2D electron density profile measurement in tokamak by laser-accelerated ion-beam probe
Chen, Y. H.; Yang, X. Y.; Lin, C. E-mail: cjxiao@pku.edu.cn; Wang, X. G.; Xiao, C. J. E-mail: cjxiao@pku.edu.cn; Wang, L.; Xu, M.
2014-11-15
A new concept of Heavy Ion Beam Probe (HIBP) diagnostic has been proposed, of which the key is to replace the electrostatic accelerator of traditional HIBP by a laser-driven ion accelerator. Due to the large energy spread of ions, the laser-accelerated HIBP can measure the two-dimensional (2D) electron density profile of tokamak plasma. In a preliminary simulation, a 2D density profile was reconstructed with a spatial resolution of about 2 cm, and with the error below 15% in the core region. Diagnostics of 2D density fluctuation is also discussed.
Reorientation of the Stripe Phase of 2D Electrons by a Minute Density Modulation
NASA Astrophysics Data System (ADS)
Mueed, M. A.; Hossain, Md. Shafayat; Pfeiffer, L. N.; West, K. W.; Baldwin, K. W.; Shayegan, M.
2016-08-01
Interacting two-dimensional electrons confined in a GaAs quantum well exhibit isotropic transport when the Fermi level resides in the first excited (N =1 ) Landau level. Adding an in-plane magnetic field (B||) typically leads to an anisotropic, stripelike (nematic) phase of electrons with the stripes oriented perpendicular to the B|| direction. Our experimental data reveal how a periodic density modulation, induced by a surface strain grating from strips of negative electron-beam resist, competes against the B||-induced orientational order of the stripe phase. Even a minute (<0.25 %) density modulation is sufficient to reorient the stripes along the direction of the surface grating.
NASA Astrophysics Data System (ADS)
Pylak, M.; Kontrym-Sznajd, G.; Dobrzyński, L.
2011-08-01
A successful application of the Maximum Entropy Method (MEM) to the reconstruction of electron-positron momentum density distribution in gadolinium out of the experimental of 2D ACAR data is presented. Formally, the algorithm used was prepared for two-dimensional reconstructions from line integrals. For the first time the results of MEM, applied to such data, are compared in detail with the ones obtained by means of Cormack's method. It is also shown how the experimental uncertainties may influence the results of the latter analysis. Preliminary calculations, using WIEN2k code, of band structure and Fermi surface have been done as well.
Oxide 2D electron gases as a route for high carrier densities on (001) Si
Kornblum, Lior; Jin, Eric N.; Kumah, Divine P.; Walker, Fred J.; Ernst, Alexis T.; Broadbridge, Christine C.; Ahn, Charles H.
2015-05-18
Two dimensional electron gases (2DEGs) formed at the interfaces of oxide heterostructures draw considerable interest owing to their unique physics and potential applications. Growing such heterostructures on conventional semiconductors has the potential to integrate their functionality with semiconductor device technology. We demonstrate 2DEGs on a conventional semiconductor by growing GdTiO{sub 3}-SrTiO{sub 3} on silicon. Structural analysis confirms the epitaxial growth of heterostructures with abrupt interfaces and a high degree of crystallinity. Transport measurements show the conduction to be an interface effect, ∼9 × 10{sup 13} cm{sup −2} electrons per interface. Good agreement is demonstrated between the electronic behavior of structures grown on Si and on an oxide substrate, validating the robustness of this approach to bridge between lab-scale samples to a scalable, technologically relevant materials system.
Electronic band structure and charge density wave transition in quasi-2D KMo6O17 purple bronze
NASA Astrophysics Data System (ADS)
Valbuena, M. A.; Avila, J.; Vyalikh, D. V.; Guyot, H.; Laubschat, C.; Molodtsov, S. L.; Asensio, M. C.
2008-03-01
High resolution angle-resolved photoemission of quasi-2D KMo6O17 purple bronze has been performed in the range from room temperature to 130 K, slightly above the charge density wave (CDW) transition (Tc = 110 K), and down to 35 K (well below Tc). In this paper we report a detailed study of how electronic band structure is affected by this transition driven by the hidden nesting scenario. The expected spectroscopic fingerprints of the CDW phase transition have been found and discussed according to the hidden one dimension and the development of a quasi-commensurate CDW. The excellent agreement between theory and our experimental results makes of potassium purple bronze a reference system for studying this type of instabilities.
