ 5 mol%), affecting especially the Bi2O3 network. In the same time the phosphate groups (structures) impose their presence by themselves, fact which is illustrated by the increasing of the intensity of characteristic 910, 1040, 1230 cm-1 bands. The IR bands belonging to the phosphate groups are strongly reduced for x ⩾ 10 mol% due to the phosphate network depolymerization and to the appearance of new vibrations characteristic for POV and VOV linkages, showing the network former role of V2O5. In the same time the changes observed in the ESR spectra of these glasses are explained supposing the superposition of two signals, one with a well-resolved hyperfine structure typical for isolated V4+ ions and a broad line characteristic for clustered ions. The line width dependence versus V2O5 content shows that dipole-dipole interactions exist between vanadium ions until x = 5 mol% and the superexchange interactions prevail at high content (x ⩾ 10 mol%).

Systematic study of electronic and magnetic properties for Cu{sub 12–x}TM{sub x}Sb{sub 4}S{sub 13} (TM = Mn, Fe, Co, Ni, and Zn) tetrahedrite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suekuni, K., E-mail: ksuekuni@hiroshima-u.ac.jp; Tomizawa, Y.; Ozaki, T.

2014-04-14

Substitution effects of 3d transition metal (TM) impurities on electronic and magnetic properties for Cu{sub 12}Sb{sub 4}S{sub 13} tetrahedrite are investigated by the combination of low-temperature experiments and first-principles electronic-structure calculations. The electrical resistivity for the cubic phase of Cu{sub 12}Sb{sub 4}S{sub 13} exhibits metallic behavior due to an electron-deficient character of the compound. Whereas that for 0.5 ≤ x ≤ 2.0 of Cu{sub 12−x}Ni{sub x}Sb{sub 4}S{sub 13} exhibits semiconducting behavior. The substituted Ni for Cu is in the divalent ionic state with a spin magnetic moment and creates impurity bands just above the Fermi level at the top of the valence band. Therefore,more » the semiconducting behavior of the electrical resistivity is attributed to the thermal excitation of electrons from the valence band to the impurity band. The substitution effect of TM on the electronic structure and the valency of TM for Cu{sub 11.0}TM{sub 1.0}Sb{sub 4}S{sub 13} are systematically studied by the calculation. The substituted Mn, Fe, and Co for Cu are found to be in the ionic states with the spin magnetic moments due to the large exchange splitting of the 3d bands between the minority- and majority-spin states.« less

Characterization and Physics-Based Modeling of Electrochemical Memristors

DTIC Science & Technology

2015-11-16

conducting films that result from electrical or optical stress. Model parameters and electrical characteristics were obtained from and validated...x- ray scattering, Conductive Bridge Random Access Memory 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME...Calculated DOS for GeSe2 in valence band and (b) conduction band .................. 43  Figure 45. DFT band structure for crystalline GeSe2

Inverse Compton Scattered Merger-nova: Late X-Ray Counterpart of Gravitational-wave Signals from NS–NS/BH Mergers

NASA Astrophysics Data System (ADS)

Ai, Shunke; Gao, He

2018-01-01

The recent observations of GW170817 and its electromagnetic (EM) counterparts show that double neutron star mergers could lead to rich and bright EM emissions. Recent numerical simulations suggest that neutron star and neutron star/black hole (NS–NS/BH) mergers would leave behind a central remnant surrounded by a mildly isotropic ejecta. The central remnant could launch a collimated jet and when the jet propagates through the ejecta, a mildly relativistic cocoon would be formed and the interaction between the cocoon and the ambient medium would accelerate electrons via external shock in a wide angle, so that the merger-nova photons (i.e., thermal emission from the ejecta) would be scattered into higher frequency via an inverse Compton (IC) process when they propagate through the cocoon shocked region. We find that the IC scattered component peaks at the X-ray band and it will reach its peak luminosity on the order of days (simultaneously with the merger-nova emission). With current X-ray detectors, such a late X-ray component could be detected out to 200 Mpc, depending on the merger remnant properties. It could serve as an important electromagnetic counterpart of gravitational-wave signals from NS–NS/BH mergers. Nevertheless, simultaneous detection of such a late X-ray signal and the merger-nova signal could shed light on the cocoon properties and the concrete structure of the jet.

X-ray photoemission study of NiS2-xSex (x=0.0 1.2)

NASA Astrophysics Data System (ADS)

Krishnakumar, S. R.; Sarma, D. D.

2003-10-01

Electronic structure of NiS2-xSex system has been investigated for various compositions (x) using x-ray photoemission spectroscopy. An analysis of the core-level as well as the valence-band spectra of NiS2 in conjunction with many-body cluster calculations provides a quantitative description of the electronic structure of this compound. With increasing Se content, the on-site Coulomb correlation strength (U) does not change, while the bandwidth W of the system increases, driving the system from a covalent insulating state to a pd-metallic state.

Nanomodulated electron beams via electron diffraction and emittance exchange for coherent x-ray generation

NASA Astrophysics Data System (ADS)

Nanni, E. A.; Graves, W. S.; Moncton, D. E.

2018-01-01

We present a new method for generation of relativistic electron beams with current modulation on the nanometer scale and below. The current modulation is produced by diffracting relativistic electrons in single crystal Si, accelerating the diffracted beam and imaging the crystal structure, then transferring the image into the temporal dimension via emittance exchange. The modulation period can be tuned by adjusting electron optics after diffraction. This tunable longitudinal modulation can have a period as short as a few angstroms, enabling production of coherent hard x-rays from a source based on inverse Compton scattering with total accelerator length of approximately ten meters. Electron beam simulations from cathode emission through diffraction, acceleration, and image formation with variable magnification are presented along with estimates of the coherent x-ray output properties.

Superconductivity and the periodic table: from elements to materials.

PubMed

Simon, Arndt

2015-03-13

Based on the normal-state electronic band structure, the necessary condition for a metal to become a superconductor is the simultaneous occurrence of flat and steep bands at the Fermi level. The sufficient condition at least for conventional superconductors is a strong enough coupling of the flat band states to the lattice, e.g. via phonons. Selected elements (Te) and compounds of the rare earth metals (RE(2)C(3), REC(2), RE(2)X(2)C(2) with X=halogen) and MgB(2) serve as examples to illustrate the idea. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

Structural, ferroelectric and magnetic study of lead free (Na{sub 0.5}Bi{sub 0.5}){sub 1-x}La{sub x}Ti{sub 0.988}Fe{sub 0.012}O{sub 3} (x=0,0.01,0.03,0.05) ceramic

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parmar, Kusum, E-mail: prmrkusum@gmail.com; Sharma, Anshu; Sharma, Hakikat

2015-05-15

Lead free (Na{sub 0.5}Bi{sub 0.5}){sub 1-x}La{sub x}Ti{sub 0.988}Fe{sub 0.012}O{sub 3} ceramic having compositions (x=0, 0.01, 0.03, 0.05) has been prepared by sol gel method using citric acid. Structural analysis has been done by X-ray diffraction and FTIR measurements. XRD patterns have been confirmed perovskite structure for all samples. FTIR absorption band at around ∼630 cm{sup −1} is observed for all samples which confirm perovskite phase formation in samples. With increasing La concentration, shifting in XRD peaks and FTIR absorption bands is observed which suggests incorporation of La on A-site in prepared (Na{sub 0.5}Bi{sub 0.5}){sub 1-x}La{sub x}Ti{sub 0.988}Fe{sub 0.012}O{sub 3} samples. Effectmore » of La substitution on Ferroelectric (Polarization vs. Electric field) and Magnetic (Magnetization vs. Magnetic field) properties have been studied at room temperature. All samples exhibit weak ferromagnetic order and also possess ferroelectric behavior which provides new insight to lead free single phase multiferroic materials.« less

First principles study on the elastic and electronic properties of CdX (X = S, Se and Te)

NASA Astrophysics Data System (ADS)

Sharma, Sheetal; Verma, Ajay Singh; Sarkar, Bimal Kumar; Bhandari, Rajiv; Jindal, Vijay Kumar

2011-12-01

Wide band gap semiconductors are emerging as a potential candidate for optically active materials in blue green spectral region and operating at high power level and high temperature. CdX, X = S, Se and Te are wide band gap semiconductors having applications in optoelectronics devices. In this paper we investigated the elastic and electronic properties of Cadmium chalcogenide (cubic zinc-blende (ZB) structure) using standard Kohn-Sham self consistent density functional theory method (DFT) that uses non conserving pseudopotentials in fully nonlocal form within the generalized gradient approximation (GGA) for the exchange-correlation potential. The independent elastic constants, C11, C12 and C44, are calculated from direct computation of stresses generated by small strains. The shear modulus and Young's modulus are estimated for CdX. Using the GGA for the exchange correlation potential, the calculated direct fundamental band gap value is in very good agreement with the measured one.

Diverse carrier mobility of monolayer BNC_x: A combined density functional theory and Boltzmann transport theory study.

PubMed

Wu, Tao; Deng, Kaiming; Deng, Wei-Qiao; Lu, Ruifeng

2017-09-19

BNCX monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNCX (x=1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC2-1 is a material with very small band gap (0.02 eV) among all the structures while other BNCX monolayers are semiconductors with band gap ranging from 0.51 to 1.32 eV. The carrier mobility of BNCX varies considerably from tens to millions of cm2 V-1 s-1. For BNC2-1, the hole mobility and electron mobility along both x and y directions can reach 105 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNCX monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC4, its hole mobility along y direction and electron mobility along x direction unexpectedly reach 106 orders of magnitude, even higher than that of graphene. Our findings suggest that BNCX layered materials with proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices. © 2017 IOP Publishing Ltd.

Structural investigation and optical properties of xMnO2-25Li2O-5Na2O-15Bi2O3-(55-x)B2O3 glasses

NASA Astrophysics Data System (ADS)

Kulkarni, Shilpa; Jali, V. M.

2018-02-01

This paper deals with the new mixed system of glass compositions Lithium sodium bismuth borate glasses doped with transition metal oxide. The technique used to prepare a sample is by melt quenching. The XRD profile pattern confirmed the amorphous phase of the present glass system. The network structure is based on BO3, BO4 units and BiO6 octahedral units. No boroxyl rings observed in the glass structure. The addition of MnO2 in small amount does not account for major structural changes. Optical band gap lies in the range 1.89 to 0.96 eV. Density, molar volume, oxygen packing density, Tg, direct optical band gap and refractive index show anomalous behavior.

