Millimeter wave coherent synchrotron radiation in a compact electron storage ring

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

Murphy, J.B.; Blum, E.; Heese, R.

1998-01-01

Installation of a 2,856 MHz RF system into the XLS compact electron storage ring would allow the generation of millimeter wave coherent synchrotron radiation. Operating at 150 MeV, one could produce bunches containing on the order of 2 {times} 10{sup 7} electrons with a bunch length {sigma}{sub L0} = 0.3 mm, resulting in coherent emission at wavelengths above 0.8 mm. The characteristics of the source and the emitted radiation are discussed. In the case of 100 mrad horizontal collection angle, the average power radiated in the wavelength band 1 mm {le} {lambda} {le} 2 mm is 0.3 mW for singlemore » bunch operation and 24 mW for 80 bunch operation. The peak power in a single pulse of a few picosecond duration is on the order of one watt. By reducing the momentum compaction, the bunch length could be reduced to {sigma}{sub L0} = 0.15 mm, resulting in coherent synchrotron radiation down to 500 {micro}m.« less

Compact localized states and flat-band generators in one dimension

NASA Astrophysics Data System (ADS)

Maimaiti, Wulayimu; Andreanov, Alexei; Park, Hee Chul; Gendelman, Oleg; Flach, Sergej

2017-03-01

Flat bands (FB) are strictly dispersionless bands in the Bloch spectrum of a periodic lattice Hamiltonian, recently observed in a variety of photonic and dissipative condensate networks. FB Hamiltonians are fine-tuned networks, still lacking a comprehensive generating principle. We introduce a FB generator based on local network properties. We classify FB networks through the properties of compact localized states (CLS) which are exact FB eigenstates and occupy U unit cells. We obtain the complete two-parameter FB family of two-band d =1 networks with nearest unit cell interaction and U =2 . We discover a novel high symmetry sawtooth chain with identical hoppings in a transverse dc field, easily accessible in experiments. Our results pave the way towards a complete description of FBs in networks with more bands and in higher dimensions.

Direct measurement of the thickness-dependent electronic band structure of MoS2 using angle-resolved photoemission spectroscopy.

PubMed

Jin, Wencan; Yeh, Po-Chun; Zaki, Nader; Zhang, Datong; Sadowski, Jerzy T; Al-Mahboob, Abdullah; van der Zande, Arend M; Chenet, Daniel A; Dadap, Jerry I; Herman, Irving P; Sutter, Peter; Hone, James; Osgood, Richard M

2013-09-06

We report on the evolution of the thickness-dependent electronic band structure of the two-dimensional layered-dichalcogenide molybdenum disulfide (MoS2). Micrometer-scale angle-resolved photoemission spectroscopy of mechanically exfoliated and chemical-vapor-deposition-grown crystals provides direct evidence for the shifting of the valence band maximum from Γ to K, for the case of MoS2 having more than one layer, to the case of single-layer MoS2, as predicted by density functional theory. This evolution of the electronic structure from bulk to few-layer to monolayer MoS2 had earlier been predicted to arise from quantum confinement. Furthermore, one of the consequences of this progression in the electronic structure is the dramatic increase in the hole effective mass, in going from bulk to monolayer MoS2 at its Brillouin zone center, which is known as the cause for the decreased carrier mobility of the monolayer form compared to that of bulk MoS2.

Design of an electron-accelerator-driven compact neutron source for non-destructive assay

NASA Astrophysics Data System (ADS)

Murata, A.; Ikeda, S.; Hayashizaki, N.

2017-09-01

The threat of nuclear and radiological terrorism remains one of the greatest challenges to international security, and the threat is constantly evolving. In order to prevent nuclear terrorism, it is important to avoid unlawful import of nuclear materials, such as uranium and plutonium. Development of technologies for non-destructive measurement, detection and recognition of nuclear materials is essential for control at national borders. At Tokyo Institute of Technology, a compact neutron source system driven by an electron-accelerator has been designed for non-destructive assay (NDA). This system is composed of a combination of an S-band (2.856 GHz) RF-gun, a tungsten target to produce photons by bremsstrahlung, a beryllium target, which is suitable for use in generating neutrons because of the low threshold energy of photonuclear reactions, and a moderator to thermalize the fast neutrons. The advantage of this system can accelerate a short pulse beam with a pulse width less than 1 μs which is difficult to produce by neutron generators. The amounts of photons and neutron produced by electron beams were simulated using the Monte Carlo simulation code PHITS 2.82. When the RF-gun is operated with an average electron beam current of 0.1 mA, it is expected that the neutron intensities are 1.19 × 109 n/s and 9.94 × 109 n/s for incident electron beam energies of 5 MeV and 10 MeV, respectively.

Compact two-beam push-pull free electron laser

DOEpatents

Hutton, Andrew [Yorktown, VA

2009-03-03

An ultra-compact free electron laser comprising a pair of opposed superconducting cavities that produce identical electron beams moving in opposite directions such that each set of superconducting cavities accelerates one electron beam and decelerates the other electron beam. Such an arrangement, allows the energy used to accelerate one beam to be recovered and used again to accelerate the second beam, thus, each electron beam is decelerated by a different structure than that which accelerated it so that energy exchange rather than recovery is achieved resulting in a more compact and highly efficient apparatus.

An ultra-thin compact polarization-independent hexa-band metamaterial absorber

NASA Astrophysics Data System (ADS)

Munaga, Praneeth; Bhattacharyya, Somak; Ghosh, Saptarshi; Srivastava, Kumar Vaibhav

2018-04-01

In this paper, an ultra-thin compact hexa-band metamaterial absorber has been presented using single layer of dielectric. The proposed design is polarization independent in nature owing to its fourfold symmetry and exhibits high angular stability up to 60° angles of incidences for both TE and TM polarizations. The structure is ultrathin in nature with 2 mm thickness, which corresponds to λ/11.4 ( λ is the operating wavelength with respect to the highest frequency of absorption). Six distinct absorption frequencies are obtained from the design, which can be distributed among three regions, namely lower band, middle band and higher band; each region consists of two closely spaced frequencies. Thereafter, the dimensions of the proposed structure are adjusted in such a way that bandwidth enhancement occurs at each region separately. Simultaneous bandwidth enhancements at middle and higher bands have also been achieved by proper optimization of the geometrical parameters. The structure with simultaneous bandwidth enhancements at X- and Ku-bands is later fabricated and the experimental absorptivity response is in agreement with the simulated one.

Compact localized states and flat bands from local symmetry partitioning

NASA Astrophysics Data System (ADS)

Röntgen, M.; Morfonios, C. V.; Schmelcher, P.

2018-01-01

We propose a framework for the connection between local symmetries of discrete Hamiltonians and the design of compact localized states. Such compact localized states are used for the creation of tunable, local symmetry-induced bound states in an energy continuum and flat energy bands for periodically repeated local symmetries in one- and two-dimensional lattices. The framework is based on very recent theorems in graph theory which are here employed to obtain a block partitioning of the Hamiltonian induced by the symmetry of a given system under local site permutations. The diagonalization of the Hamiltonian is thereby reduced to finding the eigenspectra of smaller matrices, with eigenvectors automatically divided into compact localized and extended states. We distinguish between local symmetry operations which commute with the Hamiltonian, and those which do not commute due to an asymmetric coupling to the surrounding sites. While valuable as a computational tool for versatile discrete systems with locally symmetric structures, the approach provides in particular a unified, intuitive, and efficient route to the flexible design of compact localized states at desired energies.

para-Azaquinodimethane: A Compact Quinodimethane Variant as an Ambient Stable Building Block for High-Performance Low Band Gap Polymers

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

Liu, Xuncheng; He, Bo; Anderson, Christopher L.

Quinoidal structures incorporating expanded para-quinodimethane (p-QM) units have garnered great interest as functional organic electronic, optical, and magnetic materials. The direct use of the compact p-QM unit as an electronic building block, however, has been inhibited by the high reactivity conveyed by its biradical character. Herein, we introduce a stable p-QM variant, namely p-azaquinodimethane (p-AQM), that incorporates nitrogen atoms in the central ring and alkoxy substituents on the periphery to increase the stability of the quinoidal structure. The succinct synthesis from readily available precursors leads to regio- and stereospecific p-AQMs that can be readily integrated into the backbone of conjugatedmore » polymers. The quinoidal character of the p-AQM unit endows the resulting polymers with narrow band gaps and high carrier transport mobilities. The study of a series of copolymers employing different numbers of thiophene units revealed an unconventional trend in band gaps, which is distinct from the widely adopted donor-acceptor approach to tuning the band gaps of conjugated polymers. Theoretical calculations have shed light on the nature of this trend, which may provide a unique class of conjugated polymers with promising optical and electronic properties.« less

para-Azaquinodimethane: A Compact Quinodimethane Variant as an Ambient Stable Building Block for High-Performance Low Band Gap Polymers

DOE PAGES

Liu, Xuncheng; He, Bo; Anderson, Christopher L.; ...

2017-05-24

Quinoidal structures incorporating expanded para-quinodimethane (p-QM) units have garnered great interest as functional organic electronic, optical, and magnetic materials. The direct use of the compact p-QM unit as an electronic building block, however, has been inhibited by the high reactivity conveyed by its biradical character. Herein, we introduce a stable p-QM variant, namely p-azaquinodimethane (p-AQM), that incorporates nitrogen atoms in the central ring and alkoxy substituents on the periphery to increase the stability of the quinoidal structure. The succinct synthesis from readily available precursors leads to regio- and stereospecific p-AQMs that can be readily integrated into the backbone of conjugatedmore » polymers. The quinoidal character of the p-AQM unit endows the resulting polymers with narrow band gaps and high carrier transport mobilities. The study of a series of copolymers employing different numbers of thiophene units revealed an unconventional trend in band gaps, which is distinct from the widely adopted donor-acceptor approach to tuning the band gaps of conjugated polymers. Theoretical calculations have shed light on the nature of this trend, which may provide a unique class of conjugated polymers with promising optical and electronic properties.« less

Compact Dual-Band Bandpass Filter Using Stubs Loaded Ring Resonator

NASA Astrophysics Data System (ADS)

Xu, Jin

2016-01-01

This paper presents a novel second-order dual-band bandpass filter (BPF) by using proposed stubs loaded ring resonator. The resonant behavior of proposed stubs loaded ring resonator is analyzed by even-/odd-mode method, which shows its multiple-mode resonant characteristic. Parameters sweep is done so as to give the design guidelines. As an example, a second-order dual-band BPF operating at 1.8/5.2 GHz for GSM and WLAN applications is designed, fabricated and measured. The fabricated filter has a very compact size of 0.05λg×0.15λg. Measured results also show that the proposed dual-band BPF has a better than 20 dB rejection upper stopband from 5.47 GHz to 12.56 GHz. Good agreement is shown between the simulated and measured results.

Compact Short-Pulsed Electron Linac Based Neutron Sources for Precise Nuclear Material Analysis

NASA Astrophysics Data System (ADS)

Uesaka, M.; Tagi, K.; Matsuyama, D.; Fujiwara, T.; Dobashi, K.; Yamamoto, M.; Harada, H.

2015-10-01

An X-band (11.424GHz) electron linac as a neutron source for nuclear data study for the melted fuel debris analysis and nuclear security in Fukushima is under development. Originally we developed the linac for Compton scattering X-ray source. Quantitative material analysis and forensics for nuclear security will start several years later after the safe settlement of the accident is established. For the purpose, we should now accumulate more precise nuclear data of U, Pu, etc., especially in epithermal (0.1-10 eV) neutrons. Therefore, we have decided to modify and install the linac in the core space of the experimental nuclear reactor "Yayoi" which is now under the decommission procedure. Due to the compactness of the X-band linac, an electron gun, accelerating tube and other components can be installed in a small space in the core. First we plan to perform the time-of-flight (TOF) transmission measurement for study of total cross sections of the nuclei for 0.1-10 eV energy neutrons. Therefore, if we adopt a TOF line of less than 10m, the o-pulse length of generated neutrons should be shorter than 100 ns. Electronenergy, o-pulse length, power, and neutron yield are ~30 MeV, 100 ns - 1 micros, ~0.4 kW, and ~1011 n/s (~103 n/cm2/s at samples), respectively. Optimization of the design of a neutron target (Ta, W, 238U), TOF line and neutron detector (Ce:LiCAF) of high sensitivity and fast response is underway. We are upgrading the electron gun and a buncher to realize higher current and beam power with a reasonable beam size in order to avoid damage of the neutron target. Although the neutron flux is limited in case of the X-band electron linac based source, we take advantage of its short pulse aspect and availability for nuclear data measurement with a short TOF system. First, we form a tentative configuration in the current experimental room for Compton scattering in 2014. Then, after the decommissioning has been finished, we move it to the "Yayoi" room and perform

A Compact 600 GHz Electronically Tunable Vector Measurement System for Submillimeter Wave Imaging

NASA Technical Reports Server (NTRS)

Dengler, Robert J.; Maiwald, Frank; Siegel, Peter H.

2006-01-01

A compact submillimeter wave transmission / reflection measurement system has been demonstrated at 560-635 GHz, with electronic tuning over the entire band. Maximum dynamic range measured at a single frequency is 90 dB (60 dB typical), and phase noise is less than +/- 2(deg). By using a frequency steerable lens at the source output and mixer input, the frequency agility of the system can be used to scan the source and receive beams, resulting in near real-time imaging capability using only a single pixel.

Ka-band Technologies for Small Spacecraft Communications via Relays and Direct Data Downlink

NASA Technical Reports Server (NTRS)

Budinger, James M.; Niederhaus, Charles; Reinhart, Richard; Downey, Joe; Roberts, Anthony

2016-01-01

As the scientific capabilities and number of small spacecraft missions in the near Earth region increase, standard yet configurable user spacecraft terminals operating in Ka-band are needed to lower mission cost and risk and enable significantly higher data return than current UHF or S-band terminals. These compact Ka-band terminals are intended to operate with both the current and next generation of Ka-band relay satellites and via direct data communications with near Earth tracking terminals. This presentation provides an overview of emerging NASA-sponsored and commercially provided technologies in software defined radios (SDRs), transceivers, and electronically steered antennas that will enable data rates from hundreds of kbps to over 1 Gbps and operate in multiple frequency bands (such as S- and X-bands) and expand the use of NASA's common Ka-bands frequencies: 22.55-23.15 GHz for forward data or uplink; and 25.5-27.0 GHz for return data or downlink. Reductions in mass, power and volume come from integration of multiple radio functions, operations in Ka-band, high efficiency amplifiers and receivers, and compact, flat and vibration free electronically steered narrow beam antennas for up to + 60 degrees field of regard. The software defined near Earth space transceiver (SD-NEST) described in the presentation is intended to be compliant with NASA's space telecommunications radio system (STRS) standard for communications waveforms and hardware interoperability.

Optical and electronic properties of 2 H -Mo S2 under pressure: Revealing the spin-polarized nature of bulk electronic bands

NASA Astrophysics Data System (ADS)

Brotons-Gisbert, Mauro; Segura, Alfredo; Robles, Roberto; Canadell, Enric; Ordejón, Pablo; Sánchez-Royo, Juan F.

2018-05-01

Monolayers of transition-metal dichalcogenide semiconductors present spin-valley locked electronic bands, a property with applications in valleytronics and spintronics that is usually believed to be absent in their centrosymmetric (as the bilayer or bulk) counterparts. Here we show that bulk 2 H -Mo S2 hides a spin-polarized nature of states determining its direct band gap, with the spin sequence of valence and conduction bands expected for its single layer. This relevant finding is attained by investigating the behavior of the binding energy of A and B excitons under high pressure, by means of absorption measurements and density-functional-theory calculations. These results raise an unusual situation in which bright and dark exciton degeneracy is naturally broken in a centrosymmetric material. Additionally, the phonon-assisted scattering process of excitons has been studied by analyzing the pressure dependence of the linewidth of discrete excitons observed at the absorption coefficient edge of 2 H -Mo S2 . Also, the pressure dependence of the indirect optical transitions of bulk 2 H -Mo S2 has been analyzed by absorption measurements and density-functional-theory calculations. These results reflect a progressive closure of the indirect band gap as pressure increases, indicating that metallization of bulk Mo S2 may occur at pressures higher than 26 GPa.

Adjustment of Conduction Band Edge of Compact TiO2 Layer in Perovskite Solar Cells Through TiCl4 Treatment.

PubMed

Murakami, Takurou N; Miyadera, Tetsuhiko; Funaki, Takashi; Cojocaru, Ludmila; Kazaoui, Said; Chikamatsu, Masayuki; Segawa, Hiroshi

2017-10-25

Perovskite solar cells (PSCs) without a mesoporous TiO 2 layer, that is, planar-type PSCs exhibit poorer cell performance as compared to PSCs with a porous TiO 2 layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO 2 layer in the former case. The matching of the conduction band levels of perovskite and the compact TiO 2 layer is thus essential for enhancing PSC performance. In this study, we demonstrate the shifting of the conduction band edge (CBE) of the compact TiO 2 layer through a TiCl 4 treatment, with the aim of improving PSC performance. The CBE of the compact TiO 2 layer was shifted to a higher level through the TiCl 4 treatment and then shifted in the opposite direction, that is, to a lower level, through a subsequent heat treatment. These shifts in the CBE were reflected in the PSC performance. The TiCl 4 -treated PSC showed an increase in the open-circuit voltage of more than 150 mV, as well as a decrease of 100 mV after being heated at 450 °C. On the other hand, the short-circuit current decreased after the treatment but increased after heating at temperatures higher than 300 °C. The treated PSC subjected to subsequent heating at 300 °C exhibited the best performance, with the power conversion efficiency of the PSC being 17% under optimized conditions.

Electronic and spin structure of the wide-band-gap topological insulator: Nearly stoichiometric Bi2Te2S

NASA Astrophysics Data System (ADS)

Annese, E.; Okuda, T.; Schwier, E. F.; Iwasawa, H.; Shimada, K.; Natamane, M.; Taniguchi, M.; Rusinov, I. P.; Eremeev, S. V.; Kokh, K. A.; Golyashov, V. A.; Tereshchenko, O. E.; Chulkov, E. V.; Kimura, A.

2018-05-01

We have grown the phase-homogeneous ternary compound with composition Bi2Te1.85S1.15 very close to the stoichiometric Bi2Te2S . The measurements performed with spin- and angle-resolved photoelectron spectroscopy as well as density functional theory and G W calculations revealed a wide-band-gap three-dimensional topological insulator phase. The surface electronic spectrum is characterized by the topological surface state (TSS) with Dirac point located above the valence band and Fermi level lying in the band gap. TSS band dispersion and constant energy contour manifest a weak warping effect near the Fermi level along with in-plane and out-of-plane spin polarization along the Γ ¯-K ¯ line. We identified four additional states at deeper binding energies with high in-plane spin polarization.

Development of an S-band cavity-type beam position monitor for a high power THz free-electron laser

NASA Astrophysics Data System (ADS)

Noh, Seon Yeong; Kim, Eun-San; Hwang, Ji-Gwang; Heo, A.; won Jang, Si; Vinokurov, Nikolay A.; Jeong, Young UK; Hee Park, Seong; Jang, Kyu-Ha

2015-01-01

A cavity-type beam position monitor (BPM) has been developed for a compact terahertz (THz) free-electron laser (FEL) system and ultra-short pulsed electron Linac system at the Korea Atomic Energy Research Institute (KAERI). Compared with other types of BPMs, the cavity-type BPM has higher sensitivity and faster response time even at low charge levels. When electron beam passes through the cavity-type BPM, it excites the dipole mode of the cavity of which amplitude depends linearly on the beam offset from the center of the cavity. Signals from the BPM were measured as a function of the beam offset by using an oscilloscope. The microtron accelerator for the KAERI THz FEL produces the electron beam with an energy of 6.5 MeV and pulse length of 5 μs with a micropulse of 10-20 ps at the frequency of 2.801 GHz. The macropulse beam current is 40 mA. Because the microtron provides multi-bunch system, output signal would be the superposition of each single bunch. So high output signal can be obtained from superposition of each single bunch. The designed position resolution of the cavity-type BPM in multi-bunch is submicron. Our cavity-type BPM is made of aluminum and vacuum can be maintained by indium sealing without brazing process, resulting in easy modification and cost saving. The resonance frequency of the cavity-type BPM is 2.803 GHz and the cavity-type BPM dimensions are 200 × 220 mm (length × height) with a pipe diameter of 38 mm. The measured position sensitivity was 6.19 (mV/mm)/mA and the measured isolation between the X and Y axis was -39 dB. By measuring the thermal noise of system, position resolution of the cavity-type BPM was estimated to be less than 1 μm. In this article, we present the test results of the S-band cavity-type BPM and prove the feasibility of the beam position measurement with high resolution using this device.

Development of an S-band cavity-type beam position monitor for a high power THz free-electron laser.

PubMed

Noh, Seon Yeong; Kim, Eun-San; Hwang, Ji-Gwang; Heo, A; Jang, Si won; Vinokurov, Nikolay A; Jeong, Young U K; Park, Seong Hee; Jang, Kyu-Ha

2015-01-01

A cavity-type beam position monitor (BPM) has been developed for a compact terahertz (THz) free-electron laser (FEL) system and ultra-short pulsed electron Linac system at the Korea Atomic Energy Research Institute (KAERI). Compared with other types of BPMs, the cavity-type BPM has higher sensitivity and faster response time even at low charge levels. When electron beam passes through the cavity-type BPM, it excites the dipole mode of the cavity of which amplitude depends linearly on the beam offset from the center of the cavity. Signals from the BPM were measured as a function of the beam offset by using an oscilloscope. The microtron accelerator for the KAERI THz FEL produces the electron beam with an energy of 6.5 MeV and pulse length of 5 μs with a micropulse of 10-20 ps at the frequency of 2.801 GHz. The macropulse beam current is 40 mA. Because the microtron provides multi-bunch system, output signal would be the superposition of each single bunch. So high output signal can be obtained from superposition of each single bunch. The designed position resolution of the cavity-type BPM in multi-bunch is submicron. Our cavity-type BPM is made of aluminum and vacuum can be maintained by indium sealing without brazing process, resulting in easy modification and cost saving. The resonance frequency of the cavity-type BPM is 2.803 GHz and the cavity-type BPM dimensions are 200 × 220 mm (length × height) with a pipe diameter of 38 mm. The measured position sensitivity was 6.19 (mV/mm)/mA and the measured isolation between the X and Y axis was -39 dB. By measuring the thermal noise of system, position resolution of the cavity-type BPM was estimated to be less than 1 μm. In this article, we present the test results of the S-band cavity-type BPM and prove the feasibility of the beam position measurement with high resolution using this device.

Development of an S-band cavity-type beam position monitor for a high power THz free-electron laser

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

Noh, Seon Yeong; Kim, Eun-San, E-mail: eskim1@knu.ac.kr; Hwang, Ji-Gwang

2015-01-15

A cavity-type beam position monitor (BPM) has been developed for a compact terahertz (THz) free-electron laser (FEL) system and ultra-short pulsed electron Linac system at the Korea Atomic Energy Research Institute (KAERI). Compared with other types of BPMs, the cavity-type BPM has higher sensitivity and faster response time even at low charge levels. When electron beam passes through the cavity-type BPM, it excites the dipole mode of the cavity of which amplitude depends linearly on the beam offset from the center of the cavity. Signals from the BPM were measured as a function of the beam offset by using anmore » oscilloscope. The microtron accelerator for the KAERI THz FEL produces the electron beam with an energy of 6.5 MeV and pulse length of 5 μs with a micropulse of 10-20 ps at the frequency of 2.801 GHz. The macropulse beam current is 40 mA. Because the microtron provides multi-bunch system, output signal would be the superposition of each single bunch. So high output signal can be obtained from superposition of each single bunch. The designed position resolution of the cavity-type BPM in multi-bunch is submicron. Our cavity-type BPM is made of aluminum and vacuum can be maintained by indium sealing without brazing process, resulting in easy modification and cost saving. The resonance frequency of the cavity-type BPM is 2.803 GHz and the cavity-type BPM dimensions are 200 × 220 mm (length × height) with a pipe diameter of 38 mm. The measured position sensitivity was 6.19 (mV/mm)/mA and the measured isolation between the X and Y axis was −39 dB. By measuring the thermal noise of system, position resolution of the cavity-type BPM was estimated to be less than 1 μm. In this article, we present the test results of the S-band cavity-type BPM and prove the feasibility of the beam position measurement with high resolution using this device.« less

Electronic band structure study of colossal magnetoresistance in Tl 2Mn 2O 7

NASA Astrophysics Data System (ADS)

Seo, D.-K.; Whangbo, M.-H.; Subramanian, M. A.

1997-02-01

The electronic structure of Tl 2Mn 2O 7 was examined by performing tight binding band calculations. The overlap between the Mn t 2g- and Tl 6 s-block bands results in a partial filling of the Tl 6 s-block bands. The associated Fermi surface consists of 12 cigar-shape electron pockets with each electron pocket about {1}/{1000} of the first Brillouin zone in size. The Tl 6 s-block bands have orbital contributions from the Mn atoms, and the carrier density is very low. These are important for the occurrence of a colossal magnetoresistance in Tl 2Mn 2O 7.

Compaction bands in high temperature/pressure diagenetically altered unconventional shale gas reservoirs

NASA Astrophysics Data System (ADS)

Regenauer-Lieb, K.; Veveakis, M.; Poulet, T.

2014-12-01

Unconventional energy and mineral resources are typically trapped in a low porosity/permeability environment and are difficult to produce. An extreme end-member is the shale gas reservoir in the Cooper Basin (Australia) that is located at 3500-4000 m depth and ambient temperature conditions around 200oC. Shales of lacustrine origin (with high clay content) are diagenetically altered. Diagenesis involves fluid release mineral reactions of the general type Asolid ↔ Bsolid +Cfluid and switches on suddenly in the diagenetic window between 100-200oC. Diagenetic reactions can involve concentrations of smectite, aqueous silica compound, illite, potassium ions, aqueous silica, quartz, feldspar, kerogen, water and gas . In classical petroleum engineering such interlayer water/gas release reactions are considered to cause cementation and significantly reduce porosity and permeability. Yet in contradiction to the expected permeability reduction gas is successfully being produced. We propose that the success is based on the ductile equivalent of classical compaction bands in solid mechanics. The difference being that that the rate of the volumetric compaction is controlled by the diagenetic reactions. Ductile compaction bands are forming high porosity fluid channels rather than low porosity crushed grains in the solid mechanical equivalent. We show that this new type of volumetric instability appears in rate-dependent heterogenous materials as Cnoidal waves. These are nonlinear and exact periodic stationary waves, well known in the shallow water theory of fluid mechanics. Their distance is a direct function of the hydromechanical diffusivities. These instabilities only emerge in low permeability environment where the fluid diffusivity is about an order of magnitude lower than the mechanical loading. The instabilities are expected to be of the type as shown in the image below. The image shows a CT-scan of a laboratory experiment kindly provided by Papamichos (pers

Design of an S band narrow-band bandpass BAW filter

NASA Astrophysics Data System (ADS)

Gao, Yang; Zhao, Kun-li; Han, Chao

2017-11-01

An S band narrowband bandpass filter BAW with center frequency 2.460 GHz, bandwidth 41MHz, band insertion loss - 1.154 dB, the passband ripple 0.9 dB, the out of band rejection about -42.5dB@2.385 GHz; -45.5dB@2.506 GHz was designed for potential UAV measurement and control applications. According to the design specifications, the design is as follows: each FBAR's stack was designed in BAW filter by using Mason model. Each FBAR's shape was designed with the method of apodization electrode. The layout of BAW filter was designed. The acoustic-electromagnetic cosimulation model was built to validate the performance of the designed BAW filter. The presented design procedure is a common one, and there are two characteristics: 1) an A and EM co-simulation method is used for the final BAW filter performance validation in the design stage, thus ensures over-optimistic designs by the bare 1D Mason model are found and rejected in time; 2) An in-house developed auto-layout method is used to get compact BAW filter layout, which simplifies iterative error-and-try work here and output necessary in-plane geometry information to the A and EM cosimulation model.

Electronic band-gap modified passive silicon optical modulator at telecommunications wavelengths.

PubMed

Zhang, Rui; Yu, Haohai; Zhang, Huaijin; Liu, Xiangdong; Lu, Qingming; Wang, Jiyang

2015-11-13

The silicon optical modulator is considered to be the workhorse of a revolution in communications. In recent years, the capabilities of externally driven active silicon optical modulators have dramatically improved. Self-driven passive modulators, especially passive silicon modulators, possess advantages in compactness, integration, low-cost, etc. Constrained by a large indirect band-gap and sensitivity-related loss, the passive silicon optical modulator is scarce and has been not advancing, especially at telecommunications wavelengths. Here, a passive silicon optical modulator is fabricated by introducing an impurity band in the electronic band-gap, and its nonlinear optics and applications in the telecommunications-wavelength lasers are investigated. The saturable absorption properties at the wavelength of 1.55 μm was measured and indicates that the sample is quite sensitive to light intensity and has negligible absorption loss. With a passive silicon modulator, pulsed lasers were constructed at wavelengths at 1.34 and 1.42 μm. It is concluded that the sensitive self-driven passive silicon optical modulator is a viable candidate for photonics applications out to 2.5 μm.

Compact pulse transformer for 85 kV, 3.5 μs electron gun anode of compact X-ray cargo scanner

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

Patel, R.; Sharma, D.K.; Dixit, K.

Design of compact and reliable 85kV HV pulse transformer for electron gun anode pulsing is a major concern, when size and space are constraints. This paper describes design procedures and optimization of various parameters like HV insulation, step up ratio, rise time and flat top of Pulse transformer, operating with input from a 10 stage PFN of 50 ohm impedance and charged at 14kV. The transformer should deliver rated output voltage of negative polarity 85kV, 3 to 4μs pulse width, less than 2μs rise time and flat top within 10% across an electron gun load, equivalent to a parallel combinationmore » of 10kΩ and 200pF load at a PRF of 250 Hz. Since the Cargo Scanner has to operate on movable carrier, this transformer is designed to operate even in the inclined positions. This transformer has given voltage step up, rise time and flat top of 13.75, 1.5 μs and 4.5% respectively for a 10kΩ and 200pF load at 250Hz PRF and also demonstrated operation in 90{sup °} tilted transformer positions. An effort has been put to achieve maintenance free Pulse transformer by providing effective sealing in the transformer tank to stop breathing action. Also, special flexing walls of transformer tank accommodate for small changes in volume of oil due to temperature variations. (author)« less

Band-offset-induced lateral shift of valley electrons in ferromagnetic MoS2/WS2 planar heterojunctions

NASA Astrophysics Data System (ADS)

Ghadiri, Hassan; Saffarzadeh, Alireza

2018-03-01

Low-energy coherent transport and a Goos-Hänchen (GH) lateral shift of valley electrons in planar heterojunctions composed of normal MoS2 and ferromagnetic WS2 monolayers are theoretically investigated. Two types of heterojunctions in the forms of WS2/MoS2/WS2 (type-A) and MoS2/WS2/MoS2 (type-B) with incident electrons in the MoS2 region are considered in which the lateral shift of electrons is induced by band alignments of the two constituent semiconductors. It is shown that the type-A heterojunction can act as an electron waveguide due to electron confinement between the two WS2/MoS2 interfaces which cause the incident electrons with an appropriate incidence angle to propagate along the interfaces. In this case, the spin- and valley-dependent GH shifts of totally reflected electrons from the interface lead to separated electrons with distinct spin-valley indexes after traveling a sufficiently long distance. In the type-B heterojunction, however, transmission resonances occur for incident electron beams passing through the structure, and large spin- and valley-dependent lateral shift values in propagating states can be achieved. Consequently, the transmitted electrons are spatially well-separated into electrons with distinct spin-valley indexes. Our findings reveal that the planar heterojunctions of transition metal dichalcogenides can be utilized as spin-valley beam filters and/or splitters without external gating.

Mariner Venus Mercury 1973 S/X-band experiment

NASA Technical Reports Server (NTRS)

Levy, G. S.

1977-01-01

The S/X-band experiment on the Mariner Venus/Mercury 1973 spacecraft constituted a unique opportunity to demonstrate the capability of an X-band downlink coherent with the normal S-band downlink. This was both a technological and scientific experiment, and the results indicated that it was successful in both cases. Analysis of the tracking data shows that the new S/X data type was capable of reducing the miss distance at the planet Mercury by 80% (post-processed data). The use of S/X electron content was demonstrated by comparison with Faraday rotation data. An X-band turnaround telemetry experiment showed the feasibility of a planetary X-band link. In the science area, the model atmospheric environment of Venus was refined. The ionosphere of the planet was measured to a higher accuracy than before, and the value of the dual-frequency link for measuring the scale size of turbulence was demonstrated. The estimate of the scale size was increased from 100 m to above 5 km.

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

On Valence-Band Splitting in Layered MoS2.

PubMed

Zhang, Youwei; Li, Hui; Wang, Haomin; Liu, Ran; Zhang, Shi-Li; Qiu, Zhi-Jun

2015-08-25

As a representative two-dimensional semiconducting transition-metal dichalcogenide (TMD), the electronic structure in layered MoS2 is a collective result of quantum confinement, interlayer interaction, and crystal symmetry. A prominent energy splitting in the valence band gives rise to many intriguing electronic, optical, and magnetic phenomena. Despite numerous studies, an experimental determination of valence-band splitting in few-layer MoS2 is still lacking. Here, we show how the valence-band maximum (VBM) splits for one to five layers of MoS2. Interlayer coupling is found to contribute significantly to phonon energy but weakly to VBM splitting in bilayers, due to a small interlayer hopping energy for holes. Hence, spin-orbit coupling is still predominant in the splitting. A temperature-independent VBM splitting, known for single-layer MoS2, is, thus, observed for bilayers. However, a Bose-Einstein type of temperature dependence of VBM splitting prevails in three to five layers of MoS2. In such few-layer MoS2, interlayer coupling is enhanced with a reduced interlayer distance, but thermal expansion upon temperature increase tends to decouple adjacent layers and therefore decreases the splitting energy. Our findings that shed light on the distinctive behaviors about VBM splitting in layered MoS2 may apply to other hexagonal TMDs as well. They will also be helpful in extending our understanding of the TMD electronic structure for potential applications in electronics and optoelectronics.

Quasiparticle band structures and interface physics of SnS and GeS

NASA Astrophysics Data System (ADS)

Malone, Brad; Kaxiras, Efthimios

2013-03-01

Orthorhombic SnS and GeS are layered materials made of earth-abundant elements which have the potential to play a useful role in the massive scale up of renewable power necessary by 2050 to avoid unmanageable levels of climate change. We report on first principles calculations of the quasiparticle spectra of these two materials, predicting the type and magnitude of the fundamental band gap, a quantity which shows a strong degree of scatter in the experimental literature. Additionally, in order to evaluate the possible role of GeS as an electron-blocking layer in a SnS-based photovoltaic device, we investigate the band offsets of the interfaces between these materials along the three principle crystallographic directions. We find that while the valence-band offsets are similar along the three principle directions, the conduction-band offsets display a substantial amount of anisotropy.

A compact electron spectrometer for an LWFA.

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

Lumpkin, A.; Crowell, R.; Li, Y.

2007-01-01

The use of a laser wakefield accelerator (LWFA) beam as a driver for a compact free-electron laser (FEL) has been proposed recently. A project is underway at Argonne National Laboratory (ANL) to operate an LWFA in the bubble regime and to use the quasi-monoenergetic electron beam as a driver for a 3-m-long undulator for generation of sub-ps UV radiation. The Terawatt Ultrafast High Field Facility (TUHFF) in the Chemistry Division provides the 20-TW peak power laser. A compact electron spectrometer whose initial fields of 0.45 T provide energy coverage of 30-200 MeV has been selected to characterize the electron beams.more » The system is based on the Ecole Polytechnique design used for their LWFA and incorporates the 5-cm-long permanent magnet dipole, the LANEX scintillator screen located at the dispersive plane, a Roper Scientific 16-bit MCP-intensified CCD camera, and a Bergoz ICT for complementary charge measurements. Test results on the magnets, the 16-bit camera, and the ICT will be described, and initial electron beam data will be presented as available. Other challenges will also be addressed.« less

The Heidelberg compact electron beam ion traps

NASA Astrophysics Data System (ADS)

Micke, P.; Kühn, S.; Buchauer, L.; Harries, J. R.; Bücking, T. M.; Blaum, K.; Cieluch, A.; Egl, A.; Hollain, D.; Kraemer, S.; Pfeifer, T.; Schmidt, P. O.; Schüssler, R. X.; Schweiger, Ch.; Stöhlker, T.; Sturm, S.; Wolf, R. N.; Bernitt, S.; Crespo López-Urrutia, J. R.

2018-06-01

Electron beam ion traps (EBITs) are ideal tools for both production and study of highly charged ions (HCIs). In order to reduce their construction, maintenance, and operation costs, we have developed a novel, compact, room-temperature design, the Heidelberg Compact EBIT (HC-EBIT). Four already commissioned devices operate at the strongest fields (up to 0.86 T) reported for such EBITs using permanent magnets, run electron beam currents up to 80 mA, and energies up to 10 keV. They demonstrate HCI production, trapping, and extraction of pulsed Ar16+ bunches and continuous 100 pA ion beams of highly charged Xe up to charge state 29+, already with a 4 mA, 2 keV electron beam. Moreover, HC-EBITs offer large solid-angle ports and thus high photon count rates, e.g., in x-ray spectroscopy of dielectronic recombination in HCIs up to Fe24+, achieving an electron-energy resolving power of E/ΔE > 1500 at 5 keV. Besides traditional on-axis electron guns, we have also implemented a novel off-axis gun for laser, synchrotron, and free-electron laser applications, offering clear optical access along the trap axis. We report on its first operation at a synchrotron radiation facility demonstrating the resonant photoexcitation of highly charged oxygen.

A compact electron gun for time-resolved electron diffraction

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

Robinson, Matthew S.; Lane, Paul D.; Wann, Derek A., E-mail: derek.wann@york.ac.uk

A novel compact time-resolved electron diffractometer has been built with the primary goal of studying the ultrafast molecular dynamics of photoexcited gas-phase molecules. Here, we discuss the design of the electron gun, which is triggered by a Ti:Sapphire laser, before detailing a series of calibration experiments relating to the electron-beam properties. As a further test of the apparatus, initial diffraction patterns have been collected for thin, polycrystalline platinum samples, which have been shown to match theoretical patterns. The data collected demonstrate the focusing effects of the magnetic lens on the electron beam, and how this relates to the spatial resolutionmore » of the diffraction pattern.« less

Exploring the effect of band alignment and surface states on photoinduced electron transfer from CuInS2/CdS core/shell quantum dots to TiO2 electrodes.

PubMed

Sun, Mingye; Zhu, Dehua; Ji, Wenyu; Jing, Pengtao; Wang, Xiuying; Xiang, Weidong; Zhao, Jialong

2013-12-11

Photoinduced electron transfer (ET) processes from CuInS2/CdS core/shell quantum dots (QDs) with different core sizes and shell thicknesses to TiO2 electrodes were investigated by time-resolved photoluminescence (PL) spectroscopy. The ET rates and efficiencies from CuInS2/CdS QDs to TiO2 were superior to those of CuInS2/ZnS QDs. An enhanced ET efficiency was surprisingly observed for 2.0 nm CuInS2 core QDs after growth of the CdS shell. On the basis of the experimental and theoretical analysis, the improved performances of CuInS2/CdS QDs were attributed to the passivation of nonradiative traps by overcoating shell and enhanced delocalization of electron wave function from core to CdS shell due to lower conduction band offset. These results indicated that the electron distribution regulated by the band alignment between core and shell of QDs and the passivation of surface defect states could improve ET performance between donor and acceptor.

A Very Compact and Low Profile UWB Planar Antenna with WLAN Band Rejection.

PubMed

Syed, Avez; Aldhaheri, Rabah W

2016-01-01

A low-cost coplanar waveguide fed compact ultrawideband (UWB) antenna with band rejection characteristics for wireless local area network (WLAN) is proposed. The notch band characteristic is achieved by etching half wavelength C-shaped annular ring slot in the radiating patch. By properly choosing the radius and position of the slot, the notch band can be adjusted and controlled. With an overall size of 18.7 mm × 17.6 mm, the antenna turns out to be one of the smallest UWB antennas with band-notched characteristics. It has a wide fractional bandwidth of 130% (2.9-13.7 GHz) with VSWR < 2 and rejecting IEEE 802.11a and HIPERLAN/2 frequency band of 5.1-5.9 GHz. Stable omnidirectional radiation patterns in the H plane with an average gain of 4.4 dBi are obtained. The band-notch mechanism of the proposed antenna is examined by HFSS simulator. A good agreement is found between measured and simulated results indicating that the proposed antenna is well suited for integration into portable devices for UWB applications.

Porosity localizing instability in a compacting porous layer in a pure shear flow and the evolution of porosity band wavelength

NASA Astrophysics Data System (ADS)

Butler, S. L.

2010-09-01

A porosity localizing instability occurs in compacting porous media that are subjected to shear if the viscosity of the solid matrix decreases with porosity ( Stevenson, 1989). This instability may have significant consequences for melt transport in regions of partial melt in the mantle and may significantly modify the effective viscosity of the asthenosphere ( Kohlstedt and Holtzman, 2009). Most analyses of this instability have been carried out assuming an imposed simple shear flow (e.g., Spiegelman, 2003; Katz et al., 2006; Butler, 2009). Pure shear can be realized in laboratory experiments and studying the instability in a pure shear flow allows us to test the generality of some of the results derived for simple shear and the flow pattern for pure shear more easily separates the effects of deformation from rotation. Pure shear flows may approximate flows near the tops of mantle plumes near earth's surface and in magma chambers. In this study, we present linear theory and nonlinear numerical model results for a porosity and strain-rate weakening compacting porous layer subjected to pure shear and we investigate the effects of buoyancy-induced oscillations. The linear theory and numerical model will be shown to be in excellent agreement. We will show that melt bands grow at the same angles to the direction of maximum compression as in simple shear and that buoyancy-induced oscillations do not significantly inhibit the porosity localizing instability. In a pure shear flow, bands parallel to the direction of maximum compression increase exponentially in wavelength with time. However, buoyancy-induced oscillations are shown to inhibit this increase in wavelength. In a simple shear flow, bands increase in wavelength when they are in the orientation for growth of the porosity localizing instability. Because the amplitude spectrum is always dominated by bands in this orientation, band wavelengths increase with time throughout simple shear simulations until the

X-ray imaging of water motion during capillary imbibition: A study on how compaction bands impact fluid flow in Bentheim sandstone

NASA Astrophysics Data System (ADS)

Pons, A.; David, C.; Fortin, J.; Stanchits, S.; MenéNdez, B.; Mengus, J. M.

2011-03-01

To investigate the effect of compaction bands (CB) on fluid flow, capillary imbibition experiments were performed on Bentheim sandstone specimens (initial porosity ˜22.7%) using an industrial X-ray scanner. We used a three-step procedure combining (1) X-ray imaging of capillary rise in intact Bentheim sandstone, (2) formation of compaction band under triaxial tests, at 185 MPa effective pressure, with acoustic emissions (AE) recording for localization of the induced damage, and (3) again X-ray imaging of capillary rise in the damaged specimens after the unloading. The experiments were performed on intact cylindrical specimens, 5 cm in diameter and 10.5 cm in length, cored in different orientations (parallel or perpendicular to the bedding). Analysis of the images obtained at different stages of the capillary imbibition shows that the presence of CB slows down the imbibition and disturbs the geometry of water flow. In addition, we show that the CB geometry derived from X-ray density maps analysis is well correlated with the AE location obtained during triaxial test. The analysis of the water front kinetics was conducted using a simple theoretical model, which allowed us to confirm that compaction bands act as a barrier for fluid flow, not fully impermeable though. We estimate a contrast of permeability of a factor of ˜3 between the host rock and the compaction bands. This estimation of the permeability inside the compaction band is consistent with estimations done in similar sandstones from field studies but differs by 1 order of magnitude from estimations from previous laboratory measurements.

Multi-band Electronic Structure of Ferromagnetic CeRuPO

NASA Astrophysics Data System (ADS)

Takahashi, Masaya; Ootsuki, Daiki; Horio, Masafumi; Arita, Masashi; Namatame, Hirofumi; Taniguchi, Masaki; Saini, Naurang L.; Sugawara, Hitoshi; Mizokawa, Takashi

2018-04-01

We have studied the multi-band electronic structure of ferromagnetic CeRuPO (TC = 15 K) by means of angle-resolved photoemission spectroscopy (ARPES). The ARPES results show that three hole bands exist around the zone center and two of them cross the Fermi level (EF). Around the zone corner, two electron bands are observed and cross EF. These hole and electron bands, which can be assigned to the Ru 4d bands, are basically consistent with the band-structure calculation including their orbital characters. However, one of the electron bands with Ru 4d 3z2 - r2 character is strongly renormalized indicating correlation effect due to hybridization with the Ce 4f orbitals. The Ru 4d 3z2 - r2 band changes across TC suggesting that the out-of-plane 3z2 - r2 orbital channel plays essential roles in the ferromagnetism.