2D microwave imaging reflectometer electronics
Spear, A. G.; Domier, C. W. Hu, X.; Muscatello, C. M.; Ren, X.; Luhmann, N. C.; Tobias, B. J.
2014-11-15
A 2D microwave imaging reflectometer system has been developed to visualize electron density fluctuations on the DIII-D tokamak. Simultaneously illuminated at four probe frequencies, large aperture optics image reflections from four density-dependent cutoff surfaces in the plasma over an extended region of the DIII-D plasma. Localized density fluctuations in the vicinity of the plasma cutoff surfaces modulate the plasma reflections, yielding a 2D image of electron density fluctuations. Details are presented of the receiver down conversion electronics that generate the in-phase (I) and quadrature (Q) reflectometer signals from which 2D density fluctuation data are obtained. Also presented are details on the control system and backplane used to manage the electronics as well as an introduction to the computer based control program.
2D microwave imaging reflectometer electronics
NASA Astrophysics Data System (ADS)
Spear, A. G.; Domier, C. W.; Hu, X.; Muscatello, C. M.; Ren, X.; Tobias, B. J.; Luhmann, N. C.
2014-11-01
A 2D microwave imaging reflectometer system has been developed to visualize electron density fluctuations on the DIII-D tokamak. Simultaneously illuminated at four probe frequencies, large aperture optics image reflections from four density-dependent cutoff surfaces in the plasma over an extended region of the DIII-D plasma. Localized density fluctuations in the vicinity of the plasma cutoff surfaces modulate the plasma reflections, yielding a 2D image of electron density fluctuations. Details are presented of the receiver down conversion electronics that generate the in-phase (I) and quadrature (Q) reflectometer signals from which 2D density fluctuation data are obtained. Also presented are details on the control system and backplane used to manage the electronics as well as an introduction to the computer based control program.
2D microwave imaging reflectometer electronics.
Spear, A G; Domier, C W; Hu, X; Muscatello, C M; Ren, X; Tobias, B J; Luhmann, N C
2014-11-01
A 2D microwave imaging reflectometer system has been developed to visualize electron density fluctuations on the DIII-D tokamak. Simultaneously illuminated at four probe frequencies, large aperture optics image reflections from four density-dependent cutoff surfaces in the plasma over an extended region of the DIII-D plasma. Localized density fluctuations in the vicinity of the plasma cutoff surfaces modulate the plasma reflections, yielding a 2D image of electron density fluctuations. Details are presented of the receiver down conversion electronics that generate the in-phase (I) and quadrature (Q) reflectometer signals from which 2D density fluctuation data are obtained. Also presented are details on the control system and backplane used to manage the electronics as well as an introduction to the computer based control program. PMID:25430247
Positron 2D-ACAR experiments and electron-positron momentum density in YBa{sub 2}Cu{sub 3}O{sub 7-x}
Smedskjaer, L.C.; Welp, U.; Fang, Y.; Bailey, K.G.; Bansil, A.
1991-12-01
We discuss positron annihilation (2D-ACAR) measurements in the C- projection on an untwinned metallic single crystal of YBa{sub 2}Cu{sub 3}O{sub 7-x} as a function of temperature, for five temperatures ranging from 30K to 300K. The measured 2D-ACAR intensities are interpreted in terms of the electron-positron momentum density obtained within the KKR-band theory framework. The temperature dependence of the 2D-ACAR spectra is used to extract a ``background corrected`` experimental spectrum which is in remarkable accord with the corresponding band theory predictions, and displays in particular clear signatures of the electron ridge Fermi surface.