Electronic structure and optical properties of Cs2HgI4: Experimental study and band-structure DFT calculations

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Shkumat, P. N.; Myronchuk, G. L.; Khvyshchun, M.; Fedorchuk, A. O.; Parasyuk, O. V.; Khyzhun, O. Y.

2015-04-01

High-quality single crystal of cesium mercury tetraiodide, Cs2HgI4, has been synthesized by the vertical Bridgman-Stockbarger method and its crystal structure has been refined. In addition, electronic structure and optical properties of Cs2HgI4 have been studied. For the crystal under study, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces have been measured. The present X-ray photoelectron spectroscopy (XPS) results indicate that the Cs2HgI4 single crystal surface is very sensitive with respect to Ar+ ion-irradiation. In particular, Ar+ bombardment of the single crystal surface alters the elemental stoichiometry of the Cs2HgI4 surface. To elucidate peculiarities of the energy distribution of the electronic states within the valence-band and conduction-band regions of the Cs2HgI4 compound, we have performed first-principles band-structure calculations based on density functional theory (DFT) as incorporated in the WIEN2k package. Total and partial densities of states for Cs2HgI4 have been calculated. The DFT calculations reveal that the I p states make the major contributions in the upper portion of the valence band, while the Hg d, Cs p and I s states are the dominant contributors in its lower portion. Temperature dependence of the light absorption coefficient and specific electrical conductivity has been explored for Cs2HgI4 in the temperature range of 77-300 K. Main optical characteristics of the Cs2HgI4 compound have been elucidated by the first-principles calculations.

Novel schemes for the optimization of the SPARC narrow band THz source

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marchetti, B., E-mail: barbara.marchetti@desy.de; Zagorodnov, I.; Bacci, A.

2015-07-15

A pulsed, tunable, narrow band radiation source with frequency in the THz region can be obtained collecting the coherent transition radiation produced by a train of ultra-short electron bunches having picosecond scale inter-distance. In this paper, we review the techniques feasible at the SPARC-LAB test facility to produce and manipulate the requested train of electron bunches and we examine the dynamics of their acceleration and compression. In addition, we show how the performances of the train compression and the radiation intensity and bandwidth can be significantly improved through the insertion of a fourth order harmonic cavity, working in the X-bandmore » and acting as a longitudinal phase space linearizer.« less

Performance analysis of high efficiency InxGa1-xN/GaN intermediate band quantum dot solar cells

NASA Astrophysics Data System (ADS)

Chowdhury, Injamam Ul Islam; Sarker, Jith; Shifat, A. S. M. Zadid; Shuvro, Rezoan A.; Mitul, Abu Farzan

2018-06-01

In this subsistent fifth generation era, InxGa1-xN/GaN based materials have played an imperious role and become promising contestant in the modernistic fabrication technology because of some of their noteworthy attributes. On our way of illustrating the performance, the structure of InxGa1-xN/GaN quantum dot (QD) intermediate band solar cell (IBSC) is investigated by solving the Schrödinger equation in light of the Kronig-Penney model. In comparison with p-n homojunction and heterojunction solar cells, InxGa1-xN/GaN IBQD solar cell manifests larger power conversion efficiency (PCE). PCE strongly depends on position and width of the intermediate bands (IB). Position of IBs can be controlled by tuning the size of QDs and the Indium content of InxGa1-xN whereas, width of IB can be controlled by tuning the interdot distance. PCE can also be controlled by tuning the position of fermi energy bands as well as changing the doping concentration. In this work, maximum conversion efficiency is found approximately 63.2% for a certain QD size, interdot distance, Indium content and doping concentration.

THE tilde{A}-tilde{X} AND tilde{B}-tilde{X} ABSORPTIONS OF NO_3 TRAPPED IN SOLID NEON

NASA Astrophysics Data System (ADS)

Jacox, Marilyn E.; Thompson, Warren E.

2009-06-01

Absorptions arising from the tilde{A}-tilde{X} transition of normal and isotopically substituted NO_3 have been observed between 7500 and 9500 cm^{-1}. Details of the spectra will be discussed and assignments will be proposed. Absorptions arising from the tilde{B}-tilde{X} transition of NO_3, with band origin near 15 000 cm^{-1}, have also been observed for the normal species and two of its isotopologues which possess D_{3h} symmetry. As in the gas phase, the absorptions are broadened because of predissociation. The observed band structure corresponds closely with that reported for the gas-phase molecule.

Reflectivity Around the Gold M-Edges of X-ray Reflector of the Soft X-Ray Telescope Onboard ASTRO-H

NASA Technical Reports Server (NTRS)

Kurashimaa, Sho; Furuzawa, Akihiro; Sato, Toshiki; Kikuchia, Naomichi; Nakaniwaa, Nozomi; Maeda, Yoshitomo; Ishida, Manabu; Izuka, Ryo; Okajima, Takashi; Mori, Hideyuki;

Development of High Power Vacuum Tubes for Accelerators and Plasma Heating

NASA Astrophysics Data System (ADS)

Srivastava, Vishnu

2012-11-01

High pulsed power magnetrons and klystrons for medical and industrial accelerators, and high CW power klystrons and gyrotrons for plasma heating in tokamak, are being developed at CEERI. S-band 2.0MW pulsed tunable magnetrons of centre frequency 2856MHz and 2998 MHz were developed, and S-band 2.6MW pulsed tunable magnetron is being developed for medical LINAC, and 3MW pulsed tunable magnetron is being developed for industrial accelerator. S-band (2856MHz), 5MW pulsed klystron was developed for particle accelerator, and S-band 6MW pulsed klystron is under development for 10MeV industrial accelerator. 350MHz, 100kW (CW) klystron is being developed for proton accelerator, and C-band 250kW (CW) klystron is being developed for plasma heating. 42GHz, 200kW (CW/Long pulse) gyrotron is under development for plasma heating. Plasma filled tubes are also being developed for switching. 25kV/1kA and 40kV/3kA thyratrons were developed for high voltage high current switching in pulse modulators for magnetrons and klystrons. 25kV/3kA Pseudospark switch of current rise time of 1kA/|a-sec and pulse repetition rate of 500Hz is being developed. Plasma assisted high power microwave device is also being investigated.

Growth and applications of GeSn-related group-IV semiconductor materials

PubMed Central

Zaima, Shigeaki; Nakatsuka, Osamu; Taoka, Noriyuki; Kurosawa, Masashi; Takeuchi, Wakana; Sakashita, Mitsuo

2015-01-01

We review the technology of Ge1−xSnx-related group-IV semiconductor materials for developing Si-based nanoelectronics. Ge1−xSnx-related materials provide novel engineering of the crystal growth, strain structure, and energy band alignment for realising various applications not only in electronics, but also in optoelectronics. We introduce our recent achievements in the crystal growth of Ge1−xSnx-related material thin films and the studies of the electronic properties of thin films, metals/Ge1−xSnx, and insulators/Ge1−xSnx interfaces. We also review recent studies related to the crystal growth, energy band engineering, and device applications of Ge1−xSnx-related materials, as well as the reported performances of electronic devices using Ge1−xSnx related materials. PMID:27877818

Near-infrared Fourier-transform and millimeterwave spectra of the BiS radical

NASA Astrophysics Data System (ADS)

Izumi, K.; Cohen, E. A.; Setzer, K. D.; Fink, E. H.; Kawaguchi, K.

2008-12-01

This paper reports the 6400-7400 cm -1 Fourier-transform (FT) near-infrared (NIR) emission spectrum of the BiS X22Π 3/2 → X12Π 1/2 fine structure bands as well as the millimeterwave rotational spectrum of the X12Π 1/2 state. For the FTNIR observations, BiS was produced by reaction of bismuth with sulfur vapor and excited by energy transfer from metastable oxygen, O 2( a1Δ g), in a fast-flow system. As was the case for BiO [O. Shestakov, R. Breidohr, H. Demes, K.D. Setzer, E.H. Fink, J. Mol. Spectrosc. 190 (1998) 28-77], the 0.5 cm -1resolution spectrum revealed a number of strong bands in the Δv = 0 and ±1 sequences which showed perturbed band spacings, band shapes, and intensities due to avoided crossing of the X22Π 3/2 and A14Π 3/2 potential curves for v' ⩾ 4 of X22Π 3/2. The millimeterwave rotational spectrum of BiS in its X12Π 1/2 state was observed when BiS was produced in a high-temperature oven by a discharge in a mixture of Bi vapor and CS 2. The signal to noise ratio was markedly improved by using a White-type multipath cell. Ninety seven features from J' = 23.5 to J' = 41.5 were measured between 150 and 300 GHz. Analysis of the 0.5 cm -1 resolution FT spectrum yielded the fine structure splitting and vibrational constants of the states. A simultaneous analysis of millimeterwave and a 0.005 cm -1 FT spectrum of the 0-0 band of the NIR system was carried out to give precise rotational, fine, and hyperfine constants for the X12Π 1/2 and X22Π 3/2 states. The results are consistent with those reported earlier for BiO and indicate only a slight decrease in the unpaired electron density in the 6p(π ∗) orbital on the Bi atom.

Structural, optical, physical and electrical properties of V2O5.SrO.B2O3 glasses.

PubMed

Sindhu, S; Sanghi, S; Agarwal, A; Seth, V P; Kishore, N

2006-05-01

The present work aims to study the structure and variation of optical band gap, density and dc electrical conductivity in vanadium strontium borate glasses. The glass systems xV2O5.(40-x)SrO.60B2O3 and xV2O5.(60-x)B2O3.40SrO with x varying from 0 to 20 mol% were prepared by normal melt quench technique. Structural studies were made by recording IR transmission spectra. The fundamental absorption edge for all the glasses was analyzed in terms of the theory proposed by Davis and Mott. The position of absorption edge and hence the value of the optical band gap was found to depend on the semiconducting glass composition. The absorption in these glasses is believed to be associated with indirect transitions. The origin of Urbach energy is associated with the phonon-assisted indirect transitions. The change in both density and molar volume was discussed in terms of the structural modifications that take place in the glass matrix on addition of V2O5. dc conductivity of the glass systems is also reported. The change of conductivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic one.