Shuttle Ku-band and S-band communications implementations study

NASA Technical Reports Server (NTRS)

Huth, G. K.; Nessibou, T.; Nilsen, P. W.; Simon, M. K.; Weber, C. L.

1979-01-01

The interfaces between the Ku-band system and the TDRSS, between the S-band system and the TDRSS, GSTDN and SGLS networks, and between the S-band payload communication equipment and the other Orbiter avionic equipment were investigated. The principal activities reported are: (1) performance analysis of the payload narrowband bent-pipe through the Ku-band communication system; (2) performance evaluation of the TDRSS user constraints placed on the S-band and Ku-band communication systems; (3) assessment of the shuttle-unique S-band TDRSS ground station false lock susceptibility; (4) development of procedure to make S-band antenna measurements during orbital flight; (5) development of procedure to make RFI measurements during orbital flight to assess the performance degradation to the TDRSS S-band communication link; and (6) analysis of the payload interface integration problem areas.

A megawatt-level surface wave oscillator in Y-band with large oversized structure driven by annular relativistic electron beam.

PubMed

Wang, Jianguo; Wang, Guangqiang; Wang, Dongyang; Li, Shuang; Zeng, Peng

2018-05-03

High power vacuum electronic devices of millimeter wave to terahertz regime are attracting extensive interests due to their potential applications in science and technologies. In this paper, the design and experimental results of a powerful compact oversized surface wave oscillator (SWO) in Y-band are presented. The cylindrical slow wave structure (SWS) with rectangular corrugations and large diameter about 6.8 times the radiation wavelength is proposed to support the surface wave interacting with annular relativistic electron beam. By choosing appropriate beam parameters, the beam-wave interaction takes place near the π-point of TM 01 mode dispersion curve, giving high coupling impedance and temporal growth rate compared with higher TM 0n modes. The fundamental mode operation of the device is verified by the particle-in-cell (PIC) simulation results, which also indicate its capability of tens of megawatts power output in the Y-band. Finally, a compact experimental setup is completed to validate our design. Measurement results show that a terahertz pulse with frequency in the range of 0.319-0.349 THz, duration of about 2 ns and radiation power of about 2.1 MW has been generated.

A compact D-band monolithic APDP-based sub-harmonic mixer

NASA Astrophysics Data System (ADS)

Zhang, Shengzhou; Sun, Lingling; Wang, Xiang; Wen, Jincai; Liu, Jun

2017-11-01

The paper presents a compact D-band monolithic sub-harmonic mixer (SHM) with 3 μm planar hyperabrupt schottky-varactor diodes offered by 70 nm GaAs mHEMT technology. According to empirical equivalent-circuit models, a wide-band large signal equivalent circuit model of the diode is proposed. Based on the extracted model, the mixer is implemented and optimized with a shunt-mounted anti-parallel diode pair (APDP) to fulfill the sub-harmonic mixing mechanism. Furthermore, a modified asymmetric three-transmission-line coupler is devised to achieve high-level coupling and minimize the chip size. The measured results show that the conversion gain varies between -13.9 dB and -17.5 dB from 110 GHz to 145 GHz, with a local oscillator (LO) power level of 14 dBm and an intermediate frequency (IF) of 1 GHz. The total chip size including probe GSG pads is 0.57 × 0.68mm2. In conclusion, the mixer exhibits outstanding figure-of-merits.

Core Levels, Band Alignments, and Valence-Band States in CuSbS 2 for Solar Cell Applications

DOE PAGES

Whittles, Thomas J.; Veal, Tim D.; Savory, Christopher N.; ...

2017-11-10

The earth-abundant material CuSbS 2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInxGa (1-x)Se 2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from themore » antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.« less

Core Levels, Band Alignments, and Valence-Band States in CuSbS2 for Solar Cell Applications.

PubMed

Whittles, Thomas J; Veal, Tim D; Savory, Christopher N; Welch, Adam W; de Souza Lucas, Francisco Willian; Gibbon, James T; Birkett, Max; Potter, Richard J; Scanlon, David O; Zakutayev, Andriy; Dhanak, Vinod R

2017-12-06

The earth-abundant material CuSbS 2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuIn x Ga (1-x) Se 2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.

A Very Compact and Low Profile UWB Planar Antenna with WLAN Band Rejection

PubMed Central

Syed, Avez; Aldhaheri, Rabah W.

2016-01-01

A low-cost coplanar waveguide fed compact ultrawideband (UWB) antenna with band rejection characteristics for wireless local area network (WLAN) is proposed. The notch band characteristic is achieved by etching half wavelength C-shaped annular ring slot in the radiating patch. By properly choosing the radius and position of the slot, the notch band can be adjusted and controlled. With an overall size of 18.7 mm × 17.6 mm, the antenna turns out to be one of the smallest UWB antennas with band-notched characteristics. It has a wide fractional bandwidth of 130% (2.9–13.7 GHz) with VSWR < 2 and rejecting IEEE 802.11a and HIPERLAN/2 frequency band of 5.1–5.9 GHz. Stable omnidirectional radiation patterns in the H plane with an average gain of 4.4 dBi are obtained. The band-notch mechanism of the proposed antenna is examined by HFSS simulator. A good agreement is found between measured and simulated results indicating that the proposed antenna is well suited for integration into portable devices for UWB applications. PMID:27088125

Frequency tunable electronic sources working at room temperature in the 1 to 3 THz band

NASA Astrophysics Data System (ADS)

Maestrini, Alain; Mehdi, Imran; Siles, José V.; Lin, Robert; Lee, Choonsup; Chattopadhyay, Goutam; Pearson, John; Siegel, Peter

2012-10-01

Compact, room temperature terahertz sources are much needed in the 1 to 3 THz band for developing multi-pixel heterodyne receivers for astrophysics and planetary science or for building short-range high spatial resolution THz imaging systems able to see through low water content and non metallic materials, smoke or dust for a variety of applications ranging from the inspection of art artifacts to the detection of masked or concealed objects. All solid-sate electronic sources based on a W-band synthesizer followed by a high-power W-band amplifier and a cascade of Schottky diode based THz frequency multipliers are now capable of producing more than 1 mW at 0.9THz, 50 μW at 2 THz and 18 μW at 2.6 THz without the need of any cryogenic system. These sources are frequency agile and have a relative bandwidth of 10 to 15%, limited by the high power W-band amplifiers. The paper will present the latest developments of this technology and its perspective in terms of frequency range, bandwidth and power.

Strain-induced enhancement of thermoelectric performance of TiS2 monolayer based on first-principles phonon and electron band structures

NASA Astrophysics Data System (ADS)

Li, Guanpeng; Yao, Kailun; Gao, Guoying

2018-01-01

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI2-type) structural TiS2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

Strain-induced enhancement of thermoelectric performance of TiS2 monolayer based on first-principles phonon and electron band structures.

PubMed

Li, Guanpeng; Yao, Kailun; Gao, Guoying

2018-01-05

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI 2 -type) structural TiS 2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS 2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

Compact triple band-stop filter using novel epsilon-shaped metamaterial with lumped capacitor

NASA Astrophysics Data System (ADS)

Ali, W. A. E.; Hamdalla, M. Z. M.

2018-04-01

This paper presents the design of a novel epsilon-shaped metamaterial unit cell structure that is applicable for single-band and multi-band applications. A closed-form formulas to control the resonance frequencies of the proposed design are included. The proposed unit cell, which exhibits negative permeability at its frequency bands, is etched from the ground plane to form a band-stop filter. The filter design is constructed to validate the band-notched characteristics of the proposed unit cell. A lumped capacitor is inserted for size reduction purpose in addition to multi-resonance generation. The fundamental resonance frequency is translated from 3.62 GHz to 2.45 GHz, which means that the filter size will be more compact (more than 32% size reduction). The overall size of the proposed filter is 13 × 6 × 1.524 mm3, where the electrical size is 0.221λg × 0.102λg × 0.026λg at the lower frequency band (2.45 GHz). Two other resonance frequencies are generated at 5.3 GHz and 9.2 GHz, which confirm the multi-band behavior of the proposed filter. Good agreement between simulated and measured characteristics of the fabricated filter prototype is achieved.

Electron currents associated with an auroral band

NASA Technical Reports Server (NTRS)

Spiger, R. J.; Anderson, H. R.

1975-01-01

Measurements of electron pitch angle distributions and energy spectra over a broad auroral band were used to calculate net electric current carried by auroral electrons in the vicinity of the band. The particle energy spectrometers were carried by a Nike-Tomahawk rocket launched from Poker Flat, Alaska, at 0722 UT on February 25, 1972. Data are presented which indicate the existence of upward field-aligned currents of electrons in the energy range 0.5-20 keV. The spatial relationship of these currents to visual structure of the auroral arc and the characteristics of the electrons carrying the currents are discussed.

Li-S batteries: Firing for compactness

NASA Astrophysics Data System (ADS)

Li, Yanguang; Chen, Fengjiao

2017-07-01

Conventional Li-S batteries have a non-compact cathode structure containing low areal loading of active materials. Now, a strategy of burning Li foils in a CS2 vapour is presented, which leads to the formation of highly compact Li2S nanoparticles as a lithiated sulfur cathode, offering promising battery performance.

Compaction dynamics of crunchy granular material

NASA Astrophysics Data System (ADS)

Guillard, François; Golshan, Pouya; Shen, Luming; Valdès, Julio R.; Einav, Itai

2017-06-01

Compaction of brittle porous material leads to a wide variety of densification patterns. Static compaction bands occurs naturally in rocks or bones, and have important consequences in industry for the manufacturing of powder tablets or metallic foams for example. Recently, oscillatory compaction bands have been observed in brittle porous media like snow or cereals. We will discuss the great variety of densification patterns arising during the compaction of puffed rice, including erratic compaction at low velocity, one or several travelling compaction bands at medium velocity and homogeneous compaction at larger velocity. The conditions of existence of each pattern are studied thanks to a numerical spring lattice model undergoing breakage and is mapped to the phase diagram of the patterns based on dimensionless characteristic quantities. This also allows to rationalise the evolution of the compaction behaviour during a single test. Finally, the localisation of compaction bands is linked to the strain rate sensitivity of the material.

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.

Compact low-cost detection electronics for optical coherence imaging

PubMed Central

Akcay, A. C.; Lee, K. S.; Furenlid, L. R.; Costa, M. A.; Rolland, J. P.

2015-01-01

A compact and low-cost detection electronics scheme for optical coherence imaging is demonstrated. The performance of the designed electronics is analyzed in comparison to a commercial lock-in amplifier of equal bandwidth. Images of a fresh-onion sample are presented for each detection configuration. PMID:26617422

Layer specific optical band gap measurement at nanoscale in MoS{sub 2} and ReS{sub 2} van der Waals compounds by high resolution electron energy loss spectroscopy

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

Dileep, K., E-mail: dileep@jncasr.ac.in, E-mail: ranjan@jncasr.ac.in; Sahu, R.; Datta, R., E-mail: dileep@jncasr.ac.in, E-mail: ranjan@jncasr.ac.in

2016-03-21

Layer specific direct measurement of optical band gaps of two important van der Waals compounds, MoS{sub 2} and ReS{sub 2}, is performed at nanoscale by high resolution electron energy loss spectroscopy. For monolayer MoS{sub 2}, the twin excitons (1.8 and 1.95 eV) originating at the K point of the Brillouin zone are observed. An indirect band gap of 1.27 eV is obtained from the multilayer regions. Indirect to direct band gap crossover is observed which is consistent with the previously reported strong photoluminescence from the monolayer MoS{sub 2}. For ReS{sub 2}, the band gap is direct, and a value of 1.52 andmore » 1.42 eV is obtained for the monolayer and multilayer, respectively. The energy loss function is dominated by features due to high density of states at both the valence and conduction band edges, and the difference in analyzing band gap with respect to ZnO is highlighted. Crystalline 1T ReS{sub 2} forms two dimensional chains like superstructure due to the clustering between four Re atoms. The results demonstrate the power of HREELS technique as a nanoscale optical absorption spectroscopy tool.« less

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.

Status of the Northrop Grumman Compact Infrared Free-Electron Laser

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

Lehrman, I.S.; Krishnaswamy, J.; Hartley, R.A.

1995-12-31

The Compact Infrared Free Electron Laser (CIRFEL) was built as part of a joint collaboration between the Northrop Grumman Corporation and Princeton University to develop FEL`s for use by researchers in the materials, medical and physical sciences. The CIRFEL was designed to lase in the Mid-IR and Far-IR regimes with picosecond pulses, megawatt level peak powers and an average power of a few watts. The micropulse separation is 7 nsec which allows a number of relaxation phenomenon to be observed. The CIRFEL utilizes an RF photocathode gun to produce high-brightness time synchronized electron bunches. The operational status and experimental resultsmore » of the CERFEL will be presented.« 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.

Compact dewar and electronics for large-format infrared detectors

NASA Astrophysics Data System (ADS)

Manissadjian, A.; Magli, S.; Mallet, E.; Cassaigne, P.

2011-06-01

Infrared systems cameras trend is to require higher performance (thanks to higher resolution) and in parallel higher compactness for easier integration in systems. The latest developments at SOFRADIR / France on HgCdTe (Mercury Cadmium Telluride / MCT) cooled IR staring detectors do show constant improvements regarding detector performances and compactness, by reducing the pixel pitch and optimizing their encapsulation. Among the latest introduced detectors, the 15μm pixel pitch JUPITER HD-TV format (1280×1024) has to deal with challenging specifications regarding dewar compactness, low power consumption and reliability. Initially introduced four years ago in a large dewar with a more than 2kg split Stirling cooler compressor, it is now available in a new versatile compact dewar that is vacuum-maintenance-free over typical 18 years mission profiles, and that can be integrated with the different available Stirling coolers: K548 microcooler for light solution (less than 0.7 kg), K549 or LSF9548 for split cooler and/or higher reliability solution. The IDDCAs are also required with simplified electrical interface enabling to shorten the system development time and to standardize the electronic boards definition with smaller volumes. Sofradir is therefore introducing MEGALINK, the new compact Command & Control Electronics compatible with most of the Sofradir IDDCAs. MEGALINK provides all necessary input biases and clocks to the FPAs, and digitizes and multiplexes the video outputs to provide a 14 bit output signal through a cameralink interface, in a surface smaller than a business card.

Origin of band bending at domain boundaries of MoS2: First-principles study

NASA Astrophysics Data System (ADS)

Kaneko, Tomoaki; Saito, Riichiro

2018-04-01

Using first-principles calculations based on density functional theory, the energetics and electronic structure of domain boundaries of MoS2, in which the same polar edges face each other, are investigated. We find that the interface model with homoelemental bonds is not energetically preferred in this system. The domain boundaries have defect levels that have wide distributions inside the band gap of MoS2. The upshift (or downshift) of the MoS2 energy band occurs around the domain boundaries when the occupation number of electrons in the defect levels increases (or decreases). The charge transfer of electrons from the graphite substrate plays an important role in band bending, which is observed in the recent experiments by scanning tunneling microscopy/spectroscopy.

Excitonic and band-band transitions of Cu2ZnSiS4 determined from reflectivity spectra

NASA Astrophysics Data System (ADS)

Guc, M.; Levcenko, S.; Dermenji, L.; Gurieva, G.; Schorr, S.; Syrbu, N. N.; Arushanov, E.

2014-07-01

Exciton spectra of Cu2ZnSiS4 single crystals are investigated by reflection spectroscopy at 10 and 300 K for light polarized perpendicular (E⊥c) and parallel (E∥c) to the optical axis. The parameters of the excitons and dielectric constant are determined. The free carriers effective masses have been estimated. The room temperature reflectivity spectra at photon energies higher than the fundamental band gap in the polarization Е⊥с and E∥с were measured and related to the electronic band structure of Cu2ZnSiS4.

High-perveance W-band Sheet-beam Electron Gun Design

DTIC Science & Technology

2008-04-01

APR 2008 2. REPORT TYPE 3. DATES COVERED 00-00-2008 to 00-00-2008 4. TITLE AND SUBTITLE High -perveance W- band Sheet-beam Electron Gun Design 5a...8-98) Prescribed by ANSI Std Z39-18 10.1: High -perveance W- band Sheet-beam Electron Gun Design Khanh T. Nguyen1, John Pasour, Edward L. Wright1...effects due to cathode temperature are also included in the simulation. Keywords: Sheet beam; W- band ; electron gun; high perveance; amplifiers

A Ka-Band (26 GHz) Circularly Polarized 2x2 Microstrip Patch Sub-Array with Compact Feed

NASA Technical Reports Server (NTRS)

Chrysler, Andrew; Furse, Cynthia; Simons, Rainee N.; Miranda, Felix A.

2017-01-01

A Ka-Band (26 gigahertz) 2 by 2 sub-array with square-shaped microstrip patch antenna elements having two truncated corners for circular polarization (CP) is presented. In addition, the layout for a new compact microstrip feed network for the sub-array is also presented. The compact feed network offers a footprint size reduction of near 60 percent over traditional sub-array at 26 gigahertz. Experimental data indicates that a truncation amount a equals 0.741 millimeters for an isolated patch element results in a return loss (S (sub II)) of minus 35 decibels at 26.3 gigahertz. Furthermore, the measured S (sub II) for the proof-of-concept sub-array with the above elements is better than minus 10.0 decibels at 27.7 gigahertz. However, the impedance match and the operating frequency can be fine-tuned to 26 gigahertz by adjusting the feed network dimensions. Lastly, good agreement is observed between the measured and simulated S (sub II) for the subarray for both right hand and left hand CP. The goal of this effort is utilize the above sub-array as a building block for a larger N by N element array, which would serve as a feed for a reflector antenna for satellite communications.

X-band RF gun and linac for medical Compton scattering X-ray source

NASA Astrophysics Data System (ADS)

Dobashi, Katsuhito; Uesaka, Mitsuru; Fukasawa, Atsushi; Sakamoto, Fumito; Ebina, Futaro; Ogino, Haruyuki; Urakawa, Junji; Higo, Toshiyasu; Akemoto, Mitsuo; Hayano, Hitoshi; Nakagawa, Keiichi

2004-12-01

Compton scattering hard X-ray source for 10-80 keV are under construction using the X-band (11.424 GHz) electron linear accelerator and YAG laser at Nuclear Engineering Research laboratory, University of Tokyo. This work is a part of the national project on the development of advanced compact medical accelerators in Japan. National Institute for Radiological Science is the host institute and U.Tokyo and KEK are working for the X-ray source. Main advantage is to produce tunable monochromatic hard (10-80 keV) X-rays with the intensities of 108-1010 photons/s (at several stages) and the table-top size. Second important aspect is to reduce noise radiation at a beam dump by adopting the deceleration of electrons after the Compton scattering. This realizes one beamline of a 3rd generation SR source at small facilities without heavy shielding. The final goal is that the linac and laser are installed on the moving gantry. We have designed the X-band (11.424 GHz) traveling-wave-type linac for the purpose. Numerical consideration by CAIN code and luminosity calculation are performed to estimate the X-ray yield. X-band thermionic-cathode RF-gun and RDS(Round Detuned Structure)-type X-band accelerating structure are applied to generate 50 MeV electron beam with 20 pC microbunches (104) for 1 microsecond RF macro-pulse. The X-ray yield by the electron beam and Q-switch Nd:YAG laser of 2 J/10 ns is 107 photons/RF-pulse (108 photons/sec at 10 pps). We design to adopt a technique of laser circulation to increase the X-ray yield up to 109 photons/pulse (1010 photons/s). 50 MW X-band klystron and compact modulator have been constructed and now under tuning. The construction of the whole system has started. X-ray generation and medical application will be performed in the early next year.

High Peak Power Test and Evaluation of S-band Waveguide Switches

NASA Astrophysics Data System (ADS)

Nassiri, A.; Grelick, A.; Kustom, R. L.; White, M.

1997-05-01

The injector and source of particles for the Advanced Photon Source is a 2856-MHz S-band electron-positron linear accelerator (linac) which produces electrons with energies up to 650 MeV or positrons with energies up to 450 MeV. To improve the linac rf system availability, an additional modulator-klystron subsystem is being constructed to provide a switchable hot spare unit for each of the five exsisting S-band transmitters. The switching of the transmitters will require the use of SF6-pressurized S-band waveguide switches at a peak operating power of 35 MW. Such rf switches have been successfully operated at other accelerator facilities but at lower peak powers. A test stand has been set up at the Stanford Linear Accelerator Center (SLAC) Klystron Factory to conduct tests comparing the power handling characteristics of two WR-284 and one WR-340 switches. Test results are presented and their implications for the design of the switching system are discussed.

Permanent magnet focused X-band photoinjector

DOEpatents

Yu, David U. L.; Rosenzweig, James

2002-09-10

A compact high energy photoelectron injector integrates the photocathode directly into a multicell linear accelerator with no drift space between the injection and the linac. High electron beam brightness is achieved by accelerating a tightly focused electron beam in an integrated, multi-cell, X-band rf linear accelerator (linac). The photoelectron linac employs a Plane-Wave-Transformer (PWT) design which provides strong cell-to-cell coupling, easing manufacturing tolerances and costs.

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.

Ultra-compact UHF Band-pass Filter Designed by Archimedes Spiral Capacitor and Shorted-loaded Stubs

NASA Astrophysics Data System (ADS)

Peng, Lin; Jiang, Xing

2015-01-01

UHF microstrip band-pass filters (BPFs) that much smaller than the referred BPFs are proposed in this communication. For the designing purpose of compactness, archimedes spiral capacitor and ground-loaded stubs are utilized to enhance capacitances and inductance of a filter. Two compact BPFs denoted as BPF 1 and BPF 2 are designed by applying these techniques. The size of BPF 1 and BPF 2 are 0.062 λg × 0.056 λg and 0.047 λg × 0.043 λg, respectively, where λg are guided wavelengths of the centre frequencies of the corresponding filters. The proposed filters were constructed and measured, and the measured results are in good agreement with the simulated ones.

Shuttle Ku-band and S-band communications implementation study

NASA Technical Reports Server (NTRS)

Dodds, J. G.; Huth, G. K.; Nilsen, P. W.; Polydoros, A.; Simon, M. K.; Weber, C. L.

1980-01-01

Various aspects of the shuttle orbiter S-band network communication system, the S-band payload communication system, and the Ku-band communication system are considered. A method is proposed for obtaining more accurate S-band antenna patterns of the actual shuttle orbiter vehicle during flight because the preliminary antenna patterns using mock-ups are not realistic that they do not include the effects of additional appendages such as wings and tail structures. The Ku-band communication system is discussed especially the TDRS antenna pointing accuracy with respect to the orbiter and the modifications required and resulting performance characteristics of the convolutionally encoded high data rate return link to maintain bit synchronizer lock on the ground. The TDRS user constraints on data bit clock jitter and data asymmetry on unbalanced QPSK with noisy phase references are included. The S-band payload communication system study is outlined including the advantages and experimental results of a peak regulator design built and evaluated by Axiomatrix for the bent-pipe link versus the existing RMS-type regulator. The nominal sweep rate for the deep-space transponder of 250 Hz/s, and effects of phase noise on the performance of a communication system are analyzed.

Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS2

PubMed Central

Trainer, Daniel J.; Putilov, Aleksei V.; Di Giorgio, Cinzia; Saari, Timo; Wang, Baokai; Wolak, Mattheus; Chandrasena, Ravini U.; Lane, Christopher; Chang, Tay-Rong; Jeng, Horng-Tay; Lin, Hsin; Kronast, Florian; Gray, Alexander X.; Xi, Xiaoxing X.; Nieminen, Jouko; Bansil, Arun; Iavarone, Maria

2017-01-01

Recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here, we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green’s function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS2. PMID:28084465

Inter-layer coupling induced valence band edge shift in mono- to few-layer MoS 2

DOE PAGES

Trainer, Daniel J.; Putilov, Aleksei V.; Di Giorgio, Cinzia; ...

2017-01-13

In this study, recent progress in the synthesis of monolayer MoS 2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here,we report a study of highly crystalline islands of MoS 2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS 2 as a function of the number of layers at the nanoscale and show in-depth how themore » band gap is affected by a shift of the valence band edge as a function of the layer number. Green’s function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS 2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS 2.« less

An 'X-banded' Tidbinbilla interferometer

NASA Technical Reports Server (NTRS)

Batty, Michael J.; Gardyne, R. G.; Gay, G. J.; Jauncy, David L.; Gulkis, S.; Kirk, A.

1986-01-01

The recent upgrading of the Tidbinbilla two-element interferometer to simultaneous S-band (2.3 GHz) and X-band (8.4 GHz) operation has provided a powerful new astronomical facility for weak radio source measurement in the Southern Hemisphere. The new X-band system has a minimum fringe spacing of 38 arcsec, and about the same positional measurement capability (approximately 2 arcsec) and sensitivity (1 s rms noise of 10 mJy) as the previous S-band system. However, the far lower confusion limit will allow detection and accurate positional measurements for sources as weak as a few millijanskys. This capability will be invaluable for observations of radio stars, X-ray sources and other weak, compact radio sources.

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

Determination of band offsets at GaN/single-layer MoS{sub 2} heterojunction

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

Tangi, Malleswararao; Mishra, Pawan; Ng, Tien Khee

2016-07-18

We report the band alignment parameters of the GaN/single-layer (SL) MoS{sub 2} heterostructure where the GaN thin layer is grown by molecular beam epitaxy on CVD deposited SL-MoS{sub 2}/c-sapphire. We confirm that the MoS{sub 2} is an SL by measuring the separation and position of room temperature micro-Raman E{sup 1}{sub 2g} and A{sup 1}{sub g} modes, absorbance, and micro-photoluminescence bandgap studies. This is in good agreement with HRTEM cross-sectional analysis. The determination of band offset parameters at the GaN/SL-MoS{sub 2} heterojunction is carried out by high-resolution X-ray photoelectron spectroscopy accompanying with electronic bandgap values of SL-MoS{sub 2} and GaN. Themore » valence band and conduction band offset values are, respectively, measured to be 1.86 ± 0.08 and 0.56 ± 0.1 eV with type II band alignment. The determination of these unprecedented band offset parameters opens up a way to integrate 3D group III nitride materials with 2D transition metal dichalcogenide layers for designing and modeling of their heterojunction based electronic and photonic devices.« less

Convergence of electronic bands for high performance bulk thermoelectrics.

PubMed

Pei, Yanzhong; Shi, Xiaoya; LaLonde, Aaron; Wang, Heng; Chen, Lidong; Snyder, G Jeffrey

2011-05-05

Thermoelectric generators, which directly convert heat into electricity, have long been relegated to use in space-based or other niche applications, but are now being actively considered for a variety of practical waste heat recovery systems-such as the conversion of car exhaust heat into electricity. Although these devices can be very reliable and compact, the thermoelectric materials themselves are relatively inefficient: to facilitate widespread application, it will be desirable to identify or develop materials that have an intensive thermoelectric materials figure of merit, zT, above 1.5 (ref. 1). Many different concepts have been used in the search for new materials with high thermoelectric efficiency, such as the use of nanostructuring to reduce phonon thermal conductivity, which has led to the investigation of a variety of complex material systems. In this vein, it is well known that a high valley degeneracy (typically ≤6 for known thermoelectrics) in the electronic bands is conducive to high zT, and this in turn has stimulated attempts to engineer such degeneracy by adopting low-dimensional nanostructures. Here we demonstrate that it is possible to direct the convergence of many valleys in a bulk material by tuning the doping and composition. By this route, we achieve a convergence of at least 12 valleys in doped PbTe(1-x)Se(x) alloys, leading to an extraordinary zT value of 1.8 at about 850 kelvin. Band engineering to converge the valence (or conduction) bands to achieve high valley degeneracy should be a general strategy in the search for and improvement of bulk thermoelectric materials, because it simultaneously leads to a high Seebeck coefficient and high electrical conductivity. ©2011 Macmillan Publishers Limited. All rights reserved

Band alignment at the CdS/FeS2 interface based on the first-principles calculation

NASA Astrophysics Data System (ADS)

Ichimura, Masaya; Kawai, Shoichi

2015-03-01

FeS2 is potentially well-suited for the absorber layer of a thin-film solar cell. Since it usually has p-type conductivity, a pn heterojunction cell can be fabricated by combining it with an n-type material. In this work, the band alignment in the heterostructure based on FeS2 is investigated on the basis of the first-principles calculation. CdS, the most popular buffer-layer material for thin-film solar cells, is selected as the partner in the heterostructure. The results indicate that there is a large conduction band offset (0.65 eV) at the interface, which will hinder the flow of photogenerated electrons from FeS2 to CdS. Thus an n-type material with the conduction band minimum positioned lower than that of CdS will be preferable as the partner in the heterostructure.

Probability of Two-Step Photoexcitation of Electron from Valence Band to Conduction Band through Doping Level in TiO2.

PubMed

Nishikawa, Masami; Shiroishi, Wataru; Honghao, Hou; Suizu, Hiroshi; Nagai, Hideyuki; Saito, Nobuo

2017-08-17

For an Ir-doped TiO 2 (Ir:TiO 2 ) photocatalyst, we examined the most dominant electron-transfer path for the visible-light-driven photocatalytic performance. The Ir:TiO 2 photocatalyst showed a much higher photocatalytic activity under visible-light irradiation than nondoped TiO 2 after grafting with the cocatalyst of Fe 3+ . For the Ir:TiO 2 photocatalyst, the two-step photoexcitation of an electron from the valence band to the conduction band through the Ir doping level occurred upon visible-light irradiation, as observed by electron spin resonance spectroscopy. The two-step photoexcitation through the doping level was found to be a more stable process with a lower recombination rate of hole-electron pairs than the two-step photoexcitation process through an oxygen vacancy. Once electrons are photoexcited to the conduction band by the two-step excitation, the electrons can easily transfer to the surface because the conduction band is a continuous electron path, whereas the electrons photoexcited at only the doping level could not easily transfer to the surface because of the discontinuity of this path. The observed two-step photoexcitation from the valence band to the conduction band through the doping level significantly contributes to the enhancement of the photocatalytic performance.

Shuttle S-band communications technical concepts

NASA Technical Reports Server (NTRS)

Seyl, J. W.; Seibert, W. W.; Porter, J. A.; Eggers, D. S.; Novosad, S. W.; Vang, H. A.; Lenett, S. D.; Lewton, W. A.; Pawlowski, J. F.

1985-01-01

Using the S-band communications system, shuttle orbiter can communicate directly with the Earth via the Ground Spaceflight Tracking and Data Network (GSTDN) or via the Tracking and Data Relay Satellite System (TDRSS). The S-band frequencies provide the primary links for direct Earth and TDRSS communications during all launch and entry/landing phases of shuttle missions. On orbit, S-band links are used when TDRSS Ku-band is not available, when conditions require orbiter attitudes unfavorable to Ku-band communications, or when the payload bay doors are closed. the S-band communications functional requirements, the orbiter hardware configuration, and the NASA S-band communications network are described. The requirements and implementation concepts which resulted in techniques for shuttle S-band hardware development discussed include: (1) digital voice delta modulation; (2) convolutional coding/Viterbi decoding; (3) critical modulation index for phase modulation using a Costas loop (phase-shift keying) receiver; (4) optimum digital data modulation parameters for continuous-wave frequency modulation; (5) intermodulation effects of subcarrier ranging and time-division multiplexing data channels; (6) radiofrequency coverage; and (7) despreading techniques under poor signal-to-noise conditions. Channel performance is reviewed.

The temperature-dependency of the optical band gap of ZnO measured by electron energy-loss spectroscopy in a scanning transmission electron microscope

NASA Astrophysics Data System (ADS)

Granerød, Cecilie S.; Galeckas, Augustinas; Johansen, Klaus Magnus; Vines, Lasse; Prytz, Øystein

2018-04-01

The optical band gap of ZnO has been measured as a function of temperature using Electron Energy-Loss Spectroscopy (EELS) in a (Scanning) Transmission Electron Microscope ((S)TEM) from approximately 100 K up towards 1000 K. The band gap narrowing shows a close to linear dependency for temperatures above 250 K and is accurately described by Varshni, Bose-Einstein, Pässler and Manoogian-Woolley models. Additionally, the measured band gap is compared with both optical absorption measurements and photoluminescence data. STEM-EELS is here shown to be a viable technique to measure optical band gaps at elevated temperatures, with an available temperature range up to 1500 K and the benefit of superior spatial resolution.

Model Development for MODIS Thermal Band Electronic Crosstalk

NASA Technical Reports Server (NTRS)

Chang, Tiejun; Wu, Aisheng; Geng, Xu; Li, Yonghonh; Brinkman, Jake; Keller, Graziela; Xiong, Xiaoxiong

2016-01-01

MODerate-resolution Imaging Spectroradiometer (MODIS) has 36 bands. Among them, 16 thermal emissive bands covering a wavelength range from 3.8 to 14.4 m. After 16 years on-orbit operation, the electronic crosstalk of a few Terra MODIS thermal emissive bands developed substantial issues that cause biases in the EV brightness temperature measurements and surface feature contamination. The crosstalk effects on band 27 with center wavelength at 6.7 m and band 29 at 8.5 m increased significantly in recent years, affecting downstream products such as water vapor and cloud mask. The crosstalk effect is evident in the near-monthly scheduled lunar measurements, from which the crosstalk coefficients can be derived. The development of an alternative approach is very helpful for independent verification.In this work, a physical model was developed to assess the crosstalk impact on calibration as well as in Earth view brightness temperature retrieval. This model was applied to Terra MODIS band 29 empirically to correct the Earth brightness temperature measurements. In the model development, the detectors nonlinear response is considered. The impact of the electronic crosstalk is assessed in two steps. The first step consists of determining the impact on calibration using the on-board blackbody (BB). Due to the detectors nonlinear response and large background signal, both linear and nonlinear coefficients are affected by the crosstalk from sending bands. The second step is to calculate the effects on the Earth view brightness temperature retrieval. The effects include those from affected calibration coefficients and the contamination of Earth view measurements. This model links the measurement bias with crosstalk coefficients, detector non-linearity, and the ratio of Earth measurements between the sending and receiving bands. The correction of the electronic cross talk can be implemented empirically from the processed bias at different brightness temperature. The implementation

Electronic structure of Ag7GeS5I superionic compound

NASA Astrophysics Data System (ADS)

Bletskan, Dmytro; Studenyak, Ihor; Bletskan, Mykhailo; Vakulchak, Vasyl

2018-05-01

This paper presents the originally results of ab initio calculations of electronic structure, total and partial densities of electronic states as well as electronic charge density distribution of Ag7GeS5I crystal performed within the density functional theory (DFT) in the local density approximation (LDA) for exchange-correlation potential. According to performed calculations, Ag7GeS5I is the direct-gap semiconductor with the valence band top and the conductivity band bottom in the Γ point of Brillouin zone. The band gap width calculated in the LDA-approximation is Egd = 0.73 eV. The analysis of total and partial densities of electronic states allow us to identify the atomic orbital contributions into the crystal orbitals as well as the formation data of chemical bond in the studied crystal. In the top part of Ag7GeS5I valence band it was revealed the considerable mixing (hybridization) of the occupied d-states of Ag noble metal and the delocalized p-states of sulfur and iodine, which is undoubtedly associated with the covalent character of chemical bond between S, I atoms and noble metal atom.

Ultrathin triple-band polarization-insensitive wide-angle compact metamaterial absorber

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

Shang, Shuai; Yang, Shizhong; Tao, Lu

2016-07-15

In this study, the design, realization, and characterization of an ultrathin triple-band polarization-insensitive wide-angle metamaterial absorber are reported. The metamaterial absorber comprises a periodic array of modified six-fold symmetric snowflake-shaped resonators with strip spiral line load, which is printed on a dielectric substrate backed by a metal ground plane. It is shown that the absorber exhibits three distinct near-unity absorption peaks, which are distributed across C, X, Ku bands, respectively. Owing to the six-fold symmetry, the absorber is insensitive to the polarization of the incident radiation. In addition, the absorber shows excellent absorption performance over wide oblique incident angles formore » both transverse electric and transverse magnetic polarizations. Simulated surface current and field distributions at the three absorption peaks are demonstrated to understand the absorption mechanism. Particularly, the absorption modes come from the fundamental and high-order dipole resonances. Furthermore, the experimental verification of the designed absorber is conducted, and the measured results are in reasonable agreement with the simulated ones. The proposed ultrathin (∼0.018λ{sub 0}, λ{sub 0} corresponding to the lowest peak absorption frequency) compact (0.168λ{sub 0}×0.168λ{sub 0} corresponding to the area of a unit cell) absorber enables potential applications such as stealth technology, electromagnetic interference and spectrum identification.« less

Correlation effects and electronic properties of Cr-substituted SZn with an intermediate band.

PubMed

Tablero, C

2005-09-15

A study using first principles of the electronic properties of S32Zn31Cr, a material derived from the SZn host semiconductor where a Cr atom has been substituted for each of the 32 Zn atoms, is presented. This material has an intermediate band sandwiched between the valence and conduction bands of the host semiconductor, which in a formal band-theoretic picture is metallic because the Fermi energy is located within the impurity band. The potential technological application of these materials is that when they are used to absorb photons in solar cells, the efficiency increases significantly with respect to the host semiconductor. An analysis of the gaps, bandwidths, density of states, total and orbital charges, and electronic density is carried out. The main effects of the local-density approximation with a Hubbard term corrections are an increase in the bandwidth, a modification of the relative composition of the five d and p transition-metal orbitals, and a splitting of the intermediate band. The results demonstrate that the main contribution to the intermediate band is the Cr atom. For values of U greater than 6 eV, where U is the empirical Hubbard term U parameter, this band is unfolded, thus creating two bands, a full one below the Fermi energy and an empty one above it, i.e., a metal-insulator transition.

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.

Band-to-Band Tunneling-Dominated Thermo-Enhanced Field Electron Emission from p-Si/ZnO Nanoemitters.

PubMed

Huang, Zhizhen; Huang, Yifeng; Xu, Ningsheng; Chen, Jun; She, Juncong; Deng, Shaozhi

2018-06-13

Thermo-enhancement is an effective way to achieve high performance field electron emitters, and enables the individually tuning on the emission current by temperature and the electron energy by voltage. The field emission current from metal or n-doped semiconductor emitter at a relatively lower temperature (i.e., < 1000 K) is less temperature sensitive due to the weak dependence of free electron density on temperature, while that from p-doped semiconductor emitter is restricted by its limited free electron density. Here, we developed full array of uniform individual p-Si/ZnO nanoemitters and demonstrated the strong thermo-enhanced field emission. The mechanism of forming uniform nanoemitters with well Si/ZnO mechanical joint in the nanotemplates was elucidated. No current saturation was observed in the thermo-enhanced field emission measurements. The emission current density showed about ten-time enhancement (from 1.31 to 12.11 mA/cm 2 at 60.6 MV/m) by increasing the temperature from 323 to 623 K. The distinctive performance did not agree with the interband excitation mechanism but well-fit to the band-to-band tunneling model. The strong thermo-enhancement was proposed to be benefit from the increase of band-to-band tunneling probability at the surface portion of the p-Si/ZnO nanojunction. This work provides promising cathode for portable X-ray tubes/panel, ionization vacuum gauges and low energy electron beam lithography, in where electron-dose control at a fixed energy is needed.

Probing the band structure and local electronic properties of low-dimensional semiconductor structures

NASA Astrophysics Data System (ADS)

Walrath, Jenna Cherie

Low-dimensional semiconductor structures are important for a wide variety of applications, and recent advances in nanoscale fabrication are paving the way for increasingly precise nano-engineering of a wide range of materials. It is therefore essential that the physics of materials at the nanoscale are thoroughly understood to unleash the full potential of nanotechnology, requiring the development of increasingly sophisticated instrumentation and modeling. Of particular interest is the relationship between the local density of states (LDOS) of low-dimensional structures and the band structure and local electronic properties. This dissertation presents the investigation of the band structure, LDOS, and local electronic properties of nanostructures ranging from zero-dimensional (0D) quantum dots (QDs) to two-dimensional (2D) thin films, synthesizing computational and experimental approaches including Poisson-Schrodinger band structure calculations, scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and scanning thermoelectric microscopy (SThEM). A method is presented for quantifying the local Seebeck coefficient (S) with SThEM, using a quasi-3D conversion matrix approach to directly convert temperature gradient-induced voltages S. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces. The local carrier concentration, n, is profiled across epitaxial InAs/GaAs QDs, where SThEM is used to profile the temperature gradient-induced voltage, which is converted to a profile of the local S and finally to an n profile. The S profile is converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. The combined computational-experimental approach suggests a reduced n in the QD center in

Electronic band structure effects in the stopping of protons in copper [Electronic band structure non-linear effects in the stopping of protons in copper

DOE PAGES

Quashie, Edwin E.; Saha, Bidhan C.; Correa, Alfredo A.

2016-10-05

Here, we present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities v = 0.02–10a.u. where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results aremore » in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of ~1.5) in the velocity range v = 0.07–0.3a.u., which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping v → 0 is not as simple as phenomenological models suggest and it is plagued by band-structure effects.« less

Design and testing of focusing magnets for a compact electron linac

NASA Astrophysics Data System (ADS)

Chen, Qushan; Qin, Bin; Liu, Kaifeng; Liu, Xu; Fu, Qiang; Tan, Ping; Hu, Tongning; Pei, Yuanji

2015-10-01

Solenoid field errors have great influence on electron beam qualities. In this paper, design and testing of high precision solenoids for a compact electron linac is presented. We proposed an efficient and practical method to solve the peak field of the solenoid for relativistic electron beams based on the reduced envelope equation. Beam dynamics involving space charge force were performed to predict the focusing effects. Detailed optimization methods were introduced to achieve an ultra-compact configuration as well as high accuracy, with the help of the POISSON and OPERA packages. Efforts were attempted to restrain system errors in the off-line testing, which showed the short lens and the main solenoid produced a peak field of 0.13 T and 0.21 T respectively. Data analysis involving central and off axes was carried out and demonstrated that the testing results fitted well with the design.

The dependence of the tunneling characteristic on the electronic energy bands and the carrier’s states of Graphene superlattice

NASA Astrophysics Data System (ADS)

Yang, C. H.; Shen, G. Z.; Ao, Z. M.; Xu, Y. W.

2016-09-01

Using the transfer matrix method, the carrier tunneling properties in graphene superlattice generated by the Thue-Morse sequence and Kolakoski sequence are investigated. The positions and strength of the transmission can be modulated by the barrier structures, the incident energy and angle, the height and width of the potential. These carriers tunneling characteristic can be understood from the energy band structures in the corresponding superlattice systems and the carrier’s states in well/barriers. The transmission peaks above the critical incident angle rely on the carrier’s resonance in the well regions. The structural diversity can modulate the electronic and transport properties, thus expanding its applications.

High resolution emission Fourier transform infrared spectra of the 4p-5s and 5p-6s bands of ArH.

PubMed

Baskakov, O I; Civis, S; Kawaguchi, K

2005-03-15

In the 2500-8500 cm(-1) region several strong emission bands of (40)ArH were observed by Fourier transform spectroscopy through a dc glow discharge in a mixture of argon and hydrogen. Rotational-electronic transitions of the two previously unstudied 4p-5s and 5p-6s,v = 0-0, bands of (40)ArH were measured and assigned in the 6060 and 3770 cm(-1) regions, respectively. A simultaneous fit of the emission transitions of the 4p-5s and 5p-6s bands and an extended set of transitions of the 6s-4p band observed by Dabrowski, Tokaryk, and Watson [J. Mol. Spectrosc. 189, 95 (1998)] and remeasured in the present work yielded consistent values of the spectroscopic parameters of the electronic states under investigation. In the branch of the 4p-5s band with transitions of type (Q)Q(f(3)e) we observed a narrowing in the linewidths with increasing rotational quantum number N. The rotational dependence of the linewidth is caused by predissociation of the 5s state by the repulsive ground 4s state through homogeneous coupling and changes in overlap integrals of the vibrational wave functions with the rotational level. Analysis was based on the Fermi's golden rule approximation model. In the 4p-5s band region a vibrational sequence ofv(')-v(")=1-1, 2-2, and 3-3 were recorded and a number of transitions belonging to the strongest (Q)Q(f(3)e) form branch of the 1-1 band were analyzed.

Beam Dynamics a Integrated Plane Wave Transformer Photoinjector at S- and X- band

NASA Astrophysics Data System (ADS)

Rosenzweig, J. B.; Ding, X.; Pellegrini, X.; Serafini, L.; Yu, D.