Positron 2D-ACAR experiments and electron-positron momentum density in YBa sub 2 Cu sub 3 O sub 7-x
Smedskjaer, L.C.; Welp, U.; Fang, Y.; Bailey, K.G. ); Bansil, A. . Dept. of Physics)
1991-12-01
We discuss positron annihilation (2D-ACAR) measurements in the C- projection on an untwinned metallic single crystal of YBa{sub 2}Cu{sub 3}O{sub 7-x} as a function of temperature, for five temperatures ranging from 30K to 300K. The measured 2D-ACAR intensities are interpreted in terms of the electron-positron momentum density obtained within the KKR-band theory framework. The temperature dependence of the 2D-ACAR spectra is used to extract a background corrected'' experimental spectrum which is in remarkable accord with the corresponding band theory predictions, and displays in particular clear signatures of the electron ridge Fermi surface.
NASA Astrophysics Data System (ADS)
Kononenko, O.; Lopes, N. C.; Cole, J. M.; Kamperidis, C.; Mangles, S. P. D.; Najmudin, Z.; Osterhoff, J.; Poder, K.; Rusby, D.; Symes, D. R.; Warwick, J.; Wood, J. C.; Palmer, C. A. J.
2016-09-01
In this work, two-dimensional (2D) hydrodynamic simulations of a variable length gas cell were performed using the open source fluid code OpenFOAM. The gas cell was designed to study controlled injection of electrons into a laser-driven wakefield at the Astra Gemini laser facility. The target consists of two compartments: an accelerator and an injector section connected via an aperture. A sharp transition between the peak and plateau density regions in the injector and accelerator compartments, respectively, was observed in simulations with various inlet pressures. The fluid simulations indicate that the length of the down-ramp connecting the sections depends on the aperture diameter, as does the density drop outside the entrance and the exit cones. Further studies showed, that increasing the inlet pressure leads to turbulence and strong fluctuations in density along the axial profile during target filling, and consequently, is expected to negatively impact the accelerator stability.
2D electronic materials for army applications
NASA Astrophysics Data System (ADS)
O'Regan, Terrance; Perconti, Philip
2015-05-01
The record electronic properties achieved in monolayer graphene and related 2D materials such as molybdenum disulfide and hexagonal boron nitride show promise for revolutionary high-speed and low-power electronic devices. Heterogeneous 2D-stacked materials may create enabling technology for future communication and computation applications to meet soldier requirements. For instance, transparent, flexible and even wearable systems may become feasible. With soldier and squad level electronic power demands increasing, the Army is committed to developing and harnessing graphene-like 2D materials for compact low size-weight-and-power-cost (SWAP-C) systems. This paper will review developments in 2D electronic materials at the Army Research Laboratory over the last five years and discuss directions for future army applications.
Correlated Electron Phenomena in 2D Materials
NASA Astrophysics Data System (ADS)
Lambert, Joseph G.
In this thesis, I present experimental results on coherent electron phenomena in layered two-dimensional materials: single layer graphene and van der Waals coupled 2D TiSe2. Graphene is a two-dimensional single-atom thick sheet of carbon atoms first derived from bulk graphite by the mechanical exfoliation technique in 2004. Low-energy charge carriers in graphene behave like massless Dirac fermions, and their density can be easily tuned between electron-rich and hole-rich quasiparticles with electrostatic gating techniques. The sharp interfaces between regions of different carrier densities form barriers with selective transmission, making them behave as partially reflecting mirrors. When two of these interfaces are set at a separation distance within the phase coherence length of the carriers, they form an electronic version of a Fabry-Perot cavity. I present measurements and analysis of multiple Fabry-Perot modes in graphene with parallel electrodes spaced a few hundred nanometers apart. Transition metal dichalcogenide (TMD) TiSe2 is part of the family of materials that coined the term "materials beyond graphene". It contains van der Waals coupled trilayer stacks of Se-Ti-Se. Many TMD materials exhibit a host of interesting correlated electronic phases. In particular, TiSe2 exhibits chiral charge density waves (CDW) below TCDW ˜ 200 K. Upon doping with copper, the CDW state gets suppressed with Cu concentration, and CuxTiSe2 becomes superconducting with critical temperature of T c = 4.15 K. There is still much debate over the mechanisms governing the coexistence of the two correlated electronic phases---CDW and superconductivity. I will present some of the first conductance spectroscopy measurements of proximity coupled superconductor-CDW systems. Measurements reveal a proximity-induced critical current at the Nb-TiSe2 interfaces, suggesting pair correlations in the pure TiSe2. The results indicate that superconducting order is present concurrently with CDW in