Effects of doping on photocatalytic activity for water splitting of metal oxides and nitride

NASA Astrophysics Data System (ADS)

Arai, Naoki; Saito, Nobuo; Nishiyama, Hiroshi; Kadowaki, Haruhiko; Kobayashi, Hisayoshi; Sato, Kazunori; Inoue, Yasunobu

2007-09-01

The effects of metal-ion doping or replacement on the photocatalytic performance for water splitting of d 10 and d 0 metal oxides and d 10 metal nitride were studied. The photocatalysts examined were (1) α-Ga 2-2xIn 2xO 3 and ZnGa 2-2xIn 2xO 4 in which In 3+ was added to Ga IIO 3 and ZnGa IIO 4, respectively, (2) Y xIn 2-xO 3 being a solid solution of In IIO 3 and Y IIO 3, (3) metal ion doped CeO II, and (4) metal ion doped GaN. The photocatalytic activity of 1 wt % RuO II-loaded α-Ga 2-2xIn 2xO 3 increased sharply with increasing x, reached a maximum at around x=0.02, and considerably decreased with further increase in x. The DFT calculation showed that the band structures of α-Ga 2-2xIn 2xO 3 had the contribution of In 4d orbital to the valence band and of In5s orbital to the conduction band. Similar effects were observed for ZnGa 2-2xIn 2xO 4. RuO II-dispersed Y xIn 2-xO 3 had a capability of producing H II and O II in the range x=1.0-1.5 in which the highest activity was obtained at x=1.3. The structures of both InO 6 and YO 6 octahedra were deformed in the solid solution,, and the hybridization of In5s5p and Y4d orbitals in the conduction band was enhanced. Undoped CeO II was photocatalytically inactive, but metal ion-doped CeO II showed a considerable photocatalytic activity. The activation occurred in the case that metal ions doped had larger ion sizes than that of Ce 4+. The small amount doping of divalent metal ions (Zn 2+ and Mg 2+) converted photocatalytically inactive GaN to an efficient photocatalyst. The doping was shown to produce p-type GaN which had the large concentration and high mobility of holes. The roles of metal ion doping and replacement in the photocatalytic properties are discussed.

Investigation of electronic band structure and charge transfer mechanism of oxidized three-dimensional graphene as metal-free anodes material for dye sensitized solar cell application

NASA Astrophysics Data System (ADS)

Loeblein, Manuela; Bruno, Annalisa; Loh, G. C.; Bolker, Asaf; Saguy, Cecile; Antila, Liisa; Tsang, Siu Hon; Teo, Edwin Hang Tong

2017-10-01

Dye-sensitized solar cells (DSSCs) offer an optimal trade-off between conversion-efficiency and low-cost fabrication. However, since all its electrodes need to fulfill stringent work-function requirements, its materials have remained unchanged since DSSC's first report early-90s. Here we describe a new material, oxidized-three-dimensional-graphene (o-3D-C), with a band gap of 0.2 eV and suitable electronic band-structure as alternative metal-free material for DSSCs-anodes. o-3D-C/dye-complex has a strong chemical bonding via carboxylic-group chemisorption with full saturation after 12 sec at capacity of ∼450 mg/g (600x faster and 7x higher than optimized metal surfaces). Furthermore, fluorescence quenching of life-time by 28-35% was measured demonstrating charge-transfer from dye to o-3D-C.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahlawat, Navneet; Aghamkar, Praveen; Ahlawat, Neetu

Lithium lead silicate glasses with composition 30Li{sub 2}O{center_dot}(70-x)PbO{center_dot}xSiO{sub 2}(where, x = 10, 20, 30, 40, 50 mol %)(LPS glasses) were prepared by normal melt quench technique at 1373 K for half an hour in air to understand their structure. Compositional dependence of density, molar volume and glass transition temperature of these glasses indicates more compactness of the glass structure with increasing SiO{sub 2} content. Fourier transform infrared (FTIR) spectroscopic data obtained for these glasses was used to investigate the changes induced in the local structure of samples as the ratio between PbO and SiO{sub 2} content changes from 6.0 tomore » 0.4. The observed absorption band around 450-510 cm{sup -1} in IR spectra of these glasses indicates the presence of network forming PbO{sub 4} tetrahedral units in glass structure. The increase in intensity with increasing SiO{sub 2} content (upto x = 30 mol %) suggests superposition of Pb-O and Si-O bond vibrations in absorption band around 450-510 cm{sup -1}. The values of optical basicity in these glasses were found to be dependent directly on PbO/SiO{sub 2} ratio.« less

Structural and optical properties of magnetron sputtered MgxZn1-xO thin films

NASA Astrophysics Data System (ADS)

Kumar, Sanjeev; Gupte, Vinay; Sreenivas, K.

2006-04-01

MgxZn1-xO (MZO) thin films prepared by an rf magnetron sputtering technique are reported. The films were grown at room temperature and at relatively low rf power of 50 W. MZO thin films were found to possess preferred c-axis orientation and exhibited hexagonal wurtzite structure of ZnO up to a Mg concentration of 42 mol%. A small variation in the c-axis lattice parameter of around 0.3% was observed with increasing Mg composition, showing the complete solubility of Mg in ZnO. The band gap of the MZO films in the wurtzite phase varied linearly with the Mg concentration and a maximum band gap ~4.19 eV was achieved at x = 0.42. The refractive indices of the MgO films were found to decrease with increasing Mg content. The observed optical dispersion data are in agreement with the single oscillator model. A photoluminescence study revealed a blue shift in the near band edge emission peak with increasing Mg content in the MZO films. The results show the potential of MZO films in various opto-electronic applications.

High resolution absolute absorption cross sections of the B ̃(1)A'-X ̃(1)A' transition of the CH2OO biradical.

PubMed

Foreman, Elizabeth S; Kapnas, Kara M; Jou, YiTien; Kalinowski, Jarosław; Feng, David; Gerber, R Benny; Murray, Craig

2015-12-28

Carbonyl oxides, or Criegee intermediates, are formed from the gas phase ozonolysis of alkenes and play a pivotal role in night-time and urban area atmospheric chemistry. Significant discrepancies exist among measurements of the strong B ̃(1)A'-X ̃(1)A' electronic transition of the simplest Criegee intermediate, CH2OO in the visible/near-UV. We report room temperature spectra of the B ̃(1)A'-X ̃(1)A' electronic absorption band of CH2OO acquired at higher resolution using both single-pass broadband absorption and cavity ring-down spectroscopy. The new absorption spectra confirm the vibrational structure on the red edge of the band that is absent from ionization depletion measurements. The absolute absorption cross sections over the 362-470 nm range are in good agreement with those reported by Ting et al. Broadband absorption spectra recorded over the temperature range of 276-357 K were identical within their mutual uncertainties, confirming that the vibrational structure is not due to hot bands.

Electronic structure of α-SrB4O7: experiment and theory

NASA Astrophysics Data System (ADS)

Atuchin, V. V.; Kesler, V. G.; Zaitsev, A. I.; Molokeev, M. S.; Aleksandrovsky, A. S.; Kuzubov, A. A.; Ignatova, N. Y.

2013-02-01

The investigation of valence band structure and electronic parameters of constituent element core levels of α-SrB4O7 has been carried out with x-ray photoemission spectroscopy. Optical-quality crystal α-SrB4O7 has been grown by the Czochralski method. Detailed photoemission spectra of the element core levels have been recorded from the powder sample under excitation by nonmonochromatic Al Kα radiation (1486.6 eV). The band structure of α-SrB4O7 has been calculated by ab initio methods and compared to XPS measurements. It has been found that the band structure of α-SrB4O7 is weakly dependent on the Sr-related states.

Effective performance for undoped and boron-doped double-layered nanoparticles-copper telluride and manganese telluride on tungsten oxide photoelectrodes for solar cell devices.

PubMed

Srathongluan, Pornpimol; Vailikhit, Veeramol; Teesetsopon, Pichanan; Choopun, Supab; Tubtimtae, Auttasit

2016-11-01

This work demonstrates the synthesis of a novel double-layered Cu2-xTe/MnTe structure on a WO3 photoelectrode as a solar absorber for photovoltaic devices. Each material absorber is synthesized using a successive ionic layer adsorption and reaction (SILAR) method. The synthesized individual particle sizes are Cu2-xTe(17) ∼5-10nm and MnTe(3) ∼2nm, whereas, the aggregated particle sizes of undoped and boron-doped Cu2-xTe(17)/MnTe(11) are ∼50 and 150nm, respectively. The larger size after doping is due to the interconnecting of nanoparticles as a network-like structure. A new alignment of the energy band is constructed after boron/MnTe(11) is coated on boron/Cu2-xTe nanoparticles (NPs), leading to a narrower Eg equal to 0.58eV. Then, the valence band maximum (VBM) and conduction band minimum (CBM) with a trap state are also up-shifted to near the CBM of WO3, leading to the shift of a Fermi level for ease of electron injection. The best efficiency of 1.41% was yielded for the WO3/boron-doped [Cu2-xTe(17)/MnTe(11)] structure with a photocurrent density (Jsc)=16.43mA/cm(2), an open-circuit voltage (Voc)=0.305V and a fill factor (FF)=28.1%. This work demonstrates the feasibility of this double-layered structure with doping material as a solar absorber material. Copyright © 2016 Elsevier Inc. All rights reserved.

Influence of Y doping concentration on the properties of nanostructured MxZn1-xO (M=Y) thin film deposited by nebulizer spray pyrolysis technique

NASA Astrophysics Data System (ADS)

Mariappan, R.; Ponnuswamy, V.; Chandra Bose, A.; Suresh, R.; Ragavendar, M.