1997-05-01

The beam dynamics of an integrated S-band rf photoinjector based on the plane wave transformer concept, proposed as part of an SBIR collaboration between UCLA and DULY Research, are studied. The intial design, which calls for an 11.5 cell structure run at a peak on-axis accelerating field of 60 MV/m, and has a compact solenoid around the intial 2.5 cells, is based on the recently developed theory of emittance compensation(L.Serafini, and J.B. Rosenzweig, submitted to Physical Review E.). It calls for matching the beam onto an envelope which is a generalized Brillouin flow, producing a beam which diminishes in transverse size as the square root of the accelerating beam energy. This condition produces a minimized emittance, which for the S-band case is 1 mm-rad at at charge of 1 nC. This design is also scaled to produce nearly identical performance at X-band, giving an injector appropriate to running an FEL at the SLAC NLCTA. It is noted that these designs are insensitive to rf emittance increase, allowign a choice of injection phase, and the option to compress the emitted pulse.

A Compact L-band Radiometer for High Resolution sUAS-based Imaging of Soil Moisture and Surface Salinity Variations

NASA Astrophysics Data System (ADS)

Gasiewski, A. J.; Stachura, M.; Dai, E.; Elston, J.; McIntyre, E.; Leuski, V.

2014-12-01

Due to the long electrical wavelengths required along with practical aperture size limitations the scaling of passive microwave remote sensing of soil moisture and salinity from spaceborne low-resolution (~10-100 km) applications to high resolution (~10-1000 m) applications requires use of low flying aerial vehicles. This presentation summarizes the status of a project to develop a commercial small Unmanned Aerial System (sUAS) hosting a microwave radiometer for mapping of soil moisture in precision agriculture and sea surface salinity studies. The project is based on the Tempest electric-powered UAS and a compact L-band (1400-1427 MHz) radiometer developed specifically for extremely small and lightweight aerial platforms or man-portable, tractor, or tower-based applications. Notable in this combination are a highly integrated sUAS/radiometer antenna design and use of both the upwelling emitted signal from the surface and downwelling cold space signal for precise calibration using a unique lobe-differencing correlating radiometer architecture. The system achieves a spatial resolution comparable to the altitude of the UAS above the surface while referencing upwelling measurements to the constant and well-known background temperature of cold space. The radiometer has been tested using analog correlation detection, although future builds will include infrared, near-infrared, and visible (red) sensors for surface temperature and vegetation biomass correction and digital sampling for radio frequency interference mitigation. This NASA-sponsored project is being developed for commercial application in cropland water management (for example, high-value shallow root-zone crops), landslide risk assessment, NASA SMAP satellite validation, and NASA Aquarius salinity stratification studies. The system will ultimately be capable of observing salinity events caused by coastal glacier and estuary fresh water outflow plumes and open ocean rainfall events.

Design of Compact Flower Shape Dual Notched-Band Monopole Antenna for Extended UWB Wireless Applications

NASA Astrophysics Data System (ADS)

Sharma, Manish; Awasthi, Y. K.; Singh, Himanshu; Kumar, Raj; Kumari, Sarita

2016-11-01

In this letter, a compact monopole antenna for ultra wideband (UWB) applications is proposed with small size of 18×20=360 mm2. Antenna consist of a flower shape radiating patch with a pair of C-shaped slots which offer two notch bands for WiMAX (3.04-3.68 GHz) & WLAN (4.73-5.76 GHz) and two rectangular shaped slots in the ground plane which provides a wide measured usable fractional extended bandwidth of 163 % (2.83-14.0 GHz) with improved VSWR. Moreover, it is also convenient for other wireless application as close range radar, 8-12 GHz in X-band. Measured radiation patterns exhibits nearly omnidirectional in H-plane and dipole like pattern in E-plane across the bandwidth and furthermore exhibits good time domain performance.

Band structure and orbital character of monolayer MoS2 with eleven-band tight-binding model

NASA Astrophysics Data System (ADS)

Shahriari, Majid; Ghalambor Dezfuli, Abdolmohammad; Sabaeian, Mohammad

2018-02-01

In this paper, based on a tight-binding (TB) model, first we present the calculations of eigenvalues as band structure and then present the eigenvectors as probability amplitude for finding electron in atomic orbitals for monolayer MoS2 in the first Brillouin zone. In these calculations we are considering hopping processes between the nearest-neighbor Mo-S, the next nearest-neighbor in-plan Mo-Mo, and the next nearest-neighbor in-plan and out-of-plan S-S atoms in a three-atom based unit cell of two-dimensional rhombic MoS2. The hopping integrals have been solved in terms of Slater-Koster and crystal field parameters. These parameters are calculated by comparing TB model with the density function theory (DFT) in the high-symmetry k-points (i.e. the K- and Γ-points). In our TB model all the 4d Mo orbitals and the 3p S orbitals are considered and detailed analysis of the orbital character of each energy level at the main high-symmetry points of the Brillouin zone is described. In comparison with DFT calculations, our results of TB model show a very good agreement for bands near the Fermi level. However for other bands which are far from the Fermi level, some discrepancies between our TB model and DFT calculations are observed. Upon the accuracy of Slater-Koster and crystal field parameters, on the contrary of DFT, our model provide enough accuracy to calculate all allowed transitions between energy bands that are very crucial for investigating the linear and nonlinear optical properties of monolayer MoS2.

Electronic band structure of ReS2 by high-resolution angle-resolved photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Webb, James L.; Hart, Lewis S.; Wolverson, Daniel; Chen, Chaoyu; Avila, Jose; Asensio, Maria C.

2017-09-01

The rhenium-based transition metal dichalcogenides (TMDs) are atypical of the TMD family due to their highly anisotropic crystalline structure and are recognized as promising materials for two-dimensional heterostructure devices. The nature of the band gap (direct or indirect) for bulk, few-, and single-layer forms of ReS2 is of particular interest, due to its comparatively weak interplanar interaction. However, the degree of interlayer interaction and the question of whether a transition from indirect to direct gap is observed on reducing thickness (as in other TMDs) are controversial. We present a direct determination of the valence band structure of bulk ReS2 using high-resolution angle-resolved photoemission spectroscopy. We find a clear in-plane anisotropy due to the presence of chains of Re atoms, with a strongly directional effective mass which is larger in the direction orthogonal to the Re chains (2.2 me ) than along them (1.6 me ). An appreciable interplane interaction results in an experimentally measured difference of ≈100 -200 meV between the valence band maxima at the Z point (0,0,1/2 ) and the Γ point (0,0,0) of the three-dimensional Brillouin zone. This leads to a direct gap at Z and a close-lying but larger gap at Γ , implying that bulk ReS2 is marginally indirect. This may account for recent conflicting transport and photoluminescence measurements and the resulting uncertainty about the nature of the band gap in this material.

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.

Electronic properties of ZnPSe3-MoS2 Van der Waals heterostructure

NASA Astrophysics Data System (ADS)

Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.

2018-04-01

We present a comparative study of electronic properties of ZnPSe3-MoS2 heterostructure using GGA-PBE functional and DFT-D2 method within the framework of density functional theory (DFT). Electronic band structure for the considered heterostructure shows a direct band gap semiconducting character. A decrease in band gap is observed with the heterostructuring as compared to their constituent pristine monolayers. The alignment of valance band maxima and conduction band minima on different layers in heterostructure indicate the physical separation of charge carriers. A work function of 5.31 eV has been calculated for ZnPSe3-MoS2 heterostructure. These results provide a physical basis for the potential applications of these ZnPSe3-MoS2 heterostructure in optoelectronic devices.

Electronic materials with a wide band gap: recent developments

PubMed Central

Klimm, Detlef

2014-01-01

The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap E g = 0.66 eV) after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (E g = 1.12 eV). This improved the signal-to-noise ratio for classical electronic applications. Both semiconductors have a tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or various anions and cations, other semiconductors with wider E g were obtained. These are transparent to visible light and belong to the group of wide band gap semiconductors. Nowadays, some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue regions. Oxide crystals, such as ZnO and β-Ga2O3, offer similarly good electronic properties but still suffer from significant difficulties in obtaining stable and technologically adequate p-type conductivity. PMID:25295170

Band gap and electronic structure of MgSiN2

NASA Astrophysics Data System (ADS)

Quirk, J. B.; Râsander, M.; McGilvery, C. M.; Palgrave, R.; Moram, M. A.

2014-09-01

Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN2 is similar to that of wurtzite AlN. A band gap of 5.7 eV was calculated for both MgSiN2 (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN2 is 6.2 eV. MgSiN2 has an additional direct gap of 6.3 eV at the Γ point.

Passive and active plasma deceleration for the compact disposal of electron beams

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

Bonatto, A., E-mail: abonatto@lbl.gov; CAPES Foundation, Ministry of Education of Brazil, Brasília, DF 700040-020; Schroeder, C. B.

2015-08-15

Plasma-based decelerating schemes are investigated as compact alternatives for the disposal of high-energy beams (beam dumps). Analytical solutions for the energy loss of electron beams propagating in passive and active (laser-driven) schemes are derived. These solutions, along with numerical modeling, are used to investigate the evolution of the electron distribution, including energy chirp and total beam energy. In the active beam dump scheme, a laser-driver allows a more homogeneous beam energy extraction and drastically reduces the energy chirp observed in the passive scheme. These concepts could benefit applications requiring overall compactness, such as transportable light sources, or facilities operating atmore » high beam power.« less

Saturation of side-band instabilities in a free-electron laser

NASA Astrophysics Data System (ADS)

Lin, A. T.

The efficiency of a free electron laser is intrinsically limited because the growth of the ponderomotive force produced by the interaction of the rippled magnetic field and the signal wave will eventually trap the electrons. There are a number of approaches for enhancing the efficiency of a free electron laser (FEL). One approach employs a dc field. Most of the efficiency enhancement calculations use a single-mode approximation which prohibits the side band waves to grow. In the present investigation, a particle simulation procedure is employed to demonstrate that the enhancement process is ultimately terminated by the generation of side band instabilities due to the interaction of the trapped electrons and the signal wave. The side band instability will play an important part in determining the maximum output power which can be obtained from a FEL. It is also shown that a considerable improvement in output power can still be achieved by carefully choosing the strength and the turn-on time of the dc electric field.

Compact Modules for Wireless Communication Systems in the E-Band (71-76 GHz)

NASA Astrophysics Data System (ADS)

Montero-de-Paz, Javier; Oprea, Ion; Rymanov, Vitaly; Babiel, Sebastian; García-Muñoz, Luis Enrique; Lisauskas, Alvydas; Hoefle, Matthias; Jimenez, Álvaro; Cojocari, Oleg; Segovia-Vargas, Daniel; Palandöken, Merih; Tekin, Tolga; Stöhr, Andreas; Carpintero, Guillermo

2013-04-01

The millimeter-wave spectrum above 70 GHz provides a cost-effective solution to increase the wireless communications data rates by increasing the carrier wave frequencies. We report on the development of two key components of a wireless transmission system, a high-speed photodiode (HS-PD) and a Schottky Barrier Diode (SBD). Both components operate uncooled, a key issue in the development of compact modules. On the transmitter side, an improved design of the HS-PD allows it to deliver an output RF power exceeding 0 dBm (1 mW). On the receiver side, we present the design process and achieved results on the development of a compact direct envelope detection receiver based on a quasi-optical SDB module. Different resonant (meander dipole) and broadband (Log-Spiral and Log-Periodic) planar antenna solutions are designed, matching the antenna and Schottky diode impedances at high frequency. Impedance matching at baseband is also provided by means of an impedance transition to a 50 Ohm output. From this comparison, we demonstrate the excellent performance of the broadband antennas over the entire E-band by setting up a short-range wireless link transmitting a 1 Gbps data signal.

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

Viking S-band Doppler RMS phase fluctuations used to calibrate the mean 1976 equatorial corona

NASA Technical Reports Server (NTRS)

Berman, A. L.; Wackley, J. A.

1977-01-01

Viking S-band Doppler RMS phase fluctuations (noise) and comparisons of Viking Doppler noise to Viking differenced S-X range measurements are used to construct a mean equatorial electron density model for 1976. Using Pioneer Doppler noise results (at high heliographic latitudes, also from 1976), an equivalent nonequatorial electron density model is approximated.

Multi-Band Miniaturized Patch Antennas for a Compact, Shielded Microwave Breast Imaging Array.

PubMed

Aguilar, Suzette M; Al-Joumayly, Mudar A; Burfeindt, Matthew J; Behdad, Nader; Hagness, Susan C

2013-12-18

We present a comprehensive study of a class of multi-band miniaturized patch antennas designed for use in a 3D enclosed sensor array for microwave breast imaging. Miniaturization and multi-band operation are achieved by loading the antenna with non-radiating slots at strategic locations along the patch. This results in symmetric radiation patterns and similar radiation characteristics at all frequencies of operation. Prototypes were fabricated and tested in a biocompatible immersion medium. Excellent agreement was obtained between simulations and measurements. The trade-off between miniaturization and radiation efficiency within this class of patch antennas is explored via a numerical analysis of the effects of the location and number of slots, as well as the thickness and permittivity of the dielectric substrate, on the resonant frequencies and gain. Additionally, we compare 3D quantitative microwave breast imaging performance achieved with two different enclosed arrays of slot-loaded miniaturized patch antennas. Simulated array measurements were obtained for a 3D anatomically realistic numerical breast phantom. The reconstructed breast images generated from miniaturized patch array data suggest that, for the realistic noise power levels assumed in this study, the variations in gain observed across this class of multi-band patch antennas do not significantly impact the overall image quality. We conclude that these miniaturized antennas are promising candidates as compact array elements for shielded, multi-frequency microwave breast imaging systems.

Atomic-Monolayer MoS2 Band-to-Band Tunneling Field-Effect Transistor.

PubMed

Lan, Yann-Wen; Torres, Carlos M; Tsai, Shin-Hung; Zhu, Xiaodan; Shi, Yumeng; Li, Ming-Yang; Li, Lain-Jong; Yeh, Wen-Kuan; Wang, Kang L

2016-11-01

The experimental observation of band-to-band tunneling in novel tunneling field-effect transistors utilizing a monolayer of MoS 2 as the conducting channel is demonstrated. Our results indicate that the strong gate-coupling efficiency enabled by two-dimensional materials, such as monolayer MoS 2 , results in the direct manifestation of a band-to-band tunneling current and an ambipolar transport. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Model development for MODIS thermal band electronic cross-talk

NASA Astrophysics Data System (ADS)

Chang, Tiejun; Wu, Aisheng; Geng, Xu; Li, Yonghong; Brinkmann, Jake; Keller, Graziela; Xiong, Xiaoxiong (Jack)

2016-10-01

MODerate-resolution Imaging Spectroradiometer (MODIS) has 36 bands. Among them, 16 thermal emissive bands covering a wavelength range from 3.8 to 14.4 μm. After 16 years on-orbit operation, the electronic crosstalk of a few Terra MODIS thermal emissive bands develop substantial issues which cause biases in the EV brightness temperature measurements and surface feature contamination. The crosstalk effects on band 27 with center wavelength at 6.7 μm and band 29 at 8.5 μm increased significantly in recent years, affecting downstream products such as water vapor and cloud mask. The crosstalk issue can be observed from nearly monthly scheduled lunar measurements, from which the crosstalk coefficients can be derived. Most of MODIS thermal bands are saturated at moon surface temperatures and the development of an alternative approach is very helpful for verification. In this work, a physical model was developed to assess the crosstalk impact on calibration as well as in Earth view brightness temperature retrieval. This model was applied to Terra MODIS band 29 empirically for correction of Earth brightness temperature measurements. In the model development, the detector nonlinear response is considered. The impacts of the electronic crosstalk are assessed in two steps. The first step consists of determining the impact on calibration using the on-board blackbody (BB). Due to the detector nonlinear response and large background signal, both linear and nonlinear coefficients are affected by the crosstalk from sending bands. The crosstalk impact on calibration coefficients was calculated. The second step is to calculate the effects on the Earth view brightness temperature retrieval. The effects include those from affected calibration coefficients and the contamination of Earth view measurements. This model links the measurement bias with crosstalk coefficients, detector nonlinearity, and the ratio of Earth measurements between the sending and receiving bands. The correction

Vibrational renormalisation of the electronic band gap in hexagonal and cubic ice

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

Engel, Edgar A., E-mail: eae32@cam.ac.uk; Needs, Richard J.; Monserrat, Bartomeu

2015-12-28

Electron-phonon coupling in hexagonal and cubic water ice is studied using first-principles quantum mechanical methods. We consider 29 distinct hexagonal and cubic ice proton-orderings with up to 192 molecules in the simulation cell to account for proton-disorder. We find quantum zero-point vibrational corrections to the minimum electronic band gaps ranging from −1.5 to −1.7 eV, which leads to improved agreement between calculated and experimental band gaps. Anharmonic nuclear vibrations play a negligible role in determining the gaps. Deuterated ice has a smaller band-gap correction at zero-temperature of −1.2 to −1.4 eV. Vibrations reduce the differences between the electronic band gapsmore » of different proton-orderings from around 0.17 eV to less than 0.05 eV, so that the electronic band gaps of hexagonal and cubic ice are almost independent of the proton-ordering when quantum nuclear vibrations are taken into account. The comparatively small reduction in the band gap over the temperature range 0 − 240 K of around 0.1 eV does not depend on the proton ordering, or whether the ice is protiated or deuterated, or hexagonal, or cubic. We explain this in terms of the atomistic origin of the strong electron-phonon coupling in ice.« less

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.

S-Band propagation measurements

NASA Technical Reports Server (NTRS)

Briskman, Robert D.

1994-01-01

A geosynchronous satellite system capable of providing many channels of digital audio radio service (DARS) to mobile platforms within the contiguous United States using S-band radio frequencies is being implemented. The system is designed uniquely to mitigate both multipath fading and outages from physical blockage in the transmission path by use of satellite spatial diversity in combination with radio frequency and time diversity. The system also employs a satellite orbital geometry wherein all mobile platforms in the contiguous United States have elevation angles greater than 20 deg to both of the diversity satellites. Since implementation of the satellite system will require three years, an emulation has been performed using terrestrial facilities in order to allow evaluation of DARS capabilities in advance of satellite system operations. The major objective of the emulation was to prove the feasibility of broadcasting from satellites 30 channels of CD quality programming using S-band frequencies to an automobile equipped with a small disk antenna and to obtain quantitative performance data on S-band propagation in a satellite spatial diversity system.

The LLRF System for the S-Band RF Plants of the FERMI Linac

NASA Astrophysics Data System (ADS)

Fabris, A.; Byrd, J.; D'Auria, G.; Doolittle, L.; Gelmetti, F.; Huang, G.; Jones, J.; Milloch, M.; Predonzani, M.; Ratti, A.; Rohlev, T.; Salom, A.; Serrano, C.; Stettler, M.

2016-04-01

Specifications on electron beam quality for the operation of a linac-based free-electron laser (FEL), as FERMI in Trieste (Italy), impose stringent requirements on the stability of the electromagnetic fields of the accelerating sections. These specifications can be met only with state-of-the-art low-level RF (LLRF) systems based on advanced digital technologies. Design considerations, construction, and performance results of the FERMI digital LLRF are presented in this paper. The stability requirements derived by simulations are better than 0.1% in amplitude and 0.1° S-band in phase. The system installed in the FERMI Linac S-band RF plants has met these specifications and is in operation on a 24-h basis as a user facility. Capabilities of the system allow planning for new developments that are also described here.

Band-filling of solution-synthesized CdS nanowires.

PubMed

Puthussery, James; Lan, Aidong; Kosel, Thomas H; Kuno, Masaru

2008-02-01

The band edge optical characterization of solution-synthesized CdS nanowires (NWs) is described. Investigated wires are made through a solution-liquid-solid approach that entails the use of low-melting bimetallic catalyst particles to seed NW growth. Resulting diameters are approximately 14 nm, and lengths exceed 1 microm. Ensemble diameter distributions are approximately 13%, with corresponding intrawire diameter variations of approximately 5%. High-resolution transmission electron micrographs show that the wires are highly crystalline and have the wurtzite structure with growth along at least two directions: [0001] and [1010]. Band edge emission is observed with estimated quantum yields between approximately 0.05% and 1%. Complementary photoluminescence excitation spectra show structure consistent with the linear absorption. Carrier cooling dynamics are subsequently examined through ensemble lifetime and transient differential absorption measurements. The former reveals unexpectedly long band edge decays that extend beyond tens of nanoseconds. The latter indicates rapid intraband carrier cooling on time scales of 300-400 fs. Subsequent recovery at the band edge contains significant Auger contributions at high intensities which are usurped by other, possibly surface-related, carrier relaxation pathways at lower intensities. Furthermore, an unusual intensity-dependent transient broadening is seen, connected with these long decays. The effect likely stems from band-filling on the basis of an analysis of observed spectral shifts and line widths.

Reconfiguring crystal and electronic structures of MoS 2 by substitutional doping

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

Suh, Joonki; Tan, Teck Leong; Zhao, Weijie

Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS 2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valencemore » band maximum of multilayer MoS 2 at the Γ point pushed upward by hybridization with the Nb states. Finally, when thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.« less

Reconfiguring crystal and electronic structures of MoS 2 by substitutional doping

DOE PAGES

Suh, Joonki; Tan, Teck Leong; Zhao, Weijie; ...

2018-01-15

Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS 2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valencemore » band maximum of multilayer MoS 2 at the Γ point pushed upward by hybridization with the Nb states. Finally, when thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.« less

Investigation of the Capability of Compact Polarimetric SAR Interferometry to Estimate Forest Height

NASA Astrophysics Data System (ADS)

Zhang, Hong; Xie, Lei; Wang, Chao; Chen, Jiehong

2013-08-01

The main objective of this paper is to investigate the capability of compact Polarimetric SAR Interferometry (C-PolInSAR) on forest height estimation. For this, the pseudo fully polarimetric interferomteric (F-PolInSAR) covariance matrix is firstly reconstructed, then the three- stage inversion algorithm, hybrid algorithm, Music and Capon algorithm are applied to both C-PolInSAR covariance matrix and pseudo F-PolInSAR covariance matrix. The availability of forest height estimation is demonstrated using L-band data generated by simulator PolSARProSim and X-band airborne data acquired by East China Research Institute of Electronic Engineering, China Electronics Technology Group Corporation.

Phonon dispersions, band structures, and dielectric functions of BeO and BeS polymorphs

NASA Astrophysics Data System (ADS)

Wang, Ke-Long; Gao, Shang-Peng

2018-07-01

Structures, phonon dispersions, electronic structures, and dielectric functions of beryllium oxide (BeO) and beryllium sulfide (BeS) polymorphs are investigated by density functional theory and many-body perturbation theory. Phonon calculations indicate that both wurtzite (w-) and zincblende (zb-) structures are dynamically stable for BeO and BeS, whereas rocksalt (rs-) structures for both BeO and BeS have imaginary phonon frequencies and thus are dynamically unstable at zero pressure. Band structures for the 4 dynamically stable phases show that only w-BeO has a direct band gap. Both the one-shot G0W0 and quasiparticle self-consistent GW methods are used to correct band energies at high symmetry k-points. Bethe-Salpeter equation (BSE), which considers Coulomb correlated electron-hole pairs, is employed to deal with the computation of macroscopic dielectric functions. It is shown that BSE calculation, employing scissors operator derived by self-consistent GW method, can give dielectric functions agreeing very well with experimental measurement of w-BeO. Weak anisotropic characters can be observed for w-BeO and w-BeS. Both zb-BeS and w-BeS show high optical transition probabilities within a narrow ultraviolet energy range.

Edge effects on band gap energy in bilayer 2H-MoS{sub 2} under uniaxial strain

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

Dong, Liang; Wang, Jin; Dongare, Avinash M., E-mail: dongare@uconn.edu

2015-06-28

The potential of ultrathin MoS{sub 2} nanostructures for applications in electronic and optoelectronic devices requires a fundamental understanding in their electronic structure as a function of strain. Previous experimental and theoretical studies assume that an identical strain and/or stress state is always maintained in the top and bottom layers of a bilayer MoS{sub 2} film. In this study, a bilayer MoS{sub 2} supercell is constructed differently from the prototypical unit cell in order to investigate the layer-dependent electronic band gap energy in a bilayer MoS{sub 2} film under uniaxial mechanical deformations. The supercell contains an MoS{sub 2} bottom layer andmore » a relatively narrower top layer (nanoribbon with free edges) as a simplified model to simulate the as-grown bilayer MoS{sub 2} flakes with free edges observed experimentally. Our results show that the two layers have different band gap energies under a tensile uniaxial strain, although they remain mutually interacting by van der Waals interactions. The deviation in their band gap energies grows from 0 to 0.42 eV as the uniaxial strain increases from 0% to 6% under both uniaxial strain and stress conditions. The deviation, however, disappears if a compressive uniaxial strain is applied. These results demonstrate that tensile uniaxial strains applied to bilayer MoS{sub 2} films can result in distinct band gap energies in the bilayer structures. Such variations need to be accounted for when analyzing strain effects on electronic properties of bilayer or multilayered 2D materials using experimental methods or in continuum models.« less

Trimming of silicon ring resonator by electron beam induced compaction and strain.

PubMed

Schrauwen, J; Van Thourhout, D; Baets, R

2008-03-17

Silicon is becoming the preferable platform for future integrated components, mostly due to the mature and reliable fabrication capabilities of electronics industry. Nevertheless, even the most advanced fabrication technologies suffer from non-uniformity on wafer scale and on chip scale, causing variations in the critical dimensions of fabricated components. This is an important issue since photonic circuits, and especially cavities such as ring resonators, are extremely sensitive to these variations. In this paper we present a way to circumvent these problems by trimming using electron beam induced compaction of oxide in silicon on insulator. Volume compaction of the oxide cladding causes both changes in the refractive index and creates strain in the silicon lattice. We demonstrate a resonance wavelength red shift 4.91 nm in a silicon ring resonator.

Design of sub-Angstrom compact free-electron laser source

NASA Astrophysics Data System (ADS)

Bonifacio, Rodolfo; Fares, Hesham; Ferrario, Massimo; McNeil, Brian W. J.; Robb, Gordon R. M.

2017-01-01

In this paper, we propose for first time practical parameters to construct a compact sub-Angstrom Free Electron Laser (FEL) based on Compton backscattering. Our recipe is based on using picocoulomb electron bunch, enabling very low emittance and ultracold electron beam. We assume the FEL is operating in a quantum regime of Self Amplified Spontaneous Emission (SASE). The fundamental quantum feature is a significantly narrower spectrum of the emitted radiation relative to classical SASE. The quantum regime of the SASE FEL is reached when the momentum spread of the electron beam is smaller than the photon recoil momentum. Following the formulae describing SASE FEL operation, realistic designs for quantum FEL experiments are proposed. We discuss the practical constraints that influence the experimental parameters. Numerical simulations of power spectra and intensities are presented and attractive radiation characteristics such as high flux, narrow linewidth, and short pulse structure are demonstrated.

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.

Band warping, band non-parabolicity, and Dirac points in electronic and lattice structures

NASA Astrophysics Data System (ADS)

Resca, Lorenzo; Mecholsky, Nicholas A.; Pegg, Ian L.

2017-10-01

We illustrate at a fundamental level the physical and mathematical origins of band warping and band non-parabolicity in electronic and vibrational structures. We point out a robust presence of pairs of topologically induced Dirac points in a primitive-rectangular lattice using a p-type tight-binding approximation. We analyze two-dimensional primitive-rectangular and square Bravais lattices with implications that are expected to generalize to more complex structures. Band warping is shown to arise at the onset of a singular transition to a crystal lattice with a larger symmetry group, which allows the possibility of irreducible representations of higher dimensions, hence band degeneracy, at special symmetry points in reciprocal space. Band warping is incompatible with a multi-dimensional Taylor series expansion, whereas band non-parabolicities are associated with multi-dimensional Taylor series expansions to all orders. Still band non-parabolicities may merge into band warping at the onset of a larger symmetry group. Remarkably, while still maintaining a clear connection with that merging, band non-parabolicities may produce pairs of conical intersections at relatively low-symmetry points. Apparently, such conical intersections are robustly maintained by global topology requirements, rather than any local symmetry protection. For two p-type tight-binding bands, we find such pairs of conical intersections drifting along the edges of restricted Brillouin zones of primitive-rectangular Bravais lattices as lattice constants vary relatively to each other, until these conical intersections merge into degenerate warped bands at high-symmetry points at the onset of a square lattice. The conical intersections that we found appear to have similar topological characteristics as Dirac points extensively studied in graphene and other topological insulators, even though our conical intersections have none of the symmetry complexity and protection afforded by the latter more

Compact Magic-T using microstrip-slotline transitions

NASA Technical Reports Server (NTRS)

U-Yen, Kongpop (Inventor); Wollack, Edward J. (Inventor); Doiron, Terence (Inventor); Moseley, Samuel H. (Inventor)

2010-01-01

The design of a compact low-loss Magic-T is described. The planar Magic-T incorporates a compact microstrip-slotline tee junction and small microstrip-slotline transition area to reduce slotline radiation. The Magic-T produces broadband in-phase and out-of-phase power combiner/divider responses, has low in-band insertion loss, and small in-band phase and amplitude imbalance.

Band structure of an electron in a kind of periodic potentials with singularities

NASA Astrophysics Data System (ADS)

Hai, Kuo; Yu, Ning; Jia, Jiangping

2018-06-01

Noninteracting electrons in some crystals may experience periodic potentials with singularities and the governing Schrödinger equation cannot be defined at the singular points. The band structure of a single electron in such a one-dimensional crystal has been calculated by using an equivalent integral form of the Schrödinger equation. Both the perturbed and exact solutions are constructed respectively for the cases of a general singular weak-periodic system and its an exactly solvable version, Kronig-Penney model. Any one of them leads to a special band structure of the energy-dependent parameter, which results in an effective correction to the previous energy-band structure and gives a new explanation for forming the band structure. The used method and obtained results could be a valuable aid in the study of energy bands in solid-state physics, and the new explanation may trigger investigation to different physical mechanism of electron band structures.

Phase and group delay of S-band megawatt Cassegrain diplexer and S-band megawatt transmit filter

NASA Technical Reports Server (NTRS)

Lay, R.

1977-01-01

The phase characteristics and group delay of the S-band Megawatt Cassegrain Diplexer (MCD) and S-band Megawatt Transmit Filter (MTF) are reported. These phase measurements on the MCD and MTF were done in response to the need to obtain the total DSS hardware ground delay required for very long baseline interferometry and ranging radio metric measurements.

Retrieval of the thickness of undeformed sea ice from C-band compact polarimetric SAR images

NASA Astrophysics Data System (ADS)

Zhang, X.; Dierking, W.; Zhang, J.; Meng, J. M.; Lang, H. T.

2015-10-01

In this paper we introduce a parameter for the retrieval of the thickness of undeformed first-year sea ice that is specifically adapted to compact polarimetric SAR images. The parameter is denoted as "CP-Ratio". In model simulations we investigated the sensitivity of CP-Ratio to the dielectric constant, thickness, surface roughness, and incidence angle. From the results of the simulations we deduced optimal conditions for the thickness retrieval. On the basis of C-band CTLR SAR data, which were generated from Radarsat-2 quad-polarization images acquired jointly with helicopter-borne sea ice thickness measurements in the region of the Sea of Labrador, we tested empirical equations for thickness retrieval. An exponential fit between CP-Ratio and ice thickness provides the most reliable results. Based on a validation using other compact polarimetric SAR images from the same region we found a root mean square (rms) error of 8 cm and a maximum correlation coefficient of 0.92 for the retrieval procedure when applying it on level ice of 0.9 m mean thickness.

Terra MODIS Band 27 Electronic Crosstalk Effect and Its Removal

NASA Technical Reports Server (NTRS)

Sun, Junqiang; Xiong, Xiaoxiong; Madhavan, Sriharsha; Wenny, Brian

2012-01-01

The MODerate-resolution Imaging Spectroradiometer (MODIS) is one of the primary instruments in the NASA Earth Observing System (EOS). The first MODIS instrument was launched in December, 1999 on-board the Terra spacecraft. MODIS has 36 bands, covering a wavelength range from 0.4 micron to 14.4 micron. MODIS band 27 (6.72 micron) is a water vapor band, which is designed to be insensitive to Earth surface features. In recent Earth View (EV) images of Terra band 27, surface feature contamination is clearly seen and striping has become very pronounced. In this paper, it is shown that band 27 is impacted by electronic crosstalk from bands 28-30. An algorithm using a linear approximation is developed to correct the crosstalk effect. The crosstalk coefficients are derived from Terra MODIS lunar observations. They show that the crosstalk is strongly detector dependent and the crosstalk pattern has changed dramatically since launch. The crosstalk contributions are positive to the instrument response of band 27 early in the mission but became negative and much larger in magnitude at later stages of the mission for most detectors of the band. The algorithm is applied to both Black Body (BB) calibration and MODIS L1B products. With the crosstalk effect removed, the calibration coefficients of Terra MODIS band 27 derived from the BB show that the detector differences become smaller. With the algorithm applied to MODIS L1B products, the Earth surface features are significantly removed and the striping is substantially reduced in the images of the band. The approach developed in this report for removal of the electronic crosstalk effect can be applied to other MODIS bands if similar crosstalk behaviors occur.

A multi-band, multi-level, multi-electron model for efficient FDTD simulations of electromagnetic interactions with semiconductor quantum wells

NASA Astrophysics Data System (ADS)

Ravi, Koustuban; Wang, Qian; Ho, Seng-Tiong

2015-08-01

We report a new computational model for simulations of electromagnetic interactions with semiconductor quantum well(s) (SQW) in complex electromagnetic geometries using the finite-difference time-domain method. The presented model is based on an approach of spanning a large number of electron transverse momentum states in each SQW sub-band (multi-band) with a small number of discrete multi-electron states (multi-level, multi-electron). This enables accurate and efficient two-dimensional (2-D) and three-dimensional (3-D) simulations of nanophotonic devices with SQW active media. The model includes the following features: (1) Optically induced interband transitions between various SQW conduction and heavy-hole or light-hole sub-bands are considered. (2) Novel intra sub-band and inter sub-band transition terms are derived to thermalize the electron and hole occupational distributions to the correct Fermi-Dirac distributions. (3) The terms in (2) result in an explicit update scheme which circumvents numerically cumbersome iterative procedures. This significantly augments computational efficiency. (4) Explicit update terms to account for carrier leakage to unconfined states are derived, which thermalize the bulk and SQW populations to a common quasi-equilibrium Fermi-Dirac distribution. (5) Auger recombination and intervalence band absorption are included. The model is validated by comparisons to analytic band-filling calculations, simulations of SQW optical gain spectra, and photonic crystal lasers.

X-Band CubeSat Communication System Demonstration

NASA Technical Reports Server (NTRS)

Altunc, Serhat; Kegege, Obadiah; Bundick, Steve; Shaw, Harry; Schaire, Scott; Bussey, George; Crum, Gary; Burke, Jacob C.; Palo, Scott; O'Conor, Darren

2015-01-01

Today's CubeSats mostly operate their communications at UHF- and S-band frequencies. UHF band is presently crowded, thus downlink communications are at lower data rates due to bandwidth limitations and are unreliable due to interference. This research presents an end-to-end robust, innovative, compact, efficient and low cost S-band uplink and X-band downlink CubeSat communication system demonstration between a balloon and a Near Earth Network (NEN) ground system. Since communication systems serve as umbilical cords for space missions, demonstration of this X-band communication system is critical for successfully supporting current and future CubeSat communication needs. This research has three main objectives. The first objective is to design, simulate, and test a CubeSat S- and X-band communication system. Satellite Tool Kit (STK) dynamic link budget calculations and HFSS Simulations and modeling results have been used to trade the merit of various designs for small satellite applications. S- and X-band antennas have been tested in the compact antenna test range at Goddard Space Flight Center (GSFC) to gather radiation pattern data. The second objective is simulate and test a CubeSat compatible X-band communication system at 12.5Mbps including S-band antennas, X-band antennas, Laboratory for Atmospheric and Space Physics (LASP) /GSFC transmitter and an S-band receiver from TRL-5 to TRL-8 by the end of this effort. Different X-band communication system components (antennas, diplexers, etc.) from GSFC, other NASA centers, universities, and private companies have been investigated and traded, and a complete component list for the communication system baseline has been developed by performing analytical and numerical analysis. This objective also includes running simulations and performing trades between different X-band antenna systems to optimize communication system performance. The final objective is to perform an end-to-end X-band CubeSat communication system

Design of a Compact Quad-Channel Diplexer

NASA Astrophysics Data System (ADS)

Xu, Jin

2016-01-01

This paper presents a compact quad-channel diplexer by using two asymmetrical coupling shorted stub loaded stepped-impedance (SSLSIR) dual-band bandpass filters (DB-BPFs) to replace two single-band BPFs in a traditional BPF-based diplexer. Part of its impedance matching circuit is implemented by using a three-element lowpass T-network to acquire the desired phase shift. Detailed design procedures are given to guide the diplexer design. The fabricated quad-channel diplexer occupies a compact circuit area of 0.168λg×0.136λg. High band-to-band isolation and wide stopband performance are achieved. Good agreement is shown between the simulated and measured results.

Direct imaging of band profile in single layer MoS2 on graphite: quasiparticle energy gap, metallic edge states, and edge band bending.

PubMed

Zhang, Chendong; Johnson, Amber; Hsu, Chang-Lung; Li, Lain-Jong; Shih, Chih-Kang

2014-05-14

Using scanning tunneling microscopy and spectroscopy, we probe the electronic structures of single layer MoS2 on graphite. The apparent quasiparticle energy gap of single layer MoS2 is measured to be 2.15 ± 0.06 eV at 77 K, albeit a higher second conduction band threshold at 0.2 eV above the apparent conduction band minimum is also observed. Combining it with photoluminescence studies, we deduce an exciton binding energy of 0.22 ± 0.1 eV (or 0.42 eV if the second threshold is use), a value that is lower than current theoretical predictions. Consistent with theoretical predictions, we directly observe metallic edge states of single layer MoS2. In the bulk region of MoS2, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/MoS2 heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states.

Design of 6 MeV X-band electron linac for dual-head gantry radiotherapy system

NASA Astrophysics Data System (ADS)

Shin, Seung-wook; Lee, Seung-Hyun; Lee, Jong-Chul; Kim, Huisu; Ha, Donghyup; Ghergherehchi, Mitra; Chai, Jongseo; Lee, Byung-no; Chae, Moonsik

2017-12-01

A compact 6 MeV electron linac is being developed at Sungkyunkwan University, in collaboration with the Korea atomic energy research institute (KAERI). The linac will be used as an X-ray source for a dual-head gantry radiotherapy system. X-band technology has been employed to satisfy the size requirement of the dual-head gantry radiotherapy machine. Among the several options available, we selected a pi/2-mode, standing-wave, side-coupled cavity. This choice of radiofrequency (RF) cavity design is intended to enhance the shunt impedance of each cavity in the linac. An optimum structure of the RF cavity with a high-performance design was determined by applying a genetic algorithm during the optimization procedure. This paper describes the detailed design process for a single normal RF cavity and the entire structure, including the RF power coupler and coupling cavity, as well as the beam dynamics results.

Electronic and phononic modulation of MoS2 under biaxial strain

NASA Astrophysics Data System (ADS)

Moghadasi, A.; Roknabadi, M. R.; Ghorbani, S. R.; Modarresi, M.

2017-12-01

Dichalcogenides of transition metals are attractive material due to its unique properties. In this work, it has been investigated the electronic band structure, phonon spectrum and heat capacity of MoS2 under the applied tensile and compressive biaxial strain using the density functional theory. The Molybdenum disulfide under compressive (tensile) strain up to 6% (10%) has stable atomic structure without any negative frequency in the phonon dispersion curves. The tensile biaxial strain reduces the energy gap in the electronic band structure and the optical-acoustic gap in phonon dispersion curves. The tensile biaxial strain also increases the specific heat capacity. On the other hand, the compressive biaxial strain in this material increases phonon gap and reduces the heat capacity and the electronic band gap. The phonon softening/hardening is reported for tensile/compressive biaxial strain in MoS2. We report phonon hardening for out of plane ZA mode in the presence of both tensile and compressive strains. Results show that the linear variation of specific heat with strain (CV ∝ε) and square dependency of specific heat with the temperature (CV ∝T2) for low temperature regime. The results demonstrate that the applied biaxial strain tunes the electronic energy gap and modifies the phonon spectrum of MoS2.

Electronic structure in 1T-ZrS2 monolayer by strain

NASA Astrophysics Data System (ADS)

Xin, Qianqian; Zhao, Xu; Ma, Xu; Wu, Ninghua; Liu, Xiaomeng; Wei, Shuyi

2017-09-01

We report electronic structure of 1T-ZrS2 monolayer with biaxial strain from -10% to 15%, basing the first principles calculations. Our calculation results indicate that the band structure of ZrS2 monolayer was changed clearly. The location of conduction band minimum (CBM) and valence band maximum (VBM) changed with the variation of isotropic strain. At compressive strain, the location of CBM and VBM retains at M and Γ point, respectively. The band gap of ZrS2 monolayer decreases from 1.111 eV to 0 eV when compressive strain increases from 0% to -8%, which means that the ZrS2 monolayer turns to metal at -8% compressive strain. Under the tensile strain, the ZrS2 monolayer also retains be an indirect band gap semiconductor. The location of CBM moves from M to Γ point and the location of VBM moves along Γ-A-K-Γ direction. The band gap of ZrS2 monolayer firstly increases and then decreases and the biggest band gap is 1.577 eV at tensile strain 6%. We can see the compression strain is more effective than tensile strain in modulating band gap of 1T-ZrS2 monolayer.

Characterization of a compact 6-band multifunctional camera based on patterned spectral filters in the focal plane

NASA Astrophysics Data System (ADS)

Torkildsen, H. E.; Hovland, H.; Opsahl, T.; Haavardsholm, T. V.; Nicolas, S.; Skauli, T.

2014-06-01

In some applications of multi- or hyperspectral imaging, it is important to have a compact sensor. The most compact spectral imaging sensors are based on spectral filtering in the focal plane. For hyperspectral imaging, it has been proposed to use a "linearly variable" bandpass filter in the focal plane, combined with scanning of the field of view. As the image of a given object in the scene moves across the field of view, it is observed through parts of the filter with varying center wavelength, and a complete spectrum can be assembled. However if the radiance received from the object varies with viewing angle, or with time, then the reconstructed spectrum will be distorted. We describe a camera design where this hyperspectral functionality is traded for multispectral imaging with better spectral integrity. Spectral distortion is minimized by using a patterned filter with 6 bands arranged close together, so that a scene object is seen by each spectral band in rapid succession and with minimal change in viewing angle. The set of 6 bands is repeated 4 times so that the spectral data can be checked for internal consistency. Still the total extent of the filter in the scan direction is small. Therefore the remainder of the image sensor can be used for conventional imaging with potential for using motion tracking and 3D reconstruction to support the spectral imaging function. We show detailed characterization of the point spread function of the camera, demonstrating the importance of such characterization as a basis for image reconstruction. A simplified image reconstruction based on feature-based image coregistration is shown to yield reasonable results. Elimination of spectral artifacts due to scene motion is demonstrated.

Development of components for an S-band phased array antenna subsystem

NASA Technical Reports Server (NTRS)

1975-01-01

The system requirements, module test data, and S-band phased array subsystem test data are discussed. Of the two approaches to achieving antenna gain (mechanically steered reflector or electronically steered phased array), the phased array approach offers the greatest simplicity and lowest cost (size, weight, power, and dollars) for this medium gain. A competitive system design is described as well as hardware evaluation which will lead to timely availability of this technology for implementing such a system. The objectives of the study were: to fabricate and test six engineering model transmit/receive microelectronics modules; to design, fabricate, and test one dc and logic multilayer manifold; and to integrate and test an S-band phased array antenna subsystem composed of antenna elements, seven T/R modules, RF manifolds and dc manifold.

Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy.

PubMed

Jobst, Johannes; van der Torren, Alexander J H; Krasovskii, Eugene E; Balgley, Jesse; Dean, Cory R; Tromp, Rudolf M; van der Molen, Sense Jan

2016-11-29

High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the 'chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of.

Compact Electron Gun Based on Secondary Emission Through Ionic Bombardment

PubMed Central

Diop, Babacar; Bonnet, Jean; Schmid, Thomas; Mohamed, Ajmal

2011-01-01

We present a new compact electron gun based on the secondary emission through ionic bombardment principle. The driving parameters to develop such a gun are to obtain a quite small electron gun for an in-flight instrument performing Electron Beam Fluorescence measurements (EBF) on board of a reentry vehicle in the upper atmosphere. These measurements are useful to characterize the gas flow around the vehicle in terms of gas chemical composition, temperatures and velocity of the flow which usually presents thermo-chemical non-equilibrium. Such an instrument can also be employed to characterize the upper atmosphere if placed on another carrier like a balloon. In ground facilities, it appears as a more practical tool to characterize flows in wind tunnel studies or as an alternative to complex electron guns in industrial processes requiring an electron beam. We describe in this paper the gun which has been developed as well as its different features which have been characterized in the laboratory. PMID:22163896

Influence of electronic band topology on phonon properties in Dirac materials

NASA Astrophysics Data System (ADS)

Garate, Ion; Saha, Kush; Légaré, Katherine

2015-03-01

In Dirac materials, the interaction between electrons and long-wavelength phonons has been shown to induce and stabilize topological insulation [1-2]. Here report on a theoretical study of the converse effect, namely the influence of band topology on phonon properties. We calculate how electron-phonon interactions change the bulk phonon dispersion as a function of pressure and temperature, in both trivial and topological phases. We find that (i) topological insulators are more prone to lattice instabilities than trivial insulators, and (ii) Raman and neutron scattering measurements can be used to determine the electronic band topology. Research funded by Canada's NSERC and Québec's RQMP.