Superfluid density through 2D superconductor junctions
NASA Astrophysics Data System (ADS)
Nam, Hyoungdo; Shih, Chih-Kang
As S. Qin et al. reported, two monolayer (2 ML) lead film on a silicon (111) substrate has one of two different atomic structures on the silicon substrate: the unstrained 1x1 and the psedumorphically strained √3x √3 (i.e. the same lattice constant as the Si √3x √3 lattice). Most interestingly, although these two different regions show the same quantum well state features, they have different Tc's (5 K and 4 K). These two different regions of 2 ML film naturally form superconductor-superconductor (SS or SS') junctions along silicon step edges. Physical connection of the junction is only 1 ML thickness because of the step height difference of substrate. We will present this study of SS (or SS') junction system using scanning tunneling microscopy/spectroscopy and in-situ double-coil mutual inductance measurement. The transition of superconducting gaps across either SS or SS' junctions should show how to locally affect each other. Double coil measurement show a global Tc close to the lower Tc region with sizable superfluid density. We will discuss the phase rigidity and its relationship to the superfluid density in this ultra-thin Pb film that is only 2 ML thick.
NASA Astrophysics Data System (ADS)
Valbuena, M. A.; Avila, J.; Drouard, S.; Guyot, H.; Asensio, M. C.
2006-01-01
We report on an angle-resolved-photoemission spectroscopy (ARPES) investigation of layered quasi-two dimensional (2D) Molybdenum purple bronze KMo6O17 in order to study and characterizes the transition to a charge-density-wave (CDW) state. We have performed photoemission temperature dependent measurements cooling down from room temperature (RT) to 32 K, well below the Peierls transition for this material, with CDW transition temperature Tc =110 K. The spectra have been taken at a selected kF point of the Fermi surface (FS) that satisfies the nesting condition of the FS, looking for the characteristic pseudo-gap opening in this kind of materials. The pseudogap has been estimated and it result to be in agreement with our previous works. The shift to lower binding energy of crossing Fermi level ARPES feature have been also confirmed and studied as a function of temperature, showing a rough like BCS behaviour. Finally we have also focused on ARPES measurements along ΓM¯ high symmetry direction for both room and low temperature states finding some insight for ‘shadow’ or back folded bands indicating the new periodicity of real lattice after the CDW lattice distortion.
Transport Experiments on 2D Correlated Electron Physics in Semiconductors
Tsui, Daniel
2014-03-24
This research project was designed to investigate experimentally the transport properties of the 2D electrons in Si and GaAs, two prototype semiconductors, in several new physical regimes that were previously inaccessible to experiments. The research focused on the strongly correlated electron physics in the dilute density limit, where the electron potential energy to kinetic energy ratio rs>>1, and on the fractional quantum Hall effect related physics in nuclear demagnetization refrigerator temperature range on samples with new levels of purity and controlled random disorder.
NASA Astrophysics Data System (ADS)
Schaub, S. C.; Hummelt, J. S.; Guss, W. C.; Shapiro, M. A.; Temkin, R. J.
2015-11-01
A 1 MW, 124.5 GHz gyrotron was used to produce a linearly polarized, quasioptical beam in 2.2 μs pulses. The beam was focused to a 2.6 mm spot size, producing a peak electric field of 70 kV/cm, after transmission losses. This electric field is great enough to produce a breakdown plasma in air at pressures ranging from a few Torr up to atmospheric pressure. The resulting breakdown plasma spontaneously forms a two-dimensional array of filaments, oriented parallel to the polarization of the beam, that propagate toward the microwave source. A needlepoint initiator was placed at the focal point of the beam, creating highly reproducible plasma arrays. An intensified CCD, with a minimum exposure of 2 ns, was combined with a two-wavelength laser interferometer, operating at 532 and 635 nm, to make spatially and temporally resolved electron density measurements of the plasma array.