2014-09-01

Yttrium doped Zinc Oxide (YxZn1-xO) thin films deposited at a substrate temperature 400 °C. The effect of substrate temperature on the structural, surface morphology, compositional, optical and electrical properties of YxZn1-xO thin films was studied. X-ray diffraction studies show that all films are polycrystalline in nature with hexagonal crystal structure having highly textured (002) plane parallel to the surface of the substrate. The structural parameters, such as lattice constants (a and c), crystallite size (D), dislocation density (δ), microstrain (σ) and texture coefficient were calculated for different yttrium doping concentrations (x). High resolution scanning electron microscopy measurements reveal that the surface morphology of the films change from platelet like grains to hexagonal structure with grain size increase due to the yttrium doping. Energy dispersive spectroscopy confirms the presence of Y, Zn and O elements in the films prepared. Optical studies showed that all samples have a strong optical transmittance higher than 70% in the visible range. A slight shift of the absorption edge towards the large wavelengths was observed as the Y doping concentration increased. This result shows that the band gap is slightly decreased from 3.10 to 2.05 eV with increase of the yttrium doping concentrations (up to 7.5%) and then slightly increased. Room temperature PL measurements were done and the band-to-band emission energies of films were determined and reported. The complex impedance of the 10%Y doped ZnO film shows two distinguished semicircles and the diameter of the arcs got decreased in diameter as the temperature increases from 70 to 175 °C.

A variable circular-plot method for estimated bird numbers

USGS Publications Warehouse

Reynolds, R.T.; Scott, J.M.; Nussbaum, R.A.

1980-01-01

A bird census method is presented that is designed for tall, structurally complex vegetation types, and rugged terrain. With this method the observer counts all birds seen or heard around a station, and estimates the horizontal distance from the station to each bird. Count periods at stations vary according to the avian community and structural complexity of the vegetation. The density of each species is determined by inspecting a histogram of the number of individuals per unit area in concentric bands of predetermined widths about the stations, choosing the band (with outside radius x) where the density begins to decline, and summing the number of individuals counted within the circle of radius x and dividing by the area (Bx2). Although all observations beyond radius x are rejected with this procedure, coefficients of maximum distance.

A polymer dataset for accelerated property prediction and design.

PubMed

Huan, Tran Doan; Mannodi-Kanakkithodi, Arun; Kim, Chiho; Sharma, Vinit; Pilania, Ghanshyam; Ramprasad, Rampi

2016-03-01

Emerging computation- and data-driven approaches are particularly useful for rationally designing materials with targeted properties. Generally, these approaches rely on identifying structure-property relationships by learning from a dataset of sufficiently large number of relevant materials. The learned information can then be used to predict the properties of materials not already in the dataset, thus accelerating the materials design. Herein, we develop a dataset of 1,073 polymers and related materials and make it available at http://khazana.uconn.edu/. This dataset is uniformly prepared using first-principles calculations with structures obtained either from other sources or by using structure search methods. Because the immediate target of this work is to assist the design of high dielectric constant polymers, it is initially designed to include the optimized structures, atomization energies, band gaps, and dielectric constants. It will be progressively expanded by accumulating new materials and including additional properties calculated for the optimized structures provided.

Five-cell superconducting RF module with a PBG coupler cell: design and cold testing of the copper prototype

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arsenyev, Sergey Andreyevich; Simakov, Evgenya Ivanovna; Shchegolkov, Dmitry

2015-04-29

We report the design and experimental data for a copper prototype of a superconducting radio-frequency (SRF) accelerator module. The five-cell module has an incorporated photonic band gap (PBG) cell with couplers. The purpose of the PBG cell is to achieve better higher order mode (HOM) damping, which is vital for preserving the quality of high-current electron beams. Better HOM damping raises the current threshold for beam instabilities in novel SRF accelerators. The PBG design also increases the real-estate gradient of the linac because both HOM damping and the fundamental power coupling can be done through the PBG cell instead ofmore » on the beam pipe via complicated end assemblies. First, we will discuss the design and accelerating properties of the structure. The five-cell module was optimized to provide good HOM damping while maintaining the same accelerating properties as conventional elliptical-cell modules. We will then discuss the process of tuning the structure to obtain the desired accelerating gradient profile. Finally, we will list measured quality factors for the accelerating mode and the most dangerous HOMs.« less

Efficient Green Emission from Wurtzite Al xIn1- xP Nanowires.

PubMed

Gagliano, L; Kruijsse, M; Schefold, J D D; Belabbes, A; Verheijen, M A; Meuret, S; Koelling, S; Polman, A; Bechstedt, F; Haverkort, J E M; Bakkers, E P A M

2018-06-13

Direct band gap III-V semiconductors, emitting efficiently in the amber-green region of the visible spectrum, are still missing, causing loss in efficiency in light emitting diodes operating in this region, a phenomenon known as the "green gap". Novel geometries and crystal symmetries however show strong promise in overcoming this limit. Here we develop a novel material system, consisting of wurtzite Al x In 1- x P nanowires, which is predicted to have a direct band gap in the green region. The nanowires are grown with selective area metalorganic vapor phase epitaxy and show wurtzite crystal purity from transmission electron microscopy. We show strong light emission at room temperature between the near-infrared 875 nm (1.42 eV) and the "pure green" 555 nm (2.23 eV). We investigate the band structure of wurtzite Al x In 1- x P using time-resolved and temperature-dependent photoluminescence measurements and compare the experimental results with density functional theory simulations, obtaining excellent agreement. Our work paves the way for high-efficiency green light emitting diodes based on wurtzite III-phosphide nanowires.

Pressure-Induced Structural Evolution and Band Gap Shifts of Organometal Halide Perovskite-Based Methylammonium Lead Chloride.

PubMed

Wang, Lingrui; Wang, Kai; Xiao, Guanjun; Zeng, Qiaoshi; Zou, Bo

2016-12-15

Organometal halide perovskites are promising materials for optoelectronic devices. Further development of these devices requires a deep understanding of their fundamental structure-property relationships. The effect of pressure on the structural evolution and band gap shifts of methylammonium lead chloride (MAPbCl 3 ) was investigated systematically. Synchrotron X-ray diffraction and Raman experiments provided structural information on the shrinkage, tilting distortion, and amorphization of the primitive cubic unit cell. In situ high pressure optical absorption and photoluminescence spectra manifested that the band gap of MAPbCl 3 could be fine-tuned to the ultraviolet region by pressure. The optical changes are correlated with pressure-induced structural evolution of MAPbCl 3 , as evidenced by band gap shifts. Comparisons between Pb-hybrid perovskites and inorganic octahedra provided insights on the effects of halogens on pressure-induced transition sequences of these compounds. Our results improve the understanding of the structural and optical properties of organometal halide perovskites.

Optical properties of Y and Ti co-substituted BiFeO{sub 3} multiferroics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Singh, Vikash, E-mail: rk.dwivedi@jiit.ac.in; Sharma, Subhash, E-mail: rk.dwivedi@jiit.ac.in; Kumar, Manoj, E-mail: rk.dwivedi@jiit.ac.in

2014-04-24

Pure and co substituted Bi{sub 1−x}Y{sub x}Fe{sub 1−x}Ti{sub x}O{sub 3} (x ≤ 0.24) ceramics were synthesized by solid state reaction method. X-ray diffraction patterns of Y and Ti codoped samples have shown single phase formation. Increasing Y and Ti concentration reveals structural transition from rhombohedral phase (R3c) for x ≤ 0.16 to orthorhombic phase (Pnma) for x = 0.24. FT-IR spectra exhibit broad absorption bands, which may be due to the overlapping of Fe-O and Bi-O vibrations. UV-visible spectroscopy results show strong absorption of light in the spectral range of 400-720 nm, indicating optical band gap in the visible regionmore » for these samples. These interesting optical properties of co-substituted BFO samples in visible region may find potential applications in optoelectronic devices.« less

Synthesis, structural and magnetic behavior studies of Zn-Al substituted cobalt ferrite nanoparticles

NASA Astrophysics Data System (ADS)

Zare, Samad; Ati, Ali A.; Dabagh, Shadab; Rosnan, R. M.; Othaman, Zulkafli

2015-06-01

A series of nano-sized Zn-Al substituted cobalt ferrite Co(1-x)Zn(x)Fe2-xAlxO4 with 0.0 ⩽ x ⩽ 1.0 have been synthesized by chemical co-precipitation technique. The XRD spectra revealed the single phase spinel structure of Co(1-x)Zn(x)Fe2-xAlxO4 with average size of nanoparticles are estimated to be 17-30 nm. These are small enough to achieve the suitable signal to noise ratio, which is important in the high-density recording media. The FTIR spectra show the characteristic of two strong absorption bands at 560-600 cm-1 corresponds to the intrinsic stretching vibrations of the metal at the tetrahedral site and lowest band is observed at 370-410 cm-1 corresponds to octahedral site. The crystalline structures of nanoparticles composite were characterized by Field Emission Scanning Electron Microscopy (FE-SEM). The magnetic properties such as saturation magnetization, remanence magnetization, and coercivity were calculated from the hysteresis loops. Saturation magnetization were found to increase up to x = 0.4 while remanence magnetization and coercivity continuously decrease with increasing Zn-Al concentration. The stability in coercivity while increase in saturation magnetization confirms that the Co0.6Zn0.4Fe1.6Al0.4O4 ferrite sample is suitable for applications in high-density recording media.

Dual-frequency radio soundings of planetary ionospheres avoid misinterpretations of ionospheric features

NASA Astrophysics Data System (ADS)

Paetzold, M.; Andert, T.; Bird, M. K.; Häusler, B.; Hinson, D. P.; Peter, K.; Tellmann, S.

2017-12-01

Planetary ionospheres are usually sounded at single frequency, e.g. S-band or X-band, or at dual-frequencies, e.g. simultaneous S-band and X-band frequencies. The differential Doppler is computed from the received dual-frequency sounding and it has the advantage that any residual motion by the spaceraft body is compensated. The electron density profile is derived from the propagation of the two radio signals through the ionospheric plasma. Vibrational motion of small amplitude by the spacecraft body may still be contained in the single frequency residuals and may be translated into electron densities. Examples from Mars Express and Venus Express shall be presented. Cases from other missions shall be presented where wave-like structures in the upper ionosphere may be a misinterpretation.