The effect of simultaneous substitution on the electronic band structure and thermoelectric properties of Se-doped Co3SnInS2 with the Kagome lattice

NASA Astrophysics Data System (ADS)

Fujioka, Masaya; Shibuya, Taizo; Nakai, Junya; Yoshiyasu, Keigo; Sakai, Yuki; Takano, Yoshihiko; Kamihara, Yoichi; Matoba, Masanori

2014-12-01

The thermoelectric properties and electronic band structures for Se-doped Co3SnInS2 were examined. The parent compound of this material (Co3Sn2S2) has two kinds of Sn sites (Sn1 and Sn2 sites). The density functional theory (DFT) calculations show that the indium substitution at the Sn2 site induces a metallic band structure, on the other hand, a semiconducting band structure is obtained from substitution at the Sn1 site. However, according to the previous reports, since the indium atom prefers to replace the tin atom at the Sn1 site rather than the Sn2 site, the resistivity of Co3SnInS2 shows semiconducting-like behavior. In this study we have demonstrated that metallic behavior and a decrease in resistivity for Se-doped Co3SnInS2 occurs without suppression of the Seebeck coefficient. From the DFT calculations, when the selenium content is above 0.5, the total crystallographic energy shows that a higher indium occupancy at Sn2 site is more stable. Therefore, it is suggested that the selenium doping suppress the site preference for indium substitution. This is one of the possible explanations for the metallic conductivity observed in Se-doped Co3SnInS2

Design of band-notched antenna with DG-CEBG

NASA Astrophysics Data System (ADS)

Jaglan, Naveen; Kanaujia, Binod Kumar; Gupta, Samir Dev; Srivastava, Shweta

2018-01-01

Ultra-wideband (UWB) disc monopole antenna with crescent shaped slot for double band-notched features is presented. Planned antenna discards worldwide interoperability for microwave access (WiMAX) band (3.3-3.6 GHz) and wireless local area network (WLAN) band (5-6 GHz). Defected ground compact electromagnetic band gap (DG-CEBG) designs are used to accomplish band notches in WiMAX and WLAN bands. Defected ground planes are utilised to achieve compactness in electromagnetic band gap (EBG) structures. The proposed WiMAX and WLAN DG-CEBG designs show a compactness of around 46% and 50%, respectively, over mushroom EBG structures. Parametric analyses of DG-CEBG design factors are carried out to control the notched frequencies. Stepwise notch transition from upper to lower frequencies is presented with incremental inductance augmentation. The proposed antenna is made-up on low-cost FR-4 substrate of complete extents as (42 × 50 × 1.6) mm3.Fabricated sample antenna shows excellent consistency in simulated and measured outcomes.

Doped Sc2C(OH)2 MXene: new type s-pd band inversion topological insulator.

PubMed

Balcı, Erdem; Akkuş, Ünal Özden; Berber, Savas

2018-04-18

The electronic structures of Si and Ge substitutionally doped Sc 2 C(OH) 2 MXene monolayers are investigated in density functional theory. The doped systems exhibit band inversion, and are found to be topological invariants in Z 2 theory. The inclusion of spin orbit coupling results in band gap openings. Our results point out that the Si and Ge doped Sc 2 C(OH) 2 MXene monolayers are topological insulators. The band inversion is observed to have a new mechanism that involves s and pd states.

Doped Sc2C(OH)2 MXene: new type s-pd band inversion topological insulator

NASA Astrophysics Data System (ADS)

Balcı, Erdem; Özden Akkuş, Ünal; Berber, Savas

2018-04-01

The electronic structures of Si and Ge substitutionally doped Sc2C(OH)2 MXene monolayers are investigated in density functional theory. The doped systems exhibit band inversion, and are found to be topological invariants in Z 2 theory. The inclusion of spin orbit coupling results in band gap openings. Our results point out that the Si and Ge doped Sc2C(OH)2 MXene monolayers are topological insulators. The band inversion is observed to have a new mechanism that involves s and pd states.

Correlation between diffuse interstellar bands (DIBs) and interstellar extinction using data from Bosscha Compact Spectrograph

NASA Astrophysics Data System (ADS)

Puspitarini, L.; Malasan, H. L.; Aprilia; Arifyanto, M. I.; Lallement, R.; Irfan, M.; Puspitaningrum, E.

2018-04-01

A longstanding challenge in astronomical spectroscopy is to uncover the carriers of diffuse interstellar bands (DIBs). They are broad absorption features due to the interstellar matter (ISM). They are seen in stellar spectra of background stars or other astronomical objects. Although we do not know utterly the carriers of the DIBs, they can be a promising tracer of the ISM. One of the interesting properties is their correlations with the interstellar (IS) extinction. For each band, the correlation has considerable dispersion and differences that possibly due to the IS physical conditions. Some DIBs are sensitive to the stellar radiation field, and some are not. To study the effect, we measured the DIB observed in Be/B stars spectra. The stars were observed by using Bosscha Compact Spectrograph at the Bosscha Observatory, Lembang, Indonesia. We performed an automated fitting of a combination of a smooth stellar continuum, the DIB profile, and a synthetic telluric transmission to the spectrum. The DIB measurements were compared to the general DIBs-extinction relationship. The correlation is found to be in good agreement with previous determinations.

Kinks in the σ band of graphene induced by electron-phonon coupling.

PubMed

Mazzola, Federico; Wells, Justin W; Yakimova, Rositza; Ulstrup, Søren; Miwa, Jill A; Balog, Richard; Bianchi, Marco; Leandersson, Mats; Adell, Johan; Hofmann, Philip; Balasubramanian, T

2013-11-22

Angle-resolved photoemission spectroscopy reveals pronounced kinks in the dispersion of the σ band of graphene. Such kinks are usually caused by the combination of a strong electron-boson interaction and the cutoff in the Fermi-Dirac distribution. They are therefore not expected for the σ band of graphene that has a binding energy of more than ≈3.5 eV. We argue that the observed kinks are indeed caused by the electron-phonon interaction, but the role of the Fermi-Dirac distribution cutoff is assumed by a cutoff in the density of σ states. The existence of the effect suggests a very weak coupling of holes in the σ band not only to the π electrons of graphene but also to the substrate electronic states. This is confirmed by the presence of such kinks for graphene on several different substrates that all show a strong coupling constant of λ≈1.

Tiny, Dusty, Galactic HI Clouds: The GALFA-HI Compact Cloud Catalog

NASA Astrophysics Data System (ADS)

Saul, Destry R.; Putman, M. E.; Peek, J. G.

2013-01-01

The recently published GALFA-HI Compact Cloud Catalog contains 2000 nearby neutral hydrogen clouds under 20' in angular size detected with a machine-vision algorithm in the Galactic Arecibo L-Band Feed Array HI survey (GALFA-HI). At a distance of 1kpc, the compact clouds would typically be 1 solar mass and 1pc in size. We observe that nearly all of the compact clouds that are classified as high velocity (> 90 km/s) are near previously-identified high velocity complexes. We separate the compact clouds into populations based on velocity, linewidth, and position. We have begun to search for evidence of dust in these clouds using IRIS and have detections in several populations.

Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy

PubMed Central

Jobst, Johannes; van der Torren, Alexander J. H.; Krasovskii, Eugene E.; Balgley, Jesse; Dean, Cory R.; Tromp, Rudolf M.; van der Molen, Sense Jan

2016-01-01

High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the ‘chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of. PMID:27897180

A PROFILE ANALYSIS OF RAMAN-SCATTERED O VI BANDS AT 6825 Å AND 7082 Å IN SANDULEAK’S STAR

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

Heo, Jeong-Eun; Lee, Hee-Won; Angeloni, Rodolfo

2016-12-20

We present a detailed modeling of the two broad bands observed at 6825 and 7082 Å in Sanduleak’s star, a controversial object in the Large Magellanic Cloud. These bands are known to originate from Raman scattering of O vi  λ λ 1032 and 1038 photons with atomic hydrogen and are only observed in bona fide symbiotic stars. Our high-resolution spectrum obtained with the Magellan Inamori Kyocera Echelle spectrograph at the Magellan-Clay Telescope reveals, quite surprisingly, that the profiles of the two bands look very different: while the Raman 6825 Å band shows a single broad profile with a redward extendedmore » bump, the Raman 7082 Å band exhibits a distinct triple-peak profile. Our model suggests that the O vi emission nebula can be decomposed into a red, blue, and central emission region from an accretion disk, a bipolar outflow, and a further compact, optically thick region. We also perform Monte Carlo simulations with the aim of fitting the observed flux ratio F (6825)/ F (7082) ∼ 4.5, which indicates that the neutral region in Sanduleak’s star is characterized by the column density N{sub Hi} ∼ 1 × 10{sup 23} cm{sup −2}.« less

Note: Self-biased voltage to suppress secondary electrons by a ZnO varistor in a compact pulsed neutron generator

NASA Astrophysics Data System (ADS)

Yang, Z.; Li, X.; Li, J.; Long, J. D.; Lan, C. H.; Wang, T.; Dong, P.; He, J. L.

2017-03-01

A large amount of back streaming electrons will bring about a part of current drain on power supply, cause sparking or high-voltage breakdowns, and affect the neutron yield and waveform for a compact sealed-tube pulsed neutron generator. A novel idea which uses a ZnO varistor to provide a constant self-biased voltage to suppress the secondary electrons is introduced. The I-V curve for the ZnO varistor was measured in the experiment. The effects of suppressing the secondary electrons were investigated using a ZnO varistor, linear resistors, and an independent power supply, respectively. The results show that the secondary electrons are suppressed effectively by the compact ZnO varistor, while not increasing the size and the component of the device. It is a promising design for compact sealed-tube neutron generators.

Electron band bending of polar, semipolar and non-polar GaN surfaces

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

Bartoš, I.; Romanyuk, O., E-mail: romanyuk@fzu.cz; Houdkova, J.

2016-03-14

The magnitudes of the surface band bending have been determined by X-ray photoelectron spectroscopy for polar, semipolar, and non-polar surfaces of wurtzite GaN crystals. All surfaces have been prepared from crystalline GaN samples grown by the hydride-vapour phase epitaxy and separated from sapphire substrates. The Ga 3d core level peak shifts have been used for band bending determination. Small band bending magnitudes and also relatively small difference between the band bendings of the surfaces with opposite polarity have been found. These results point to the presence of electron surface states of different amounts and types on surfaces of different polaritymore » and confirm the important role of the electron surface states in compensation of the bound surface polarity charges in wurtzite GaN crystals.« less

Towards TeV-scale electron-positron collisions: the Compact Linear Collider (CLIC)

NASA Astrophysics Data System (ADS)

Doebert, Steffen; Sicking, Eva

2018-02-01

The Compact Linear Collider (CLIC), a future electron-positron collider at the energy frontier, has the potential to change our understanding of the universe. Proposed to follow the Large Hardron Collider (LHC) programme at CERN, it is conceived for precision measurements as well as for searches for new phenomena.

Strain distributions and their influence on electronic structures of WSe2-MoS2 laterally strained heterojunctions

NASA Astrophysics Data System (ADS)

Zhang, Chendong; Li, Ming-Yang; Tersoff, Jerry; Han, Yimo; Su, Yushan; Li, Lain-Jong; Muller, David A.; Shih, Chih-Kang

2018-02-01

Monolayer transition metal dichalcogenide heterojunctions, including vertical and lateral p-n junctions, have attracted considerable attention due to their potential applications in electronics and optoelectronics. Lattice-misfit strain in atomically abrupt lateral heterojunctions, such as WSe2-MoS2, offers a new band-engineering strategy for tailoring their electronic properties. However, this approach requires an understanding of the strain distribution and its effect on band alignment. Here, we study a WSe2-MoS2 lateral heterojunction using scanning tunnelling microscopy and image its moiré pattern to map the full two-dimensional strain tensor with high spatial resolution. Using scanning tunnelling spectroscopy, we measure both the strain and the band alignment of the WSe2-MoS2 lateral heterojunction. We find that the misfit strain induces type II to type I band alignment transformation. Scanning transmission electron microscopy reveals the dislocations at the interface that partially relieve the strain. Finally, we observe a distinctive electronic structure at the interface due to hetero-bonding.

8-band and 14-band kp modeling of electronic band structure and material gain in Ga(In)AsBi quantum wells grown on GaAs and InP substrates

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

Gladysiewicz, M.; Wartak, M. S.; Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5

The electronic band structure and material gain have been calculated for GaAsBi/GaAs quantum wells (QWs) with various bismuth concentrations (Bi ≤ 15%) within the 8-band and 14-band kp models. The 14-band kp model was obtained by extending the standard 8-band kp Hamiltonian by the valence band anticrossing (VBAC) Hamiltonian, which is widely used to describe Bi-related changes in the electronic band structure of dilute bismides. It has been shown that in the range of low carrier concentrations n < 5 × 10{sup 18 }cm{sup −3}, material gain spectra calculated within 8- and 14-band kp Hamiltonians are similar. It means that the 8-band kp model can be usedmore » to calculate material gain in dilute bismides QWs. Therefore, it can be applied to analyze QWs containing new dilute bismides for which the VBAC parameters are unknown. Thus, the energy gap and electron effective mass for Bi-containing materials are used instead of VBAC parameters. The electronic band structure and material gain have been calculated for 8 nm wide GaInAsBi QWs on GaAs and InP substrates with various compositions. In these QWs, Bi concentration was varied from 0% to 5% and indium concentration was tuned in order to keep the same compressive strain (ε = 2%) in QW region. For GaInAsBi/GaAs QW with 5% Bi, gain peak was determined to be at about 1.5 μm. It means that it can be possible to achieve emission at telecommunication windows (i.e., 1.3 μm and 1.55 μm) for GaAs-based lasers containing GaInAsBi/GaAs QWs. For GaInAsBi/Ga{sub 0.47}In{sub 0.53}As/InP QWs with 5% Bi, gain peak is predicted to be at about 4.0 μm, i.e., at the wavelengths that are not available in current InP-based lasers.« less

Compact Planar Ultrawideband Antennas with 3.5/5.2/5.8 GHz Triple Band-Notched Characteristics for Internet of Things Applications.

PubMed

Dong, Jian; Li, Qianqian; Deng, Lianwen

2017-02-10

Ultrawideband (UWB) antennas, as core devices in high-speed wireless communication, are widely applied to mobile handsets, wireless sensor networks, and Internet of Things (IoT). A compact printed monopole antenna for UWB applications with triple band-notched characteristics is proposed in this paper. The antenna has a very compact size of 10 x 16 mm2 and is composed of a square slotted radiation patch and a narrow rectangular ground plane on the back of the substrate. First, by etching a pair of inverted T-shaped slots at the bottom of the radiation patch, one notched band at 5-6 GHz for rejecting the Wireless Local Area Network (WLAN) is generated. Then, by cutting a comb-shaped slot on the top of the radiation patch, a second notched band for rejecting 3.5 GHz Worldwide Interoperability for Microwave Access (WiMAX) is obtained. Further, by cutting a pair of rectangular slots and a C-shaped slot as well as adding a pair of small square parasitic patches at the center of the radiating patch, two separate notched bands for rejecting 5.2 GHz lower WLAN and 5.8 GHz upper WLAN are realized, respectively. Additionally, by integrating the slotted radiation patch with the narrow rectangular ground plane, an enhanced impedance bandwidth can be achieved, especially at the higher band. The antenna consists of linear symmetrical sections only and is easy for fabrication and fine-tuning. The measured results show that the designed antenna provides a wide impedance bandwidth of 150% from 2.12 to 14.80 GHz for VSWR < 2, except for three notched bands of 3.36-4.16, 4.92-5.36, and 5.68-6.0 GHz. Additionally, the antenna exhibits nearly omnidirectional radiation characteristics, low gain at the stopbands, and flat group delay over the whole UWB except at the stopbands. Simulated and experimental results show that the proposed antenna can provide good frequency-domain and time-domain performances at desired UWB frequencies and be an attractive candidate for portable Io

Status of a compact electron cyclotron resonance ion source for National Institute of Radiological Sciences-930 cyclotron.

PubMed

Hojo, S; Katagiri, K; Nakao, M; Sugiura, A; Muramatsu, M; Noda, A; Okada, T; Takahashi, Y; Komiyama, A; Honma, T; Noda, K

2014-02-01

The Kei-source is a compact electron cyclotron resonance ion source using only permanent magnets and a frequency of 10 GHz. It was developed at the National Institute of Radiological Sciences (NIRS) for producing C(4+) ions oriented for high-energy carbon therapy. It has also been used as an ion source for the NIRS-930 cyclotron. Its microwave band region for the traveling-wave-tube amplifier and maximum output power are 8-10 GHz and 350 W, respectively. Since 2006, it has provided various ion beams such as proton, deuteron, carbon, oxygen, and neon with sufficient intensity (200 μA for proton and deuteron, 50 μA for C(4+), for example) and good stability for radioisotope production, tests of radiation damage, and basic research experiments. Its horizontal and vertical emittances were measured using a screen monitor and waist-scan. The present paper reports the current status of the Kei-source.

Design of an Electronic Chest-Band

NASA Astrophysics Data System (ADS)

Atakan, R.; Acikgoz Tufan, H.; Baskan, H.; Eryuruk, S. H.; Akalin, N.; Kose, H.; Li, Y.; Kursun Bahadir, S.; Kalaoglu, F.

2017-10-01

In this study, an electronic chest strap prototype was designed for measuring fitness level, performance optimization, mobility and fall detection. Knitting technology is used for production by using highly elastic nylon yarn. In order to evaluate comfort performance of the garment, yarn strength and elongation, air permeability, moisture management and FAST tests (Fabric Assurance Fabric Testing) were carried out, respectively. After testing of textile part of the chest band, IMU sensors were integrated onto the garment by means of conductive yarns. Electrical conductivity of the circuit was also assessed at the end. Results indicated that the weight and the thickness of the product are relatively high for sports uses and it has a negative impact on comfort properties. However, it is highly stretchable and moisture management properties are still in acceptable values. From the perspective of possible application areas, developed smart chest band in this research could be used in sports facilities as well as health care applications for elderly and disabled people.

Development of an S-band cavity Beam Position Monitor for ATF2

NASA Astrophysics Data System (ADS)

Heo, A.; Kim, E.-S.; Kim, H.; Son, D.; Honda, Y.; Tauchi, T.

2013-04-01

We have developed an S-band cavity Beam Position Monitor (BPM) in order to measure the position of an electron beam in the final focus area at ATF2, which is the test facility for the final focus design for the International Linear Collider (ILC). The lattice of the ILC Beam Delivery System (BDS) has been modified, requiring a larger physical aperture of 40 mm in the final focus area. The beam orbit measurement in this area is now covered with high resolution S-Band cavity BPMs. In this paper we summarize the design of the cavity BPM and the first experimental results. The calibration slopes were measured as 0.87 counts/μm in the x-coordinate direction and 1.16 counts/μm in the y-coordinate direction.

Simultaneous Observations of Lower Band Chorus Emissions at the Equator and Microburst Precipitating Electrons in the Ionosphere

NASA Astrophysics Data System (ADS)

Mozer, F. S.; Agapitov, O. V.; Blake, J. B.; Vasko, I. Y.

2018-01-01

On 11 December 2016 at 00:12:30 UT, Van Allen Probe-B, at the equator and near midnight, and AC6-B, in the ionosphere, were on magnetic field lines whose 100 km altitude foot points were separated by 600 km. Van Allen Probe-B observed a 30 s burst of lower band chorus waves (with maximum amplitudes >1 nT) at the same time that AC6-B observed intense microburst electrons in the loss cone. One second averaged variations of the chorus intensity and the microburst electron flux were well correlated. The low-altitude electron flux expected from quasi-linear diffusion of the equatorial electrons by the equatorial chorus is in excellent agreement with the observed, 1 s averaged, low-altitude electron flux. However, the large-amplitude, <0.5 s duration, low-altitude electron pulses require nonlinear processes for their explanation.

Nonadiabatic dynamics of electron scattering from adsorbates in surface bands

NASA Astrophysics Data System (ADS)

Gumhalter, Branko; Šiber, Antonio; Buljan, Hrvoje; Fauster, Thomas

2008-10-01

We present a comparative study of nonadiabatic dynamics of electron scattering in quasi-two-dimensional surface band which is induced by the long-range component of the interactions with a random array of adsorbates. Using three complementary model descriptions of intraband spatiotemporal propagation of quasiparticles that go beyond the single-adsorbate scattering approach we are able to identify distinct subsequent regimes of evolution of an electron following its promotion into an unoccupied band state: (i) early quadratic or ballistic decay of the initial-state survival probability within the Heisenberg uncertainty window, (ii) preasymptotic exponential decay governed by the self-consistent Fermi golden rule scattering rate, and (iii) asymptotic decay described by a combined inverse power-law and logarithmic behavior. The developed models are applied to discuss the dynamics of intraband adsorbate-induced scattering of hot electrons excited into the n=1 image-potential band on Cu(100) surface during the first stage of a two-photon photoemission process. Estimates of crossovers between the distinct evolution regimes enable assessments of the lifespan of a standard quasiparticle behavior and thereby of the range of applicability of the widely used Fermi golden rule and optical Bloch equations approach for description of adsorbate-induced quasiparticle decay and dephasing in ultrafast experiments.

Band offset and electron affinity of MBE-grown SnSe2

NASA Astrophysics Data System (ADS)

Zhang, Qin; Li, Mingda Oscar; Lochocki, Edward B.; Vishwanath, Suresh; Liu, Xinyu; Yan, Rusen; Lien, Huai-Hsun; Dobrowolska, Malgorzata; Furdyna, Jacek; Shen, Kyle M.; Cheng, Guangjun; Hight Walker, Angela R.; Gundlach, David J.; Xing, Huili G.; Nguyen, N. V.

2018-01-01

SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is experimentally confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by molecular beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band minimum is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band minimum. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect transistor that can potentially achieve high on-currents.

Electronic structure investigation of MoS2 and MoSe2 using angle-resolved photoemission spectroscopy and ab initio band structure studies.

PubMed

Mahatha, S K; Patel, K D; Menon, Krishnakumar S R

2012-11-28

Angle-resolved photoemission spectroscopy (ARPES) and ab initio band structure calculations have been used to study the detailed valence band structure of molybdenite, MoS(2) and MoSe(2). The experimental band structure obtained from ARPES has been found to be in good agreement with the theoretical calculations performed using the linear augmented plane wave (LAPW) method. In going from MoS(2) to MoSe(2), the dispersion of the valence bands decreases along both k(parallel) and k(perpendicular), revealing the increased two-dimensional character which is attributed to the increasing interlayer distance or c/a ratio in these compounds. The width of the valence band and the band gap are also found to decrease, whereas the valence band maxima shift towards the higher binding energy from MoS(2) to MoSe(2).

Electronic structure of LiGaS 2

NASA Astrophysics Data System (ADS)

Atuchin, V. V.; Isaenko, L. I.; Kesler, V. G.; Lobanov, S.; Huang, H.; Lin, Z. S.

2009-04-01

X-ray photoelectron spectroscopy (XPS) measurement has been performed to determine the valence band structure of LiGaS 2 crystals. The experimental measurement is compared with the electronic structure obtained from the density functional calculations. It is found that the Ga 3d states in the XPS spectrum are much higher than the calculated results. In order to eliminate this discrepancy, the LDA+ U method is employed and reasonable agreement is achieved. Further calculations show that the difference of the linear and nonlinear optical coefficients between LDA and LDA+ U calculations is negligibly small, indicating that the Ga 3d states are actually independent of the excited properties of LiGaS 2 crystals since they are located at a very deep position in the valence bands.

Enhanced tunable narrow-band THz emission from laser-modulated electron beams

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

Xiang, D.; Stupakov, G.; /SLAC

2009-06-19

We propose and analyze a scheme to generate enhanced narrow-band terahertz (THz) radiation through down-conversion of the frequency of optical lasers using laser-modulated electron beams. In the scheme the electron beam is first energy modulated by two lasers with wave numbers k{sub 1} and k2, respectively. After passing through a dispersion section, the energy modulation is converted to density modulation. Due to the nonlinear conversion process, the beam will have density modulation at wave number k = nk{sub 1} + mk{sub 2}, where n and m are positive or negative integers. By properly choosing the parameters for the lasers andmore » dispersion section, one can generate density modulation at THz frequency in the beam using optical lasers. This density-modulated beam can be used to generate powerful narrow-band THz radiation. Since the THz radiation is in tight synchronization with the lasers, it should provide a high temporal resolution for the optical-pump THz-probe experiments. The central frequency of the THz radiation can be easily tuned by varying the wavelength of the two lasers and the energy chirp of the electron beam. The proposed scheme is in principle able to generate intense narrow-band THz radiation covering the whole THz range and offers a promising way towards the tunable intense narrow-band THz sources.« less

TDRSS multimode transponder program S-band modification

NASA Technical Reports Server (NTRS)

Mackey, J. E.

1975-01-01

The S-Band TDRS multimode transponder and its associated ground support equipment is described. The transponder demonstrates candidate modulation techniques to provide the required information for the design of an eventual S-band transponder suitable for installation in a user satellite, capable of operating as part of a Tracking and Data Relay Satellite (TDRS) system.

I. Final Report for DOE SBIR Phase I Project DE-SC0013795 Final Report for DOE SBIR Phase I Project DE-SC0013795 Microtron-based Compact, Portable Gamma-Ray Source

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

Abrams, Robert J.

Microtron-based Compact, Portable Gamma-Ray Source. The objective of Phase I of this project was to produce a conceptual design of a prototype compact microtron electron accelerator, which could be designed, built, and demonstrated in Phase II of the project. The conceptual design study included an analysis of the parameters of the microtron and its components, and the expected performance of the prototype microtron as a source of x-rays and/or RF neutrons in the MeV energy range. The major components of the microtron are the magnet, the accelerating system, the power system, the vacuum system, the control system, the beam extractionmore » system and the targets to produce x-rays (and/or neutrons). Our objectives for the design of the prototype were for it to be compact, cost-effective, capable of producing high intensity x-ray (an/or neutron) fluxes. In addition, the prototype was to be easily assembled and disassembled so that components could be easily replaced. The main parameters for the prototype are the following: the range of electron kinetic energies, the output power, the RF frequency band (X-band, C-band, or S-Band), the type of injection (Type I or Type II), the magnet type, i.e. permanent magnet, electromagnet, or a hybrid combination of permanent and electromagnet. The results of the Phase I study and analysis for a prototype microtron are the following: The electron energy range can be varied from below 6 MeV to 9 MeV, the optimal frequency range is S-Band (2-4 GHz) RF frequency, Type II injection (described below), and the magnet type is the hybrid version. The prototype version will be capable of producing gamma ray doses of ~1800 R/min-m and neutron fluxes of up to ~6 x 10 10 n/s with appropriate targets. The results of the Phase I study and analysis are provided below. The proposed Phase II plan was to demonstrate the prototype at low beam power. In the subsequent Phase III, high power tests would be performed, and the design of commercial

Electronic structure of shandite Co3Sn2S2

NASA Astrophysics Data System (ADS)

Dedkov, Y. S.; Holder, M.; Molodtsov, S. L.; Rosner, H.

2008-03-01

The electronic structure of shandite Co3Sn2S2 was determined by photoelectron spectroscopy and compared with ab initio band structure calculations. Presented results give evidence that this compound has half-metallic ferromagnetic properties.

Reconstruction of Band Structure Induced by Electronic Nematicity in an FeSe Superconductor

NASA Astrophysics Data System (ADS)

Nakayama, K.; Miyata, Y.; Phan, G. N.; Sato, T.; Tanabe, Y.; Urata, T.; Tanigaki, K.; Takahashi, T.

2014-12-01

We have performed high-resolution angle-resolved photoemission spectroscopy on an FeSe superconductor (Tc˜8 K ), which exhibits a tetragonal-to-orthorhombic structural transition at Ts˜90 K . At low temperature, we found splitting of the energy bands as large as 50 meV at the M point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to T ˜110 K , slightly above Ts, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives rise to a van Hove singularity within 5 meV of the Fermi energy. The present result strongly suggests that this unusual electronic state is responsible for the unconventional superconductivity in FeSe.

Cd-free buffer layer materials on Cu2ZnSn(SxSe1-x)4: Band alignments with ZnO, ZnS, and In2S3

NASA Astrophysics Data System (ADS)

Barkhouse, D. Aaron R.; Haight, Richard; Sakai, Noriyuki; Hiroi, Homare; Sugimoto, Hiroki; Mitzi, David B.

2012-05-01

The heterojunctions formed between Cu2ZnSn(SxSe1-x)4 (CZTSSe) and three Cd-free n-type buffers, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission and photovoltage spectroscopy. The electronic properties including the Fermi level location at the interface, band bending in the CZTSSe substrate, and valence and conduction band offsets were determined and correlated with device properties. We also describe a method for determining the band bending in the buffer layer and demonstrate this for the In2S3/CZTSSe system. The chemical bath deposited In2S3 buffer is found to have near optimal conduction band offset (0.15 eV), enabling the demonstration of Cd-free In2S3/CZTSSe solar cells with 7.6% power conversion efficiency.

Proportionality between Doppler noise and integrated signal path electron density validated by differenced S-X range

NASA Technical Reports Server (NTRS)

Berman, A. L.

1977-01-01

Observations of Viking differenced S-band/X-band (S-X) range are shown to correlate strongly with Viking Doppler noise. A ratio of proportionality between downlink S-band plasma-induced range error and two-way Doppler noise is calculated. A new parameter (similar to the parameter epsilon which defines the ratio of local electron density fluctuations to mean electron density) is defined as a function of observed data sample interval (Tau) where the time-scale of the observations is 15 Tau. This parameter is interpreted to yield the ratio of net observed phase (or electron density) fluctuations to integrated electron density (in RMS meters/meter). Using this parameter and the thin phase-changing screen approximation, a value for the scale size L is calculated. To be consistent with Doppler noise observations, it is seen necessary for L to be proportional to closest approach distance a, and a strong function of the observed data sample interval, and hence the time-scale of the observations.

Electronic transport in disordered MoS2 nanoribbons

NASA Astrophysics Data System (ADS)

Ridolfi, Emilia; Lima, Leandro R. F.; Mucciolo, Eduardo R.; Lewenkopf, Caio H.

2017-01-01

We study the electronic structure and transport properties of zigzag and armchair monolayer molybdenum disulfide nanoribbons using an 11-band tight-binding model that accurately reproduces the material's bulk band structure near the band gap. We study the electronic properties of pristine zigzag and armchair nanoribbons, paying particular attention to the edges states that appear within the MoS2 bulk gap. By analyzing both their orbital composition and their local density of states, we find that in zigzag-terminated nanoribbons these states can be localized at a single edge for certain energies independent of the nanoribbon width. We also study the effects of disorder in these systems using the recursive Green's function technique. We show that for the zigzag nanoribbons, the conductance due to the edge states is strongly suppressed by short-range disorder such as vacancies. In contrast, the local density of states still shows edge localization. We also show that long-range disorder has a small effect on the transport properties of nanoribbons within the bulk gap energy window.

Retrieval of the thickness of undeformed sea ice from simulated C-band compact polarimetric SAR images

NASA Astrophysics Data System (ADS)

Zhang, Xi; Dierking, Wolfgang; Zhang, Jie; Meng, Junmin; Lang, Haitao

2016-07-01

In this paper we introduce a parameter for the retrieval of the thickness of undeformed first-year sea ice that is specifically adapted to compact polarimetric (CP) synthetic aperture radar (SAR) images. The parameter is denoted as the "CP ratio". In model simulations we investigated the sensitivity of the CP ratio to the dielectric constant, ice thickness, ice surface roughness, and radar incidence angle. From the results of the simulations we deduced optimal sea ice conditions and radar incidence angles for the ice thickness retrieval. C-band SAR data acquired over the Labrador Sea in circular transmit and linear receive (CTLR) mode were generated from RADARSAT-2 quad-polarization images. In comparison with results from helicopter-borne measurements, we tested different empirical equations for the retrieval of ice thickness. An exponential fit between the CP ratio and ice thickness provides the most reliable results. Based on a validation using other compact polarimetric SAR images from the same region, we found a root mean square (rms) error of 8 cm and a maximum correlation coefficient of 0.94 for the retrieval procedure when applying it to level ice between 0.1 and 0.8 m thick.

Compact multi-band fluorescent microscope with an electrically tunable lens for autofocusing

PubMed Central

Wang, Zhaojun; Lei, Ming; Yao, Baoli; Cai, Yanan; Liang, Yansheng; Yang, Yanlong; Yang, Xibin; Li, Hui; Xiong, Daxi

2015-01-01

Autofocusing is a routine technique in redressing focus drift that occurs in time-lapse microscopic image acquisition. To date, most automatic microscopes are designed on the distance detection scheme to fulfill the autofocusing operation, which may suffer from the low contrast of the reflected signal due to the refractive index mismatch at the water/glass interface. To achieve high autofocusing speed with minimal motion artifacts, we developed a compact multi-band fluorescent microscope with an electrically tunable lens (ETL) device for autofocusing. A modified searching algorithm based on equidistant scanning and curve fitting is proposed, which no longer requires a single-peak focus curve and then efficiently restrains the impact of external disturbance. This technique enables us to achieve an autofocusing time of down to 170 ms and the reproductivity of over 97%. The imaging head of the microscope has dimensions of 12 cm × 12 cm × 6 cm. This portable instrument can easily fit inside standard incubators for real-time imaging of living specimens. PMID:26601001

Excitation of the Werner bands of H2 by electron impact

NASA Technical Reports Server (NTRS)

Stone, E. J.; Zipf, E. C.

1972-01-01

Absolute cross sections for the excitation of the H2 Werner band system were measured from energy threshold to 300 eV for electron impact on H2. The bands were observed in emission in the wavelength region 1100A to 1250A. The measured cross sections were compared with published transition probabilities, leading to the conclusion that the Werner bands are suitable as the basis for a relative spectral response calibration only when the bands are observed under sufficiently high resolution. The effect of the perturbation between the C 1Pi u and B 1 Sigma-u states of the hydrogen molecule was clearly observed in anomalies in the rotational intensity distribution in bands of the (3 v '') progression.

Electron Microscopic Examination of Irradiated TRISO Coated Particles of Compact 6-3-2 of AGR-1 Experiment

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

Van Rooyen, Isabella Johanna; Demkowicz, Paul Andrew; Riesterer, Jessica Lori

2012-12-01

The electron microscopic examination of selected irradiated TRISO coated particles of the AGR-1 experiment of fuel compact 6-3-2 are presented in this report. Compact 6-3-2 refers to the compact in Capsule 6 at level 3 of Stack 2. The fuel used in capsule 6 compacts, are called the “baseline” fuel as it is fabricated with refined coating process conditions used to fabricate historic German fuel, because of its excellent irradiation performance with UO2 kernels. The AGR-1 fuel is however made of low-enriched uranium oxycarbide (UCO). Kernel diameters are approximately 350 µm with a U-235 enrichment of approximately 19.7%. Compact 6-3-2more » has been irradiated to 11.3% FIMA compact average burn-up with a time average, volume average temperature of 1070.2°C and with a compact average fast fluence of 2.38E21 n/cm« less

Compact Cr:ZnS Channel Waveguide Laser Operating at 2333 nm

DTIC Science & Technology

2014-03-24

B. Mirov and V. V. Federov, “Mid-IR microchip laser : ZnS:Cr2+ laser with saturable absorber material,” (US Patent No 6,960,486., 2009). 23. A...Compact Cr:ZnS channel waveguide laser operating at 2333 nm John R. Macdonald,1* Stephen J. Beecher,2 Adam Lancaster,1 Patrick A. Berry,3 Kenneth...35294, USA *J.R.Macdonald@hw.ac.uk Abstract: A compact mid-infrared channel waveguide laser is demonstrated in Cr:ZnS with a view to power scaling

High quality single shot ultrafast MeV electron diffraction from a photocathode radio-frequency gun.

PubMed

Fu, Feichao; Liu, Shengguang; Zhu, Pengfei; Xiang, Dao; Zhang, Jie; Cao, Jianming

2014-08-01

A compact ultrafast electron diffractometer, consisting of an s-band 1.6 cell photocathode radio-frequency gun, a multi-function changeable sample chamber, and a sensitive relativistic electron detector, was built at Shanghai Jiao Tong University. High-quality single-shot transmission electron diffraction patterns have been recorded by scattering 2.5 MeV electrons off single crystalline gold and polycrystalline aluminum samples. The high quality diffraction pattern indicates an excellent spatial resolution, with the ratio of the diffraction ring radius over the ring rms width beyond 10. The electron pulse width is estimated to be about 300 fs. The high temporal and spatial resolution may open new opportunities in various areas of sciences.

High quality single shot ultrafast MeV electron diffraction from a photocathode radio-frequency gun

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

Fu, Feichao; Liu, Shengguang; Zhu, Pengfei

2014-08-15

A compact ultrafast electron diffractometer, consisting of an s-band 1.6 cell photocathode radio-frequency gun, a multi-function changeable sample chamber, and a sensitive relativistic electron detector, was built at Shanghai Jiao Tong University. High-quality single-shot transmission electron diffraction patterns have been recorded by scattering 2.5 MeV electrons off single crystalline gold and polycrystalline aluminum samples. The high quality diffraction pattern indicates an excellent spatial resolution, with the ratio of the diffraction ring radius over the ring rms width beyond 10. The electron pulse width is estimated to be about 300 fs. The high temporal and spatial resolution may open new opportunities inmore » various areas of sciences.« less

Electron Elevator: Excitations across the Band Gap via a Dynamical Gap State.

PubMed

Lim, A; Foulkes, W M C; Horsfield, A P; Mason, D R; Schleife, A; Draeger, E W; Correa, A A

2016-01-29

We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap.

Space shuttle engineering and operations support. Isolation between the S-band quad antenna and the S-band payload antenna. Engineering systems analysis

NASA Technical Reports Server (NTRS)

Lindsey, J. F.

1976-01-01

The isolation between the upper S-band quad antenna and the S-band payload antenna on the shuttle orbiter is calculated using a combination of plane surface and curved surface theories along with worst case values. A minimum value of 60 db isolation is predicted based on recent antenna pattern data, antenna locations on the orbiter, curvature effects, dielectric covering effects and edge effects of the payload bay. The calculated value of 60 db is significantly greater than the baseline value of 40 db. Use of the new value will result in the design of smaller, lighter weight and less expensive filters for S-band transponder and the S-band payload interrogator.

Band alignment of ZnO/multilayer MoS{sub 2} interface determined by x-ray photoelectron spectroscopy

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

Liu, Xinke, E-mail: xkliu@szu.edu.cn, E-mail: liuwj@szu.edu.cn; He, Jiazhu; Chen, Le

2016-08-15

The energy band alignment between ZnO and multilayer (ML)-MoS{sub 2} was characterized using high-resolution x-ray photoelectron spectroscopy. The ZnO film was deposited using an atomic layer deposition tool, and ML-MoS{sub 2} was grown by chemical vapor deposition. A valence band offset (VBO) of 3.32 eV and a conduction band offset (CBO) of 1.12 eV were obtained for the ZnO/ML-MoS{sub 2} interface without any treatment. With CHF{sub 3} plasma treatment, a VBO and a CBO across the ZnO/ML-MoS{sub 2} interface were found to be 3.54 eV and 1.34 eV, respectively. With the CHF{sub 3} plasma treatment, the band alignment of the ZnO/ML-MoS{sub 2} interface hasmore » been changed from type II or staggered band alignment to type III or misaligned one, which favors the electron-hole pair separation. The band alignment difference is believed to be dominated by the down-shift in the core level of Zn 2p or the interface dipoles, which is caused by the interfacial layer rich in F.« less

A Wide-Band High-Gain Compact SIS Receiver Utilizing a 300-μW SiGe IF LNA

NASA Astrophysics Data System (ADS)

Montazeri, Shirin; Grimes, Paul K.; Tong, Cheuk-Yu Edward; Bardin, Joseph C.

2017-06-01

Low-power low-noise amplifiers integrated with superconductor-insulator-superconductor (SIS) mixers are required to enable implementation of large-scale focal plane arrays. In this work, a 220-GHz SIS mixer has been integrated with a high-gain broad-band low-power IF amplifier into a compact receiver module. The low noise amplifier (LNA) was specifically designed to match to the SIS output impedance and contributes less than 7 K to the system noise temperature over the 4-8 GHz IF frequency range. A receiver noise temperature of 30-45 K was measured for a local oscillator frequency of 220 GHz over an IF spanning 4-8 GHz. The LNA power dissipation was only 300-μW. To the best of the authors' knowledge, this is the lowest power consumption reported for a high-gain wide-band LNA directly integrated with an SIS mixer.

Multi-Band Multi-Tone Tunable Millimeter-Wave Frequency Synthesizer For Satellite Beacon Transmitter

NASA Technical Reports Server (NTRS)

Simons, Rainee N.; Wintucky, Edwin G.

2016-01-01

This paper presents the design and test results of a multi-band multi-tone tunable millimeter-wave frequency synthesizer, based on a solid-state frequency comb generator. The intended application of the synthesizer is in a satellite beacon transmitter for radio wave propagation studies at K-band (18 to 26.5 GHz), Q-band (37 to 42 GHz), and E-band (71 to 76 GHz). In addition, the architecture for a compact beacon transmitter, which includes the multi-tone synthesizer, polarizer, horn antenna, and power/control electronics, has been investigated for a notional space-to-ground radio wave propagation experiment payload on a small satellite. The above studies would enable the design of robust high throughput multi-Gbps data rate future space-to-ground satellite communication links.

Discrete Electronic Bands in Semiconductors and Insulators: Potential High-Light-Yield Scintillators

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

Shi, Hongliang; Du, Mao-Hua

Bulk semiconductors and insulators typically have continuous valence and conduction bands. In this paper, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., Cs 2NaInBr 6, Cs 2NaBiCl 6, and Tl 2NaBiCl 6. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use Cs 2NaInBr 6 as an example tomore » show that the narrow bands can stabilize self-trapped and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. Finally, these characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.« less

Discrete Electronic Bands in Semiconductors and Insulators: Potential High-Light-Yield Scintillators

DOE PAGES

Shi, Hongliang; Du, Mao-Hua

2015-05-12

Bulk semiconductors and insulators typically have continuous valence and conduction bands. In this paper, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., Cs 2NaInBr 6, Cs 2NaBiCl 6, and Tl 2NaBiCl 6. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use Cs 2NaInBr 6 as an example tomore » show that the narrow bands can stabilize self-trapped and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. Finally, these characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.« less

GEOS-2 C-band radar system project. Spectral analysis as related to C-band radar data analysis

NASA Technical Reports Server (NTRS)

1972-01-01

Work performed on spectral analysis of data from the C-band radars tracking GEOS-2 and on the development of a data compaction method for the GEOS-2 C-band radar data is described. The purposes of the spectral analysis study were to determine the optimum data recording and sampling rates for C-band radar data and to determine the optimum method of filtering and smoothing the data. The optimum data recording and sampling rate is defined as the rate which includes an optimum compromise between serial correlation and the effects of frequency folding. The goal in development of a data compaction method was to reduce to a minimum the amount of data stored, while maintaining all of the statistical information content of the non-compacted data. A digital computer program for computing estimates of the power spectral density function of sampled data was used to perform the spectral analysis study.

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

Compact Planar Ultrawideband Antennas with 3.5/5.2/5.8 GHz Triple Band-Notched Characteristics for Internet of Things Applications

PubMed Central

Dong, Jian; Li, Qianqian; Deng, Lianwen

2017-01-01

Ultrawideband (UWB) antennas, as core devices in high-speed wireless communication, are widely applied to mobile handsets, wireless sensor networks, and Internet of Things (IoT). A compact printed monopole antenna for UWB applications with triple band-notched characteristics is proposed in this paper. The antenna has a very compact size of 10 × 16 mm2 and is composed of a square slotted radiation patch and a narrow rectangular ground plane on the back of the substrate. First, by etching a pair of inverted T-shaped slots at the bottom of the radiation patch, one notched band at 5–6 GHz for rejecting the Wireless Local Area Network (WLAN) is generated. Then, by cutting a comb-shaped slot on the top of the radiation patch, a second notched band for rejecting 3.5 GHz Worldwide Interoperability for Microwave Access (WiMAX) is obtained. Further, by cutting a pair of rectangular slots and a C-shaped slot as well as adding a pair of small square parasitic patches at the center of the radiating patch, two separate notched bands for rejecting 5.2 GHz lower WLAN and 5.8 GHz upper WLAN are realized, respectively. Additionally, by integrating the slotted radiation patch with the narrow rectangular ground plane, an enhanced impedance bandwidth can be achieved, especially at the higher band. The antenna consists of linear symmetrical sections only and is easy for fabrication and fine-tuning. The measured results show that the designed antenna provides a wide impedance bandwidth of 150% from 2.12 to 14.80 GHz for VSWR < 2, except for three notched bands of 3.36–4.16, 4.92–5.36, and 5.68–6.0 GHz. Additionally, the antenna exhibits nearly omnidirectional radiation characteristics, low gain at the stopbands, and flat group delay over the whole UWB except at the stopbands. Simulated and experimental results show that the proposed antenna can provide good frequency-domain and time-domain performances at desired UWB frequencies and be an attractive candidate for portable Io

Trends on band alignments: Validity of Anderson's rule in SnS2- and SnSe2-based van der Waals heterostructures

NASA Astrophysics Data System (ADS)

Koda, Daniel S.; Bechstedt, Friedhelm; Marques, Marcelo; Teles, Lara K.