Wheeler, J; Mariani, E; Piazolo, S; Prior, D J; Trimby, P; Drury, M R
2009-03-01
The Weighted Burgers Vector (WBV) is defined here as the sum, over all types of dislocations, of [(density of intersections of dislocation lines with a map) x (Burgers vector)]. Here we show that it can be calculated, for any crystal system, solely from orientation gradients in a map view, unlike the full dislocation density tensor, which requires gradients in the third dimension. No assumption is made about gradients in the third dimension and they may be non-zero. The only assumption involved is that elastic strains are small so the lattice distortion is entirely due to dislocations. Orientation gradients can be estimated from gridded orientation measurements obtained by EBSD mapping, so the WBV can be calculated as a vector field on an EBSD map. The magnitude of the WBV gives a lower bound on the magnitude of the dislocation density tensor when that magnitude is defined in a coordinate invariant way. The direction of the WBV can constrain the types of Burgers vectors of geometrically necessary dislocations present in the microstructure, most clearly when it is broken down in terms of lattice vectors. The WBV has three advantages over other measures of local lattice distortion: it is a vector and hence carries more information than a scalar quantity, it has an explicit mathematical link to the individual Burgers vectors of dislocations and, since it is derived via tensor calculus, it is not dependent on the map coordinate system. If a sub-grain wall is included in the WBV calculation, the magnitude of the WBV becomes dependent on the step size but its direction still carries information on the Burgers vectors in the wall. The net Burgers vector content of dislocations intersecting an area of a map can be simply calculated by an integration round the edge of that area, a method which is fast and complements point-by-point WBV calculations. PMID:19250469
Graphene as a platform to study 2D electronic transitions
NASA Astrophysics Data System (ADS)
Bouchiat, Vincent; Kessler, Brian; Girit, Caglar; Zettl, Alex
2010-03-01
The easily accessible 2D electron gas in graphene provides an ideal platform on which to tune, via application of an electrostatic gate, the coupling between electronically ordered dopants deposited on its surface. To demonstrate this concept, we have measured arrays of superconducting clusters deposited on Graphene capable to induce via the proximity effect a gate-tunable superconducting transition. Using a simple fabrication procedure based on metal layer dewetting, doped graphene sheets can be decorated with a non percolating network on nanoscale tin clusters. This hybrid material displays a two-step superconducting transition. The higher transition step is gate independent and corresponds to the transition of the tin clusters to the superconducting state. The lower transition step towards a real zero resistance state exhibiting a well developped supercurrent, is strongly gate-tunable and is quantitatively described by Berezinskii-Kosterlitz-Thouless 2D vortex unbinding. Our simple self-assembly method and tunable coupling can readily be extended to other electronic order parameters such as ferro/antiferromagnetism, charge/spin density waves using similar decoration techniques. [1] B. M. Kessler, C.O. Girit, A. Zettl, and V. Bouchiat, Tunable Superconducting Phase Transition in Metal-Decorated Graphene Sheets submitted to PRL, arXiv:0907.3661
Materials for Flexible, Stretchable Electronics: Graphene and 2D Materials
NASA Astrophysics Data System (ADS)
Kim, Sang Jin; Choi, Kyoungjun; Lee, Bora; Kim, Yuna; Hong, Byung Hee
2015-07-01
Recently, 2D materials have been intensively studied as emerging materials for future electronics, including flexible electronics, photonics, and electrochemical energy storage devices. Among representative 2D materials (such as graphene, boron nitride, and transition metal dichalcogenides) that exhibit extraordinary properties, graphene stands out in the flexible electronics field due to its combination of high electron mobility, high thermal conductivity, high specific surface area, high optical transparency, excellent mechanical flexibility, and environmental stability. This review covers the synthesis, transfer, and characterization methods of graphene and 2D materials and graphene's application to flexible devices as well as comparison with other competing materials.