Wind-jet interaction in high-mass X-ray binaries

NASA Astrophysics Data System (ADS)

Zdziarski, Andrzej

2016-07-01

Jets in high-mass X-ray binaries can strongly interact with the stellar wind from the donor. The interaction leads, in particular, to formation of recollimation shocks. The shocks can then accelerate electrons in the jet and lead to enhanced emission, observable in the radio and gamma-ray bands. DooSoo, Zdziarski & Heinz (2016) have formulated a condition on the maximum jet power (as a function of the jet velocity and wind rate and velocity) at which such shocks form. This criterion can explain the large difference in the radio and gamma-ray loudness between Cyg X-1 and Cyg X-3. The orbital modulation of radio emission observed in Cyg X-1 and Cyg X-3 allows a measurement of the location of the height along the jet where the bulk of emission at a given frequency occurs. Strong absorption of X-rays in the wind of Cyg X-3 is required to account for properties of the correlation of the radio emission with soft and hard X-rays. That absorption can also account for the unusual spectral and timing X-ray properties of this source.

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca 1 - x La x FeAs 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kang, Chang-Jong; Birol, Turan; Kotliar, Gabriel

The recently discovered high-T c superconductor Ca 1-xLa xFeAs 2 is a unique compound not just because of its low-symmetry crystal structure but also because of its electronic structure, which hosts Dirac-like metallic bands resulting from (spacer) zigzag As chains. We present a comprehensive first-principles theoretical study of the electronic and crystal structures of Ca 1-xLa xFeAs 2. After discussing the connection between the crystal structure of the 112 family, which Ca 1-xLa xFeAs 2 is a member of, with the other known structures of Fe pnictide superconductors, we check the thermodynamic phase stability of CaFeAs 2, and similar hyphotheticalmore » compounds SrFeAs 2 and BaFeAs 2 which, we find, are slightly higher in energy. We calculate the optical conductivity of Ca 1-xLa xFeAs 2 using the DFT+DMFT method and predict a large in-plane resistivity anisotropy in the normal phase, which does not originate from electronic nematicity, but is enhanced by the electronic correlations. In particular, we predict a 0.34 eV peak in the yy component of the optical conductivity of the 30% La-doped compound, which corresponds to coherent interband transitions within a fast-dispersing band arising from the zigzag As chains, which are unique to this compound. We also study the Landau free energy for Ca 1-xLa xFeAs 2 including the order parameter relevant for the nematic transition and find that the free energy does not have any extra terms that could induce ferro-orbital order. This explains why the presence of As chains does not broaden the nematic transition in Ca 1-xLa xFeAs 2.« less

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca 1 - x La x FeAs 2

DOE PAGES

Kang, Chang-Jong; Birol, Turan; Kotliar, Gabriel

2017-01-17

The recently discovered high-T c superconductor Ca 1-xLa xFeAs 2 is a unique compound not just because of its low-symmetry crystal structure but also because of its electronic structure, which hosts Dirac-like metallic bands resulting from (spacer) zigzag As chains. We present a comprehensive first-principles theoretical study of the electronic and crystal structures of Ca 1-xLa xFeAs 2. After discussing the connection between the crystal structure of the 112 family, which Ca 1-xLa xFeAs 2 is a member of, with the other known structures of Fe pnictide superconductors, we check the thermodynamic phase stability of CaFeAs 2, and similar hyphotheticalmore » compounds SrFeAs 2 and BaFeAs 2 which, we find, are slightly higher in energy. We calculate the optical conductivity of Ca 1-xLa xFeAs 2 using the DFT+DMFT method and predict a large in-plane resistivity anisotropy in the normal phase, which does not originate from electronic nematicity, but is enhanced by the electronic correlations. In particular, we predict a 0.34 eV peak in the yy component of the optical conductivity of the 30% La-doped compound, which corresponds to coherent interband transitions within a fast-dispersing band arising from the zigzag As chains, which are unique to this compound. We also study the Landau free energy for Ca 1-xLa xFeAs 2 including the order parameter relevant for the nematic transition and find that the free energy does not have any extra terms that could induce ferro-orbital order. This explains why the presence of As chains does not broaden the nematic transition in Ca 1-xLa xFeAs 2.« less

Electronic structure of the dilute magnetic semiconductor G a1 -xM nxP from hard x-ray photoelectron spectroscopy and angle-resolved photoemission

NASA Astrophysics Data System (ADS)

Keqi, A.; Gehlmann, M.; Conti, G.; Nemšák, S.; Rattanachata, A.; Minár, J.; Plucinski, L.; Rault, J. E.; Rueff, J. P.; Scarpulla, M.; Hategan, M.; Pálsson, G. K.; Conlon, C.; Eiteneer, D.; Saw, A. Y.; Gray, A. X.; Kobayashi, K.; Ueda, S.; Dubon, O. D.; Schneider, C. M.; Fadley, C. S.

2018-04-01

We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) G a0.98M n0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimental data, as well as theoretical calculations, to understand the role of the Mn dopant in the emergence of ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between G a0.98M n0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The G a0.98M n0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of G a0.97M n0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012), 10.1038/nmat3450], demonstrating the strong similarity between these two materials. The Mn 2 p and 3 s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

Significant reduction in NiO band gap upon formation of Lix Ni1-x O alloys: applications to solar energy conversion.

PubMed

Alidoust, Nima; Toroker, Maytal Caspary; Keith, John A; Carter, Emily A

2014-01-01

Long-term sustainable solar energy conversion relies on identifying economical and versatile semiconductor materials with appropriate band structures for photovoltaic and photocatalytic applications (e.g., band gaps of ∼ 1.5-2.0 eV). Nickel oxide (NiO) is an inexpensive yet highly promising candidate. Its charge-transfer character may lead to longer carrier lifetimes needed for higher efficiencies, and its conduction band edge is suitable for driving hydrogen evolution via water-splitting. However, NiO's large band gap (∼ 4 eV) severely limits its use in practical applications. Our first-principles quantum mechanics calculations show band gaps dramatically decrease to ∼ 2.0 eV when NiO is alloyed with Li2O. We show that Lix Ni1-x O alloys (with x=0.125 and 0.25) are p-type semiconductors, contain states with no impurity levels in the gap and maintain NiO's desirable charge-transfer character. Lastly, we show that the alloys have potential for photoelectrochemical applications, with band edges well-placed for photocatalytic hydrogen production and CO2 reduction, as well as in tandem dye-sensitized solar cells as a photocathode. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

An Intriguing Convex Break in the EGRET SED of Mrk 421

NASA Technical Reports Server (NTRS)

Nandikotkur, Giridhar; Jahoda, Keith M.; Georganopoulos, M.; Hartman, R. C.; Mukherjee, R.; Thompson, D. J.; Swank, Jean H.

2007-01-01

Based upon analysis of the entire EGRET data from Mrk 421, it is found that the time-averaged spectra are inconsistent with the predictions of current theoretical models that have had success in describing simultaneous X-ray/TeV observations, and suggest additional components in the GeV band, as well as complex time variability. Current theoretical pictures explain the GeV emission as comptonization of the synchrotron photons in the jet, and predict hard spectra that should join smoothly with the TeV emission. Our analysis shows that the situation is more complex. The spectrum ranges from hard to soft during individual epochs, and shows a convext break in the aggregated data. We also present the mission-averaged EGRET spectrum for PKS 2155-304, which shows a similar (but not as pronounced) convex curvature. We discuss a series of possible explanations for the 10(exp 22) - 10(exp 23) HZ declining part of the EGRET nu F(sub nu), spectrum for Mrk 421, and suggest that it is synchrotron emission from the high energy tail of the electron population that produces the X-rays during the highest X-ray states. Such multi-MeV photons are produced by electrons accelerated close to the limit of diffusive shock acceleration. Simultaneous GLAST and X-ray observations of high X-ray states will address the issue of the convex curvature in the future.

Global minimum-energy structure and spectroscopic properties of I2(*-) x n H2O clusters: a Monte Carlo simulated annealing study.

PubMed

Pathak, Arup Kumar; Mukherjee, Tulsi; Maity, Dilip Kumar

2010-01-18

The vibrational (IR and Raman) and photoelectron spectral properties of hydrated iodine-dimer radical-anion clusters, I(2)(*-) x n H(2)O (n=1-10), are presented. Several initial guess structures are considered for each size of cluster to locate the global minimum-energy structure by applying a Monte Carlo simulated annealing procedure including spin-orbit interaction. In the Raman spectrum, hydration reduces the intensity of the I-I stretching band but enhances the intensity of the O-H stretching band of water. Raman spectra of more highly hydrated clusters appear to be simpler than the corresponding IR spectra. Vibrational bands due to simultaneous stretching vibrations of O-H bonds in a cyclic water network are observed for I(2)(*-) x n H(2)O clusters with n > or = 3. The vertical detachment energy (VDE) profile shows stepwise saturation that indicates closing of the geometrical shell in the hydrated clusters on addition of every four water molecules. The calculated VDE of finite-size small hydrated clusters is extrapolated to evaluate the bulk VDE value of I(2)(*-) in aqueous solution as 7.6 eV at the CCSD(T) level of theory. Structure and spectroscopic properties of these hydrated clusters are compared with those of hydrated clusters of Cl(2)(*-) and Br(2)(*-).

Optical study of gamma irradiated sodium metaphosphate glasses containing divalent metal oxide MO (ZnO or CdO)

NASA Astrophysics Data System (ADS)

Nabhan, E.; Abd-Allah, W. M.; Ezz-El-Din, F. M.

Sodium metaphosphate glasses containing divalent metal oxide, ZnO or CdO with composition 50 P2O5 - (50 - x) Na2O - x MO (ZnO, or CdO) where x = 0, 10, 20 (mol%) were prepared by conventional melt method. UV/visible spectroscopy and FTIR spectroscopy are measured before and after exposing to successive gamma irradiation doses (5-80 kGy). The optical absorption spectra results of the samples before irradiation reveal a strong UV absorption band at (∼230 nm) which is related to unavoided iron impurities. The effects of gamma irradiation on the optical spectral properties of the various glasses have been compared. From the optical absorption spectral data, the optical band gap is evaluated. The main structural groups and the influence of both divalent metal oxide and gamma irradiation effect on the structural vibrational groups are realized through IR spectroscopy. The FTIR spectra of γ-irradiated samples are characterized by the stability of the number and position for the main characteristic band of phosphate groups. To better understood the structural changes during γ-irradiation, a deconvolution of FTIR spectra in the range 650-1450 cm-1 is made. The FTIR deconvolution results found evidence that, the changes occurring after gamma irradiation have been related to irradiation induced structural defects and compositional changes.