2018-04-01

Van der Waals (vdW) heterostructures are promising candidates for building blocks in novel electronic and optoelectronic devices with tailored properties, since their electronic action is dominated by the band alignments upon their contact. In this work, we analyze 10 vdW heterobilayers based on tin dichalcogenides by first-principles calculations. Structural studies show that all systems are stable, and that commensurability leads to smaller interlayer distances. Using hybrid functional calculations, we derive electronic properties and band alignments for all the heterosystems and isolated two-dimensional (2D) crystals. Natural band offsets are derived from calculated electron affinities and ionization energies of 11 freestanding 2D crystals. They are compared with band alignments in true heterojunctions, using a quantum mechanical criterion, and available experimental data. For the hBN/SnSe 2 system, we show that hBN suffers an increase in band gap, while leaving almost unchanged the electronic properties of SnSe2. Similarly, MX2 (M = Mo, W; X = S, Se) over SnX2 preserve the natural discontinuities from each side of the heterobilayer. Significant charge transfer occurs in junctions with graphene, which becomes p-doped and forms an Ohmic contact with SnX2. Zirconium and hafnium dichalcogenides display stronger interlayer interactions, leading to larger shifts in band alignments with tin dichalcogenides. Significant orbital overlap is found, which creates zero conduction band offset systems. The validity of the Anderson electron affinity rule is discussed. Failures of this model are traced back to interlayer interaction, band hybridization, and quantum dipoles. The systematic work sheds light on interfacial engineering for future vdW electronic and optoelectronic devices.

V-doped SnS2: a new intermediate band material for a better use of the solar spectrum.

PubMed

Wahnón, Perla; Conesa, José C; Palacios, Pablo; Lucena, Raquel; Aguilera, Irene; Seminovski, Yohanna; Fresno, Fernando

2011-12-07

Intermediate band materials can boost photovoltaic efficiency through an increase in photocurrent without photovoltage degradation thanks to the use of two sub-bandgap photons to achieve a full electronic transition from the valence band to the conduction band of a semiconductor structure. After having reported in previous works several transition metal-substituted semiconductors as able to achieve the electronic structure needed for this scheme, we propose at present carrying out this substitution in sulfides that have bandgaps of around 2.0 eV and containing octahedrally coordinated cations such as In or Sn. Specifically, the electronic structure of layered SnS(2) with Sn partially substituted by vanadium is examined here with first principles quantum methods and seen to give favourable characteristics in this respect. The synthesis of this material in nanocrystalline powder form is then undertaken and achieved using solvothermal chemical methods. The insertion of vanadium in SnS(2) is found to produce an absorption spectrum in the UV-Vis-NIR range that displays a new sub-bandgap feature in agreement with the quantum calculations. A photocatalytic reaction-based test verifies that this sub-bandgap absorption produces highly mobile electrons and holes in the material that may be used for the solar energy conversion, giving experimental support to the quantum calculations predictions.

Importance of doping, dopant distribution, and defects on electronic band structure alteration of metal oxide nanoparticles: Implications for reactive oxygen species.

PubMed

Saleh, Navid B; Milliron, Delia J; Aich, Nirupam; Katz, Lynn E; Liljestrand, Howard M; Kirisits, Mary Jo

2016-10-15

Metal oxide nanoparticles (MONPs) are considered to have the potency to generate reactive oxygen species (ROS), one of the key mechanisms underlying nanotoxicity. However, the nanotoxicology literature demonstrates a lack of consensus on the dominant toxicity mechanism(s) for a particular MONP. Moreover, recent literature has studied the correlation between band structure of pristine MONPs to their ability to introduce ROS and thus has downplayed the ROS-mediated toxicological relevance of a number of such materials. On the other hand, material science can control the band structure of these materials to engineer their electronic and optical properties and thereby is constantly modulating the pristine electronic structure. Since band structure is the fundamental material property that controls ROS-producing ability, band tuning via introduction of dopants and defects needs careful consideration in toxicity assessments. This commentary critically evaluates the existing material science and nanotoxicity literature and identifies the gap in our understanding of the role of important crystal structure features (i.e., dopants and defects) on MONPs' electronic structure alteration as well as their ROS-generation capability. Furthermore, this commentary provides suggestions on characterization techniques to evaluate dopants and defects on the crystal structure and identifies research needs for advanced theoretical predictions of their electronic band structures and ROS-generation abilities. Correlation of electronic band structure and ROS will not only aid in better mechanistic assessment of nanotoxicity but will be impactful in designing and developing ROS-based applications ranging from water disinfection to next-generation antibiotics and even cancer therapeutics. Copyright © 2016 Elsevier B.V. All rights reserved.

Electron elevator: Excitations across the band gap via a dynamical gap state

DOE PAGES

Lim, Anthony; Foulkes, W. M. C.; Horsfield, A. P.; ...

2016-01-27

We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. Lastly, an analysis of the time dependence of the transition rates using coupled linear rate equations enables one of themore » excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap.« less

Role of electron-phonon coupling and thermal expansion on band gaps, carrier mobility, and interfacial offsets in kesterite thin-film solar cells

NASA Astrophysics Data System (ADS)

Monserrat, Bartomeu; Park, Ji-Sang; Kim, Sunghyun; Walsh, Aron

2018-05-01

The efficiencies of solar cells based on kesterite Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are limited by a low open-circuit voltage due to high rates of non-radiative electron-hole recombination. To probe the origin of this bottleneck, we calculate the band offset of CZTS(Se) with CdS, confirming a weak spike of 0.1 eV for CZTS/wurtzite-CdS and a strong spike of 0.4 eV for CZTSe/wurtzite-CdS. We also consider the effects of temperature on the band alignment, finding that increasing temperature significantly enhances the spike-type offset. We further resolve an outstanding discrepancy between the measured and calculated phonon frequencies for the kesterites, and use these to estimate the upper limit of electron and hole mobilities based on optic phonon Fröhlich scattering, which uncovers an intrinsic asymmetry with faster (minority carrier) electron mobility.

Electron Microscopic Examination of Irradiated TRISO Coated Particles of Compact 6-3-2 of AGR-1 Experiment

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

Van Rooyen, Isabella Johanna; Demkowicz, Paul Andrew; Riesterer, Jessica Lori

2012-12-01

The electron microscopic examination of selected irradiated TRISO coated particles of the AGR-1 experiment of fuel compact 6-3-2 are presented in this report. Compact 6-3-2 refers to the compact in Capsule 6 at level 3 of Stack 2. The fuel used in capsule 6 compacts, are called the “baseline” fuel as it is fabricated with refined coating process conditions used to fabricate historic German fuel, because of its excellent irradiation performance with UO 2 kernels. The AGR-1 fuel is however made of low-enriched uranium oxycarbide (UCO). Kernel diameters are approximately 350 µm with a U-235 enrichment of approximately 19.7%. Compactmore » 6-3-2 has been irradiated to 11.3% FIMA compact average burn-up with a time average, volume average temperature of 1070.2°C and with a compact average fast fluence of 2.38E21 n/cm« less

Experimental Study of Electronic Quantum Interference, Photonic Crystal Cavity, Photonic Band Edge Effects for Optical Amplification

DTIC Science & Technology

2016-01-26

MONITOR’S REPORT Kirtland AFB, NM 87117-5776 NUMBER(S) AFRL -RV-PS-TR-2016-0003 12. DISTRIBUTION / AVAILABILITY STATEMENT Approved for public release...22060-6218 1 cy AFRL /RVIL Kirtland AFB, NM 87117-5776 2 cys Official Record Copy AFRL /RVSS/David Cardimona 1 cy 12 Approved for... AFRL -RV-PS- AFRL -RV-PS- TR-2016-0003 TR-2016-0003 EXPERIMENTAL STUDY OF ELECTRONIC QUANTUM INTERFERENCE, PHOTONIC CRYSTAL CAVITY, PHOTONIC BAND

Theoretical analysis and simulation of the influence of self-bunching effects and longitudinal space charge effects on the propagation of keV electron bunch produced by a novel S-band Micro-Pulse electron Gun

NASA Astrophysics Data System (ADS)

Zhao, Jifei; Lu, Xiangyang; Zhou, Kui; Yang, Ziqin; Yang, Deyu; Luo, Xing; Tan, Weiwei; Yang, Yujia

2016-06-01

As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL). The stable output of S-band MPG has been achieved in many labs. To establish reliable foundation for the future application of it, the propagation of picosecond electron bunch produced by MPG should be studied in detail. In this article, the MPG which was working on the rising stage of total effective Secondary Electron Yield (SEY) curve was introduced. The self-bunching mechanism was discussed in depth both in the multipacting amplifying state and the steady working state. The bunch length broadening induced by the longitudinal space-charge (SC) effects was investigated by different theoretical models in different regions. The 2D PIC codes MAGIC and beam dynamic codes TraceWin simulations were also performed in the propagation. The result shows an excellent agreement between the simulation and the theoretical analysis for bunch length evolution.

Theoretical analysis and simulation of the influence of self-bunching effects and longitudinal space charge effects on the propagation of keV electron bunch produced by a novel S-band Micro-Pulse electron Gun

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

Zhao, Jifei; Lu, Xiangyang, E-mail: xylu@pku.edu.cn; Yang, Ziqin

As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL). The stable output of S-band MPG has been achieved in many labs. To establish reliable foundation for the future application of it, the propagation of picosecond electron bunch produced by MPG should be studied in detail. In this article, the MPG which was working on the rising stage of total effective Secondary Electron Yield (SEY) curve was introduced. Themore » self-bunching mechanism was discussed in depth both in the multipacting amplifying state and the steady working state. The bunch length broadening induced by the longitudinal space-charge (SC) effects was investigated by different theoretical models in different regions. The 2D PIC codes MAGIC and beam dynamic codes TraceWin simulations were also performed in the propagation. The result shows an excellent agreement between the simulation and the theoretical analysis for bunch length evolution.« less

Effects of van der Waals interaction and electric field on the electronic structure of bilayer MoS2.

PubMed

Xiao, Jin; Long, Mengqiu; Li, Xinmei; Zhang, Qingtian; Xu, Hui; Chan, K S

2014-10-08

The modification of the electronic structure of bilayer MoS2 by an external electric field can have potential applications in optoelectronics and valleytronics. Nevertheless, the underlying physical mechanism is not clearly understood, especially the effects of the van der Waals interaction. In this study, the spin orbit-coupled electronic structure of bilayer MoS2 has been investigated using the first-principle density functional theory. We find that the van der Waals interaction as well as the interlayer distance has significant effects on the band structure. When the interlayer distance of bilayer MoS2 increases from 0.614 nm to 0.71 nm, the indirect gap between the Γ and Λ points increases from 1.25 eV to 1.70 eV. Meanwhile, the energy gap of bilayer MoS2 transforms from an indirect one to a direct one. An external electric field can shift down (up) the energy bands of the bottom (top) MoS2 layer and also breaks the inversion symmetry of bilayer MoS2. As a result, the electric field can affect the band gaps, the spin-orbit interaction and splits the valance bands into two groups. The present study can help us understand more about the electronic structures of MoS2 materials for potential applications in electronics and optoelectronics.

Theoretical study on strain induced variations in electronic properties of 2H-MoS{sub 2} bilayer sheets

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

Dong, Liang; Dongare, Avinash M., E-mail: dongare@uconn.edu; Namburu, Raju R.

2014-02-03

The strain dependence of the electronic properties of bilayer sheets of 2H-MoS{sub 2} is studied using ab initio simulations based on density functional theory. An indirect band gap for bilayer MoS{sub 2} is observed for all variations of strain along the basal plane. Several transitions for the indirect band gap are observed for various strains for the bilayer structure. The variation of the band gap and the carrier effective masses for the holes and the electrons for the bilayer MoS{sub 2} structure under conditions of uniaxial strain, biaxial strain, as well as uniaxial stress is investigated.

A Compact Multiple Notched Ultra-Wide Band Antenna with an Analysis of the CSRR-TO-CSRR Coupling for Portable UWB Applications.

PubMed

Rahman, MuhibUr; Ko, Dong-Sik; Park, Jung-Dong

2017-09-25

We present a compact ultra-wideband (UWB) antenna integrated with sharp notches with a detailed analysis of the mutual coupling of the multiple notch resonators. By utilizing complementary split ring resonators (CSRR) on the radiating semi-circular patch, we achieve the sharp notch-filtering of various bands within the UWB band without increasing the antenna size. The notched frequency bands include WiMAX, INSAT, and lower and upper WLAN. In order to estimate the frequency shifts of the notch due to the coupling of the nearby CSRRs, an analysis of the coupling among the multiple notch resonators is carried out and we construct the lumped-circuit equivalent model. The time domain analysis of the proposed antenna is performed to show its validity on the UWB application. The measured frequency response of the input port corresponds quite well with the calculations and simulations. The radiation pattern of the implemented quad-notched UWB antenna is nearly omnidirectional in the passband.

A Compact Multiple Notched Ultra-Wide Band Antenna with an Analysis of the CSRR-TO-CSRR Coupling for Portable UWB Applications

PubMed Central

Ko, Dong-Sik

2017-01-01

We present a compact ultra-wideband (UWB) antenna integrated with sharp notches with a detailed analysis of the mutual coupling of the multiple notch resonators. By utilizing complementary split ring resonators (CSRR) on the radiating semi-circular patch, we achieve the sharp notch-filtering of various bands within the UWB band without increasing the antenna size. The notched frequency bands include WiMAX, INSAT, and lower and upper WLAN. In order to estimate the frequency shifts of the notch due to the coupling of the nearby CSRRs, an analysis of the coupling among the multiple notch resonators is carried out and we construct the lumped-circuit equivalent model. The time domain analysis of the proposed antenna is performed to show its validity on the UWB application. The measured frequency response of the input port corresponds quite well with the calculations and simulations. The radiation pattern of the implemented quad-notched UWB antenna is nearly omnidirectional in the passband. PMID:28946658

Effective High-Frequency Permeability of Compacted Metal Powders

NASA Astrophysics Data System (ADS)

Volkovskaya, I. I.; Semenov, V. E.; Rybakov, K. I.

2018-03-01

We propose a model for determination of the effective complex permeability of compacted metal-powder media. It is based on the equality of the magnetic moment in a given volume of the media with the desired effective permeability to the total magnetic moment of metal particles in the external high-frequency magnetic field, which arises due to excitation of electric eddy currents in the particles. Calculations within the framework of the proposed model allow us to refine the values of the real and imaginary components of the permeability of metal powder compacts in the microwave band. The conditions of applicability of the proposed model are formulated, and their fulfillment is verified for metal powder compacts in the microwave and millimeter wavelength bands.

Design and analysis of O-S-C triple band wavelength division demultiplexer using cascaded MMI couplers

NASA Astrophysics Data System (ADS)

Chack, Devendra; Kumar, V.; Raghuwanshi, Sanjeev Kumar; Singh, Dev Prakash

2017-01-01

Compact triple O-S-C band wavelength demultiplexer, which consists of series cascaded multimode interference (MMI) couplers has been carried out in this paper. The MMI coupler has been used to drop the wavelengths of 1510 nm and 1550 nm at bar port while the wavelength 1300 nm into the cross port. Then another MMI coupler has been designed to separate the wavelength 1510 nminto one port and wavelength 1550 nm into another port. The triple wavelength demultiplexer function has been performed by choosing a suitable refractive index of the guiding region and geometrical parameters such as the width and length of MMI coupler. Numerical simulation with finite difference beam propagation method (BPM) has been utilized to design and optimize the operation of the proposed triple wavelength demultiplexer. The simulation results show that insertion losses of wavelength O, S and C, bands are 1.884 dB, 1.452 dB and 2.568 dB, respectively, with isolations for each output waveguide ranging from 10 dB to 28.72 dB. The 3-dB bandwidth of insertion loss for 1300 nm, 1510 nm and 1550 nm are 80 nm, 20 nm and 10 nm, respectively.

Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation.

PubMed

Chirayath, V A; Callewaert, V; Fairchild, A J; Chrysler, M D; Gladen, R W; Mcdonald, A D; Imam, S K; Shastry, K; Koymen, A R; Saniz, R; Barbiellini, B; Rajeshwar, K; Partoens, B; Weiss, A H

2017-07-13

Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated background resulting from primary beam-induced secondary electrons. Here, we report the direct measurement of the energy spectra of electrons emitted from single layer graphene as a result of the decay of deep holes in the valence band. These measurements were made possible by eliminating competing backgrounds by employing low-energy positrons (<1.25 eV) to create valence-band holes by annihilation. Our experimental results, supported by theoretical calculations, indicate that between 80 and 100% of the deep valence-band holes in graphene are filled via an Auger transition.

Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation

PubMed Central

Chirayath, V. A.; Callewaert, V.; Fairchild, A. J.; Chrysler, M. D.; Gladen, R. W.; Mcdonald, A. D.; Imam, S. K.; Shastry, K.; Koymen, A. R.; Saniz, R.; Barbiellini, B.; Rajeshwar, K.; Partoens, B.; Weiss, A. H.

2017-01-01

Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated background resulting from primary beam-induced secondary electrons. Here, we report the direct measurement of the energy spectra of electrons emitted from single layer graphene as a result of the decay of deep holes in the valence band. These measurements were made possible by eliminating competing backgrounds by employing low-energy positrons (<1.25 eV) to create valence-band holes by annihilation. Our experimental results, supported by theoretical calculations, indicate that between 80 and 100% of the deep valence-band holes in graphene are filled via an Auger transition. PMID:28703225

Sub-band gap photo-enhanced secondary electron emission from high-purity single-crystal chemical-vapor-deposited diamond

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

Yater, J. E., E-mail: joan.yater@nrl.navy.mil; Shaw, J. L.; Pate, B. B.

2016-02-07

Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distributionmore » as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ∼0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum

Electron and hole photoemission detection for band offset determination of tunnel field-effect transistor heterojunctions

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

Li, Wei; Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871; Zhang, Qin

2014-11-24

We report experimental methods to ascertain a complete energy band alignment of a broken-gap tunnel field-effect transistor based on an InAs/GaSb hetero-junction. By using graphene as an optically transparent electrode, both the electron and hole barrier heights at the InAs/GaSb interface can be quantified. For a Al{sub 2}O{sub 3}/InAs/GaSb layer structure, the barrier height from the top of the InAs and GaSb valence bands to the bottom of the Al{sub 2}O{sub 3} conduction band is inferred from electron emission whereas hole emissions reveal the barrier height from the top of the Al{sub 2}O{sub 3} valence band to the bottom ofmore » the InAs and GaSb conduction bands. Subsequently, the offset parameter at the broken gap InAs/GaSb interface is extracted and thus can be used to facilitate the development of predicted models of electron quantum tunneling efficiency and transistor performance.« less

Band structure and unconventional electronic topology of CoSi

NASA Astrophysics Data System (ADS)

Pshenay-Severin, D. A.; Ivanov, Y. V.; Burkov, A. A.; Burkov, A. T.

2018-04-01

Semimetals with certain crystal symmetries may possess unusual electronic structure topology, distinct from that of the conventional Weyl and Dirac semimetals. Characteristic property of these materials is the existence of band-touching points with multiple (higher than two-fold) degeneracy and nonzero Chern number. CoSi is a representative of this group of materials exhibiting the so-called ‘new fermions’. We report on an ab initio calculation of the electronic structure of CoSi using density functional methods, taking into account the spin-orbit interactions. The linearized \

Hydrogen production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2

NASA Astrophysics Data System (ADS)

Waterhouse, G. I. N.; Wahab, A. K.; Al-Oufi, M.; Jovic, V.; Anjum, D. H.; Sun-Waterhouse, D.; Llorca, J.; Idriss, H.

2013-10-01

Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

A vacuum-sealed compact x-ray tube based on focused carbon nanotube field-emission electrons

NASA Astrophysics Data System (ADS)

Jeong, Jin-Woo; Kim, Jae-Woo; Kang, Jun-Tae; Choi, Sungyoul; Ahn, Seungjoon; Song, Yoon-Ho

2013-03-01

We report on a fully vacuum-sealed compact x-ray tube based on focused carbon nanotube (CNT) field-emission electrons for various radiography applications. The specially designed two-step brazing process enabled us to accomplish a good vacuum level for the stable and reliable operation of the x-ray tube without any active vacuum pump. Also, the integrated focusing electrodes in the field-emission electron gun focused electron beams from the CNT emitters onto the anode target effectively, giving a small focal spot of around 0.3 mm with a large current of above 50 mA. The active-current control through the cathode electrode of the x-ray tube led a fast digital modulation of x-ray dose with a low voltage of below 5 V. The fabricated compact x-ray tube showed a stable and reliable operation, indicating good maintenance of a vacuum level of below 5 × 10-6 Torr and the possibility of field-emission x-ray tubes in a stand-alone device without an active pumping system.

Compact microwave imaging system to measure spatial distribution of plasma density

NASA Astrophysics Data System (ADS)

Ito, H.; Oba, R.; Yugami, N.; Nishida, Y.

2004-10-01

We have developed an advanced microwave interferometric system operating in the K band (18-27 GHz) with the use of a fan-shaped microwave based on a heterodyne detection system for measuring the spatial distribution of the plasma density. In order to make a simple, low-cost, and compact microwave interferometer with better spatial resolution, a microwave scattering technique by a microstrip antenna array is employed. Experimental results show that the imaging system with the microstrip antenna array can have finer spatial resolution than one with the diode antenna array and reconstruct a good spatially resolved image of the finite size dielectric phantoms placed between the horn antenna and the micro strip antenna array. The precise two-dimensional electron density distribution of the cylindrical plasma produced by an electron cyclotron resonance has been observed. As a result, the present imaging system is more suitable for a two- or three-dimensional display of the objects or stationary plasmas and it is possible to realize a compact microwave imaging system.

Band Alignment in MoS2/WS2 Transition Metal Dichalcogenide Heterostructures Probed by Scanning Tunneling Microscopy and Spectroscopy.

PubMed

Hill, Heather M; Rigosi, Albert F; Rim, Kwang Taeg; Flynn, George W; Heinz, Tony F

2016-08-10

Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we examine the electronic structure of transition metal dichalcogenide heterostructures (TMDCHs) composed of monolayers of MoS2 and WS2. STS data are obtained for heterostructures of varying stacking configuration as well as the individual monolayers. Analysis of the tunneling spectra includes the influence of finite sample temperature, yield information about the quasi-particle bandgaps, and the band alignment of MoS2 and WS2. We report the band gaps of MoS2 (2.16 ± 0.04 eV) and WS2 (2.38 ± 0.06 eV) in the materials as measured on the heterostructure regions and the general type II band alignment for the heterostructure, which shows an interfacial band gap of 1.45 ± 0.06 eV.

Physics of compact nonthermal sources. III - Energetic considerations. [electron synchrotron radiation

NASA Technical Reports Server (NTRS)

Burbidge, G. R.; Jones, T. W.; Odell, S. L.

1974-01-01

The energy content of the compact incoherent electron-synchrotron sources 3C 84, 3C 120, 3C 273, 3C 279, 3C 454.3, CTA 102, 3C 446, PKS 2134+004, VRO 42.22.01 and OJ 287 is calculated on the assumption that the low-frequency turnovers in the radio spectrum are due to self-absorption and that the electron distribution is isotropic. The dependence of the source parameters on various modifications of the standard assumptions is determined. These involve relativistic motions, alternate explanations for the low-frequency turnover, proton-synchrotron radiation, and distance to the source. The canonical interpretation is found to be accurate in many respects; some of the difficulties and ways of dealing with them are discussed in detail.

Snapshot hyperspectral retinal imaging using compact spectral resolving detector array.

PubMed

Li, Hao; Liu, Wenzhong; Dong, Biqin; Kaluzny, Joel V; Fawzi, Amani A; Zhang, Hao F

2017-06-01

Hyperspectral retinal imaging captures the light spectrum from each imaging pixel. It provides spectrally encoded retinal physiological and morphological information, which could potentially benefit diagnosis and therapeutic monitoring of retinal diseases. The key challenges in hyperspectral retinal imaging are how to achieve snapshot imaging to avoid motions between the images from multiple spectral bands, and how to design a compact snapshot imager suitable for clinical use. Here, we developed a compact, snapshot hyperspectral fundus camera for rodents using a novel spectral resolving detector array (SRDA), on which a thin-film Fabry-Perot cavity filter was monolithically fabricated on each imaging pixel. We achieved hyperspectral retinal imaging with 16 wavelength bands (460 to 630 nm) at 20 fps. We also demonstrated false-color vessel contrast enhancement and retinal oxygen saturation (sO 2 ) measurement through spectral analysis. This work could potentially bring hyperspectral retinal imaging from bench to bedside. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Tuning to the band gap by complex defects engineering: insights from hybrid functional calculations in CuInS2

NASA Astrophysics Data System (ADS)

Yang, Pei; Shi, Li-Jie; Zhang, Jian-Min; Liu, Gui-Bin; Yang, Shengyuan A.; Guo, Wei; Yao, Yugui

2018-01-01

Tuning band gaps of semiconductors in terms of defect control is essential for the optical and electronic properties of photon emission or photon harvesting devices. By using first-principles calculations, we study the stability condition of bulk CuInS2 and formation energies of point and complex defects in CuInS2 with hybrid exchange-correlation functionals. We find that at Cu-rich and In-poor conditions, 2Cui  +  CuIn is the main complex defect, while InCu  +  2VCu is the main complex defect at In-rich and Cu-poor conditions. Such stable complex defects provide the feasibility of tuning band gaps by varying the [Cu]/[In] molar ratios. These results present how the off-stoichiometry CuInS2 crystal structures, and electronic and optical properties can be optimized by tuning the [Cu]/[In] ratio and Fermi level, and highlight the importance of complex defects in achieving better photoelectric performance in CuInS2. Such band gap tuning in terms of complex defect engineering is a general approach and thus applicable to other photo-harvest or light-emission semiconductors.

Band-structure tailoring and surface passivation for highly efficient near-infrared responsive PbS quantum dot photovoltaics

NASA Astrophysics Data System (ADS)

Zhou, Ru; Niu, Haihong; Ji, Fengwei; Wan, Lei; Mao, Xiaoli; Guo, Huier; Xu, Jinzhang; Cao, Guozhong

2016-11-01

PbS is a promising light harvester for near-infrared (NIR) responsive quantum dot (QD) photovoltaics due to its narrow bulk band gap (0.41 eV) and large exciton Bohr radius (18 nm). However, the relatively low conduction band (CB) and high-density surface defects of PbS as two major drawbacks for its use in solar cells severely hamper the photovoltaic performance enhancement. In this work, a modified solution-based successive ionic layer adsorption and reaction (SILAR) utilizing mixed cationic precursors of Pb2+ and Cd2+ is explored, and such a scheme offers two benefits, band-structure tailoring and surface passivation. In-situ deposited CdS suppresses the excessive growth of PbS in the mesopores, thereby facilitating the favorable electron injection from PbS to TiO2 in view of the up-shifted CB level of QDs; the intimate interpenetration of two sulfides with each other leads to superior passivation of trap state defects on PbS, which suppresses the interfacial charge recombination. With the construction of photovoltaics based on such a hybrid (Pb,Cd)S/CdS configuration, impressive power conversion efficiency up to 4.08% has been reached, outperforming that of the conventional PbS/CdS pattern (2.95%). This work highlights the great importance of band-structure tailoring and surface passivation for constructing highly efficient PbS QD photovoltaics.

Hydrogen production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2

PubMed Central

Waterhouse, G. I. N.; Wahab, A. K.; Al-Oufi, M.; Jovic, V.; Anjum, D. H.; Sun-Waterhouse, D.; Llorca, J.; Idriss, H.

2013-01-01

Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability. PMID:24108361

Differential compaction behaviour of roller compacted granules of clopidogrel bisulphate polymorphs.

PubMed

Khomane, Kailas S; Bansal, Arvind K

2014-09-10

In the present work, in-die and out-of-die compaction behaviour of dry-granulated powders of clopidogrel bisulphate (CLP) polymorphs, form I and form II, was investigated using a fully instrumented rotary tablet press. Each polymorph was compacted at three different roller pressures [70.3 (S1), 105.5 (S2) and 140.6 (S3)kgf/cm(2)], and obtained granules were characterized for their physico-mechanical properties. Compaction data were analyzed for out-of-die compressibility, tabletability and compactibility profiles, and in-die Heckel, Kawakita and Walker analysis. The roller compacted granules of both forms showed markedly different tabletting behaviour. Roller pressure exhibited a trend on compaction behaviour of form I granules, whereas, in case of form II, the effect was insignificant. Tabletability of the six granule batches follows the order; I_S1>I_S2>I_S3>II_S1≈II_S2≈II_S3. In case of form I, the reduced tabletability of the granules compacted at higher roller pressure was attributed to the decreased compressibility and plastic deformation. This was confirmed by compressibility plot and various mathematical parameters derived from Heckel (Py), Kawakita (1/b) and Walker (W) equations. The reduced tabletability of form I granules was due to 'granule hardening' during roller compaction. On the other hand, insignificant effect of roller compaction on tabletting behaviour of form II granules was attributed to brittle fragmentation. The extensive fragmentation of granules offered new 'clean' surfaces and higher contact points that negated the effect of granule hardening. Copyright © 2014 Elsevier B.V. All rights reserved.

Tuning of electronic band gaps and optoelectronic properties of binary strontium chalcogenides by means of doping of magnesium atom(s)- a first principles based theoretical initiative with mBJ, B3LYP and WC-GGA functionals

NASA Astrophysics Data System (ADS)

Debnath, Bimal; Sarkar, Utpal; Debbarma, Manish; Bhattacharjee, Rahul; Chattopadhyaya, Surya

2018-02-01

First principle based theoretical initiative is taken to tune the optoelectronic properties of binary strontium chalcogenide semiconductors by doping magnesium atom(s) into their rock-salt unit cells at specific concentrations x = 0.0, 0.25, 0.50, 0.75 and 1.0 and such tuning is established by studying structural, electronic and optical properties of designed binary compounds and ternary alloys employing WC-GGA, B3LYP and mBJ exchange-correlation functionals. Band structure of each compound is constructed and respective band gaps under all the potential schemes are measured. The band gap bowing and its microscopic origin are calculated using quadratic fit and Zunger's approach, respectively. The atomic and orbital origins of electronic states in the band structure of any compound are explored from its density of states. The nature of chemical bonds between the constituent atoms in each compound is explored from the valence electron density contour plots. Optical properties of any specimen are explored from the computed spectra of its dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity optical absorption and energy loss function. Several calculated results are compared with available experimental and earlier theoretical data.

Asymmetry induces Q-band split in the electronic excitations of magnesium porphyrin

NASA Astrophysics Data System (ADS)

Jiang, Xiankai; Gao, Yi; Lal, Ratnesh; Hu, Jun; Song, Bo

2018-07-01

The electronic excitations of magnesium porphyrin (MgP), a molecular model for understanding the physics in light harvesting by biological systems, have been studied extensively. However, the theoretical underpinning of experimental measurements is still lacking, especially about the sub-bands in absorption spectrum. Here we propose that an asymmetry of MgP based on the uneven charge distribution of pyrrole rings and the linear structure of sp hybridised orbitals in Mg can largely influence the electronic excitations. Upon a very weak asymmetry of Mg-pyrrole bindings in MgP being introduced through the uneven distribution of charge, three different excitations are observed in the Q-band region of the experimental spectrum. Additionally, the predicted B-band excitations are highly correlated (10-2 eV level) with experimental measurements. In contrast, without this asymmetry, there are only two degenerate excitations in the Q-band region, and low agreement (10-1 eV level) of the B-band excitations with the experiment. The key physics of the unexpected and observable asymmetry in MgP is the ability of Mg to form sp hybridised orbitals on the third shell upon Mg binding to the nitrogen of pyrrole ring. Our findings provide new insight for high-energy efficiency of natural as well as artificial light-harvesting system for energy challenge.

Band Alignment and Controllable Electron Migration between Rutile and Anatase TiO2

PubMed Central

Mi, Yang; Weng, Yuxiang

2015-01-01

TiO2 is the most promising semiconductor for photocatalytic splitting of water for hydrogen and degradation of pollutants. The highly photocatalytic active form is its mixed phase of two polymorphs anatase and rutile rather than their pristine compositions. Such a synergetic effect is understood by the staggered band alignment favorable to spatial charge separation. However, electron migration in either direction between the two phases has been reported, the reason of which is still unknown. We determined the band alignment by a novel method, i.e., transient infrared absorption-excitation energy scanning spectra, showing their conduction bands being aligned, thus the electron migration direction is controlled by dynamical factors, such as varying the particle size of anatase, putting electron or hole scavengers on either the surface of anatase or rutile phases, or both. A quantitative criterion capable of predicting the migration direction under various conditions including particle size and surface chemical reactions is proposed, the predictions have been verified experimentally in several typical cases. This would give rise to a great potential in designing more effective titania photocatalysts. PMID:26169699

Resonant enhancement of band-to-band tunneling in in-plane MoS2/WS2 heterojunctions

NASA Astrophysics Data System (ADS)

Kuroda, Tatsuya; Mori, Nobuya

2018-04-01

The band-to-band (BTB) tunneling current J through in-plane MoS2/WS2 heterojunctions is calculated by the nonequilibrium Green function method combined with tight-binding approximation. Types A and B of band configurations are considered. For type-A (type-B) heterojunctions, a potential notch exists (or is absent) at the heterointerface. Both type-A and type-B MoS2/WS2 heterojunctions can support a higher BTB current than MoS2 and WS2 homojunctions. For type-A heterojunctions, the resonant enhancement of J occurs resulting in a significantly higher BTB tunneling current.

Quasiparticle Energies and Band Gaps in Graphene Nanoribbons

NASA Astrophysics Data System (ADS)

Yang, Li; Park, Cheol-Hwan; Son, Young-Woo; Cohen, Marvin L.; Louie, Steven G.

2007-11-01

We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green’s function approach within the GW approximation. Because of the quasi-one-dimensional nature of a GNR, electron-electron interaction effects due to the enhanced screened Coulomb interaction and confinement geometry greatly influence the quasiparticle band gap. Compared with previous tight-binding and density functional theory studies, our calculated quasiparticle band gaps show significant self-energy corrections for both armchair and zigzag GNRs, in the range of 0.5 3.0 eV for ribbons of width 2.4 0.4 nm. The quasiparticle band gaps found here suggest that use of GNRs for electronic device components in ambient conditions may be viable.

Fabrication of compact electron gun for 6 MeV X-ray source

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

Ghodke, S.R.; Barnwal, Rajesh; Kumar, Mahendra, E-mail: ghodke_barc@yahoo.co.in

The 6 MeV X-Ray source for container cargo scanning application has been designed and developed by the Accelerator and Pulse Power Division, BARC, Mumbai. This compact linac has been designed as a mobile system, to be mounted on a moving container. In linac-based cargo-scanning system, to work electron gun on a movable container, it has to be robust. Electron gun is to work at 10{sup -7} mbar vacuum and 2000 degree Celsius temperature. An effort is made to engineer the gun assembly to make it more robust and aligned. The linac acts as the source of X-rays, which fall onmore » the cargo and are then detected by the detector system. Many components are indigenously developed like grid, insulating ring, Tungsten filament and filament guide, which are made from alumina ceramic and Tantalum which is to work at 1500 degree Celsius. Filament connector is made from Invar to reduce heat loss and to make rigid connection. It was CNC machined and wire cut by EDM. Invar and Copper electrode feed through is shrink fitted with the help of liquid Nitrogen. Shrink fit tolerances of 15 micrometer are achieved by jig boring machining processes. Tantalum cup for LaB6 cathode and heat shield are made from die and punch mechanism. For alignment of electron emitter with beam axis this Tantalum cup is a crucial component. Electron gun is assembled and aligned its components with the help of precision jigs. The whole assembly was Helium leak tested by MSLD up to 4 x 10{sup -10} mbar.l/s vacuum, no leak was found. This paper will describe the machining, Tantalum cup forming, ceramic components development, heat shields, ceramic feed through etc of electron gun. (author)« less

Time-Resolved IR-Absorption Spectroscopy of Hot-Electron Dynamics in Satellite and Upper Conduction Bands in GaP

NASA Technical Reports Server (NTRS)

Cavicchia, M. A.; Alfano, R. R.

1995-01-01

The relaxation dynamics of hot electrons in the X6 and X7 satellite and upper conduction bands in GaP was directly measured by femtosecond UV-pump-IR-probe absorption spectroscopy. From a fit to the induced IR-absorption spectra the dominant scattering mechanism giving rise to the absorption at early delay times was determined to be intervalley scattering of electrons out of the X7 upper conduction-band valley. For long delay times the dominant scattering mechanism is electron-hole scattering. Electron transport dynamics of the upper conduction band of GaP has been time resolved.

First-principles studies of electronic, transport and bulk properties of pyrite FeS2

NASA Astrophysics Data System (ADS)

Banjara, Dipendra; Malozovsky, Yuriy; Franklin, LaShounda; Bagayoko, Diola

2018-02-01

We present results from first principle, local density approximation (LDA) calculations of electronic, transport, and bulk properties of iron pyrite (FeS2). Our non-relativistic computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbitals (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). We discuss the electronic energy bands, total and partial densities of states, electron effective masses, and the bulk modulus. Our calculated indirect band gap of 0.959 eV (0.96), using an experimental lattice constant of 5.4166 Å, at room temperature, is in agreement with the measured indirect values, for bulk samples, ranging from 0.84 eV to 1.03 ± 0.05 eV. Our calculated bulk modulus of 147 GPa is practically in agreement with the experimental value of 145 GPa. The calculated, partial densities of states reproduced the splitting of the Fe d bands to constitute the dominant upper most valence and lower most conduction bands, separated by the generally accepted, indirect, experimental band gap of 0.95 eV.

Design and development of compact pulsed power driver for electron beam experiments

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

Deb, Pankaj; Sharma, S.K.; Adhikary, B.

2014-07-01

Pulsed electron beam generation requires high power pulses of fast rise, short duration pulse with flat top. With this objective we have designed a low cost compact pulsed power driver based on water dielectric transmission line. The paper describes the design aspects and construction of the pulse power driver and its experimental results. The pulsed power driver consist of a capacitor bank and its charging power supply, high voltage generator, high voltage switch and pulse compression system. (author)

Vacuum currents in braneworlds on AdS bulk with compact dimensions

NASA Astrophysics Data System (ADS)

Bellucci, S.; Saharian, A. A.; Vardanyan, V.

2015-11-01

The two-point function and the vacuum expectation value (VEV) of the current density are investigated for a massive charged scalar field with arbitrary curvature coupling in the geometry of a brane on the background of AdS spacetime with partial toroidal compactification. The presence of a gauge field flux, enclosed by compact dimensions, is assumed. On the brane the field obeys Robin boundary condition and along compact dimensions periodicity conditions with general phases are imposed. There is a range in the space of the values for the coefficient in the boundary condition where the Poincaré vacuum is unstable. This range depends on the location of the brane and is different for the regions between the brane and AdS boundary and between the brane and the horizon. In models with compact dimensions the stability condition is less restrictive than that for the AdS bulk with trivial topology. The vacuum charge density and the components of the current along non-compact dimensions vanish. The VEV of the current density along compact dimensions is a periodic function of the gauge field flux with the period equal to the flux quantum. It is decomposed into the boundary-free and brane-induced contributions. The asymptotic behavior of the latter is investigated near the brane, near the AdS boundary and near the horizon. It is shown that, in contrast to the VEVs of the field squared an denergy-momentum tensor, the current density is finite on the brane and vanishes for the special case of Dirichlet boundary condition. Both the boundary-free and brane-induced contributions vanish on the AdS boundary. The brane-induced contribution vanishes on the horizon and for points near the horizon the current is dominated by the boundary-free part. In the near-horizon limit, the latter is connected to the corresponding quantity for a massless field in the Minkowski bulk by a simple conformal relation. Depending on the value of the Robin coefficient, the presence of the brane can either

Band alignments of different buffer layers (CdS, Zn(O,S), and In{sub 2}S{sub 3}) on Cu{sub 2}ZnSnS{sub 4}

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

Yan, Chang; Liu, Fangyang; Song, Ning

2014-04-28

The heterojunctions of different n-type buffers, i.e., CdS, Zn(O,S), and In{sub 2}S{sub 3} on p-type Cu{sub 2}ZnSnS{sub 4} (CZTS) were investigated using X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) Measurements. The band alignment of the heterojunctions formed between CZTS and the buffer materials was carefully measured. The XPS data were used to determine the Valence Band Offsets (VBO) of different buffer/CZTS heterojunctions. The Conduction Band Offset (CBO) was calculated indirectly by XPS data and directly measured by NEXAFS characterization. The CBO of the CdS/CZTS heterojunction was found to be cliff-like with CBO{sub XPS} = −0.24 ± 0.10 eV and CBO{submore » NEXAFS} = −0.18 ± 0.10 eV, whereas those of Zn(O,S) and In{sub 2}S{sub 3} were found to be spike-like with CBO{sub XPS} = 0.92 ± 0.10 eV and CBO{sub NEXAFS} = 0.87 ± 0.10 eV for Zn(O,S)/CZTS and CBO{sub XPS} = 0.41 ± 0.10 eV for In{sub 2}S{sub 3}/CZTS, respectively. The CZTS photovoltaic device using the spike-like In{sub 2}S{sub 3} buffer was found to yield a higher open circuit voltage (Voc) than that using the cliff-like CdS buffer. However, the CBO of In{sub 2}S{sub 3}/CZTS is slightly higher than the optimum level and thus acts to block the flow of light-generated electrons, significantly reducing the short circuit current (Jsc) and Fill Factor (FF) and thereby limiting the efficiency. Instead, the use of a hybrid buffer for optimization of band alignment is proposed.« less

Compactly supported Wannier functions and algebraic K -theory

NASA Astrophysics Data System (ADS)

Read, N.

2017-03-01

In a tight-binding lattice model with n orbitals (single-particle states) per site, Wannier functions are n -component vector functions of position that fall off rapidly away from some location, and such that a set of them in some sense span all states in a given energy band or set of bands; compactly supported Wannier functions are such functions that vanish outside a bounded region. They arise not only in band theory, but also in connection with tensor-network states for noninteracting fermion systems, and for flat-band Hamiltonians with strictly short-range hopping matrix elements. In earlier work, it was proved that for general complex band structures (vector bundles) or general complex Hamiltonians—that is, class A in the tenfold classification of Hamiltonians and band structures—a set of compactly supported Wannier functions can span the vector bundle only if the bundle is topologically trivial, in any dimension d of space, even when use of an overcomplete set of such functions is permitted. This implied that, for a free-fermion tensor network state with a nontrivial bundle in class A, any strictly short-range parent Hamiltonian must be gapless. Here, this result is extended to all ten symmetry classes of band structures without additional crystallographic symmetries, with the result that in general the nontrivial bundles that can arise from compactly supported Wannier-type functions are those that may possess, in each of d directions, the nontrivial winding that can occur in the same symmetry class in one dimension, but nothing else. The results are obtained from a very natural usage of algebraic K -theory, based on a ring of polynomials in e±i kx,e±i ky,..., which occur as entries in the Fourier-transformed Wannier functions.

Circularly Polarized S Band Dual Frequency Square Patch Antenna Using Glass Microfiber Reinforced PTFE Composite

PubMed Central

Samsuzzaman, M.; Islam, M. T.; Arshad, Haslina; Mandeep, J. S.; Misran, N.

2014-01-01

Circularly polarized (CP) dual frequency cross-shaped slotted patch antenna on 1.575 mm thick glass microfiber reinforced polytetrafluoroethylene (PTFE) composite material substrate is designed and fabricated for satellite applications. Asymmetric cross-shaped slots are embedded in the middle of the square patch for CP radiation and four hexagonal slots are etched on the four sides of the square patch for desired dual frequency. Different substrate materials have been analysed to achieve the desired operating band. The experimental results show that the impedance bandwidth is approximately 30 MHz (2.16 GHz to 2.19 GHz) for lower band and 40 MHz (3.29 GHz to 3.33 GHz) for higher band with an average peak gain of 6.59 dBiC and 5.52 dBiC, respectively. Several optimizations are performed to obtain the values of the antenna physical parameters. Moreover, the proposed antenna possesses compactness, light weight, simplicity, low cost, and circularly polarized. It is an attractive candidate for dual band satellite antennas where lower band can be used for uplink and upper band can be used for downlink. PMID:24982943

Electron transfer dynamics and yield from gold nanoparticle to different semiconductors induced by plasmon band excitation

NASA Astrophysics Data System (ADS)

Du, L. C.; Xi, W. D.; Zhang, J. B.; Matsuzaki, H.; Furube, A.