2-D Imaging of Electron Temperature in Tokamak Plasmas
T. Munsat; E. Mazzucato; H. Park; C.W. Domier; M. Johnson; N.C. Luhmann Jr.; J. Wang; Z. Xia; I.G.J. Classen; A.J.H. Donne; M.J. van de Pol
2004-07-08
By taking advantage of recent developments in millimeter wave imaging technology, an Electron Cyclotron Emission Imaging (ECEI) instrument, capable of simultaneously measuring 128 channels of localized electron temperature over a 2-D map in the poloidal plane, has been developed for the TEXTOR tokamak. Data from the new instrument, detailing the MHD activity associated with a sawtooth crash, is presented.
Resonances of piezoelectric plate with embedded 2D electron system
NASA Astrophysics Data System (ADS)
Suslov, A. V.
2009-02-01
A thin GaAs/AlGaAs plate was studied by the resonant ultrasound spectroscopy (RUS) in the temperature range 0.3-10 K and in magnetic fields of up to 18 T. The resonance frequencies and linewidths were measured. Quantum oscillations of both these values were observed and were associated with the quantum Hall effect occurred in the 2D electron system. For an analysis the sample was treated as a dielectric piezoelectric plate covered on one side by a film with a field dependent conductivity. Screening of the strain-driven electric field was changed due to the variation of the electron relaxation time in the vicinity of the metal-dielectric transitions caused by the magnetic field in the 2D system. The dielectric film does not affect properties of GaAs and thus the resonance frequencies are defined only by the elastic, piezoelectric and dielectric constants of GaAs. A metallic 2D sheet effectively screens the parallel electric field, so the ultrasound wave velocities and resonance frequencies decrease when the sheet conductivity increases. Oscillations of the resonance linewidth reflect the influence of the 2D system on the ultrasound attenuation, which is proportional to the linewidth. A metallic film as well as a dielectric one does not affect this attenuation but at some finite nonzero value of the conductivity the linewidth approaches a maximum. In high magnetic field each oscillation of the conductivity produces one oscillation of a resonance frequency and two linewidth peaks. The observed phenomena can be described by the relaxation type equations and the resonant ultrasound spectroscopy opens another opportunity for contactless studies on 2D electron systems.
Universal Fabrication of 2D Electron Systems in Functional Oxides.
Rödel, Tobias Chris; Fortuna, Franck; Sengupta, Shamashis; Frantzeskakis, Emmanouil; Fèvre, Patrick Le; Bertran, François; Mercey, Bernard; Matzen, Sylvia; Agnus, Guillaume; Maroutian, Thomas; Lecoeur, Philippe; Santander-Syro, Andrés Felipe
2016-03-01
2D electron systems (2DESs) in functional oxides are promising for applications, but their fabrication and use, essentially limited to SrTiO3 -based heterostructures, are hampered by the need for growing complex oxide overlayers thicker than 2 nm using evolved techniques. It is demonstrated that thermal deposition of a monolayer of an elementary reducing agent suffices to create 2DESs in numerous oxides. PMID:26753522
Dual-mode operation of 2D material-base hot electron transistors
Lan, Yann-Wen; Torres, Jr., Carlos M.; Zhu, Xiaodan; Qasem, Hussam; Adleman, James R.; Lerner, Mitchell B.; Tsai, Shin-Hung; Shi, Yumeng; Li, Lain-Jong; Yeh, Wen-Kuan; Wang, Kang L.
2016-01-01
Vertical hot electron transistors incorporating atomically-thin 2D materials, such as graphene or MoS2, in the base region have been proposed and demonstrated in the development of electronic and optoelectronic applications. To the best of our knowledge, all previous 2D material-base hot electron transistors only considered applying a positive collector-base potential (VCB > 0) as is necessary for the typical unipolar hot-electron transistor behavior. Here we demonstrate a novel functionality, specifically a dual-mode operation, in our 2D material-base hot electron transistors (e.g. with either graphene or MoS2 in the base region) with the application of a negative collector-base potential (VCB < 0). That is, our 2D material-base hot electron transistors can operate in either a hot-electron or a reverse-current dominating mode depending upon the particular polarity of VCB. Furthermore, these devices operate at room temperature and their current gains can be dynamically tuned by varying VCB. We anticipate our multi-functional dual-mode transistors will pave the way towards the realization of novel flexible 2D material-based high-density and low-energy hot-carrier electronic applications. PMID:27581550
Dual-mode operation of 2D material-base hot electron transistors.