Formation of Dirac point and the topological surface states inside the strained gap for mixed 3D Hg1-xCdx Te

NASA Astrophysics Data System (ADS)

Marchewka, Michał

2016-10-01

In this paper the results of the numerical calculation obtained for the three-dimensional (3D) strained Hg1-xCdx Te layers for the x-Cd composition from 0.1 to 0.155 and a different mismatch of the lattice constant are presented. For the investigated region of the Cd composition (x value) the negative energy gap (Eg =Γ8 -Γ6) in the Hg1-xCdx Te is smaller than in the case of pure HgTe which, as it turns out, has a significant influence on the topological surface states (TSS) and the position of the Dirac point. The numerical calculation based on the finite difference method applied for the 8×8 kp model with the in-plane tensile strain for (001) growth oriented structure shows that the Dirac cone inside the induced insulating band gap for non zero of the Cd composition and a bigger strain caused by the bigger lattice mismatch (than for the 3D HgTe TI) can be obtained. It was also shown how different x-Cd compounds move the Dirac cone from the valence band into the band gap. The presented results show that 75 nm wide 3D Hg1-xCdx Te structures with x ≈ 0.155 and 1.6% lattice mismatch make the system a true topological insulator with the dispersion of the topological surface states similar to those ones obtained for the strained CdTe/HgTe QW.

Comparative research on the influence of varied Al component on the active layer of AlGaN photocathode

NASA Astrophysics Data System (ADS)

He, Minyou; Chen, Liang; Su, Lingai; Yin, Lin; Qian, Yunsheng

2017-06-01

To theoretically research the influence of a varied Al component on the active layer of AlGaN photocathodes, the first principle based on density functional theory is used to calculate the formation energy and band structure of Al x Ga1-x N with x at 0, 0.125, 0.25, 0.325, and 0.5. The calculation results show that the formation energy declines along with the Al component rise, while the band gap is increasing with Al component increasing. Al x Ga1-x N with x at 0, 0.125, 0.25, 0.325, and 0.5 are direct band gap semiconductors, and their absorption coefficient curves have the same variation tendency. For further study, we designed two kinds of reflection-mode AlGaN photocathode samples. Sample 1 has an Al x Ga1-x N active layer with varied Al component ranging from 0.5 to 0 and decreasing from the bulk to the surface, while sample 2 has an Al x Ga1-x N active layer with the fixed Al component of 0.25. Using the multi-information measurement system, we measured the spectral response of the activated samples at room temperature. Their photocathode parameters were obtained by fitting quantum efficiency curves. Results show that sample 1 has a better spectral response than sample 2 at the range of short-wavelength. This work provides a reference for the structure design of the AlGaN photocathode. Project supported by the National Natural Science Foundation of China (Nos. 61308089, 6144005) and the Public Technology Applied Research Project of Zhejiang Province (No. 2013C31068).

S-band 1.4 cell photoinjector design for high brightness beam generation

NASA Astrophysics Data System (ADS)

Pirez, E.; Musumeci, P.; Maxson, J.; Alesini, D.

2017-09-01

In this paper we study in detail the design of a novel S-band radiofrequency photogun structure to maximize the accelerating field experienced by the particles at injection. This is a critical quantity for electron sources as it has a direct impact on the maximum brightness achievable. The proposed design is based on a modification of the latest generation of S-band RF photoinjectors to include novel fabrication approaches. The gun is designed to operate at a 120 MV/m gradient and at an optimal injection phase of 70° providing the beam quality required to enable novel electron beam applications such as single shot time-resolved transmission electron microscopy and ultrafast electron nanodiffraction.

Rhombohedral to Cubic Conversion of GeTe via MnTe Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence, and High Thermoelectric Performance.

PubMed

Zheng, Zheng; Su, Xianli; Deng, Rigui; Stoumpos, Constantinos; Xie, Hongyao; Liu, Wei; Yan, Yonggao; Hao, Shiqiang; Uher, Ctirad; Wolverton, Chris; Kanatzidis, Mercouri G; Tang, Xinfeng

2018-02-21

In this study, a series of Ge 1-x Mn x Te (x = 0-0.21) compounds were prepared by a melting-quenching-annealing process combined with spark plasma sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge 1-x Mn x Te is profound. With increasing content of MnTe, the structure of the Ge 1-x Mn x Te compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and Fm-3m structures, enhancing a multiband character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m 0 (GeTe) to 6.15 m 0 (Ge 0.85 Mn 0.15 Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 W m -1 K -1 at 823 K. Subsequent doping of the Ge 0.9 Mn 0.1 Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yields a maximum ZT value of 1.61 at 823 K (for Ge 0.86 Mn 0.10 Sb 0.04 Te). The average ZT value of the compound over the temperature range 400-800 K is 1.09, making it the best GeTe-based thermoelectric material.

High-resolution continuum and Br (gamma) imaging observations of M82

NASA Technical Reports Server (NTRS)

Larkin, J. E.; Graham, J. R.; Matthews, K.; Soifer, B. T.; Beckwith, S.; Herbst, T. M.; Quillen, A. C.

1994-01-01

We report high angular resolution (about 0.6 sec), broad-band imaging at 1.2 microns (J band), 1.6 microns (H band), 2.2 microns (K band), and 3.7 microns (L' band) of the central 110 sec x 21 sec (1.65 kpc x 0.32 kpc) of the nearby starburst galaxy M82. We also present spectral imaging with 90 km/s resolution in the Br (gamma) (2.17 microns) hydrogen recombination line covering the central 16 sec x 16 sec (240 pc x 240 pc) of this edge-on, disk galaxy. The broad-band mosaics reveal two plateaus of emission indicative of an inner disk of stars and perhaps a larger bar structure. Color maps reveal an extinction ridge running along the central kiloparsec which is strongest at the nucleus and on the western side. The dust emission is more symmetric, suggesting that a dust lane is in front of the stellar population to the west of the nucleus, and behind the stars to the east; this is again suggestive of a stellar bar with leading dust lanes. Channel maps and a position-velocity image of the Br (gamma) reveal two lobes and are consistent with the interpretation that the ionized gas, and hence, the young massive stars are distributed in a toroid of H II regions surrounding the nucleus.

Features of the band structure and conduction mechanisms of n-HfNiSn heavily doped with Y

DOE Office of Scientific and Technical Information (OSTI.GOV)

Romaka, V. A., E-mail: vromaka@polynet.lviv.ua; Rogl, P.; Romaka, V. V.

The crystalline and electronic structures, energy, kinetic, and magnetic characteristics of n-HfNiSn semiconductor heavily doped with Y acceptor impurity are studied in the ranges: T = 80–400 K, N{sub A}{sup Y} ≈ 1.9 × 10{sup 20}–5.7 × 10{sup 21} cm{sup –3} (x = 0.01–0.30), and H ≤ 10 kG. The nature of the mechanism of structural defect generation is determined, which leads to a change in the band gap and the degree of semiconductor compensation, the essence of which is the simultaneous reduction and elimination of structural donor-type defects as a result of the displacement of ~1% of Ni atomsmore » from the Hf (4a) site, and the generation of structural acceptor-type defects by substituting Hf atoms with Y atoms at the 4a site. The results of calculations of the electronic structure of Hf{sub 1–x}Y{sub x}NiSn are in agreement with the experimental data. The discussion is performed within the Shklovskii–Efros model of a heavily doped and compensated semiconductor.« less

Electronic Structure Approach to Tunable Electronic Properties of Hybrid Organic-Inorganic Perovskites

NASA Astrophysics Data System (ADS)

Liu, Garnett; Huhn, William; Mitzi, David B.; Kanai, Yosuke; Blum, Volker

We present a study of the electronic structure of layered hybrid organic-inorganic perovskite (HOIP) materials using all-electron density-functional theory. Varying the nature of the organic and inorganic layers should enable systematically fine-tuning the carrier properties of each component. Using the HSE06 hybrid density functional including spin-orbit coupling (SOC), we validate the principle of tuning subsystem-specific parts of the electron band structures and densities of states in CH3NH3PbX3 (X=Cl, Br, I) compared to a modified organic component in layered (C6H5C2H4NH3) 2PbX4 (X=Cl, Br, I) and C20H22S4N2PbX4 (X=Cl, Br, I). We show that tunable shifts of electronic levels indeed arise by varying Cl, Br, I as the inorganic components, and CH3NH3+ , C6H5C2H4NH3+ , C20H22S4N22 + as the organic components. SOC is found to play an important role in splitting the conduction bands of the HOIP compounds investigated here. The frontier orbitals of the halide shift, increasing the gap, when Cl is substituted for Br and I.

Origin of the X-ray Spectral Variation and Seemingly Broad Iron Line Strucuture in the Seyfert Galaxies

NASA Astrophysics Data System (ADS)

Ebisawa, Ken; Naoki, Iso

2012-07-01

X-ray intensities and spectra of the Seyfert galaxies are known to be variable. Some of the sources have characteristic seemingly broad iron line structure, and their spectral variations are small in the iron line energy band. MCG-6-30-15 is such an archetypal source, and Miyakawa (2011) proposed a "Variable Partial Covering (VPC)" model to explain its continuum spectral variation, seemingly broad iron line structure, and small spectral variation in the iron energy band simultaneously, only due to variation of a single parameter. That single parameter is the "partial covering fraction" to describe the geometrical fraction of the X-ray emitting area covered by the ionized absorbers in the line of sight. The intrinsic X-ray luminosity is hardly variable in this model. We have applied the VPC model to the 27 Seyfert galaxies observed with Suzaku, and found that spectral variations of the 22 sources are successfully explained by this model only varying the partial covering fraction. Intrinsic X-ray luminosities of Seyfert galaxies are not variable, as opposed to what they apparently seem, and gravitationally red-shifted iron line is not necessary. Those ionized absorbing clouds are most likely to be Broad Line Region (BLR) clouds, and we will be able to constrain the BLR structure from X-ray observations.

Warp-X: A new exascale computing platform for beam–plasma simulations

DOE PAGES

Vay, J. -L.; Almgren, A.; Bell, J.; ...