2018-06-01

Photoinduced electron transfer from gold nanoparticles (NPs) to semiconductor under plasmon excitation is an important phenomenon in photocatalysis and solar cell applications. Femtosecond plasmon-induced electron transfer from gold NPs to the conduction band of different semiconductor like TiO2, SnO2, and ZnO was monitored at 3440 nm upon optical excitation of the surface plasmon band of gold NPs. It was found that electron injection was completed within 240 fs and the electron injection yield reached 10-30% under 570 nm excitation. It means TiO2 is not the only proper semiconductor as electron acceptors in such gold/semiconductor nanoparticle systems.

Transition-metal-substituted indium thiospinels as novel intermediate-band materials: prediction and understanding of their electronic properties.

PubMed

Palacios, P; Aguilera, I; Sánchez, K; Conesa, J C; Wahnón, P

2008-07-25

Results of density-functional calculations for indium thiospinel semiconductors substituted at octahedral sites with isolated transition metals (M=Ti,V) show an isolated partially filled narrow band containing three t2g-type states per M atom inside the usual semiconductor band gap. Thanks to this electronic structure feature, these materials will allow the absorption of photons with energy below the band gap, in addition to the normal light absorption of a semiconductor. To our knowledge, we demonstrate for the first time the formation of an isolated intermediate electronic band structure through M substitution at octahedral sites in a semiconductor, leading to an enhancement of the absorption coefficient in both infrared and visible ranges of the solar spectrum. This electronic structure feature could be applied for developing a new third-generation photovoltaic cell.

S-band antenna phased array communications system

NASA Technical Reports Server (NTRS)

Delzer, D. R.; Chapman, J. E.; Griffin, R. A.

1975-01-01

The development of an S-band antenna phased array for spacecraft to spacecraft communication is discussed. The system requirements, antenna array subsystem design, and hardware implementation are examined. It is stated that the phased array approach offers the greatest simplicity and lowest cost. The objectives of the development contract are defined as: (1) design of a medium gain active phased array S-band communications antenna, (2) development and test of a model of a seven element planar array of radiating elements mounted in the appropriate cavity matrix, and (3) development and test of a breadboard transmit/receive microelectronics module.

Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption

NASA Astrophysics Data System (ADS)

Rahman, Altaf Ur; Rahman, Gul; Kratzer, Peter

2018-05-01

The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F–Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices.

Fluorinated graphene oxide for enhanced S and X-band microwave absorption

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

Sudeep, P. M.; TIFR-Centre for Interdisciplinary Sciences, Tata Institute of Fundamental Research, Hyderabad 500075; Vinayasree, S.

2015-06-01

Here we report the microwave absorbing properties of three graphene derivatives, namely, graphene oxide (GO), fluorinated GO (FGO, containing 5.6 at. % Fluorine (F)), and highly FGO (HFGO, containing 23 at. % F). FGO is known to be exhibiting improved electrochemical and electronic properties when compared to GO. Fluorination modifies the dielectric properties of GO and hence thought of as a good microwave absorber. The dielectric permittivities of GO, FGO, and HFGO were estimated in the S (2 GHz to 4 GHz) and X (8 GHz to 12 GHz) bands by employing cavity perturbation technique. For this, suspensions containing GO/FGO/HFGO were made in N-Methylmore » Pyrrolidone (NMP) and were subjected to cavity perturbation. The reflection loss was then estimated and it was found that −37 dB (at 3.2 GHz with 6.5 mm thickness) and −31 dB (at 2.8 GHz with 6 mm thickness) in the S band and a reflection loss of −18 dB (at 8.4 GHz with 2.5 mm thickness) and −10 dB (at 11 GHz with 2 mm thickness) in the X band were achieved for 0.01 wt. % of FGO and HFGO in NMP, respectively, suggesting that these materials can serve as efficient microwave absorbers even at low concentrations.« less

Topologically nontrivial electronic bands and tunable Dirac cones in graphynes with spin-orbit coupling

NASA Astrophysics Data System (ADS)

Juricic, Vladimir; van Miert, Guido; Morais Smith, Cristiane

2015-03-01

Graphynes represent an emerging family of carbon allotropes that differ from graphene by the presence of the triple bonds (-C ≡C-) in their band structure. They have recently attracted much interest due to the tunability of the Dirac cones in the band structure. I will show that the spin-orbit coupling in β-graphyne could produce various effects related to the topological properties of its electronic bands. Intrinsic spin-orbit coupling yields high- and tunable Chern-number bands, which may host both topological and Chern insulators, in the presence and absence of time-reversal symmetry, respectively. Furthermore, Rashba spin-orbit coupling can be used to control the position and the number of Dirac cones in the Brillouin zone. Finally, I will also discuss the electronic properties of α - and γ - graphyne in the presence of the spin-orbit coupling within recently developed general theory of spin-orbit couplings in graphynes. Work supported by the Netherlands Organization for Scientific Research (NWO).

A study on the enhancement of opto-electronic properties of CdS thin films: seed-assisted fabrication

NASA Astrophysics Data System (ADS)

Kumarage, W. G. C.; Wijesundera, R. P.; Seneviratne, V. A.; Jayalath, C. P.; Dassanayake, B. S.

2017-04-01

A novel method of fabricating chemical bath deposited CdS thin films (CBD-CdS) by using electrodeposited CdS (ED-CdS) as a seed layer is reported. The resulting thin, compact, uniform and adherent seed-assisted CdS films (ED/CBD-CdS) show enhanced effective surface area compared to both ED-CdS and CBD-CdS. The phase of these CdS films was determined to be hexagonal. The fabricated ED/CBD-CdS films show higher photoelectrochemical (PEC) cell efficiency than either ED-CdS and CBD-CdS thin films. Carrier concentration and flat band potential values for ED/CBD-CdS systems are also found to be superior compared to both ED-CdS and CBD-CdS systems.

Compact quantum gates on electron-spin qubits assisted by diamond nitrogen-vacancy centers inside cavities

NASA Astrophysics Data System (ADS)

Wei, Hai-Rui; Deng, Fu-Guo

2013-10-01

Constructing compact quantum circuits for universal quantum gates on solid-state systems is crucial for quantum computing. We present some compact quantum circuits for a deterministic solid-state quantum computing, including the cnot, Toffoli, and Fredkin gates on the diamond NV centers confined inside cavities, achieved by some input-output processes of a single photon. Our quantum circuits for these universal quantum gates are simple and economic. Moreover, additional electron qubits are not employed, but only a single-photon medium. These gates have a long coherent time. We discuss the feasibility of these universal solid-state quantum gates, concluding that they are feasible with current technology.

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%.

Spectroscopic evidence for temperature-dependent convergence of light- and heavy-hole valence bands of PbQ (Q = Te, Se, S)

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

Zhao, J.; Malliakas, C. D.; Wijayaratne, K.

2017-01-01

We have conducted a temperature- dependent angle-resolved photoemission spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the light-hole upper valence bands (UVBs) and hitherto undetected heavy-hole lower valence bands (LVBs) in these materials. An unusual temperature-dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is known as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

Spectroscopic evidence for temperature-dependent convergence of light- and heavy-hole valence bands of PbQ (Q = Te, Se, S)

NASA Astrophysics Data System (ADS)

Zhao, J.; Malliakas, C. D.; Wijayaratne, K.; Karlapati, V.; Appathurai, N.; Chung, D. Y.; Rosenkranz, S.; Kanatzidis, M. G.; Chatterjee, U.

2017-01-01

We have conducted a temperature-dependent angle-resolved photoemission spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the light-hole upper valence bands (UVBs) and hitherto undetected heavy-hole lower valence bands (LVBs) in these materials. An unusual temperature-dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is known as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

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.

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

Compact fiber optic gyroscopes for platform stabilization

NASA Astrophysics Data System (ADS)

Dickson, William C.; Yee, Ting K.; Coward, James F.; McClaren, Andrew; Pechner, David A.

2013-09-01

SA Photonics has developed a family of compact Fiber Optic Gyroscopes (FOGs) for platform stabilization applications. The use of short fiber coils enables the high update rates required for stabilization applications but presents challenges to maintain high performance. We are able to match the performance of much larger FOGs by utilizing several innovative technologies. These technologies include source noise reduction to minimize Angular Random Walk (ARW), advanced digital signal processing that minimizes bias drift at high update rates, and advanced passive thermal packaging that minimizes temperature induced bias drift while not significantly affecting size, weight, or power. In addition, SA Photonics has developed unique distributed FOG packaging technologies allowing the FOG electronics and photonics to be packaged remotely from the sensor head or independent axis heads to minimize size, weight, and power at the sensing location(s). The use of these technologies has resulted in high performance, including ARW less than 0.001 deg/rt-hr and bias drift less than 0.004 deg/hr at an update rate of 10 kHz, and total packaged volume less than 30 cu. in. for a 6 degree of freedom FOG-based IMU. Specific applications include optical beam stabilization for LIDAR and LADAR, beam stabilization for long-range free-space optical communication, Optical Inertial Reference Units for HEL stabilization, and Ka band antenna pedestal pointing and stabilization. The high performance of our FOGs also enables their use in traditional navigation and positioning applications. This paper will review the technologies enabling our high-performance compact FOGs, and will provide performance test results.

Superconductivity in an electron band just above the Fermi level: possible route to BCS-BEC superconductivity.

PubMed

Okazaki, K; Ito, Y; Ota, Y; Kotani, Y; Shimojima, T; Kiss, T; Watanabe, S; Chen, C-T; Niitaka, S; Hanaguri, T; Takagi, H; Chainani, A; Shin, S

2014-02-28

Conventional superconductivity follows Bardeen-Cooper-Schrieffer(BCS) theory of electrons-pairing in momentum-space, while superfluidity is the Bose-Einstein condensation(BEC) of atoms paired in real-space. These properties of solid metals and ultra-cold gases, respectively, are connected by the BCS-BEC crossover. Here we investigate the band dispersions in FeTe(0.6)Se(0.4)(Tc = 14.5 K ~ 1.2 meV) in an accessible range below and above the Fermi level(EF) using ultra-high resolution laser angle-resolved photoemission spectroscopy. We uncover an electron band lying just 0.7 meV (~8 K) above EF at the Γ-point, which shows a sharp superconducting coherence peak with gap formation below Tc. The estimated superconducting gap Δ and Fermi energy [Symbol: see text]F indicate composite superconductivity in an iron-based superconductor, consisting of strong-coupling BEC in the electron band and weak-coupling BCS-like superconductivity in the hole band. The study identifies the possible route to BCS-BEC superconductivity.

Intelligent Controller for a Compact Wide-Band Compositional Infrared Fourier Transform Spectrometer

NASA Astrophysics Data System (ADS)

Yiu, P.; Keymeulen, D.; Berisford, D. F.; Hand, K. P.; Carlson, R. W.

2013-12-01

This paper presents the design and integration of an intelligent controller for CIRIS (Compositional InfraRed Interferometric Spectrometer) on a stand-alone field programmable gate array (FPGA) architecture. CIRIS is a novel take on traditional Fourier Transform Spectrometers (FTS) and replaces linearly moving mirrors (characteristic of Michelson interferometers) with a constant-velocity rotating refractor to variably phase shift and alter the path length of incoming light. This design eliminates the need for periodically accelerating/decelerating mirrors inherent to canonical Michelson designs and allows for a compact and robust device that is intrinsically radiation-hard, making it ideal for spaceborne measurements in the near-IR to thermal-IR band (2-12 μm) on planetary exploration missions. A traditional Michelson FTS passes a monochromatic light source (incident light from the sample) through a system of refractors/mirrors followed by a mirror moving linearly in the plane of the incident light. This process selectively blocks certain wavelengths and permits measurement of the sample's absorption rates as a function of the wavelengths blocked to produce an 'inteferogram.' This is subsequently processed using a Fourier transform to obtain the sample's spectrum and ascertain the sample's composition. With our prototype CIRIS instrument in development at Design and Prototype Inc. and NASA-JPL, we propose the use of a rotating refractor spinning at a constant velocity to variably phase shift incident light to the detector as an alternative to a linearly moving mirror. This design eliminates sensitivity to vibrations, minimizing path length and non-linear errors due to minor perturbations to the system, in addition to facilitating compact design critical to meeting the strict volume requirements of spacecraft. Further, this is done without sacrificing spectral resolution or throughput when compared to Michelson or diffractive designs. While Michelson designs

Efficient evaluation of epitaxial MoS2 on sapphire by direct band structure imaging

NASA Astrophysics Data System (ADS)

Kim, Hokwon; Dumcenco, Dumitru; Fregnaux, Mathieu; Benayad, Anass; Kung, Yen-Cheng; Kis, Andras; Renault, Olivier; Lanes Group, Epfl Team; Leti, Cea Team

The electronic band structure evaluation of two-dimensional metal dichalcogenides is critical as the band structure can be greatly influenced by the film thickness, strain, and substrate. Here, we performed a direct measurement of the band structure of as-grown monolayer MoS2 on single crystalline sapphire by reciprocal-space photoelectron emission microscopy with a conventional laboratory ultra-violet He I light source. Arrays of gold electrodes were deposited onto the sample in order to avoid charging effects due to the insulating substrate. This allowed the high resolution mapping (ΔE = 0.2 eV Δk = 0.05 Å-1) of the valence states in momentum space down to 7 eV below the Fermi level. The high degree of the epitaxial alignment of the single crystalline MoS2 nuclei was verified by the direct momentum space imaging over a large area containing multiple nuclei. The derived values of the hole effective mass were 2.41 +/-0.05 m0 and 0.81 +/-0.05 m0, respectively at Γ and K points, consistent with the theoretical values of the freestanding monolayer MoS2 reported in the literature. HK acknowledges the french CEA Basic Technological Research program (RTB) for funding.

Quasiparticle interference in ZrSiS: Strongly band-selective scattering depending on impurity lattice site

NASA Astrophysics Data System (ADS)

Butler, Christopher J.; Wu, Yu-Mi; Hsing, Cheng-Rong; Tseng, Yi; Sankar, Raman; Wei, Ching-Ming; Chou, Fang-Cheng; Lin, Minn-Tsong

2017-11-01

Scanning tunneling microscopy visualizations of quasiparticle interference (QPI) enable powerful insights into the k -space properties of superconducting, topological, Rashba, and other exotic electronic phases, but their reliance on impurities acting as scattering centers is rarely scrutinized. Here, we investigate QPI at the vacuum-cleaved (001) surface of the Dirac semimetal ZrSiS. We find that interference patterns around impurities located on the Zr and S lattice sites appear very different, and can be ascribed to selective scattering of different subsets of the predominantly Zr 4 d -derived band structure, namely, the m =0 and ±1 components. We show that the selectivity of scattering channels requires an explanation beyond the different bands' orbital characteristics and their respective charge density distributions over Zr and S lattice sites. Importantly, this result shows that the usual assumption of generic scattering centers allowing observations of quasiparticle interference to shed light indiscriminately and isotropically upon the q space of scattering events does not hold, and that the scope and interpretation of QPI observations can therefore be be strongly contingent on the material defect chemistry. This finding promises to spur new investigations into the quasiparticle scattering process itself, to inform future interpretations of quasiparticle interference observations, and ultimately to aid the understanding and engineering of quantum electronic transport properties.

Electronic structure descriptor for the discovery of narrow-band red-emitting phosphors

DOE PAGES

Wang, Zhenbin; Chu, Iek -Heng; Zhou, Fei; ...

2016-05-09

Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu 2+-activated emission identified through a comparison of the electronic structures of known narrow-band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu 2+ 4 f 7 bands. By incorporating this descriptor in a high-throughput first-principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu 2+-activated red-emitting phosphors thatmore » are predicted to exhibit good chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Lastly, our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth.« less

Electronic structure descriptor for the discovery of narrow-band red-emitting phosphors

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

Wang, Zhenbin; Chu, Iek -Heng; Zhou, Fei

Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu 2+-activated emission identified through a comparison of the electronic structures of known narrow-band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu 2+ 4 f 7 bands. By incorporating this descriptor in a high-throughput first-principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu 2+-activated red-emitting phosphors thatmore » are predicted to exhibit good chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Lastly, our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth.« less

Substitutional Electron and Hole Doping of WSe2 : Synthesis, Electrical Characterization, and Observation of Band-to-Band Tunneling

NASA Astrophysics Data System (ADS)

Mukherjee, R.; Chuang, H. J.; Koehler, M. R.; Combs, N.; Patchen, A.; Zhou, Z. X.; Mandrus, D.

2017-03-01

Transition-metal dichalcogenides (TMDs) such as MoS2 , MoSe2 , and WSe2 have emerged as promising two-dimensional semiconductors. Many anticipated applications of these materials require both p -type and n -type TMDs with long-term doping stability. Here, we report on the synthesis of substitutionally doped WSe2 crystals using Nb and Re as p - and n -type dopants, respectively. Hall coefficient and gate-dependent transport measurements reveal drastically different doping properties between nominally 0.5% Nb- and 0.5% Re-doped WSe2 . While 0.5% Nb-doped WSe2 (WSe2∶Nb ) is degenerately hole doped with a nearly temperature-independent carrier density of approximately 1019 cm-3 , electrons in 0.5% Re-doped WSe2 (WSe2 ∶Re ) are largely trapped in localized states below the mobility edge and exhibit thermally activated behavior. Charge transport in both WSe2∶Nb and WSe2 ∶Re is found to be limited by Coulomb scattering from ionized impurities. Furthermore, we fabricate vertical van der Waals-junction diodes consisting of multilayers of WSe2∶Nb and WSe2 ∶Re . Finally, we demonstrate reverse rectifying behavior as a direct proof of band-to-band tunneling in our WSe2∶Nb /WSe2∶Re diodes.

Inflatable Antenna for CubeSat: Extension of the Previously Developed S-Band Design to the X-Band

NASA Technical Reports Server (NTRS)

Babuscia, Alessandra; Choi, Thomas; Cheung, Kar-Ming; Thangavelautham, Jekan; Ravichandran, Mithun; Chandra, Aman

2015-01-01

The inflatable antenna for CubeSat is a 1 meter antenna reflector designed with one side reflective Mylar, another side clear Mylar with a patch antenna at the focus. The development of this technology responds to the increasing need for more capable communication systems to allow CubeSats to operate autonomously in interplanetary missions. An initial version of the antenna for the S-Band was developed and tested in both anechoic chamber and vacuum chamber. Recent developments in transceivers and amplifiers for CubeSat at X-band motivated the extension from the S-Band to the X-Band. This paper describes the process of extending the design of the antenna to the X-Band focusing on patch antenna redesign, new manufacturing challenges and initial results of experimental tests.

NAOMI instrument: a product line of compact and versatile cameras designed for high resolution missions in Earth observation

NASA Astrophysics Data System (ADS)

Luquet, Ph.; Chikouche, A.; Benbouzid, A. B.; Arnoux, J. J.; Chinal, E.; Massol, C.; Rouchit, P.; De Zotti, S.

2017-11-01

EADS Astrium is currently developing a new product line of compact and versatile instruments for high resolution missions in Earth Observation. First version has been developed in the frame of the ALSAT-2 contract awarded by the Algerian Space Agency (ASAL) to EADS Astrium. The Silicon Carbide Korsch-type telescope coupled with a multilines detector array offers a 2.5 m GSD in PAN band at Nadir @ 680 km altitude (10 m GSD in the four multispectral bands) with a 17.5 km swath width. This compact camera - 340 (W) x 460 (L) x 510 (H) mm3, 13 kg - is embarked on a Myriade-type small platform. The electronics unit accommodates video, housekeeping, and thermal control functions and also a 64 Gbit mass memory. Two satellites are developed; the first one is planned to be launched on mid 2009. Several other versions of the instrument have already been defined with enhanced resolution or/and larger field of view.

A Novel Compact Wideband TSA Array for Near-Surface Ice Sheet Penetrating Radar Applications

NASA Astrophysics Data System (ADS)

Zhang, Feng; Liu, Xiaojun; Fang, Guangyou

2014-03-01

A novel compact tapered slot antenna (TSA) array for near-surface ice sheet penetrating radar applications is presented. This TSA array is composed of eight compact antenna elements which are etched on two 480mm × 283mm FR4 substrates. Each antenna element is fed by a wideband coplanar waveguide (CPW) to coupled strip-line (CPS) balun. The two antenna substrates are connected together with a metallic baffle. To obtain wideband properties, another two metallic baffles are used along broadsides of the array. This array is fed by a 1 × 8 wideband power divider. The measured S11 of the array is less than -10dB in the band of 500MHz-2GHz, and the measured gain is more than 6dBi in the whole band which agrees well with the simulated results.

Valence-band electronic structure evolution of graphene oxide upon thermal annealing for optoelectronics

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

Yamaguchi, Hisato; Ogawa, Shuichi; Watanabe, Daiki

We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 °C. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a setmore » of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 °C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart.« less

Valence-band electronic structure evolution of graphene oxide upon thermal annealing for optoelectronics

DOE PAGES

Yamaguchi, Hisato; Ogawa, Shuichi; Watanabe, Daiki; ...

2016-09-01

We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 °C. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a setmore » of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 °C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart.« less

Experimental and Theoretical Investigations on Intermediate Band in Doped Nano-SnS2

NASA Astrophysics Data System (ADS)

Heiba, Zein K.; Mohamed, Mohamed Bakr; Abdel Kader, M. H.

2018-03-01

Nano-SnS2 and Sn0.75 X 0.25S2 (X = Cr, Fe, Y) have been prepared by thermolysis method. Phase analysis of x-ray diffraction data confirmed the single-phase nature of all prepared samples, with some residual carbon contributing to the background. Rietveld refinement revealed high anisotropy in crystallite size, signifying a cylindrical structure for the particle shape, as confirmed by transmission electron microscopy. The refined occupancies obtained for the doped cations were found to be smaller than the nominal target doping ratio (25%). Fourier-transform infrared spectra showed presence of Sn-S bond in all samples. The energy was found to be 3.42 eV, 3.33 eV, 2.1 eV and 3.14 eV, and 3.62 eV for undoped SnS2 and when doped with Cr, Fe, and Y, respectively. Density functional theory calculations illustrated that Fe-doped SnS2 has two bandgaps [normal and intermediate (IB) bands]. Meanwhile, Sn0.75Fe0.25S2 sample showed anti-Stokes and an extra photoluminescence peak related to the newly created intermediate band (IB) inside the energy gap. On the other hand, pure SnS2 and Sn0.75 X 0.25S2 (X = Cr, Y) samples emitted four photoluminescence subspectra in ultraviolet, violet, and blue regions.

Direct band gap measurement of Cu(In,Ga)(Se,S){sub 2} thin films using high-resolution reflection electron energy loss spectroscopy

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

Heo, Sung; College of Information and Communication Engineering, Sungkyunkwan University, Cheoncheon-dong 300, Jangan-gu, Suwon 440-746; Lee, Hyung-Ik

2015-06-29

To investigate the band gap profile of Cu(In{sub 1−x},Ga{sub x})(Se{sub 1−y}S{sub y}){sub 2} of various compositions, we measured the band gap profile directly as a function of in-depth using high-resolution reflection energy loss spectroscopy (HR-REELS), which was compared with the band gap profile calculated based on the auger depth profile. The band gap profile is a double-graded band gap as a function of in-depth. The calculated band gap obtained from the auger depth profile seems to be larger than that by HR-REELS. Calculated band gaps are to measure the average band gap of the spatially different varying compositions with respectmore » to considering its void fraction. But, the results obtained using HR-REELS are to be affected by the low band gap (i.e., out of void) rather than large one (i.e., near void). Our findings suggest an analytical method to directly determine the band gap profile as function of in-depth.« less

A Compact Low-loss Magic-T using Microstrip-Slotline Transitions

NASA Technical Reports Server (NTRS)

U-yen, Kongpop; Wollack, Edward J.; Moseley, Samuel H.; Papapolymerou, John; Laskar, Joy

2007-01-01

The design of a compact low-loss magic-T is proposed. The planar magic-T incorporates the compact microstrip-slotline tee junction and small microstrip-slotline transition area to reduce slotline radiation. The experimental results show that the magic-T produces broadband in-phase and out-of-phase power combiner/divider responses, has an average in-band insertion loss of 0.3 dB and small in-band phase and amplitude imbalance of less than plus or minus 1.6 deg. and plus or minus 0.3 dB, respectively.

The angular electronic band structure and free particle model of aromatic molecules: High-frequency photon-induced ring current

NASA Astrophysics Data System (ADS)

Öncan, Mehmet; Koç, Fatih; Şahin, Mehmet; Köksal, Koray

2017-05-01

This work introduces an analysis of the relationship of first-principles calculations based on DFT method with the results of free particle model for ring-shaped aromatic molecules. However, the main aim of the study is to reveal the angular electronic band structure of the ring-shaped molecules. As in the case of spherical molecules such as fullerene, it is possible to observe a parabolic dispersion of electronic states with the variation of angular quantum number in the planar ring-shaped molecules. This work also discusses the transition probabilities between the occupied and virtual states by analyzing the angular electronic band structure and the possibility of ring currents in the case of spin angular momentum (SAM) or orbital angular momentum (OAM) carrying light. Current study focuses on the benzene molecule to obtain its angular electronic band structure. The obtained electronic band structure can be considered as a useful tool to see the transition probabilities between the electronic states and possible contribution of the states to the ring currents. The photoinduced current due to the transfer of SAM into the benzene molecule has been investigated by using analytical calculations within the frame of time-dependent perturbation theory.

Photoemission and Auger-electron spectroscopic study of the Chevrel-phase compound FexMo6S8

NASA Astrophysics Data System (ADS)

Fujimori, A.; Sekita, M.; Wada, H.

1986-05-01

The electronic structure of the Chevrel-phase compound FexMo6S8 has been studied by photoemission and Auger-electron spectroscopy. Core-level shifts suggest a large charge transfer from the Fe atoms to the Mo6S8 clusters and a small Mo-to-S charge transfer within the cluster. Line-shape asymmetry in the core levels indicates that the density of states (DOS) at the Fermi level has a finite S 3p component as well as the dominant Mo 3d character. Satellite structure and exchange splitting in the Fe core levels point to weak Fe 3d-S 3p hybridization in spite of the short Fe-S distances comparable to that in FeS. The x-ray and ultraviolet valence-band photoemission spectra and the Mo 4d partial DOS obtained by deconvoluting the Mo M4,5VV Auger spectrum are compared with existing band-structure calculations, and the Mo 4d-S 3p bonding character, the structure of the Mo 4d-derived conduction band etc., are discussed. In particular, it is shown that the conduction-band structure is sensitive to the noncubic distortion of the crystal through changes in the intercluster Mo 4d-S 3p hybridization. A pronounced final-state effect is found in the Mo M4,5N2,3V Auger spectrum and is attributed to strong 4p-4d intershell coupling.

A Survey Study of U.S. Collegiate and K-12 Steel Band Directors' Attitudes Relating to Steel Band Curriculum and Pedagogy

ERIC Educational Resources Information Center

Haskett, Brandon L.

2016-01-01

Steel bands were introduced into U.S. schools and universities during the 1950s and 1960s. There are now more than 600 U.S. school and university steel bands. The range of teaching methods and repertoire choices must be examined to more fully understand the variety of steel band traditions present in K-12 schools and universities. U.S. steel band…

Electron momentum density and band structure calculations of α- and β-GeTe

NASA Astrophysics Data System (ADS)

Vadkhiya, Laxman; Arora, Gunjan; Rathor, Ashish; Ahuja, B. L.

2011-12-01

We have measured isotropic experimental Compton profile of α-GeTe by employing high energy (662 keV) γ-radiation from a 137Cs isotope. To compare our experiment, we have also computed energy bands, density of states, electron momentum densities and Compton profiles of α- and β-phases of GeTe using the linear combination of atomic orbitals method. The electron momentum density is found to play a major role in understanding the topology of bands in the vicinity of the Fermi level. It is seen that the density functional theory (DFT) with generalised gradient approximation is relatively in better agreement with the experiment than the local density approximation and hybrid Hartree-Fock/DFT.

Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption.

PubMed

Ur Rahman, Altaf; Rahman, Gul; Kratzer, Peter

2018-05-16

The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (N Ga ), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (F S or F Ga ) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for F Ga and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, F Ga induces p-type conductivity in GaS, whereas F S induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N 2 molecules. While N Ga induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic N S impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices.

Compact photonic crystal circulator with flat-top transmission band created by cascading magneto-optical resonance cavities.

PubMed

Wang, Qiong; Ouyang, Zhengbiao; Lin, Mi; Liu, Qiang

2015-11-20

A new type of compact three-port circulator with flat-top transmission band (FTTB) in a two-dimensional photonic crystal has been proposed, through coupling the cascaded magneto-optical resonance cavities to waveguides. The coupled-mode theory is applied to investigate the coupled structure and analyze the condition to achieve FTTB. According to the theoretical analysis, the structure is further optimized to ensure that the condition for achieving FTTB can be satisfied for both cavity-cavity coupling and cavity-waveguide coupling. Through the finite-element method, it is demonstrated that the design can realize a high quality, nonreciprocal circulating propagation of waves with an insertion loss of 0.023 dB and an isolation of 23.3 dB, covering a wide range of operation frequency. Such a wideband circulator has potential applications in large-scale integrated photonic circuits for guiding or isolating harmful optical reflections from load elements.

Banded Structures in Electron Pitch Angle Diffusion Coefficients from Resonant Wave Particle Interactions

NASA Technical Reports Server (NTRS)

Tripathi, A. K.; Singhal, R. P.; Khazanov, G. V.; Avanov, L. A.

2016-01-01

Electron pitch angle (D (alpha)) and momentum (D(pp)) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L = 4.6 and 6.8 for electron energies 10 keV. Landau (n = 0) resonance and cyclotron harmonic resonances n = +/-1, +/-2,...+/-5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (alpha) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n = +1 and n = +2. A major contribution to momentum diffusion coefficients appears from n = +2. However, the banded structures in D alpha and Dpp coefficients appear only in the profile of diffusion coefficients for n = +2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The Dpp diffusion coefficient for ECH waves is one to two orders smaller than D alpha coefficients. For chorus waves, Dpp coefficients are about an order of magnitude smaller than D alpha coefficients for the case n does not = 0. In case of Landau resonance, the values of Dpp coefficient are generally larger than the values of D alpha coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89 deg and harmonic resonances n = +1, +2, and +3, whereas for whistler mode waves, the frequencies have been calculated for angle

Adaptive Optics System with Deformable Composite Mirror and High Speed, Ultra-Compact Electronics

NASA Astrophysics Data System (ADS)

Chen, Peter C.; Knowles, G. J.; Shea, B. G.

2006-06-01

We report development of a novel adaptive optics system for optical astronomy. Key components are very thin Deformable Mirrors (DM) made of fiber reinforced polymer resins, subminiature PMN-PT actuators, and low power, high bandwidth electronics drive system with compact packaging and minimal wiring. By using specific formulations of fibers, resins, and laminate construction, we are able to fabricate mirror face sheets that are thin (< 2mm), have smooth surfaces and excellent optical shape. The mirrors are not astigmatic and do not develop surface irregularities when cooled. The actuators are small footprint multilayer PMN-PT ceramic devices with large stroke (2- 20 microns), high linearity, low hysteresis, low power, and flat frequency response to >2 KHz. By utilizing QorTek’s proprietary synthetic impendence power supply technology, all the power, control, and signal extraction for many hundreds to 1000s of actuators and sensors can be implemented on a single matrix controller printed circuit board co-mounted with the DM. The matrix controller, in turn requires only a single serial bus interface, thereby obviating the need for massive wiring harnesses. The technology can be scaled up to multi-meter aperture DMs with >100K actuators.

Fermionic currents in AdS spacetime with compact dimensions

NASA Astrophysics Data System (ADS)

Bellucci, S.; Saharian, A. A.; Vardanyan, V.

2017-09-01

We derive a closed expression for the vacuum expectation value (VEV) of the fermionic current density in a (D +1 )-dimensional locally AdS spacetime with an arbitrary number of toroidally compactified Poincaré spatial dimensions and in the presence of a constant gauge field. The latter can be formally interpreted in terms of a magnetic flux treading the compact dimensions. In the compact subspace, the field operator obeys quasiperiodicity conditions with arbitrary phases. The VEV of the charge density is zero and the current density has nonzero components along the compact dimensions only. They are periodic functions of the magnetic flux with the period equal to the flux quantum and tend to zero on the AdS boundary. Near the horizon, the effect of the background gravitational field is small and the leading term in the corresponding asymptotic expansion coincides with the VEV for a massless field in the locally Minkowski bulk. Unlike the Minkowskian case, in the system consisting of an equal number of fermionic and scalar degrees of freedom, with same masses, charges and phases in the periodicity conditions, the total current density does not vanish. In these systems, the leading divergences in the scalar and fermionic contributions on the horizon are canceled and, as a consequence of that, the charge flux, integrated over the coordinate perpendicular to the AdS boundary, becomes finite. We show that in odd spacetime dimensions the fermionic fields realizing two inequivalent representations of the Clifford algebra and having equal phases in the periodicity conditions give the same contribution to the VEV of the current density. Combining the contributions from these fields, the current density in odd-dimensional C -,P - and T -symmetric models are obtained. As an application, we consider the ground state current density in curved carbon nanotubes described in terms of a (2 +1 )-dimensional effective Dirac model.

UWB Filtering Power Divider with Two Narrow Notch-bands and Wide Stop-band

NASA Astrophysics Data System (ADS)

Wei, Feng; Wang, Xin-Yi; Zou, Xin Tong; Shi, Xiao Wei

2017-12-01

A compact filtering ultra-wideband (UWB) microstrip power divider (PD) with two sharply rejected notch-bands and wide stopband is analyzed and designed in this paper. The proposed UWB PD is based on a conventional Wilkinson power divider, while two stub loaded resonators (SLRs) are coupled into two symmetrical output ports to achieve a bandpass filtering response. The simplified composite right/left-handed (SCRLH) resonators are employed to generate the dual notched bands. Defected ground structure (DGS) is introduced to improve the passband performance. Good insertion/return losses, isolation and notch-band rejection are achieved as demonstrated in both simulation and experiment.

Simulations of the temporal and spatial resolution for a compact time-resolved electron diffractometer

NASA Astrophysics Data System (ADS)

Robinson, Matthew S.; Lane, Paul D.; Wann, Derek A.

2016-02-01

A novel compact electron gun for use in time-resolved gas electron diffraction experiments has recently been designed and commissioned. In this paper we present and discuss the extensive simulations that were performed to underpin the design in terms of the spatial and temporal qualities of the pulsed electron beam created by the ionisation of a gold photocathode using a femtosecond laser. The response of the electron pulses to a solenoid lens used to focus the electron beam has also been studied. The simulated results show that focussing the electron beam affects the overall spatial and temporal resolution of the experiment in a variety of ways, and that factors that improve the resolution of one parameter can often have a negative effect on the other. A balance must, therefore, be achieved between spatial and temporal resolution. The optimal experimental time resolution for the apparatus is predicted to be 416 fs for studies of gas-phase species, while the predicted spatial resolution of better than 2 nm-1 compares well with traditional time-averaged electron diffraction set-ups.

Single layer of MX3(M = Ti, Zr; X = S, Se, Te): a new platform for nano-electronics and optics

NASA Astrophysics Data System (ADS)

Jin, Yingdi; Li, Xingxing; Yang, Jinlong

A serial of two dimensional titanium and zirconium trichalcogenides nanosheets MX3 (M=Ti, Zr; X=S, Se, Te) are investigated based on first-principles calculations. The evaluated low cleavage energy indicates that stable two dimensional monolayers can be exfoliated from their bulk crystals in experiment. Electronic studies reveal very rich electronic properties in these monolayers, including metallic TiTe3 and ZrTe3, direct band gap semiconductor TiS3 and indirect band gap semiconductors TiSe3, ZrS3 and ZrSe3. The band gaps of all the semiconductors are between 0.57~1.90 eV, which implies their potential applications in nano-electronics. And the calculated effective masses demonstrate highly anisotropic conduction properties for all the semiconductors. Optically, TiS3 and TiSe3 monolayers exhibit good light absorption in the visible and near-infrared region respectively, indicating their potential applications in optical devices. In particular, the highly anisotropic optical absorption of TiS3 monolayer suggests it could be used in designing nano optical waveguide polarizers.

Strong interplay between structure and electronic properties in CuIn(S,Se){2}: a first-principles study.

PubMed

Vidal, Julien; Botti, Silvana; Olsson, Pär; Guillemoles, Jean-François; Reining, Lucia

2010-02-05

We present a first-principles study of the electronic properties of CuIn(S,Se){2} (CIS) using state-of-the-art self-consistent GW and hybrid functionals. The calculated band gap depends strongly on the anion displacement u, an internal structural parameter that measures lattice distortion. This contrasts with the observed stability of the band gap of CIS solar panels under operating conditions, where a relatively large dispersion of values for u occurs. We solve this apparent paradox considering the coupled effect on the band gap of copper vacancies and lattice distortions. The correct treatment of d electrons in these materials requires going beyond density functional theory, and GW self-consistency is critical to evaluate the quasiparticle gap and the valence band maximum.

Interface Engineering of Monolayer MoS2/GaN Hybrid Heterostructure: Modified Band Alignment for Photocatalytic Water Splitting Application by Nitridation Treatment.

PubMed

Zhang, Zhaofu; Qian, Qingkai; Li, Baikui; Chen, Kevin J

2018-05-23

Interface engineering is a key strategy to deal with the two-dimensional (2D)/three-dimensional (3D) hybrid heterostructure, since the properties of this atomic-layer-thick 2D material can easily be impacted by the substrate environment. In this work, the structural, electronic, and optical properties of the 2D/3D heterostructure of monolayer MoS 2 on wurtzite GaN surface without and with nitridation interfacial layer are systematically investigated by first-principles calculation and experimental analysis. The nitridation interfacial layer can be introduced into the 2D/3D heterostructure by remote N 2 plasma treatment to GaN sample surface prior to stacking monolayer MoS 2 on top. The calculation results reveal that the 2D/3D integrated heterostructure is energetically favorable with a negative formation energy. Both interfaces demonstrate indirect band gap, which is a benefit for longer lifetime of the photoexcited carriers. Meanwhile, the conduction band edge and valence band edge of the MoS 2 side increases after nitridation treatment. The modification to band alignment is then verified by X-ray photoelectron spectroscopy measurement on MoS 2 /GaN heterostructures constructed by a modified wet-transfer technique, which indicates that the MoS 2 /GaN heterostructure without nitridation shows a type-II alignment with a conduction band offset (CBO) of only 0.07 eV. However, by the deployment of interface nitridation, the band edges of MoS 2 move upward for ∼0.5 eV as a result of the nitridized substrate property. The significantly increased CBO could lead to better electron accumulation capability at the GaN side. The nitridized 2D/3D heterostructure with effective interface treatment exhibits a clean band gap and substantial optical absorption ability and could be potentially used as practical photocatalyst for hydrogen generation by water splitting using solar energy.

Concept Design of a Multi-Band Shared Aperture Reflectarray/Reflector Antenna

NASA Technical Reports Server (NTRS)

Spence, Thomas; Cooley, Michael E.; Stenger, Peter; Park, Richard; Li, Lihua; Racette, Paul; Heymsfield, Gerald; Mclinden, Matthew

2016-01-01

A scalable dual-band (Ka/W) shared-aperture antenna system design has been developed as a proposed solution to meet the needs of the planned NASA Earth Science Aerosol, Clouds, and Ecosystem (ACE) mission. The design is comprised of a compact Cassegrain reflector/reflectarray with a fixed pointing W-band feed and a cross track scanned Ka-band Active Electronically Scanned Array (AESA). Critical Sub-scale prototype testing and flight tests have validated some of the key aspects of this innovative antenna design, including the low loss reflector/reflectarray surface. More recently the science community has expressed interest in a mission that offers the ability to measure precipitation in addition to clouds and aerosols. In this paper we present summaries of multiple designs that explore options for realizing a tri-frequency (Ku/Ka/W), shared-aperture antenna system to meet these science objectives. Design considerations include meeting performance requirements while emphasizing payload size, weight, prime power, and cost. The extensive trades and lessons learned from our previous dual-band ACE system development were utilized as the foundation for this work.

Concept Design of a Multi-Band Shared Aperture Reflectarray/Reflector Antenna

NASA Technical Reports Server (NTRS)

Spence, Thomas; Cooley, Michael; Stenger, Peter; Park, Richard; Li, Lihua; Racette, Paul; Heymsfield, Gerald; Mclinden, Matthew

2016-01-01

A scalable dual-band (KaW) shared-aperture antenna system design has been developed as a proposed solution to meet the needs of the planned NASA Earth Science Aerosol, Clouds, and Ecosystem (ACE) mission. The design is comprised of a compact Cassegrain reflector/reflectarray with a fixed pointing W-band feed and a cross track scanned Ka-band Active Electronically Scanned Array (AESA). Critical Sub-scale prototype testing and flight tests have validated some of the key aspects of this innovative antenna design, including the low loss reflector/reflectarray surface.More recently the science community has expressed interest in a mission that offers the ability to measure precipitation in addition to clouds and aerosols. In this paper we present summaries of multiple designs that explore options for realizing a tri-frequency (KuKaW), shared-aperture antenna system to meet these science objectives. Design considerations include meeting performance requirements while emphasizing payload size, weight, prime power, and cost. The extensive trades and lessons learned from our previous dual-band ACE system development were utilized as the foundation for this work.

Simultaneous measurements of ion and electron currents using a novel compact electrostatic end-loss-current detector

NASA Astrophysics Data System (ADS)

Hirata, M.; Miyake, Y.; Cho, T.; Kohagura, J.; Numakura, T.; Shimizu, K.; Ito, M.; Kiminami, S.; Morimoto, N.; Hirai, K.; Yamagishi, T.; Miyata, Y.; Nakashima, Y.; Miyoshi, S.; Ogura, K.; Kondoh, T.; Kariya, T.

2006-10-01

For the purpose of end-loss-ion and -electron analyses in open-field plasmas, a compact-sized electrostatic end-loss-current detector is proposed on the basis of a self-collection principle for suppressing the effects of secondary-electron emission from a metal collector. For employing this specific method, it is worth noting that no further additional magnetic systems except the ambient open-ended magnetic fields are required in the detector operation. This characteristic property provides a compactness of the total detection system and availability for its use in plasma confinement devices without disturbing plasma-confining magnetic fields. The detector consists of a set of parallel metal plates with respect to lines of ambient magnetic forces of a plasma device for analyzing incident ion currents along with a grid for shielding the collector against strays due to the metal-plate biasing. The characterization experiments are carried out by the use of a test-ion-beam line along with an additional use of a Helmholtz coil system for the formation of open magnetic fields similar to those in the GAMMA 10 end region. The applications of the developed end-loss-current detector in the GAMMA 10 plasma experiments are demonstrated under the conditions with simultaneous incidence of energetic electrons produced by electron-cyclotron heatings for end-loss-plugging potential formation.

Theoretical modeling of low-energy electronic absorption bands in reduced cobaloximes

DOE PAGES

Bhattacharjee, Anirban; Chavarot-Kerlidou, Murielle; Dempsey, Jillian L.; ...

2014-08-11

Here, we report that the reduced Co(I) states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we have analyzed the low energy electronic absorption bands of two cobaloxime systems experimentally and using a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.

Toward Revealing the Critical Role of Perovskite Coverage in Highly Efficient Electron-Transport Layer-Free Perovskite Solar Cells: An Energy Band and Equivalent Circuit Model Perspective.

PubMed

Huang, Like; Xu, Jie; Sun, Xiaoxiang; Du, Yangyang; Cai, Hongkun; Ni, Jian; Li, Juan; Hu, Ziyang; Zhang, Jianjun

2016-04-20

Currently, most efficient perovskite solar cells (PVKSCs) with a p-i-n structure require simultaneously electron transport layers (ETLs) and hole transport layers (HTLs) to help collecting photogenerated electrons and holes for obtaining high performance. ETL free planar PVKSC is a relatively new and simple structured solar cell that gets rid of the complex and high temperature required ETL (such as compact and mesoporous TiO2). Here, we demonstrate the critical role of high coverage of perovskite in efficient ETL free PVKSCs from an energy band and equivalent circuit model perspective. From an electrical point of view, we confirmed that the low coverage of perovskite does cause localized short circuit of the device. With coverage optimization, a planar p-i-n(++) device with a power conversion efficiency of over 11% was achieved, implying that the ETL layer may not be necessary for an efficient device as long as the perovskite coverage is approaching 100%.

Hot super-dense compact object with particular EoS

NASA Astrophysics Data System (ADS)

Tito, E. P.; Pavlov, V. I.