2018-01-31

Turning the current experimental plasma accelerator state-of-the-art from a promising technology into mainstream scientific tools depends critically on high-performance, high-fidelity modeling of complex processes that develop over a wide range of space and time scales. As part of the U.S. Department of Energy's Exascale Computing Project, a team from Lawrence Berkeley National Laboratory, in collaboration with teams from SLAC National Accelerator Laboratory and Lawrence Livermore National Laboratory, is developing a new plasma accelerator simulation tool that will harness the power of future exascale supercomputers for high-performance modeling of plasma accelerators. We present the various components of the codes such asmore » the new Particle-In-Cell Scalable Application Resource (PICSAR) and the redesigned adaptive mesh refinement library AMReX, which are combined with redesigned elements of the Warp code, in the new WarpX software. Lastly, the code structure, status, early examples of applications and plans are discussed.« less

Warp-X: A new exascale computing platform for beam–plasma simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vay, J. -L.; Almgren, A.; Bell, J.

Turning the current experimental plasma accelerator state-of-the-art from a promising technology into mainstream scientific tools depends critically on high-performance, high-fidelity modeling of complex processes that develop over a wide range of space and time scales. As part of the U.S. Department of Energy's Exascale Computing Project, a team from Lawrence Berkeley National Laboratory, in collaboration with teams from SLAC National Accelerator Laboratory and Lawrence Livermore National Laboratory, is developing a new plasma accelerator simulation tool that will harness the power of future exascale supercomputers for high-performance modeling of plasma accelerators. We present the various components of the codes such asmore » the new Particle-In-Cell Scalable Application Resource (PICSAR) and the redesigned adaptive mesh refinement library AMReX, which are combined with redesigned elements of the Warp code, in the new WarpX software. Lastly, the code structure, status, early examples of applications and plans are discussed.« less

The Design and Analysis of a Novel Split-H-Shaped Metamaterial for Multi-Band Microwave Applications

PubMed Central

Islam, Sikder Sunbeam; Faruque, Mohammad Rashed Iqbal; Islam, Mohammad Tariqul

2014-01-01

This paper presents the design and analysis of a novel split-H-shaped metamaterial unit cell structure that is applicable in a multi-band frequency range and that exhibits negative permeability and permittivity in those frequency bands. In the basic design, the separate split-square resonators are joined by a metal link to form an H-shaped unit structure. Moreover, an analysis and a comparison of the 1 × 1 array and 2 × 2 array structures and the 1 × 1 and 2 × 2 unit cell configurations were performed. All of these configurations demonstrate multi-band operating frequencies (S-band, C-band, X-band and Ku-band) with double-negative characteristics. The equivalent circuit model and measured result for each unit cell are presented to validate the resonant behavior. The commercially available finite-difference time-domain (FDTD)-based simulation software, Computer Simulation Technology (CST) Microwave Studio, was used to obtain the reflection and transmission parameters of each unit cell. This is a novel and promising design in the electromagnetic paradigm for its simplicity, scalability, double-negative characteristics and multi-band operation. PMID:28788116

The Design and Analysis of a Novel Split-H-Shaped Metamaterial for Multi-Band Microwave Applications.

PubMed

Islam, Sikder Sunbeam; Faruque, Mohammad Rashed Iqbal; Islam, Mohammad Tariqul

2014-07-02

This paper presents the design and analysis of a novel split-H-shaped metamaterial unit cell structure that is applicable in a multi-band frequency range and that exhibits negative permeability and permittivity in those frequency bands. In the basic design, the separate split-square resonators are joined by a metal link to form an H-shaped unit structure. Moreover, an analysis and a comparison of the 1 × 1 array and 2 × 2 array structures and the 1 × 1 and 2 × 2 unit cell configurations were performed. All of these configurations demonstrate multi-band operating frequencies (S-band, C-band, X-band and K u -band) with double-negative characteristics. The equivalent circuit model and measured result for each unit cell are presented to validate the resonant behavior. The commercially available finite-difference time-domain (FDTD)-based simulation software, Computer Simulation Technology (CST) Microwave Studio, was used to obtain the reflection and transmission parameters of each unit cell. This is a novel and promising design in the electromagnetic paradigm for its simplicity, scalability, double-negative characteristics and multi-band operation.

Nanodopant-Induced Band Modulation in AgPbmSbTe2+m-Type Nanocomposites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yi; Ke, Xuezhi; Chen, Changfeng

2011-01-01

We elucidate the fundamental physics of nanoscale dopants in narrow band-gap thermoelectric nanocomposites XPbmYTe2+m (X=Ag,Na; Y=Sb,Bi) using first-principles calculations. Our re- sults unveil distinct band-structure modulations, most notably a sizable band-gap widening driven by nanodopant-induced lattice strain and a band split-off at the conduction band minimum caused by the spin-orbit interaction of the dopant Sb or Bi atoms. Boltzmann transport calculations demon- strate that these band modulations have significant but competing effects on high-temperature elec- tron transport behavior. These results offer insights for understanding recent experimental findings and suggest principles for optimizing thermoelectric properties of narrow band-gap semiconductors.

Energy dependence of the band-limited noise in black hole X-ray binaries★

NASA Astrophysics Data System (ADS)

Stiele, H.; Yu, W.

2015-10-01

Black hole low-mass X-ray binaries show a variety of variability features, which manifest as narrow peak-like structures superposed on broad noise components in power density spectra in the hard X-ray emission. In this work, we study variability properties of the band-limited noise component during the low-hard state for a sample of black hole X-ray binaries. We investigate the characteristic frequency and amplitude of the band-limited noise component and study covariance spectra. For observations that show a noise component with a characteristic frequency above 1 Hz in the hard energy band (4-8 keV), we found this very same component at a lower frequency in the soft band (1-2 keV). This difference in characteristic frequency is an indication that while both the soft and the hard band photons contribute to the same band-limited noise component, which likely represents the modulation of the mass accretion rate, the origin of the soft photons is actually further away from the black hole than the hard photons. Thus, the soft photons are characterized by larger radii, lower frequencies and softer energies, and are probably associated with a smaller optical depth for Comptonization up-scattering from the outer layer of the corona, or suggest a temperature gradient of the corona. We interpret this energy dependence within the picture of energy-dependent power density states as a hint that the contribution of the up-scattered photons originating in the outskirts of the Comptonizing corona to the overall emission in the soft band is becoming significant.

Theoretical investigation of structural, electronic and optical properties of MgxBa1-xS, MgxBa1-xSe and MgxBa1-xTe ternary alloys using DFT based FP-LAPW approach

NASA Astrophysics Data System (ADS)

Bhattacharjee, Rahul; Chattopadhyaya, Surya

2017-11-01

Density functional theory (DFT) based full-potential linearized augmented plane wave (FP-LAPW) methodology has been employed to investigate theoretically the structural, electronic and optical properties of MgxBa1-xS, MgxBa1-xSe and MgxBa1-xTe ternary alloys for 0 ≤ x ≤ 1 in their rock-salt (B1) crystallographic phase. The exchange-correlation potentials for the structural properties have been computed using the Wu-Cohen generalized-gradient approximation (WC-GGA) scheme, while those for the electronic and optical properties have been computed using both the WC-GGA and the recently developed Tran-Blaha modified Becke-Johnson (TB-mBJ) schemes. The thermodynamic stability of all the ternary alloys have been investigated by calculating their respective enthalpy of formation. The atomic and orbital origin of different electronic states in the band structure of the compounds have been identified from the respective density of states (DOS). Using the approach of Zunger and co-workers, the microscopic origin of band gap bowing has been discussed in term of volume deformation, charge exchange and structural relaxation. Bonding characteristics among the constituent atoms of each of the specimens have been discussed from their charge density contour plots. Optical properties of the binary compounds and ternary alloys have been investigated theoretically in terms of their respective dielectric function, refractive index, normal incidence reflectivity and optical conductivity. Several calculated results have been compared with available experimental and other theoretical data.

Lattice dynamics, phase transition, and tunable fundamental band gap of photovoltaic (K,Ba)(Ni,Nb)O3 -δ ceramics from spectral measurements and first-principles calculations

NASA Astrophysics Data System (ADS)

Li, Chuanqian; Wang, Fang; Sun, Yuyun; Jiang, Kai; Gong, Shijing; Hu, Zhigao; Zhou, Zhiyong; Dong, Xianlin; Chu, Junhao

2018-03-01

Ferroelectrics have long been recognized as one of the candidate class of materials for applications in photovoltaic devices. Recently, ferroelectric perovskite (K,Ba) (Ni,Nb) O3 -δ has been successfully synthesized and demonstrated to have a near-optimal band gap (1.39 eV), exhibiting good photovoltaic performance. However, the connection between the structural order-disorder transformation, electronic structure, bulk photovoltaic, and photocatalytic properties remains not well understood. Here, we investigate the phase transition evolutions of lead-free [KNbO3]1-x[BaNi1/2Nb1/2O3 -δ] x (KBNNO x , x =0 -0.5 ) ceramics via x-ray diffraction (XRD), Raman scattering, and computational evidences. The lattice dynamics and the origin of the successive rhombohedral→orthorhombic→tetragonal→cubic phase transitions have been systemically explored based on temperature-dependent XRD peak positions and phonon modes under different geometries. Moreover, the differences in the phase transition temperature and interior structure between the solid solution x =0.2 and the end member x =0 highlight local and nonlocal characteristics, which are helpful for understanding the photovoltaic mechanisms. Additionally, the robust photocatalytic decoloration effect on methylene blue can further confirm the photon-generated carrier behavior in the partly structural disordered orthorhombic phase. This identification of structural phases, combined with the ability to perform photocatalytic decoloration, give some insights on promising oxide applications as semiconducting ferroelectric absorbers and carrier-separating layers in photocatalytic or photovoltaic devices.

Independence of solitary-cation properties on the atomic neighborhood in In1 -xGaxN alloys: A novel perspective for material engineering

NASA Astrophysics Data System (ADS)

Filippone, Francesco; Mattioli, Giuseppe; Amore Bonapasta, Aldo

2017-11-01

In InN, a genuine band gap opening observed after hydrogenation has been explained by means of the "solitary cation" model, a multi-H complex in which the central cation, In*, is fully separated from the structure [Pettinari et al., Adv. Funct. Mater. 25, 5353 (2015), 10.1002/adfm.201501858]. Similar effects of H on the host band gap have been observed in In-rich In1-xGaxN alloys. Paying attention to these materials, we have theoretically investigated the In* properties against three kinds of disorder, structural, compositional, and configurational, all of them possibly occurring in In1-xGaxN alloys. As a first major result we have found that a same, general solitary-cation model and mechanism explain the effects of hydrogenation on the electronic properties of both InN and In-rich In1-xGaxN alloys. Even more interestingly, in these alloys, both the energetics of the In* solitary cations and their effects on the band gap result to be thoroughly independent of their atomic neighborhood, in particular, of the number and spatial distribution of their cation neighbors. Significantly, this implies that band-gap opening effects can be safely predicted in whatever hydrogenated In-rich nitride alloy containing different In companions (e.g., B, Al, or Ga) as well as in InN-containing, unconventional compounds (e.g., ZnO-InN), thus offering novel opportunities for material engineering.