2018-03-01

We show the possibility of existence of a self-gravitating spherically-symmetric equilibrium configuration for a neutral matter with neutron-like density, small mass M ≪ M_{⊙}, and small radius R ≪ R_{⊙}. We incorporate the effects of both the special and general theories of relativity. Such object may be formed in a cosmic cataclysm, perhaps an exotic one. Since the base equations of hydrostatic equilibrium are completed by the equation of state (EoS) for the matter of the object, we offer a novel, interpolating experimental data from high-energy physics, EoS which permits the existence of such compact system of finite radius. This EoS model possesses a critical state characterized by density ρc and temperature Tc. For such an object, we derive a radial distribution for the super-dense matter in "liquid" phase using Tolman-Oppenheimer-Volkoff equations for hydrostatic equilibrium. We demonstrate that a stable configuration is indeed possible (only) for temperatures smaller than the critical one. We derive the mass-radius relation (adjusted for relativistic corrections) for such small (M ≪ M_{⊙}) super-dense compact objects. The results are within the constraints established by both heavy-ion collision experiments and theoretical studies of neutron-rich matter.

41 CFR 101-26.508 - Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band).

Code of Federal Regulations, 2013 CFR

2013-07-01

... processing (EDP) tape and instrumentation tape (wide and intermediate band). 101-26.508 Section 101-26.508... Programs § 101-26.508 Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band). Procurement by Federal agencies of EDP tape and instrumentation tape (wide and intermediate band...

41 CFR 101-26.508 - Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band).

Code of Federal Regulations, 2014 CFR

2014-07-01

... processing (EDP) tape and instrumentation tape (wide and intermediate band). 101-26.508 Section 101-26.508... Programs § 101-26.508 Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band). Procurement by Federal agencies of EDP tape and instrumentation tape (wide and intermediate band...

41 CFR 101-26.508 - Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band).

Code of Federal Regulations, 2010 CFR

2010-07-01

... processing (EDP) tape and instrumentation tape (wide and intermediate band). 101-26.508 Section 101-26.508... Programs § 101-26.508 Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band). Procurement by Federal agencies of EDP tape and instrumentation tape (wide and intermediate band...

41 CFR 101-26.508 - Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band).

Code of Federal Regulations, 2012 CFR

2012-07-01

... processing (EDP) tape and instrumentation tape (wide and intermediate band). 101-26.508 Section 101-26.508... Programs § 101-26.508 Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band). Procurement by Federal agencies of EDP tape and instrumentation tape (wide and intermediate band...

41 CFR 101-26.508 - Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band).

Code of Federal Regulations, 2011 CFR

2011-07-01

... processing (EDP) tape and instrumentation tape (wide and intermediate band). 101-26.508 Section 101-26.508... Programs § 101-26.508 Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band). Procurement by Federal agencies of EDP tape and instrumentation tape (wide and intermediate band...

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.

Radio Frequency Compatibility Evaluation of S Band Navigation Signals for Future BeiDou.

PubMed

Sun, Yanbo; Xue, Rui; Zhao, Danfeng; Wang, Dun

2017-05-05

With L band frequency allocations for satellite navigation getting more crowded, S band (2483.5-2500 MHz) is already allocated for navigation services, where Globalstar broadcasts downlink communications to user terminals. The Indian Regional Navigation Satellite System (IRNSS) is transmitting navigation signals and Galileo exploits some potential signals in S band. Also, several candidate S band signals based on binary offset carrier (BOC), binary phase shift keying (BPSK), continuous phase modulation (CPM) and minimum shift keying-BOC (MSK-BOC) are suggested for BeiDou system (BDS). In quite narrow S band, mutual interference among these systems is inevitable, thus the compatibility issue is particularly significant for S band signal design. To explore desired S band signals for BDS, the paper firstly describes a comprehensive compatibility evaluation methods based on effective carrier-to-noise ratio degradation for acquisition and code tracking. Then a real simulation is established using space constellations, modulation schemes and received power. Finally, the worst mutual interference of BDS candidate signals with Galileo, IRNSS and Globalstar is calculated and compared. The results indicate that CPM signal is easier to allow peaceful coexistence of other systems with minimal mutual interference in S band compared to other BDS candidates.

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.

Ab initio electronic structure calculations for metallic intermediate band formation in photovoltaic materials

NASA Astrophysics Data System (ADS)

Wahnón, P.; Tablero, C.

2002-04-01

A metallic isolated band in the middle of the band gap of several III-V semiconductors has been predicted as photovoltaic materials with the possibility of providing substantially enhanced efficiencies. We have investigated the electronic band structures and lattice constants of GanAsmM and GanPmM with M=Sc, Ti, V, and Cr, to identify whether this isolated band is likely to exist by means of accurate calculations. For this task, we use the SIESTA program, an ab initio periodic density-functional method, fully self consistent in the local-density approximation. Norm-conserving, nonlocal pseudopotentials and confined linear combination of atomic orbitals have been used. We have carried out a case study of GanAsmTi and GanPmTi energy-band structure including analyses of the effect of the basis set, fine k-point mesh to ensure numerical convergence, structural parameters, and generalized gradient approximation for exchange and correlation corrections. We find the isolated intermediate band when one Ti atom replaces the position of one As (or P) atom in the crystal structure. For this kind of compound we show that the intermediate band relative position inside the band gap and width are sensitive to the dynamic relaxation of the crystal and the size of the basis set.

Electron-Electron and Electron-Phonon interactions effects on the tunnel electronic spectrum of PbS quantum dots

NASA Astrophysics Data System (ADS)

Wang, Hongyue; Lhuillier, Emmanuel; Yu, Qian; Mottaghizadeh, Alireza; Ulysse, Christian; Zimmers, Alexandre; Dubertret, Benoit; Aubin, Herve

2015-03-01

We present a tunnel spectroscopy study of the electronic spectrum of single PbS Quantum Dots (QDs) trapped between nanometer-spaced electrodes, measured at low temperature T=5 K. The carrier filling of the QD can be controlled either by the drain voltage in the shell filling regime or by a gate voltage. In the empty QD, the tunnel spectrum presents the expected signature of the 8x degenerated excited levels. In the drain controlled shell filling regime, the levels degeneracies are lifted by the global electrostatic Coulomb energy of the QD; in the gate controlled shell filling regime, the levels degeneracies are lifted by the intra-Coulomb interactions. In the charged quantum dot, electron-phonons interactions lead to the apparition of Franck-Condon side bands on the single excited levels and possibly Franck Condon blockade at low energy. The sharpening of excited levels at higher gate voltage suggests that the magnitude of electron-phonon interactions is decreased upon increasing the electron filling in the quantum dot. This work was supported by the French ANR Grants 10-BLAN-0409-01, 09-BLAN-0388-01, by the Region Ile-de-France in the framework of DIM Nano-K and by China Scholarship Council.

Radio Frequency Compatibility Evaluation of S Band Navigation Signals for Future BeiDou

PubMed Central

Sun, Yanbo; Xue, Rui; Zhao, Danfeng; Wang, Dun

2017-01-01

With L band frequency allocations for satellite navigation getting more crowded, S band (2483.5–2500 MHz) is already allocated for navigation services, where Globalstar broadcasts downlink communications to user terminals. The Indian Regional Navigation Satellite System (IRNSS) is transmitting navigation signals and Galileo exploits some potential signals in S band. Also, several candidate S band signals based on binary offset carrier (BOC), binary phase shift keying (BPSK), continuous phase modulation (CPM) and minimum shift keying-BOC (MSK-BOC) are suggested for BeiDou system (BDS). In quite narrow S band, mutual interference among these systems is inevitable, thus the compatibility issue is particularly significant for S band signal design. To explore desired S band signals for BDS, the paper firstly describes a comprehensive compatibility evaluation methods based on effective carrier-to-noise ratio degradation for acquisition and code tracking. Then a real simulation is established using space constellations, modulation schemes and received power. Finally, the worst mutual interference of BDS candidate signals with Galileo, IRNSS and Globalstar is calculated and compared. The results indicate that CPM signal is easier to allow peaceful coexistence of other systems with minimal mutual interference in S band compared to other BDS candidates. PMID:28475142

Banded structures in electron pitch angle diffusion coefficients from resonant wave-particle interactions

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

Tripathi, A. K., E-mail: aktrip2001@yahoo.co.in; Singhal, R. P., E-mail: rpsiitbhu@yahoo.com; Khazanov, G. V., E-mail: George.V.Khazanov@nasa.gov

2016-04-15

Electron pitch angle (D{sub αα}) and momentum (D{sub pp}) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L = 4.6 and 6.8 for electron energies ≤10 keV. Landau (n = 0) resonance and cyclotron harmonic resonances n = ±1, ±2, … ±5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (α) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusionmore » coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n = +1 and n = +2. A major contribution to momentum diffusion coefficients appears from n = +2. However, the banded structures in D{sub αα} and D{sub pp} coefficients appear only in the profile of diffusion coefficients for n = +2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The D{sub pp} diffusion coefficient for ECH waves is one to two orders smaller than D{sub αα} coefficients. For chorus waves, D{sub pp} coefficients are about an order of magnitude smaller than D{sub αα} coefficients for the case n ≠ 0. In case of Landau resonance, the values of D{sub pp} coefficient are generally larger than the values of D{sub αα} coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89° and harmonic resonances n = +1, +2, and

Banded Structures in Electron Pitch Angle Diffusion Coefficients from Resonant Wave-Particle Interactions

NASA Technical Reports Server (NTRS)

Tripathi, A. K.; Singhal, R. P.; Khazanov, G. V.; Avanov, L. A.

2016-01-01

Electron pitch angle (D(sub (alpha alpha))) and momentum (D(sub pp)) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L=4.6 and 6.8 for electron energies less than or equal to 10 keV. Landau (n=0) resonance and cyclotron harmonic resonances n= +/- 1, +/-2, ... +/-5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (alpha) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n=+1 and n=+2. A major contribution to momentum diffusion coefficients appears from n=+2. However, the banded structures in D(sub alpha alpha) and D(sub pp) coefficients appear only in the profile of diffusion coefficients for n=+2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The D(sub pp) diffusion coefficient for ECH waves is one to two orders smaller than D(sub alpha alpha) coefficients. For chorus waves, D(sub pp) coefficients are about an order of magnitude smaller than D(sub alpha alpha) coefficients for the case n does not equal 0. In case of Landau resonance, the values of D(sub pp) coefficient are generally larger than the values of D(sub alpha alpha) coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89 deg and harmonic resonances

The electronic and transport properties of monolayer transition metal dichalcogenides: a complex band structure analysis

NASA Astrophysics Data System (ADS)

Szczesniak, Dominik

Recently, monolayer transition metal dichalcogenides have attracted much attention due to their potential use in both nano- and opto-electronics. In such applications, the electronic and transport properties of group-VIB transition metal dichalcogenides (MX2 , where M=Mo, W; X=S, Se, Te) are particularly important. Herein, new insight into these properties is presented by studying the complex band structures (CBS's) of MX2 monolayers while accounting for spin-orbit coupling effects. By using the symmetry-based tight-binding model a nonlinear generalized eigenvalue problem for CBS's is obtained. An efficient method for solving such class of problems is presented and gives a complete set of physically relevant solutions. Next, these solutions are characterized and classified into propagating and evanescent states, where the latter states present not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gaps, which describe the tunneling currents in the MX2 materials. The importance of CBS's and tunneling currents is demonstrated by the analysis of the quantum transport across MX2 monolayers within phase field matching theory. Present work has been prepared within the Qatar Energy and Environment Research Institute (QEERI) grand challenge ATHLOC project (Project No. QEERI- GC-3008).

Analysis of optical and electronic properties of MoS2 for optoelectronics and FET applications

NASA Astrophysics Data System (ADS)

Ullah, Muhammad S.; Yousuf, Abdul Hamid Bin; Es-Sakhi, Azzedin D.; Chowdhury, Masud H.

2018-04-01

Molybdenum disulfide (MoS2) is considered as a promising alternative to conventional semiconductor materials that used in the IC industry because of its novel properties. In this paper, we explore the optical and electronic properties of MoS2 for photodetector and transistors applications. This simulation is done using `DFT materials properties simulator'. Our findings show that mono- and multi-layer MoS2 is suitable for conventional and tunnel FET applications due to direct and indirect band-gap respectively. The bulk MoS2 crystal, which are composed of stacked layers have indirect bandgap and mono-layer MoS2 crystal form direct bandgap at the K-point of Brillouin zone. Indirect bandgap of bulk MoS2 crystal implies that phonons need to be involved in band-to-band tunneling (BTBT) process. Degenerately doped semiconductor, which is basically spinning the Fermi level, changing the DOS profile, and thinning the indirect bandgap that allow tunneling from valence band to conduction band. The optical properties of MoS2 is explored in terms of Absorption coefficient, extinction coefficient and refractive index. Our results shows that a MoS2 based photodetector can be fabricate to detect light in the visible range (below 500nm). It is also observed that the MoS2 is most sensitive for the light of wavelength 450nm.

Generation of lower and upper bands of electrostatic electron cyclotron harmonic waves in the Van Allen radiation belts

DOE PAGES

Zhou, Qinghua; Xiao, Fuliang; Yang, Chang; ...

2017-05-22

Electrostatic electron cyclotron harmonic (ECH) waves generated by the electron loss cone distribution can produce efficient scattering loss of plasma sheet electrons, which has a significant effect on the dynamics in the outer magnetosphere. Here we report two ECH emission events around the same location L≈ 5.7–5.8, MLT ≈ 12 from Van Allen Probes on 11 February (event A) and 9 January 2014 (event B), respectively. The spectrum of ECH waves was centered at the lower half of the harmonic bands during event A, but the upper half during event B. The observed electron phase space density in both eventsmore » is fitted by the subtracted bi-Maxwellian distribution, and the fitting functions are used to evaluate the local growth rates of ECH waves based on a linear theory for homogeneous plasmas. ECH waves are excited by the loss cone instability of 50 eV–1 keV electrons in the lower half of harmonic bands in the low-density plasmasphere in event A, and 1–10 keV electrons in the upper half of harmonic bands in a relatively high-density region in event B. Here, the current results successfully explain observations and provide a first direct evidence on how ECH waves are generated in the lower and upper half of harmonic frequency bands.« less

Generation of lower and upper bands of electrostatic electron cyclotron harmonic waves in the Van Allen radiation belts

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

Zhou, Qinghua; Xiao, Fuliang; Yang, Chang

Electrostatic electron cyclotron harmonic (ECH) waves generated by the electron loss cone distribution can produce efficient scattering loss of plasma sheet electrons, which has a significant effect on the dynamics in the outer magnetosphere. Here we report two ECH emission events around the same location L≈ 5.7–5.8, MLT ≈ 12 from Van Allen Probes on 11 February (event A) and 9 January 2014 (event B), respectively. The spectrum of ECH waves was centered at the lower half of the harmonic bands during event A, but the upper half during event B. The observed electron phase space density in both eventsmore » is fitted by the subtracted bi-Maxwellian distribution, and the fitting functions are used to evaluate the local growth rates of ECH waves based on a linear theory for homogeneous plasmas. ECH waves are excited by the loss cone instability of 50 eV–1 keV electrons in the lower half of harmonic bands in the low-density plasmasphere in event A, and 1–10 keV electrons in the upper half of harmonic bands in a relatively high-density region in event B. Here, the current results successfully explain observations and provide a first direct evidence on how ECH waves are generated in the lower and upper half of harmonic frequency bands.« less

Revivals of electron currents and topological-band insulator transitions in 2D gapped Dirac materials

NASA Astrophysics Data System (ADS)

Romera, E.; Bolívar, J. C.; Roldán, J. B.; de los Santos, F.

2016-07-01

We have studied the time evolution of electron wave packets in silicene under perpendicular magnetic and electric fields to characterize topological-band insulator transitions. We have found that at the charge neutrality points, the periodicities exhibited by the wave packet dynamics (classical and revival times) reach maximum values, and that the electron currents reflect the transition from a topological insulator to a band insulator. This provides a signature of topological phase transition in silicene that can be extended to other 2D Dirac materials isostructural to graphene and with a buckled structure and a significant spin-orbit coupling.

Installation of new Generation General Purpose Computer (GPC) compact unit

NASA Technical Reports Server (NTRS)

1991-01-01

In the Kennedy Space Center's (KSC's) Orbiter Processing Facility (OPF) high bay 2, Spacecraft Electronics technician Ed Carter (right), wearing clean suit, prepares for (26864) and installs (26865) the new Generation General Purpose Computer (GPC) compact IBM unit in Atlantis', Orbiter Vehicle (OV) 104's, middeck avionics bay as Orbiter Systems Quality Control technician Doug Snider looks on. Both men work for NASA contractor Lockheed Space Operations Company. All three orbiters are being outfitted with the compact IBM unit, which replaces a two-unit earlier generation computer.

Polygonal deformation bands in sandstone

NASA Astrophysics Data System (ADS)

Antonellini, Marco; Nella Mollema, Pauline

2017-04-01

We report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are dm-wide zones of shear deformation bands that developed under shallow burial conditions in the lower portion of the Jurassic Entrada Fm (Utah, USA). The edges of the polygons are 1 to 5 meters long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain < 0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. Density inversion, that takes place where under-compacted and over-pressurized layers (Carmel Fm) lay below normally compacted sediments (Entrada Sandstone), may be an important process for polygonal deformation bands formation. The gravitational sliding and soft sediment structures typically observed within the Carmel Fm support this hypothesis. Soft sediment deformation may induce polygonal faulting in the section of the Entrada Sandstone just above the Carmel Fm. The permeability of the polygonal deformation bands is approximately 10-14 to 10-13 m2, which is less than the permeability of the host, Entrada Sandstone (range 10-12 to 10-11 m2). The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes.

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.

Modulators for the S-band test linac at DESY

NASA Astrophysics Data System (ADS)

Bieler, M.; Choroba, S.; Hameister, J.; Lewin, H.-Ch.

1995-07-01

The development of adequate modulators for high peak power klystrons is one of the focus points for linear collider R&D programs. For the DESY/THD S-band linear collider study 150 MW rf-pulse power at 50 Hz repetition rate and 3 μs pulse duration is required [1]. Two different modulator schemes are under investigation. One is the conventional line type pulser, using a pulse forming network and a step up transformer, the other one is a hard tube pulser, using a dc power source at the full klystron voltage and a switch tube. This paper is focused on the modulator development for the S-band Test Linac at DESY. After a short overview over the test linac and a brief description of the 150 MW S-band klystron the circuitry of the line type pulse (LTP) is given. A hard tube pulser (HTP), which switches the high voltage directly from a storage capacitor to the klystron, has been built up at DESY. Circuitry and the results of the commissioning of the switch tube are reported.

Gd3+ spin-lattice relaxation via multi-band conduction electrons in Y(1-x)Gd(x)In3: an electron spin resonance study.

PubMed

Cabrera-Baez, M; Iwamoto, W; Magnavita, E T; Osorio-Guillén, J M; Ribeiro, R A; Avila, M A; Rettori, C

2014-04-30

Interest in the electronic structure of the intermetallic compound YIn3 has been renewed with the recent discovery of superconductivity at T ∼ 1 K, which may be filamentary in nature. In this work we perform electron spin resonance (ESR) experiments on Gd(3+) doped YIn3 (Y1-xGdxIn3; 0.001 ⪅ x ⩽̸ 0.08), showing that the spin-lattice relaxation of the Gd(3+) ions, due to the exchange interaction between the Gd(3+) localized magnetic moment and the conduction electrons (ce), is processed via the presence of s-, p- and d-type ce at the YIn3 Fermi level. These findings are revealed by the Gd(3+) concentration dependence of the Korringa-like relaxation rate d(ΔH)/dT and g-shift (Δg = g - 1.993), that display bottleneck relaxation behavior for the s-electrons and unbottleneck behavior for the p- and d-electrons. The Korringa-like relaxation rates vary from 22(2) Oe/K for x ⪅ 0.001 to 8(2) Oe/K for x = 0.08 and the g-shift values change, respectively, from a positive Δg = +0.047(10) to a negative Δg = -0.008(4). Analysis in terms of a three-band ce model allows the extraction of the corresponding exchange interaction parameters Jfs, Jfp and Jfd.

Slant path L- and S-Band tree shadowing measurements

NASA Technical Reports Server (NTRS)

Vogel, Wolfhard J.; Torrence, Geoffrey W.

1994-01-01

This contribution presents selected results from simultaneous L- and S-Band slant-path fade measurements through a pecan, a cottonwood, and a pine tree employing a tower-mounted transmitter and dual-frequency receiver. A single, circularly-polarized antenna was used at each end of the link. The objective was to provide information for personal communications satellite design on the correlation of tree shadowing between frequencies near 1620 and 2500 MHz. Fades were measured along 10 m lateral distance with 5 cm spacing. Instantaneous fade differences between L- and S-Band exhibited normal distribution with means usually near 0 dB and standard deviations from 5.2 to 7.5 dB. The cottonwood tree was an exception, with 5.4 dB higher average fading at S- than at L-Band. The spatial autocorrelation reduced to near zero with lags of about 10 lambda. The fade slope in dB/MHz is normally distributed with zero mean and standard deviation increasing with fade level.

Slant path L- and S-Band tree shadowing measurements

NASA Astrophysics Data System (ADS)

Vogel, Wolfhard J.; Torrence, Geoffrey W.

1994-08-01

This contribution presents selected results from simultaneous L- and S-Band slant-path fade measurements through a pecan, a cottonwood, and a pine tree employing a tower-mounted transmitter and dual-frequency receiver. A single, circularly-polarized antenna was used at each end of the link. The objective was to provide information for personal communications satellite design on the correlation of tree shadowing between frequencies near 1620 and 2500 MHz. Fades were measured along 10 m lateral distance with 5 cm spacing. Instantaneous fade differences between L- and S-Band exhibited normal distribution with means usually near 0 dB and standard deviations from 5.2 to 7.5 dB. The cottonwood tree was an exception, with 5.4 dB higher average fading at S- than at L-Band. The spatial autocorrelation reduced to near zero with lags of about 10 lambda. The fade slope in dB/MHz is normally distributed with zero mean and standard deviation increasing with fade level.

Spectroscopic evidence for temperature dependent relative movement of light and heavy hole valence bands of PbQ (Q=Te,Se,S)

NASA Astrophysics Data System (ADS)

Chatterjee, Utpal; Zhao, Junjing; Kanatzidis, Mercouri; Malliakas, Christos

We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) studies of the electronic structures of PbTe, PbSe and PbS. Our ARPES measurements provide direct evidences for the light hole upper valence bands (UVBs) and the so-called heavy hole lower valence bands (LVBs), and an unusual temperature dependent relative movement between their band maxima leading to a monotonic decrease in the energy separation between LVBs and UVBs with increase in temperature. This enables convergence of these valence bands and consequently an effective increase in the valley degeneracy in PbQ at higher temperatures, which has long been believed to be the driving factor behind their extraordinary thermoelectric performance.

Electronic band structures and excitonic properties of delafossites: A GW-BSE study

NASA Astrophysics Data System (ADS)

Wang, Xiaoming; Meng, Weiwei; Yan, Yanfa

2017-08-01

We report the band structures and excitonic properties of delafossites CuMO2 (M=Al, Ga, In, Sc, Y, Cr) calculated using the state-of-the-art GW-BSE approach. We evaluate different levels of self-consistency of the GW approximations, namely G0W0, GW0, GW, and QSGW, on the band structures and find that GW0, in general, predicts the band gaps in better agreement with experiments considering the electron-hole effect. For CuCrO2, the HSE wave function is used as the starting point for the perturbative GW0 calculations, since it corrects the band orders wrongly predicted by PBE. The discrepancy about the valence band characters of CuCrO2 is classified based on both HSE and QSGW calculations. The PBE wave functions, already good, are used for other delafossites. All the delafossites are shown to be indirect band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV, in consistent with experimental findings. The excitation mechanisms are explained by examining the exciton amplitude projections on the band structures. Discrepancies compared with experiments are also addressed. The lowest and strongest exciton, mainly contributed from either Cu 3d → Cu 3p (Al, Ga, In) or Cu 3d → M 3d (M = Sc, Y, Cr) transitions, is always located at the L point of the rhombohedral Brillouin zone.

Structural analysis, electronic properties, and band gaps of a graphene nanoribbon: A new 2D materials

NASA Astrophysics Data System (ADS)

Dass, Devi

2018-03-01

Graphene nanoribbon (GNR), a new 2D carbon nanomaterial, has some unique features and special properties that offer a great potential for interconnect, nanoelectronic devices, optoelectronics, and nanophotonics. This paper reports the structural analysis, electronic properties, and band gaps of a GNR considering different chirality combinations obtained using the pz orbital tight binding model. In structural analysis, the analytical expressions for GNRs have been developed and verified using the simulation for the first time. It has been found that the total number of unit cells and carbon atoms within an overall unit cell and molecular structure of a GNR have been changed with the change in their chirality values which are similar to the values calculated using the developed analytical expressions thus validating both the simulation as well as analytical results. Further, the electronic band structures at different chirality values have been shown for the identification of metallic and semiconductor properties of a GNR. It has been concluded that all zigzag edge GNRs are metallic with very small band gaps range whereas all armchair GNRs show both the metallic and semiconductor nature with very small and high band gaps range. Again, the total number of subbands in each electronic band structure is equal to the total number of carbon atoms present in overall unit cell of the corresponding GNR. The semiconductors GNRs can be used as a channel material in field effect transistor suitable for advanced CMOS technology whereas the metallic GNRs could be used for interconnect.

Ultra-deep K S-band Imaging of the Hubble Frontier Fields

NASA Astrophysics Data System (ADS)

Brammer, Gabriel B.; Marchesini, Danilo; Labbé, Ivo; Spitler, Lee; Lange-Vagle, Daniel; Barker, Elizbeth A.; Tanaka, Masayuki; Fontana, Adriano; Galametz, Audrey; Ferré-Mateu, Anna; Kodama, Tadayuki; Lundgren, Britt; Martis, Nicholas; Muzzin, Adam; Stefanon, Mauro; Toft, Sune; van der Wel, Arjen; Vulcani, Benedetta; Whitaker, Katherine E.

2016-09-01

We present an overview of the “KIFF” project, which provides ultra-deep K s -band imaging of all six of the Hubble Frontier Fields clusters, Abell 2744, MACS-0416, Abell S1063, Abell 370, MACS-0717, and MACS-1149. All of these fields have recently been observed with large allocations of Directors’ Discretionary Time with the Hubble and Spitzer telescopes, covering 0.4\\lt λ \\lt 1.6 μ {{m}} and 3.6-4.5 μ {{m}}, respectively. VLT/HAWK-I integrations of the first four fields reach 5σ limiting depths of {K}s˜ 26.0 (AB, point sources) and have excellent image quality (FWHM ˜ 0.″4). The MACS-0717 and MACS-1149 fields are observable from the northern hemisphere, and shorter Keck/MOSFIRE integrations on those fields reach limiting depths of K s = 25.5 and 25.1, with a seeing FWHM of ˜ 0.″4 and 0\\buildrel{\\prime\\prime}\\over{.} 5. In all cases the K s -band mosaics cover the primary cluster and parallel HST/ACS+WFC3 fields. The total area of the K s -band coverage is 490 arcmin2. The K s -band at 2.2 μ {{m}} crucially fills the gap between the reddest HST filter (1.6 μ {{m}} ˜ H band) and the IRAC 3.6 μ {{m}} passband. While reaching the full depths of the space-based imaging is not currently feasible from the ground, the deep K s -band images provide important constraints on both the redshifts and the stellar population properties of galaxies extending well below the characteristic stellar mass across most of the age of the universe, down to and including the redshifts of the targeted galaxy clusters (z≲ 0.5). Reduced, aligned mosaics of all six survey fields are provided.

Pressure effects on band structures in dense lithium

NASA Astrophysics Data System (ADS)

Goto, Naoyuki; Nagara, Hitose

2012-07-01

We studied the change of the band structures in some structures of Li predicted at high pressures, using GGA and GW calculations. The width of the 1s band coming from the 1s electron of Li shows broadening by the pressurization, which is the normal behavior of bands at high pressure. The width of the band just below the Fermi level decreases by the pressurization, which is an opposite behavior to the normal bands. The character of this narrowing band is mostly p-like with a little s-like portion. The band gaps in some structures are really observed even by the GGA calculations. The gaps by the GW calculations increase to about 1.5 times the GGA values. Generally the one-shot GW calculation (diagonal only calculations) gives more reliable values than the GGA, but it may fail to predict band gaps for the case where band dispersion shows complex crossing near the Fermi level. There remains some structures for which GW calculations with off-diagonal elements taken into account are needed to identify the phase to be metallic or semiconducting.

Energies of rare-earth ion states relative to host bands in optical materials from electron photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Thiel, Charles Warren

There are a vast number of applications for rare-earth-activated materials and much of today's cutting-edge optical technology and emerging innovations are enabled by their unique properties. In many of these applications, interactions between the rare-earth ion and the host material's electronic states can enhance or inhibit performance and provide mechanisms for manipulating the optical properties. Continued advances in these technologies require knowledge of the relative energies of rare-earth and crystal band states so that properties of available materials may be fully understood and new materials may be logically developed. Conventional and resonant electron photoemission techniques were used to measure 4f electron and valence band binding energies in important optical materials, including YAG, YAlO3, and LiYF4. The photoemission spectra were theoretically modeled and analyzed to accurately determine relative energies. By combining these energies with ultraviolet spectroscopy, binding energies of excited 4fN-15d and 4fN+1 states were determined. While the 4fN ground-state energies vary considerably between different trivalent ions and lie near or below the top of the valence band in optical materials, the lowest 4f N-15d states have similar energies and are near the bottom of the conduction band. As an example for YAG, the Tb3+ 4f N ground state is in the band gap at 0.7 eV above the valence band while the Lu3+ ground state is 4.7 eV below the valence band maximum; however, the lowest 4fN-15d states are 2.2 eV below the conduction band for both ions. We found that a simple model accurately describes the binding energies of the 4fN, 4fN-1 5d, and 4fN+1 states. The model's success across the entire rare-earth series indicates that measurements on two different ions in a host are sufficient to predict the energies of all rare-earth ions in that host. This information provides new insight into electron transfer transitions, luminescence quenching, and valence

Electronic and magnetic properties of nonmetal atoms adsorbed ReS{sub 2} monolayers

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

Zhang, Xiaoou; Li, Qingfang, E-mail: qingfangli@nuist.edu.cn; Department of Physics, Nanjing University of Information Science and Technology, Nanjing 210044

2015-08-14

The stable configurations and electronic and magnetic properties of nonmetal atoms (H, N, P, O, S, F, and Cl) adsorbed ReS{sub 2} monolayers have been investigated by first-principles calculations. It is found that H, O, S, F, and Cl prefer to occupy the peak sites of S atoms, while both N and P atoms favor the valley sites of S atoms. The ReS{sub 2} sheet exhibits a good adsorption capability to nonmetal atoms. The reconstruction of the surface is pronounced in N- and P-adsorbed ReS{sub 2} monolayers. In H-adsorbed case, the Fermi level is pulled into the conduction band, whichmore » results in the semiconductor-metal transition. The same magnetic moment of 1μ{sub B} is found in the N-, P-, F-, and Cl-adsorbed ReS{sub 2} monolayers, while the mechanisms of forming magnetic moment for N (P)- and F (Cl)-adsorbed cases are different. In addition, the spatial extensions of spin density in P-, F-, and Cl-adsorbed cases are larger than that in N-adsorbed case, which is more suitable to achieve long-range magnetic coupling interaction at low defect concentrations. Our results provide insight for achieving metal-free magnetism and a tunable band gap for various electronic and spintronic devices based on ReS{sub 2}.« less

Plasmon enhanced heterogeneous electron transfer with continuous band energy model

NASA Astrophysics Data System (ADS)

Zhao, Dandan; Niu, Lu; Wang, Luxia

2017-08-01

Photoinduced charge injection from a perylene dye molecule into the conduction band of a TiO2 system decorated by a metal nanoparticles (MNP) is studied theoretically. Utilizing the density matrix theory the charge transfer dynamics is analyzed. The continuous behavior of the TiO2 conduction band is accounted for by a Legendre polynomials expansion. The simulations consider optical excitation of the dye molecule coupled to the MNP and the subsequent electron injection into the TiO2 semiconductor. Due to the energy transfer coupling between the molecule and the MNP optical excitation and subsequent charge injection into semiconductor is strongly enhanced. The respective enhancement factor can reach values larger than 103. Effects of pulse duration, coupling strength and energetic resonances are also analyzed. The whole approach offers an efficient way to increase charge injection in dye-sensitized solar cells.

Development of a novel thermionic RF electron gun applied on a compact THz-FEL facility

NASA Astrophysics Data System (ADS)

Hu, T. N.; Pei, Y. J.; Qin, B.; Liu, K. F.; Feng, G. Y.

2018-04-01

The current requirements from civil and commercial applications lead to the development of compact free-electron laser (FEL)-based terahertz (THz) radiation sources. A picosecond electron gun plays an important role in an FEL-THz facility and attracts significant attention, as machine performance is very sensitive to initial conditions. A novel thermionic gun with an external cathode (EC) and two independently tunable cavities (ITCs) has been found to be a promising alternative to conventional electron sources due to its remarkable characteristics, and correspondingly an FEL injector can achieve a balance between a compact layout and high brightness benefitting from the velocity bunching properties and RF focusing effects in the EC-ITC gun. Nevertheless, the EC-ITC gun has not been extensively examined as part of the FEL injector in the past years. In this regard, to fill this gap, a development focusing on the experimental setup of an FEL injector based on an EC-ITC gun is described in detail. Before assembly, dynamic beam simulations were performed to investigate the optimal mounting position for the Linac associated with the focusing coils, and a suitable radio-frequency (RF) system was established based on a power coupling design and allocation. The testing bench proved to be fully functional through basic experiments using typical diagnostic approaches for estimating primary parameters. Associated with dynamic beam calculations, a performance evaluation for an EC-ITC gun was established while providing indirect testing results for an FEL injector.

A compact high current pulsed electron gun with subnanosecond electron pulse widths

NASA Technical Reports Server (NTRS)

Khakoo, M. A.; Srivastava, S. K.

1984-01-01

A magnetically-collimated, double-pulsed electron gun capable of generating electron pulses with a peak instantaneous current of approximately 70 microamps and a temporal width of 0.35 ns (FWHM) has been developed. Calibration is accomplished by measuring the lifetime of the well known 2(1P)-to-1(1S) transition in helium (58.4nm) at a near-threshold electron-impact energy by use of the delayed-coincidence technique.

Solid state, S-band, power amplifier

NASA Technical Reports Server (NTRS)

Digrindakis, M.

1973-01-01

The final design and specifications for a solid state, S-band, power amplifier is reported. Modifications from a previously proposed design were incorporated to improve efficiency and meet input overdrive and noise floor requirements. Reports on the system design, driver amplifier, power amplifier, and voltage and current limiter are included along with a discussion of the testing program.

Inter-spin distance determination using L-band (1-2 GHz) non-adiabatic rapid sweep electron paramagnetic resonance (NARS EPR)

PubMed Central

Kittell, Aaron W.; Hustedt, Eric J.; Hyde, James S.

2014-01-01

Site-directed spin-labeling electron paramagnetic resonance (SDSL EPR) provides insight into the local structure and motion of a spin probe strategically attached to a molecule. When a second spin is introduced to the system, macromolecular information can be obtained through measurement of inter-spin distances either by continuous wave (CW) or pulsed electron double resonance (ELDOR) techniques. If both methodologies are considered, inter-spin distances of 8 to 80 Å can be experimentally determined. However, there exists a region at the upper limit of the conventional X-band (9.5 GHz) CW technique and the lower limit of the four-pulse double electron-electron resonance (DEER) experiment where neither method is particularly reliable. The work presented here utilizes L-band (1.9 GHz) in combination with non-adiabatic rapid sweep (NARS) EPR to address this opportunity by increasing the upper limit of the CW technique. Because L-band linewidths are three to seven times narrower than those at X-band, dipolar broadenings that are small relative to the X-band inhomogeneous linewidth become observable, but the signal loss due to the frequency dependence of the Boltzmann factor, has made L-band especially challenging. NARS has been shown to increase sensitivity by a factor of five, and overcomes much of this loss, making L-band distance determination more feasible [1]. Two different systems are presented and distances of 18–30 Å have been experimentally determined at physiologically relevant temperatures. Measurements are in excellent agreement with a helical model and values determined by DEER. PMID:22750251

Experimental and theoretical investigations of the electronic band structure of metal-organic frameworks of HKUST-1 type

NASA Astrophysics Data System (ADS)

Gu, Zhi-Gang; Heinke, Lars; Wöll, Christof; Neumann, Tobias; Wenzel, Wolfgang; Li, Qiang; Fink, Karin; Gordan, Ovidiu D.; Zahn, Dietrich R. T.

2015-11-01

The electronic properties of metal-organic frameworks (MOFs) are increasingly attracting the attention due to potential applications in sensor techniques and (micro-) electronic engineering, for instance, as low-k-dielectric in semiconductor technology. Here, the band gap and the band structure of MOFs of type HKUST-1 are studied in detail by means of spectroscopic ellipsometry applied to thin surface-mounted MOF films and by means of quantum chemical calculations. The analysis of the density of states, the band structure, and the excitation spectrum reveal the importance of the empty Cu-3d orbitals for the electronic properties of HKUST-1. This study shows that, in contrast to common belief, even in the case of this fairly "simple" MOF, the excitation spectra cannot be explained by a superposition of "intra-unit" excitations within the individual building blocks. Instead, "inter-unit" excitations also have to be considered.

Determination of the band parameters of bulk 2H-MX2 (M = Mo, W; X = S, Se) by angle-resolved photoemission spectroscopy

PubMed Central

Kim, Beom Seo; Rhim, Jun-Won; Kim, Beomyoung; Kim, Changyoung; Park, Seung Ryong

2016-01-01

Monolayer MX2 (M = Mo, W; X = S, Se) has recently been drawn much attention due to their application possibility as well as the novel valley physics. On the other hand, it is also important to understand the electronic structures of bulk MX2 for material applications since it is very challenging to grow large size uniform and sustainable monolayer MX2. We performed angle-resolved photoemission spectroscopy and tight binding calculations to investigate the electronic structures of bulk 2H-MX2. We could extract all the important electronic band parameters for bulk 2H-MX2, including the band gap, direct band gap size at K (-K) point and spin splitting size. Upon comparing the parameters for bulk 2H-MX2 (our work) with mono- and multi-layer MX2 (published), we found that stacked layers, substrates for thin films, and carrier concentration significantly affect the parameters, especially the band gap size. The origin of such effect is discussed in terms of the screening effect. PMID:27805019

Compact x-ray source and panel

DOEpatents

Sampayon, Stephen E [Manteca, CA

2008-02-12

A compact, self-contained x-ray source, and a compact x-ray source panel having a plurality of such x-ray sources arranged in a preferably broad-area pixelized array. Each x-ray source includes an electron source for producing an electron beam, an x-ray conversion target, and a multilayer insulator separating the electron source and the x-ray conversion target from each other. The multi-layer insulator preferably has a cylindrical configuration with a plurality of alternating insulator and conductor layers surrounding an acceleration channel leading from the electron source to the x-ray conversion target. A power source is connected to each x-ray source of the array to produce an accelerating gradient between the electron source and x-ray conversion target in any one or more of the x-ray sources independent of other x-ray sources in the array, so as to accelerate an electron beam towards the x-ray conversion target. The multilayer insulator enables relatively short separation distances between the electron source and the x-ray conversion target so that a thin panel is possible for compactness. This is due to the ability of the plurality of alternating insulator and conductor layers of the multilayer insulators to resist surface flashover when sufficiently high acceleration energies necessary for x-ray generation are supplied by the power source to the x-ray sources.

Assessment of the Polycyclic Aromatic Hydrocarbon-Diffuse Interstellar Band Proposal

NASA Technical Reports Server (NTRS)

Salama, Farid; Bakes, F.; Allamandola, L.; Tielens, A. G. G. M.; Witteborn, Fred C. (Technical Monitor)

1995-01-01

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high energy range of the spectrum, and possess a large oscillator strength), and seems to correlate with strong and broad DIBs. In the case of non-compact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low energy range of the spectrum, and possess a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that: (i) Only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narrow bands on the other hand can easily be interpreted in the context of the PAH proposal by a distribution between compact and non-compact PAH ions, respectively. A

A compact planar multi-broad band monopole antenna for mobile devices

NASA Astrophysics Data System (ADS)

Zhong, Xiaoqing; Yao, Bin; Zheng, Qinhong; Yang, Jikong; Cao, Xiangqi

2015-10-01

A Multiple-frequency broadband planar monopole antenna is proposed in this Paper. The antenna is stimulated and numerically optimized by HFSS13.0 (High Frequency Structure Simulator). The size of it is 39mm×22mm×1.7mm. The antenna resonates at many frequencies. The parameter S11<=-6dB means the proposed antenna matches well with its feed-line and covers many useful operation frequency bands, including 2G(DCS1800 and PCS1900), 3G(UMTS), 4G(LTE2300 and LTE2500), ISM, WLAN. It is quiet appropriate for the present ultra-thin smart phones

A compact electron cyclotron resonance proton source for the Paul Scherrer Institute's proton accelerator facility

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

Baumgarten, C.; Barchetti, A.; Einenkel, H.

2011-05-15

A compact electron cyclotron resonance proton source has been developed and installed recently at thePaul Scherrer Institute's high intensity proton accelerator. Operation at the ion source test stand and the accelerator demonstrates a high reliability and stability of the new source. When operated at a 10 - 12 mA net proton current the lifetime of the source exceeds 2000 h. The essential development steps towards the observed performance are described.

Growth of the 889 per cm infrared band in annealed electron-irradiated silicon

NASA Technical Reports Server (NTRS)

Svensson, B. G.; Lindstrom, J. L.; Corbett, J. W.

1985-01-01

Isothermal annealing of electron-irradiated Czochralski silicon has been studied at four different temperatures ranging from 304 to 350 C using infrared spectroscopy. At annealing temperatures above 300 C the irradiation-induced band at 830 per cm, usually attributed to a vacancy-oxygen complex (the A center), disappears and a new band at 889 per cm grows up. Within the experimental accuracy, the activation energy for the growth of this band is found to be identical with the value given by Stavola et al. for 'anomalous' oxygen diffusion in silicon. Also the frequency factors for the two processes are in reasonable agreement. The results show that a vacancy-assisted process may provide an explanation for enhanced motion of oxygen in silicon.

Complex band structure and electronic transmission eigenchannels

NASA Astrophysics Data System (ADS)

Jensen, Anders; Strange, Mikkel; Smidstrup, Søren; Stokbro, Kurt; Solomon, Gemma C.; Reuter, Matthew G.

2017-12-01

It is natural to characterize materials in transport junctions by their conductance length dependence, β. Theoretical estimations of β are made employing two primary theories: complex band structure and density functional theory (DFT) Landauer transport. It has previously been shown that the β value derived from total Landauer transmission can be related to the β value from the smallest |ki| complex band; however, it is an open question whether there is a deeper relationship between the two. Here we probe the details of the relationship between transmission and complex band structure, in this case individual eigenchannel transmissions and different complex bands. We present calculations of decay constants for the two most conductive states as determined by complex band structure and standard DFT Landauer transport calculations for one semi-conductor and two molecular junctions. The molecular junctions show that both the length dependence of the total transmission and the individual transmission eigenvalues can be, almost always, found through the complex band structure. The complex band structure of the semi-conducting material, however, does not predict the length dependence of the total transmission but only of the individual channels, at some k-points, due to multiple channels contributing to transmission. We also observe instances of vertical bands, some of which are the smallest |ki| complex bands, that do not contribute to transport. By understanding the deeper relationship between complex bands and individual transmission eigenchannels, we can make a general statement about when the previously accepted wisdom linking transmission and complex band structure will fail, namely, when multiple channels contribute significantly to the transmission.

Inelastic Compaction in High-Porosity Limestone Monitored Using Acoustic Emissions

NASA Astrophysics Data System (ADS)

Baud, Patrick; Schubnel, Alexandre; Heap, Michael; Rolland, Alexandra

2017-12-01

We performed a systematic investigation of mechanical compaction and strain localization in Saint-Maximin limestone, a quartz-rich, high-porosity (37%) limestone from France. Our new data show that the presence of a significant proportion of secondary mineral (i.e., quartz) did not impact the mechanical strength of the limestone in both the brittle faulting and cataclastic flow regimes, but that the presence of water exerted a significant weakening effect. In contrast to previously published studies on deformation in limestones, inelastic compaction in Saint-Maximin limestone was accompanied by abundant acoustic emission (AE) activity. The location of AE hypocenters during triaxial experiments revealed the presence of compaction localization. Two failure modes were identified in agreement with microstructural analysis and X-ray computed tomography imaging: compactive shear bands developed at low confinement and complex diffuse compaction bands formed at higher confinement. Microstructural observations on deformed samples suggest that the recorded AE activity associated with inelastic compaction, unusual for a porous limestone, could have been due to microcracking at the quartz grain interfaces. Similar to published data on high-porosity macroporous limestones, the crushing of calcite grains was the dominant micromechanism of inelastic compaction in Saint-Maximin limestone. New P wave velocity data show that the effect of microcracking was dominant near the yield point and resulted in a decrease in P wave velocity, while porosity reduction resulted in a significant increase in P wave velocity beyond a few percent of plastic volumetric strain. These new data highlight the complex interplay between mineralogy, rock microstructure, and strain localization in porous rocks.

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

Development of a Multi-Band Shared Aperture Reflectarray/Reflector Antenna Design for NASA

NASA Technical Reports Server (NTRS)

Spence, Thomas; Cooley, Michael; Stenger, Peter; Park, Richard; Li, Lihua; Racette, Paul; Heymsfield, Gerald; Mclinden, Matthew

2016-01-01

A dual-band (Ka/W) shared-aperture antenna system design has been developed as a proposed solution to meet the needs of NASA's planned Aerosol, Clouds, and Ecosystem (ACE) mission. The design is comprised of a compact Cassegrain reflector/reflect array with a fixed W-band feed and a cross track scanned Ka-band Active Electronically Scanned Array (AESA). Critical Sub-scale prototype testing and flight tests have validated some of the key aspects of this innovative antenna design, including the low loss reflector/reflect array surface. More recently the science community has expressed interest in a mission that offers the ability to measure precipitation (Ku- band with scanning) in addition to clouds and aerosols. In this paper we present findings from a design study that explores options for realizing a tri-frequency (Ku/Ka/W), shared-aperture antenna system to meet these science objectives. Design considerations included meeting performance requirements while striving to minimize payload size, weight, prime power, and cost. The extensive trades and lessons learned from the ACE system development were utilized as the foundation for this work.

Correlation between morphology, electron band structure, and resistivity of Pb atomic chains on the Si(5 5 3)-Au surface

NASA Astrophysics Data System (ADS)

Jałochowski, M.; Kwapiński, T.; Łukasik, P.; Nita, P.; Kopciuszyński, M.

2016-07-01

Structural and electron transport properties of multiple Pb atomic chains fabricated on the Si(5 5 3)-Au surface are investigated using scanning tunneling spectroscopy, reflection high electron energy diffraction, angular resolved photoemission electron spectroscopy and in situ electrical resistance. The study shows that Pb atomic chains growth modulates the electron band structure of pristine Si(5 5 3)-Au surface and hence changes its sheet resistivity. Strong correlation between chains morphology, electron band structure and electron transport properties is found. To explain experimental findings a theoretical tight-binding model of multiple atomic chains interacting on effective substrate is proposed.

The structural, electronic and magnetic properties of CoS2 under pressure

NASA Astrophysics Data System (ADS)

Feng, Zhong-Ying; Yang, Yan; Zhang, Jian-Min

2018-05-01

The structural, electronic and magnetic properties of CoS2 under pressure have been investigated by the first-principles calculations. The lattice constant and volume decrease with increasing pressure. The CoS2 is stable and behaves a brittle characteristic under the pressures of 0-5 GPa. The CoS2 presents metallic characteristic under the pressures of 1-5 GPa although it is nearly half-metal (HM) under the pressure of 0 GPa. The lowest conduction bands for spin-up and spin-down channels shift towards higher and lower energy region, respectively, with the pressure increasing from 0 to 5 GPa. In spin-up channel the conduction band minimum (CBM) is mainly contributed by Co-3d(eg) orbitals at R point but the valence band maximum (VBM) is contributed by Co-3d(t2g) orbitals near M point. While in spin-down channel the CBM is contributed by S-3p orbitals at Γ point but the VBM is contributed by Co-3d(t2g) orbitals near X point. The CoS2 is still suitable to be used in the supercapacitor under the environmental pressures of 0-5 GPa due to the high conductivity.

Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals

NASA Astrophysics Data System (ADS)

Gehlmann, Mathias; Aguilera, Irene; Bihlmayer, Gustav; Młyńczak, Ewa; Eschbach, Markus; Döring, Sven; Gospodarič, Pika; Cramm, Stefan; Kardynał, Beata; Plucinski, Lukasz; Blügel, Stefan; Schneider, Claus M.

2016-06-01

Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.

The effects of buoyancy on shear-induced melt bands in a compacting porous medium

NASA Astrophysics Data System (ADS)

Butler, S. L.

2009-03-01

It has recently been shown [Holtzman, B., Groebner, N., Zimmerman, M., Ginsberg, S., Kohlstedt, D., 2003. Stress-driven melt segregation in partially molten rocks. Geochem. Geophys. Geosyst. 4, Art. No. 8607; Holtzman, B.K., Kohlstedt, D.L., 2007. Stress-driven melt segregation and strain partitioning in partially molten rocks: effects of stress and strain. J. Petrol. 48, 2379-2406] that when partially molten rock is subjected to simple shear, bands of high and low porosity are formed at a particular angle to the direction of instantaneous maximum extension. These have been modeled numerically and it has been speculated that high porosity bands may form an interconnected network with a bulk, effective permeability that is enhanced in a direction parallel to the bands. As a result, the bands may act to focus mantle melt towards the axis of mid-ocean ridges [Katz, R.F., Spiegelman, M., Holtzman, B., 2006. The dynamics of melt and shear localization in partially molten aggregates. Nature 442, 676-679]. In this contribution, we examine the combined effects of buoyancy and matrix shear on a deforming porous layer. The linear theory of Spiegelman [Spiegelman, M., 1993. Flow in deformable porous media. Part 1. Simple analysis. J. Fluid Mech. 247, 17-38; Spiegelman, M., 2003. Linear analysis of melt band formation by simple shear. Geochem. Geophys. Geosyst. 4, doi:10.1029/2002GC000499, Article 8615] and Katz et al. [Katz, R.F., Spiegelman, M., Holtzman, B., 2006. The dynamics of melt and shear localization in partially molten aggregates. Nature 442, 676-679] is generalized to include both the effects of buoyancy and matrix shear on a deformable porous layer with strain-rate dependent rheology. The predictions of linear theory are compared with the early time evolution of our 2D numerical model and they are found to be in excellent agreement. For conditions similar to the upper mantle, buoyancy forces can be similar to or much greater than matrix shear-induced forces. The

Structural and electronic properties of CdSe/ZnS and ZnS/CdSe core/shell nanowires via first principles study

NASA Astrophysics Data System (ADS)

Rehman, Shafiq Ur; Li, H. M.; Ding, Z. J.

2018-05-01

First principles calculations have been performed to predict the structural stability and electronic structures of hydrogen passivated wurtzite CdSe/ZnS and ZnS/CdSe core/shell nanowires (CSNWs) in the [0001] direction. The calculated binding energy shows that ZnS/CdSe CSNWs are more stable than CdSe/ZnS CSNWs and the stability of ZnS/CdSe CSNWs increases with increasing the thickness of ZnS shell. The modulated electronic band gap demonstrates an increase when the size of both CSNWs is reduced, as a result of the quantum confinement effect. The core-to-shell chemical composition of atoms shows that a strong composition effect also exists in these CSNWs, which in turn affects their electronic properties. Our simulated results show that the photoemission spectra of the CSNWs can be significantly improved by tuning the energy gap of CSNWs.

Experimental and theoretical investigations of the electronic band structure of metal-organic frameworks of HKUST-1 type

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

Gu, Zhi-Gang; State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 350002 Fuzhou; Heinke, Lars, E-mail: Lars.Heinke@KIT.edu

The electronic properties of metal-organic frameworks (MOFs) are increasingly attracting the attention due to potential applications in sensor techniques and (micro-) electronic engineering, for instance, as low-k-dielectric in semiconductor technology. Here, the band gap and the band structure of MOFs of type HKUST-1 are studied in detail by means of spectroscopic ellipsometry applied to thin surface-mounted MOF films and by means of quantum chemical calculations. The analysis of the density of states, the band structure, and the excitation spectrum reveal the importance of the empty Cu-3d orbitals for the electronic properties of HKUST-1. This study shows that, in contrast tomore » common belief, even in the case of this fairly “simple” MOF, the excitation spectra cannot be explained by a superposition of “intra-unit” excitations within the individual building blocks. Instead, “inter-unit” excitations also have to be considered.« less

Apollo experience report: S-band system signal design and analysis

NASA Technical Reports Server (NTRS)

Rosenberg, H. R. (Editor)

1972-01-01

A description is given of the Apollo communications-system engineering-analysis effort that ensured the adequacy, performance, and interface compatibility of the unified S-band system elements for a successful lunar-landing mission. The evolution and conceptual design of the unified S-band system are briefly reviewed from a historical viewpoint. A comprehensive discussion of the unified S-band elements includes the salient design features of the system and serves as a basis for a better understanding of the design decisions and analyses. The significant design decisions concerning the Apollo communications-system signal design are discussed providing an insight into the role of systems analysis in arriving at the current configuration of the Apollo communications system. Analyses are presented concerning performance estimation (mathematical-model development through real-time mission support) and system deficiencies, modifications, and improvements.

Optimization of a triode-type cusp electron gun for a W-band gyro-TWA

NASA Astrophysics Data System (ADS)

Zhang, Liang; Donaldson, Craig R.; He, Wenlong

2018-04-01

A triode-type cusp electron gun was optimized through numerical simulations for a W-band gyrotron traveling wave amplifier. An additional electrode in front of the cathode could switch the electron beam on and off instantly when its electric potential is properly biased. An optimal electron beam of current 1.7 A and a velocity ratio (alpha) of 1.12 with an alpha spread of ˜10.7% was achieved when the triode gun was operated at 40 kV.

Measured temperature and pressure dependence of Vp and Vs in compacted, polycrystalline sI methane and sII methane-ethane hydrate

USGS Publications Warehouse

Helgerud, M.B.; Waite, W.F.; Kirby, S.H.; Nur, A.

2003-01-01

We report on compressional- and shear-wave-speed measurements made on compacted polycrystalline sI methane and sII methane-ethane hydrate. The gas hydrate samples are synthesized directly in the measurement apparatus by warming granulated ice to 17??C in the presence of a clathrate-forming gas at high pressure (methane for sI, 90.2% methane, 9.8% ethane for sII). Porosity is eliminated after hydrate synthesis by compacting the sample in the synthesis pressure vessel between a hydraulic ram and a fixed end-plug, both containing shear-wave transducers. Wave-speed measurements are made between -20 and 15??C and 0 to 105 MPa applied piston pressure.

Structural, optical, and photoluminescence characterization of electron beam evaporated ZnS/CdSe nanoparticles thin films

NASA Astrophysics Data System (ADS)

Mohamed, S. H.; Ali, H. M.

2011-01-01

Structural, optical, and photoluminescence investigations of ZnS capped with CdSe films prepared by electron beam evaporation are presented. X-ray diffraction analysis revealed that the ZnS/CdSe nanoparticles films contain cubic cadmium selenide and hexagonal zinc sulfide crystals and the ZnS grain sizes increased with increasing ZnS thickness. The refractive index was evaluated in terms of envelope method, which has been suggested by Swanepoel in the transparent region. The refractive index values were found to increase with increasing ZnS thickness. However, the optical band gap and the extinction coefficient were decreased with increasing ZnS thickness. Photoluminescence (PL) investigations revealed the presence of two broad emission bands. The ZnS thickness significantly influenced the PL intensities.

3rd-generation MW/LWIR sensor engine for advanced tactical systems

NASA Astrophysics Data System (ADS)

King, Donald F.; Graham, Jason S.; Kennedy, Adam M.; Mullins, Richard N.; McQuitty, Jeffrey C.; Radford, William A.; Kostrzewa, Thomas J.; Patten, Elizabeth A.; McEwan, Thomas F.; Vodicka, James G.; Wootan, John J.

2008-04-01

Raytheon has developed a 3rd-Generation FLIR Sensor Engine (3GFSE) for advanced U.S. Army systems. The sensor engine is based around a compact, productized detector-dewar assembly incorporating a 640 x 480 staring dual-band (MW/LWIR) focal plane array (FPA) and a dual-aperture coldshield mechanism. The capability to switch the coldshield aperture and operate at either of two widely-varying f/#s will enable future multi-mode tactical systems to more fully exploit the many operational advantages offered by dual-band FPAs. RVS has previously demonstrated high-performance dual-band MW/LWIR FPAs in 640 x 480 and 1280 x 720 formats with 20 μm pitch. The 3GFSE includes compact electronics that operate the dual-band FPA and variable-aperture mechanism, and perform 14-bit analog-to-digital conversion of the FPA output video. Digital signal processing electronics perform "fixed" two-point non-uniformity correction (NUC) of the video from both bands and optional dynamic scene-based NUC; advanced enhancement processing of the output video is also supported. The dewar-electronics assembly measures approximately 4.75 x 2.25 x 1.75 inches. A compact, high-performance linear cooler and cooler electronics module provide the necessary FPA cooling over a military environmental temperature range. 3GFSE units are currently being assembled and integrated at RVS, with the first units planned for delivery to the US Army.

CPM Signals for Satellite Navigation in the S and C Bands.

PubMed

Xue, Rui; Sun, Yanbo; Zhao, Danfeng

2015-06-05

Frequency allocations in the L band suitable for global navigation satellite system (GNSS) services are getting crowded and system providers face an ever tougher job when they try to bring in new signals and services while maintaining radio frequency compatibility. With the successive opening of the S and C bands to GNSS service, the multi-band combined navigation is predicted to become a key technology for future high-precision positioning navigation systems, and a single modulation scheme satisfying the requirements in each band is a promising solution for reducing user terminal complexity. A universal modulation scheme based on the continuous phase modulation (CPM) family suitable for the above bands' demands is proposed. Moreover, this paper has put forward two specific CPM signals for the S and C bands, respectively. Then the proposed modulation schemes, together with existing candidates, are comprehensively evaluated. Simulation results show that the proposed CPM signals can not only satisfy the constraint condition of compatibility in different bands well and reduce user terminal complexity, but also provide superior performance in terms of tracking accuracy, multi-path mitigation and anti-jamming compared to other candidate modulation schemes.

Laser-driven electron beam acceleration and future application to compact light sources

NASA Astrophysics Data System (ADS)

Hafz, N.; Jeong, T. M.; Lee, S. K.; Pae, K. H.; Sung, J. H.; Choi, I. W.; Yu, T. J.; Jeong, Y. U.; Lee, J.

2009-07-01

Laser-driven plasma accelerators are gaining much attention by the advanced accelerator community due to the potential these accelerators hold in miniaturizing future high-energy and medium-energy machines. In the laser wakefield accelerator (LWFA), the ponderomotive force of an ultrashort high intensity laser pulse excites a longitudinal plasma wave or bubble. Due to huge charge separation, electric fields created in the plasma bubble can be several orders of magnitude higher than those available in conventional microwave and RF-based accelerator facilities which are limited (up to ˜100 MV/m) by material breakdown. Therefore, if an electron bunch is injected into the bubble in phase with its field, it will gain relativistic energies within an extremely short distance. Here, in the LWFA we show the generation of high-quality and high-energy electron beams up to the GeV-class within a few millimeters of gas-jet plasmas irradiated by tens of terawatt ultrashort laser pulses. Thus we realize approximately four orders of magnitude acceleration gradients higher than available by conventional technology. As a practical application of the stable high-energy electron beam generation, we are planning on injecting the electron beams into a few-meters long conventional undulator in order to realize compact X-ray synchrotron (immediate) and FEL (future) light sources. Stable laser-driven electron beam and radiation devices will surely open a new era in science, medicine and technology and will benefit a larger number of users in those fields.

Energy Impacts of Wide Band Gap Semiconductors in U.S. Light-Duty Electric Vehicle Fleet.

PubMed

Warren, Joshua A; Riddle, Matthew E; Graziano, Diane J; Das, Sujit; Upadhyayula, Venkata K K; Masanet, Eric; Cresko, Joe

2015-09-01

Silicon carbide and gallium nitride, two leading wide band gap semiconductors with significant potential in electric vehicle power electronics, are examined from a life cycle energy perspective and compared with incumbent silicon in U.S. light-duty electric vehicle fleet. Cradle-to-gate, silicon carbide is estimated to require more than twice the energy as silicon. However, the magnitude of vehicle use phase fuel savings potential is comparatively several orders of magnitude higher than the marginal increase in cradle-to-gate energy. Gallium nitride cradle-to-gate energy requirements are estimated to be similar to silicon, with use phase savings potential similar to or exceeding that of silicon carbide. Potential energy reductions in the United States vehicle fleet are examined through several scenarios that consider the market adoption potential of electric vehicles themselves, as well as the market adoption potential of wide band gap semiconductors in electric vehicles. For the 2015-2050 time frame, cumulative energy savings associated with the deployment of wide band gap semiconductors are estimated to range from 2-20 billion GJ depending on market adoption dynamics.

Theoretical study on electronic properties of MoS{sub 2} antidot lattices

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

Shao, Li; Chen, Guangde; Ye, Honggang, E-mail: hgye@mail.xjtu.edu.cn

2014-09-21

Motivated by the state of the art method for etching hexagonal array holes in molybdenum disulfide (MoS{sub 2}), the electronic properties of MoS{sub 2} antidot lattices (MoS{sub 2}ALs) with zigzag edge were studied with first-principles calculations. Monolayer MoS{sub 2}ALs are semiconducting and the band gaps converge to constant values as the supercell area increases, which can be attributed to the edge effect. Multilayer MoS{sub 2}ALs and chemical adsorbed MoS{sub 2}ALs by F atoms show metallic behavior, while the structure adsorbed with H atoms remains to be semiconducting with a tiny bandgap. Our results show that forming periodically repeating structures inmore » MoS{sub 2} can develop a promising technique for engineering nano materials and offer new opportunities for designing MoS{sub 2}-based nanoscale electronic devices and chemical sensors.« less

Electronic Properties, Screening, and Efficient Carrier Transport in NaSbS 2

DOE PAGES

Sun, Jifeng; Singh, David J.

2017-02-13

NaSbS 2 is a semiconductor that was recently shown to have remarkable efficacy as a solar absorber indicating efficient charge collection even in material containing defects. We report first-principles calculations of properties that show (1) an indirect gap only slightly smaller than the direct gap, which may impede the recombination of photoexcited carriers, (2) highly anisotropic electronic and optical properties reflecting a layered crystal structure, (3) a pushed-up valence-band maximum due to repulsion from the Sb 5s states, and (4) cross-gap hybridization between the S p—derived valence bands and the Sb 5p states. This latter feature leads to enhanced Bornmore » effective charges that can provide local screening and, therefore, defect tolerance. Finally, these features are discussed in relation to the performance of the compound as a semiconductor with efficient charge collection.« less

Design and development of a 600-720 GHz receiver for ALMA Band 9

NASA Astrophysics Data System (ADS)

Baryshev, A. M.; Hesper, R.; Mena, F. P.; Jackson, B. D.; Adema, J.; Schaeffer, H.; Barkhof, J.; Wild, W.; Candotti, M.; Lodewijk, C.; Loudkov, D.; Zijlstra, T.; Noroozian, 0.; Klapwijk, T. M.

2006-05-01

This paper describes the design and development of the ALMA Band 9 receiver cartridges. The ALMA project is a collaboration between Europe, North America, and Japan to build an aperture synthesis telescope consisting of at least 64 12-m antennas located at 5000 m altitude in Chile. In its full configuration, ALMA will observe in 10 frequency bands between 30 and 950 GHz, and will provide astronomers with unprecedented sensitivity and spatial resolution at millimetre and sub-millimetre wavelengths. Band 9, covering 600-720 GHz, is the highest frequency band in the baseline ALMA project, and will thus offer the telescope's highest spatial resolutions. The ALMA Band 9 cartridge is a compact unit containing the core of a 600-720 GHz heterodyne receiver front-end that can be easily inserted into and removed from the ALMA cryostat. In particular, its core technologies include low-noise, broadband SIS mixers; an electronically-tunable solid-state local oscillator; and low-noise cryogenic IF amplifiers. These components are built into a rigid opto-mechanical structure that includes a compact optical assembly mounted on the cartridge's 4 K stage that combines the astronomical and local oscillator signals and focuses them into two SIS mixers. In this report we present the noise measurement with an emphasis on the extreme large IF bandwidth (4-12 GHz). IF-gain slope, receiver linearity/saturation, receiver beam pattern and cross polarization level measurements will be presented and compared with expectations. The receiver phase and amplitude stability measurements will be presented and the system aspects related to interferometer will be discussed. Finally, a detailed measurement of LO noise contribution will be presented. This measurement was done by comparing receiver noise measured with internal ALMA LO (multipliers power amplifiers combination) to receiver noise measured by means of Gunn diode, followed by a x2x3 multiplier.

A Compact Band-Pass Filter with High Selectivity and Second Harmonic Suppression

PubMed Central

Hadarig, Ramona Cosmina; de Cos Gomez, Maria Elena; Las-Heras, Fernando

2013-01-01

The design of a novel band-pass filter with narrow-band features based on an electromagnetic resonator at 6.4 GHz is presented. A prototype is manufactured and characterized in terms of transmission and reflection coefficient. The selective passband and suppression of the second harmonic make the filter suitable to be used in a C band frequency range for radar systems and satellite/terrestrial applications. To avoid substantial interference for this kind of applications, passive components with narrow band features and small dimensions are required. Between 3.6 GHz and 4.2 GHz the band-pass filter with harmonic suppression should have an attenuation of at least 35 dB, whereas for a passband, less than 10% is sufficient. PMID:28788412

Nonlocal Electron Coherence in MoS2 Flakes Correlated through Spatial Self Phase Modulation

NASA Astrophysics Data System (ADS)

Wu, Yanling; Wu, Qiong; Sun, Fei; Tian, Yichao; Zuo, Xu; Meng, Sheng; Zhao, Jimin

2015-03-01

Electron coherence among different flake domains of MoS2 has been generated using ultrafast or continuous wave laser beams. Such electron coherence generates characteristic far-field diffraction patterns through a purely coherent nonlinear optical effect--spatial self-phase modulation (SSPM). A wind-chime model is developed to describe the establishment of the electron coherence through correlating the photo-excited electrons among different flakes using coherent light. Owing to its finite gap band structure, we find different mechanisms, including two-photon processes, might be responsible for the SSPM in MoS2 [with a large nonlinear dielectric susceptibility χ (3) = 1.6 × 10-9 e.s.u. (SI: 2.23 × 10-17 m2/V2) per layer]. Finally, we realized all optical switching based on SSPM, demonstrating that the electron coherence generation we report here is a ubiquitous property of layered quantum materials, by which novel optical applications are accessible. National Natural Science Foundation of China (11274372).

POX 186: the ultracompact blue compact dwarf galaxy reveals its nature

NASA Astrophysics Data System (ADS)

Doublier, V.; Kunth, D.; Courbin, F.; Magain, P.

2000-01-01

High resolution, ground based R and I band observations of the ultra compact dwarf galaxy POX 186 are presented. The data, obtained with the ESO New Technology Telescope (NTT), are analyzed using a new deconvolution algorithm which allows one to resolve the innermost regions of this stellar-like object into three Super-Star Clusters (SSC). Upper limits to both masses (M ~ 105 Msun) and physical sizes (<=60pc) of the SSCs are set. In addition, and maybe most importantly, extended light emission underlying the compact star-forming region is clearly detected in both bands. The R-I color rules out nebular Hα contamination and is consistent with an old stellar population. This casts doubt on the hypothesis that Blue Compact Dwarf Galaxies (BCDG) are young galaxies. based on observations carried out at NTT in La Silla, operated by the European Southern Observatory, during Director's Discretionary Time.

Dynamical electron-phonon coupling, G W self-consistency, and vertex effect on the electronic band gap of ice and liquid water

NASA Astrophysics Data System (ADS)

Ziaei, Vafa; Bredow, Thomas

2017-06-01

We study the impact of dynamical electron-phonon (el-ph) effects on the electronic band gap of ice and liquid water by accounting for frequency-dependent Fan contributions in the el-ph mediated self-energy within the many-body perturbation theory (MBPT). We find that the dynamical el-ph coupling effects greatly reduce the static el-ph band-gap correction of the hydrogen-rich molecular ice crystal from-2.46 to -0.23 eV in great contrast to the result of Monserrat et al. [Phys. Rev. B 92, 140302 (2015), 10.1103/PhysRevB.92.140302]. This is of particular importance as otherwise the static el-ph gap correction would considerably reduce the electronic band gap, leading to considerable underestimation of the intense peaks of optical absorption spectra of ice which would be in great disagreement to experimental references. By contrast, the static el-ph gap correction of liquid water is very moderate (-0.32 eV), and inclusion of dynamical effects slightly reduces the gap correction to -0.19 eV. Further, we determine the diverse sensitivity of ice and liquid water to the G W self-consistency and show that the energy-only self-consistent approach (GnWn ) exhibits large implicit vertex character in comparison to the quasiparticle self-consistent approach, for which an explicit calculation of vertex corrections is necessary for good agreement with experiment.

High electron doping to a wide band gap semiconductor 12CaO•7Al2O3 thin film

NASA Astrophysics Data System (ADS)

Miyakawa, Masashi; Hirano, Masahiro; Kamiya, Toshio; Hosono, Hideo

2007-04-01

High-density electrons (˜1.9×1021cm-3) were doped into a polycrystalline film of a wide band gap (˜7eV) semiconductor 12CaO•7Al2O3 (C12A7) by an in situ postdeposition reduction treatment using an oxygen-deficient C12A7 overlayer. The resultant film exhibits metallic conduction with a Hall mobility of ˜2.5cm2V-1s-1 and a conductivity of ˜800Scm-1. Optical analyses indicate that most of the doped electrons behave as free carriers with an effective mass of 0.82me and the estimated in-grain mobility is 5.2cm2V-1s-1, which agrees reasonably with the value obtained for high-quality single crystals.

U.S. Aeronautical L-Band Satellite Technology Test Program : Interim Tests Results

DOT National Transportation Integrated Search

1975-06-01

The U.S. Aeronautical L-Band satellite test program was performed between September 1974 and April 1975 as part of an international ATS-6 L-Band satellite test program. The U.S. program consisted of both technology and ATC communications demonstratio...

Time Dependent Density Functional Theory Calculations of Large Compact PAH Cations: Implications for the Diffuse Interstellar Bands

NASA Technical Reports Server (NTRS)

Weisman, Jennifer L.; Lee, Timothy J.; Salama, Farid; Gordon-Head, Martin; Kwak, Dochan (Technical Monitor)

2002-01-01

We investigate the electronic absorption spectra of several maximally pericondensed polycyclic aromatic hydrocarbon radical cations with time dependent density functional theory calculations. We find interesting trends in the vertical excitation energies and oscillator strengths for this series containing pyrene through circumcoronene, the largest species containing more than 50 carbon atoms. We discuss the implications of these new results for the size and structure distribution of the diffuse interstellar band carriers.

Effect of Molecular Coupling on Ultrafast Electron-Transfer and Charge-Recombination Dynamics in a Wide-Gap ZnS Nanoaggregate Sensitized by Triphenyl Methane Dyes.

PubMed

Debnath, Tushar; Maity, Partha; Dana, Jayanta; Ghosh, Hirendra N

2016-03-03

Wide-band-gap ZnS nanocrystals (NCs) were synthesized, and after sensitizing the NCs with series of triphenyl methane (TPM) dyes, ultrafast charge-transfer dynamics was demonstrated. HRTEM images of ZnS NCs show the formation of aggregate crystals with a flower-like structure. Exciton absorption and lumimescence, due to quantum confinement of the ZnS NCs, appear at approximately 310 and 340 nm, respectively. Interestingly, all the TPM dyes (pyrogallol red, bromopyrogallol red, and aurin tricarboxylic acid) form charge-transfer complexes with the ZnS NCs, with the appearance of a red-shifted band. Electron injection from the photoexcited TPM dyes into the conduction band of the ZnS NCs is shown to be a thermodynamically viable process, as confirmed by steady-state and time-resolved emission studies. To unravel charge-transfer (both electron injection and charge recombination) dynamics and the effect of molecular coupling, femtosecond transient absorption studies were carried out in TPM-sensitized ZnS NCs. The electron-injection dynamics is pulse-width-limited in all the ZnS/TPM dye systems, however, the back electron transfer differs, depending on the molecular coupling of the sensitizers (TPM dyes). The detailed mechanisms for the above-mentioned processes are discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electron band structure of the high pressure cubic phase of AlH3

NASA Astrophysics Data System (ADS)

Shi, Hongliang; Zarifi, Niliffar; Yim, Wai-Leung; Tse, J. S.

2012-07-01

The electronic band structure of the cubic Pm3n phase of AlH3 stable above 100 GPa is examined with semi-local, Tran-Blaha modified Becke-Johnson local density approximation (TB-mBJLDA), screened hybrid density functionals and GW methods. The shift of the conduction band to higher energy with increasing pressure is predicted by all methods. However, there are significant differences in detail band structure. In the pressure range from 90 to160 GPa, semi-local, hybrid functional and TB-mBJLDA calculations predicted that AlH3 is a poor metal. In comparison, GW calculations show a gap opening at 160 GPa and AlH3 becomes a small gap semi-conductor. From the trends of the calculated band shifts, it can be concluded that the favourable conditions leading to the nesting of Fermi surfaces predicted by semi-local calculation have disappeared if the exchange term is included. The results highlight the importance of the correction to the exchange energy on the band structure of hydrogen dominant dense metal hydrides at high pressure hydrides and may help to rationalize the absence of superconductivity in AlH3 from experimental measurements.

Compact beam transport system for free-electron lasers driven by a laser plasma accelerator

DOE PAGES

Liu, Tao; Zhang, Tong; Wang, Dong; ...

2017-02-01

Utilizing laser-driven plasma accelerators (LPAs) as a high-quality electron beam source is a promising approach to significantly downsize the x-ray free-electron laser (XFEL) facility. A multi-GeV LPA beam can be generated in several-centimeter acceleration distance, with a high peak current and a low transverse emittance, which will considerably benefit a compact FEL design. However, the large initial angular divergence and energy spread make it challenging to transport the beam and realize FEL radiation. In this paper, a novel design of beam transport system is proposed to maintain the superior features of the LPA beam and a transverse gradient undulator (TGU)more » is also adopted as an effective energy spread compensator to generate high-brilliance FEL radiation. As a result, theoretical analysis and numerical simulations are presented based on a demonstration experiment with an electron energy of 380 MeV and a radiation wavelength of 30 nm.« less

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.

X-ray photoelectron spectrum and electronic properties of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4): LDA, GGA, and EV-GGA.

PubMed

Reshak, Ali Hussain; Khenata, R; Kityk, I V; Plucinski, K J; Auluck, S

2009-04-30

An all electron full potential linearized augmented plane wave method has been applied for a theoretical study of the band structure, density of states, and electron charge density of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4) using three different approximations for the exchange correlation potential. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma resulting in a direct energy gap of about 2.0, 2.2, and 2.8 eV for local density approximation (LDA), generalized gradient approximation (GGA), and Engel-Vosko (EVGGA) compared to the experimental value of 2.84 eV. We notice that EVGGA shows excellent agreement with the experimental data. This agreement is attributed to the fact that the Engel-Vosko GGA formalism optimizes the corresponding potential for band structure calculations. We make a detailed comparison of the density of states deduced from the X-ray photoelectron spectra with our calculations. We find that there is a strong covalent bond between the Hg and S atoms and Ga and S atoms. The Hg-Hg, Ga-Ga, and S-S bonds are found to be weaker than the Hg-S and Ga-S bonds showing that a covalent bond exists between Hg and S atoms and Ga and S atoms.

Special purpose modes in photonic band gap fibers

DOEpatents

Spencer, James; Noble, Robert; Campbell, Sara

2013-04-02

Photonic band gap fibers are described having one or more defects suitable for the acceleration of electrons or other charged particles. Methods and devices are described for exciting special purpose modes in the defects including laser coupling schemes as well as various fiber designs and components for facilitating excitation of desired modes. Results are also presented showing effects on modes due to modes in other defects within the fiber and due to the proximity of defects to the fiber edge. Techniques and devices are described for controlling electrons within the defect(s). Various applications for electrons or other energetic charged particles produced by such photonic band gap fibers are also described.

Wurtzite/zinc-blende electronic-band alignment in basal-plane stacking faults in semi-polar GaN

NASA Astrophysics Data System (ADS)

Monavarian, Morteza; Hafiz, Shopan; Izyumskaya, Natalia; Das, Saikat; Özgür, Ümit; Morkoç, Hadis; Avrutin, Vitaliy

2016-02-01

Heteroepitaxial semipolar and nonpolar GaN layers often suffer from high densities of extended defects including basal plane stacking faults (BSFs). BSFs which are considered as inclusions of cubic zinc-blende phase in wurtzite matrix act as quantum wells strongly affecting device performance. Band alignment in BSFs has been discussed as type of band alignment at the wurtzite/zinc blende interface governs the response in differential transmission; fast decay after the pulse followed by slow recovery due to spatial splitting of electrons and heavy holes for type- II band alignment in contrast to decay with no recovery in case of type I band alignment. Based on the results, band alignment is demonstrated to be of type II in zinc-blende segments in wurtzite matrix as in BSFs.

k - dependent Jeff=1/2 band splitting and the electron-hole asymmetry in SrIrO3

NASA Astrophysics Data System (ADS)

Singh, Vijeta; Pulikkotil, J. J.

2017-02-01

The Ir ion in Srn+1 IrnO 3 n + 1 series of compounds is octahedrally coordinated. However, unlike Sr2IrO4 (n=1) and Sr3Ir2O7 (n=2) which are insulating due to spin-orbit induced Jeff splitting of the t2g bands, SrIrO3 (n= ∞) is conducting. To explore whether such a splitting is relevant in SrIrO3, and if so to what extent, we investigate the electronic structure of orthorhombic SrIrO3 using density functional theory. Calculations reveal that the crystal field split Ir t2 g bands in SrIrO3 are indeed split into Jeff=3/2 and and Jeff=1/2 states. However, the splitting is found to be strongly k - dependent with its magnitude determined by the Ir - O orbital hybridization. Besides, we find that the spin-orbit induced pseudo-gap, into which the Fermi energy is positioned, is composed of both light electron-like and heavy hole-like bands. These features in the band structure of SrIrO3 suggest that variations in the carrier concentration control the electronic transport properties in SrIrO3, which is consistent with the experiments.

Compact electromagnetic bandgap structures for notch band in ultra-wideband applications.

PubMed

Rotaru, Mihai; Sykulski, Jan

2010-01-01

This paper introduces a novel approach to create notch band filters in the front-end of ultra-wideband (UWB) communication systems based on electromagnetic bandgap (EBG) structures. The concept presented here can be implemented in any structure that has a microstrip in its configuration. The EBG structure is first analyzed using a full wave electromagnetic solver and then optimized to work at WLAN band (5.15-5.825 GHz). Two UWB passband filters are used to demonstrate the applicability and effectiveness of the novel EBG notch band feature. Simulation results are provided for two cases studied.

Compact Electromagnetic Bandgap Structures for Notch Band in Ultra-Wideband Applications

PubMed Central

Rotaru, Mihai; Sykulski, Jan

2010-01-01

This paper introduces a novel approach to create notch band filters in the front-end of ultra-wideband (UWB) communication systems based on electromagnetic bandgap (EBG) structures. The concept presented here can be implemented in any structure that has a microstrip in its configuration. The EBG structure is first analyzed using a full wave electromagnetic solver and then optimized to work at WLAN band (5.15–5.825 GHz). Two UWB passband filters are used to demonstrate the applicability and effectiveness of the novel EBG notch band feature. Simulation results are provided for two cases studied. PMID:22163430

CdO as the archetypical transparent conducting oxide. Systematics of dopant ionic radius and electronic structure effects on charge transport and band structure.

PubMed

Yang, Yu; Jin, Shu; Medvedeva, Julia E; Ireland, John R; Metz, Andrew W; Ni, Jun; Hersam, Mark C; Freeman, Arthur J; Marks, Tobin J

2005-06-22

A series of yttrium-doped CdO (CYO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 410 degrees C by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties have been investigated. XRD data reveal that all as-deposited CYO thin films are phase-pure and polycrystalline, with features assignable to a cubic CdO-type crystal structure. Epitaxial films grown on single-crystal MgO(100) exhibit biaxial, highly textured microstructures. These as-deposited CYO thin films exhibit excellent optical transparency, with an average transmittance of >80% in the visible range. Y doping widens the optical band gap from 2.86 to 3.27 eV via a Burstein-Moss shift. Room temperature thin film conductivities of 8,540 and 17,800 S/cm on glass and MgO(100), respectively, are obtained at an optimum Y doping level of 1.2-1.3%. Finally, electronic band structure calculations are carried out to systematically compare the structural, electronic, and optical properties of the In-, Sc-, and Y-doped CdO systems. Both experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: (1) lattice parameters contract as a function of dopant ionic radii in the order Y (1.09 A) < In (0.94 A) < Sc (0.89 A); (2) the carrier mobilities and doping efficiencies decrease in the order In > Y > Sc; (3) the dopant d state has substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics.

Size effect on the magnetic and electronic properties of the monolayer lateral hetero-junction WS2-MoS2 nanoribbon

NASA Astrophysics Data System (ADS)

Wen, Yan-Ni; Xia, Ming-Gang; Zhang, Sheng-Li

2016-05-01

By using the VASP, we studied the magnetic and electronic properties of the monolayer lateral hetero-junction WS2-MoS2-nanoribbons (WS2-MoS2-NRs). Our results show that the NRs' edge chirality and width affect significantly its magnetic and electronic properties. The monolayer lateral hetero-junction ZZ-WS2-MoS2-NRs(ZZ: zigzag) exhibitmetallic behavior and have considerable magnetic moment. Their magnetic moments decrease in the order of Nz = 2, 6 and 4 (the width of NRs). While, the magnetic moment decreases with the increased rz (the number of the Mo-S chains, rz ≠ 0 and rz ≠ Nz) at the same width Nz. The NA-AC-WS2-NR (AC: armchair) and NA-AC-WS2-MoS2-NR-1 (the number of the Mo-S chain is 1) show metallic behavior when NA = 3 (the width of NRs). The other monolayer lateral hetero-junction AC-WS2-MoS2-NRs remain the nonmagnetic and semiconductingbehavior as bulk. But they are indirect band-gap except for the NA = 3, rA = 2 (the number of the Mo-S chains) and NA = 7, rA = 0 when NA < 9. However they are direct band-gap when NA ≥ 9. Their lowest and highest band gaps are 0.150 eV and 0.581 eV, respectively. These unique magnetic and electronic properties will provide guidanceon the WS2-MoS2 hetero-junction application in nanodevice.

Ab initio calculations of the electron spectrum and density of states of TlFeS{sub 2} and TlFeSe{sub 2} crystals

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

Ismayilova, N. A., E-mail: ismayilova-narmin-84@mail.ru; Orudjev, H. S.; Jabarov, S. H.

2017-04-15

The results of ab initio calculations of the electron spectrum of TlFeS{sub 2} and TlFeSe{sub 2} crystals in the antiferromagnetic phase are reported. Calculations are carried out in the context of the density functional theory. The origin of the bands of s, p, and d electron states of Tl, Fe, S, and Se atoms is studied. It is established that, in the antiferromagnetic phase, the crystals possess semiconductor properties. The band gaps are found to be 0.05 and 0.34 eV for TlFeS{sub 2} and TlFeSe{sub 2} crystals, respectively.

XANES: observation of quantum confinement in the conduction band of colloidal PbS quantum dots

NASA Astrophysics Data System (ADS)

Demchenko, I. N.; Chernyshova, M.; He, X.; Minikayev, R.; Syryanyy, Y.; Derkachova, A.; Derkachov, G.; Stolte, W. C.; Piskorska-Hommel, E.; Reszka, A.; Liang, H.

2013-04-01

The presented investigations aimed at development of inexpensive method for synthesized materials suitable for utilization of solar energy. This important issue was addressed by focusing, mainly, on electronic local structure studies with supporting x-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis of colloidal galena nano-particles (NPs) and quantum dots (QDs) synthesized using wet chemistry under microwave irradiation. Performed x-ray absorption near edge structure (XANES) analysis revealed an evidence of quantum confinement for the sample with QDs, where the bottom of the conduction band was shifted to higher energy. The QDs were found to be passivated with oxides at the surface. Existence of sulfate/sulfite and thiosulfate species in pure PbS and QDs, respectively, was identified.

Waveguide based compact silicon Schottky photodetector with enhanced responsivity in the telecom spectral band.

PubMed

Goykhman, Ilya; Desiatov, Boris; Khurgin, Jacob; Shappir, Joseph; Levy, Uriel

2012-12-17

We experimentally demonstrate an on-chip compact and simple to fabricate silicon Schottky photodetector for telecom wavelengths operating on the basis of internal photoemission process. The device is realized using CMOS compatible approach of local-oxidation of silicon, which enables the realization of the photodetector and low-loss bus photonic waveguide at the same fabrication step. The photodetector demonstrates enhanced internal responsivity of 12.5mA/W for operation wavelength of 1.55µm corresponding to an internal quantum efficiency of 1%, about two orders of magnitude higher than our previously demonstrated results [22]. We attribute this improved detection efficiency to the presence of surface roughness at the boundary between the materials forming the Schottky contact. The combination of enhanced quantum efficiency together with a simple fabrication process provides a promising platform for the realization of all silicon photodetectors and their integration with other nanophotonic and nanoplasmonic structures towards the construction of monolithic silicon opto-electronic circuitry on-chip.

Lattice structures and electronic properties of CIGS/CdS interface: First-principles calculations

NASA Astrophysics Data System (ADS)

Tang, Fu-Ling; Liu, Ran; Xue, Hong-Tao; Lu, Wen-Jiang; Feng, Yu-Dong; Rui, Zhi-Yuan; Huang, Min

2014-07-01

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective (2VCu+InCu) CuInGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of (2VCu+InCu) interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the (2VCu+InCu) interface the interface states near the Fermi level in CuInGaSe2 and CdS band gap regions are mainly composed of interfacial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CuInGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.

Performance interface document for the S-band diplexer for space users of NASA networks

NASA Technical Reports Server (NTRS)

Line, L. G.

1985-01-01

This report discusses the test results and interfacing information of the S-band diplexer development program supported by RTOP 310 funding. The program was implemented to reduce the S-band transponder noise figure by minimizing the receive channel insertion loss and to also provide Space Transportation System (STS) compatibility by providing 70-db rejection up to 16 GHz in the receive channel. This compatibility includes rejection of signals from the Shuttle S-band Data Link, the K-band Data Link, and the K-band Rendezvous Radar. The first of many projects to benefit from this accomplishment was the Earth Radiation Budget Satellite (ERBS).

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.

Single-layer MoS2 electronics.

PubMed

Lembke, Dominik; Bertolazzi, Simone; Kis, Andras

2015-01-20

allowed rapid progress in this area and was followed by demonstrations of basic digital circuits and transistors operating in the technologically relevant gigahertz range of frequencies, showing that the mobility of MoS2 and TMD materials is sufficiently high to allow device operation at such high frequencies. Monolayer MoS2 and other TMDs are also direct band gap semiconductors making them interesting for realizing optoelectronic devices. These range from simple phototransistors showing high sensitivity and low noise, to light emitting diodes and solar cells. All the electronic and optoelectronic properties of MoS2 and TMDs are accompanied by interesting mechanical properties with monolayer MoS2 being as stiff as steel and 30× stronger. This makes it especially interesting in the context of flexible electronics where it could combine the high degree of mechanical flexibility commonly associated with organic semiconductors with high levels of electrical performance. All these results show that MoS2 and TMDs are promising materials for electronic and optoelectronic applications.

Colloidal nanocrystals as LEGO® bricks for building electronic band structure models.

PubMed

Tadjine, Athmane; Delerue, Christophe

2018-03-28

The synthesis of self-assembled semiconductor nanocrystal (NC) superlattices using oriented attachment recently became a flourishing research topic. This technique already produced remarkable forms of NC superlattices, such as linear chains, mono and multilayer square lattices, and silicene-like honeycomb lattices. In the case of lead chalcogenide semiconductors where NCs are in the form of truncated nanocubes, the attachment mostly occurs via (100) facets. In this work, we show that all these structures can be seen as sub-structures of a simple cubic lattice. From this, we investigate a rich variety of one-dimensional or two-dimensional superlattices that could be built as few lines or few layers taken from the same cubic system following different crystallographic orientations. Each NC can be therefore considered as a LEGO® brick, and any superlattice can be obtained from another one by rearranging the bricks. Moreover, we show that this concept of LEGO® bricks can be extended to the calculation of the electronic band structure of the superlattices. This leads to a simple yet powerful way to build analytical Hamiltonians that present band structures in excellent agreement with more elaborate atomistic tight-binding calculations. This LEGO® concept could guide the synthesis of superlattices and LEGO® Hamiltonians should greatly simplify further studies on the (opto-)electronic properties of such structures.

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.