Spectroscopic and theoretical investigation of the electronic states of layered perovskite oxyfluoride S r2Ru O3F2 thin films

NASA Astrophysics Data System (ADS)

Chikamatsu, Akira; Kurauchi, Yuji; Kawahara, Keisuke; Onozuka, Tomoya; Minohara, Makoto; Kumigashira, Hiroshi; Ikenaga, Eiji; Hasegawa, Tetsuya

2018-06-01

We investigated the electronic structure of a layered perovskite oxyfluoride S r2Ru O3F2 thin film by hard x-ray photoemission spectroscopy (HAXPES) and soft x-ray absorption spectroscopy (XAS) as well as density functional theory (DFT)-based calculations. The core-level HAXPES spectra suggested that S r2Ru O3F2 is a Mott insulator. The DFT calculations described the total and site-projected density of states and the band dispersion for the optimized crystal structure of S r2Ru O3F2 , predicting that R u4 + takes a high-spin configuration of (xy ) ↑(yz ,z x ) ↑↑(3z2-r2 ) ↑ and that S r2Ru O3F2 has an indirect band gap of 0.7 eV with minima at the M ,A and X ,R points. HAXPES spectra near the Fermi level and the angular-dependent O 1 s XAS spectra of the S r2Ru O3F2 thin film, corresponding to the valence band and conduction band density of states, respectively, were drastically different compared to those of the S r2Ru O4 film, suggesting that the changes in the electronic states were mainly driven by the substitution of an oxygen atom coordinated to Ru by fluorine and subsequent modification of the crystal field.

Nanomodulated electron beams via electron diffraction and emittance exchange for coherent x-ray generation

DOE PAGES

Nanni, E. A.; Graves, W. S.; Moncton, D. E.

2018-01-19

We present a new method for generation of relativistic electron beams with current modulation on the nanometer scale and below. The current modulation is produced by diffracting relativistic electrons in single crystal Si, accelerating the diffracted beam and imaging the crystal structure, then transferring the image into the temporal dimension via emittance exchange. The modulation period can be tuned by adjusting electron optics after diffraction. This tunable longitudinal modulation can have a period as short as a few angstroms, enabling production of coherent hard x-rays from a source based on inverse Compton scattering with total accelerator length of approximately tenmore » meters. Electron beam simulations from cathode emission through diffraction, acceleration, and image formation with variable magnification are presented along with estimates of the coherent x-ray output properties.« less

Nanomodulated electron beams via electron diffraction and emittance exchange for coherent x-ray generation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nanni, E. A.; Graves, W. S.; Moncton, D. E.

We present a new method for generation of relativistic electron beams with current modulation on the nanometer scale and below. The current modulation is produced by diffracting relativistic electrons in single crystal Si, accelerating the diffracted beam and imaging the crystal structure, then transferring the image into the temporal dimension via emittance exchange. The modulation period can be tuned by adjusting electron optics after diffraction. This tunable longitudinal modulation can have a period as short as a few angstroms, enabling production of coherent hard x-rays from a source based on inverse Compton scattering with total accelerator length of approximately tenmore » meters. Electron beam simulations from cathode emission through diffraction, acceleration, and image formation with variable magnification are presented along with estimates of the coherent x-ray output properties.« less

Microstructure, electronic structure and optical properties of combustion synthesized Co doped ZnO nanoparticles

NASA Astrophysics Data System (ADS)

Srinatha, N.; Nair, K. G. M.; Angadi, Basavaraj

2015-10-01

We report on the microstructure, electronic structure and optical properties of nanocrystalline Zn1-xCoxO (x=0, 0.01, 0.03, 0.05 and 0.07) particles prepared by solution combustion technique using L-Valine as fuel. The detailed structural and micro-structural studies were carried out by XRD, HRTEM and TEM-SAED respectively, which confirms the formation of single phased, nano-sized particles. The electronic structure was determined through NEXAFS and atomic multiplet calculations/simulations performed for various symmetries and valence states of 'Co' to determine the valance state, symmetry and crystal field splitting. The correlations between the experimental NEXAFS spectra and atomic multiplet simulations, confirms that, 'Co' present is in the 2+ valence state and substituted at the 'Zn' site in tetrahedral symmetry with crystal field splitting, 10Dq =-0.6 eV. The optical properties and 'Co' induced defect formation of as-synthesized materials were examined by using diffuse reflectance and Photoluminescence spectroscopy, respectively. Red-shift of band gap energy (Eg) was observed in Zn1-xCoxO samples due to Co (0.58 Å) substitution at Zn (0.60 Å) site of the host ZnO. Also, in PL spectra, a prominent pre-edge peak corresponds to ultraviolet (UV) emission around 360-370 nm was observed with Co concentration along with near band edge emission (NBE) of the wide band gap ZnO and all samples show emission in the blue region.

Copper Vacancies and Heavy Holes in the Two-Dimensional Semiconductor KCu 3–xSe 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rettie, Alexander J. E.; Sturza, Mihai; Malliakas, Christos D.

The two-dimensional material KCu 3–xSe 2 was synthesized using both a K 2Se 3 flux and directly from the elements. It crystallizes in the CsAg 3S 2 structure (monoclinic space group C2/m with a = 15.417(3) Å, b = 4.0742(8) Å, c = 8.3190(17) Å, and β = 112.94(3)°), and single-crystal refinement revealed infinite copper-deficient [Cu 3–xSe 2]– layers separated by K + ions. Thermal analysis indicated that KCu 3–xSe 2 melts congruently at ~755 °C. UV–vis spectroscopy showed an optical band gap of ~1.35 eV that is direct in nature, as confirmed by electronic structure calculations. Electronic transport measurementsmore » on single crystals yielded an in-plane resistivity of ~6 × 10 –1 Ω cm at 300 K that has a complex temperature dependence. The results of Seebeck coefficient measurements were consistent with a doped p-type semiconductor (S = +214 μV K –1 at 300 K), with doping being attributed to copper vacancies. Transport is dominated by low-mobility (on the order of 1 cm 2 V –1 s –1) holes caused by relatively flat valence bands with substantial Cu 3d character and a significant concentration of Cu ion vacancy defects (p ~ 10 19 cm –3) in this material. In conclusion, electronic band structure calculations showed that electrons should be significantly more mobile in this structure type.« less

Copper Vacancies and Heavy Holes in the Two-Dimensional Semiconductor KCu 3–xSe 2

DOE PAGES

Rettie, Alexander J. E.; Sturza, Mihai; Malliakas, Christos D.; ...

2017-06-21

The two-dimensional material KCu 3–xSe 2 was synthesized using both a K 2Se 3 flux and directly from the elements. It crystallizes in the CsAg 3S 2 structure (monoclinic space group C2/m with a = 15.417(3) Å, b = 4.0742(8) Å, c = 8.3190(17) Å, and β = 112.94(3)°), and single-crystal refinement revealed infinite copper-deficient [Cu 3–xSe 2]– layers separated by K + ions. Thermal analysis indicated that KCu 3–xSe 2 melts congruently at ~755 °C. UV–vis spectroscopy showed an optical band gap of ~1.35 eV that is direct in nature, as confirmed by electronic structure calculations. Electronic transport measurementsmore » on single crystals yielded an in-plane resistivity of ~6 × 10 –1 Ω cm at 300 K that has a complex temperature dependence. The results of Seebeck coefficient measurements were consistent with a doped p-type semiconductor (S = +214 μV K –1 at 300 K), with doping being attributed to copper vacancies. Transport is dominated by low-mobility (on the order of 1 cm 2 V –1 s –1) holes caused by relatively flat valence bands with substantial Cu 3d character and a significant concentration of Cu ion vacancy defects (p ~ 10 19 cm –3) in this material. In conclusion, electronic band structure calculations showed that electrons should be significantly more mobile in this structure type.« less

Electronic and optical properties of α-InX (X = S, Se and Te) monolayer: Under strain conditions

NASA Astrophysics Data System (ADS)

Jalilian, Jaafar; Safari, Mandana

2017-04-01

Using ab initio study, the structural, electronic and optical properties of α-InX (X = S, Se and Te) are investigated under tensile and compressive strain conditions. The results illustrate that exerting biaxial tensile and compressive strain conditions can lead to a tunable energy gap with a linear trend. The shape of valence band maximum (VBM) and conduction band minimum (CBM) is so sensitive to applying tensile and compressive strain. Besides, a shift in optical spectra toward shorter wavelength (blue shift) occurs under compression. The exerting tensile strain, on the other hand, gives rise to a red shift in optical spectra correspondingly. The results have been presented that InX monolayers can be good candidates for optoelectronic applications as well.

Infrared spectral studies of pre- and post-irradiated Li{sub 0.5(1+x)}Ti{sub x}Al{sub 0.1}Fe{sub 2.4-1.5x}O{sub 4} system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chhantbar, M. C.; Joshi, H. H., E-mail: joshihh@gmail.com

2015-06-24

The consequences of 50 MeV Li{sup 3+} ions irradiation on the structural behaviour of polycrystalline samples of spinel ferrite system Li{sub 0.5(1+x)}Ti{sub x}Al{sub 0.1}Fe{sub 2.4-1.5x}O{sub 4} are investigated by using infrared absorption spectroscopy. The IR spectrum showed two main absorption bands ν{sub 1} and ν{sub 2} in the range 400-700 cm{sup −1} arising from tetrahedral and octahedral interstitial sites in the spinel lattice. The shifting and splitting observed in IR frequency bands ν{sub 1} and ν{sub 2} for tetrahedral and octahedral sites under irradiation effect are ascribed to redistribution of cations.