Lan, Yann-Wen; Torres, Carlos M; Zhu, Xiaodan; Qasem, Hussam; Adleman, James R; Lerner, Mitchell B; Tsai, Shin-Hung; Shi, Yumeng; Li, Lain-Jong; Yeh, Wen-Kuan; Wang, Kang L
2016-01-01
Vertical hot electron transistors incorporating atomically-thin 2D materials, such as graphene or MoS2, in the base region have been proposed and demonstrated in the development of electronic and optoelectronic applications. To the best of our knowledge, all previous 2D material-base hot electron transistors only considered applying a positive collector-base potential (VCB > 0) as is necessary for the typical unipolar hot-electron transistor behavior. Here we demonstrate a novel functionality, specifically a dual-mode operation, in our 2D material-base hot electron transistors (e.g. with either graphene or MoS2 in the base region) with the application of a negative collector-base potential (VCB < 0). That is, our 2D material-base hot electron transistors can operate in either a hot-electron or a reverse-current dominating mode depending upon the particular polarity of VCB. Furthermore, these devices operate at room temperature and their current gains can be dynamically tuned by varying VCB. We anticipate our multi-functional dual-mode transistors will pave the way towards the realization of novel flexible 2D material-based high-density and low-energy hot-carrier electronic applications. PMID:27581550
Quantum Oscillations in an Interfacial 2D Electron Gas.
Zhang, Bingop; Lu, Ping; Liu, Henan; Lin, Jiao; Ye, Zhenyu; Jaime, Marcelo; Balakirev, Fedor F.; Yuan, Huiqiu; Wu, Huizhen; Pan, Wei; Zhang, Yong
2016-01-01
Recently, it has been predicted that topological crystalline insulators (TCIs) may exist in SnTe and Pb_{1-x}Sn_{x}Te thin films [1]. To date, most studies on TCIs were carried out either in bulk crystals or thin films, and no research activity has been explored in heterostructures. We present here the results on electronic transport properties of the 2D electron gas (2DEG) realized at the interfaces of PbTe/ CdTe (111) heterostructures. Evidence of topological state in this interfacial 2DEG was observed.
Electron Energy Levels in the 1D-2D Transition
NASA Astrophysics Data System (ADS)
Pepper, Michael; Sanjeev, Kumar; Thomas, Kalarikad; Creeth, Graham; English, David; Ritchie, David; Griffiths, Jonathan; Farrer, Ian; Jones, Geraint
Using GaAs-AlGaAs heterostructures we have investigated the behaviour of electron energy levels with relaxation of the potential confining a 2D electron gas into a 1D configuration. In the ballistic regime of transport, when the conductance shows quantized plateaux, different types of behaviour are found according to the spins of interacting levels, whether a magnetic field is applied and lifting of the momentum degeneracy with a source-drain voltage. We have observed both crossing and anti-crossing of levels and have investigated the manner in which they can be mutually converted. In the presence of a magnetic field levels can cross and lock together as the confinement is altered in a way which is characteristic of parallel channels. The overall behaviour is discussed in terms of electron interactions and the wavefunction flexibility allowed by the increasing two dimensionality of the electron distribution as the confinement is weakened. Work supported by UK EPSRC.
NASA Astrophysics Data System (ADS)
Peng, Cheng; Efetov, Dmitri; Shiue, Ren-Jye; Nanot, Sebastien; Hempel, Marek; Kong, Jing; Koppens, Frank; Englund, Dirk
Strong spatial tunability of the charge carrier density at nanoscale is essential to many 2D-material-based electronic and optoelectronic applications. As an example, plasmonic metamaterials with nanoscale dimensions would make graphene plasmonics at visible and near-infrared wavelengths possible. However, existing gating techniques based on conventional dielectric gating geometries limit the spatial resolution and achievable carrier concentration, strongly restricting the available wavelength, geometry, and quality of the devices. Here, we present a novel spatially selective electrolyte gating approach that allows for in-plane spatial Fermi energy modulation of 2D materials of more than 1 eV (carrier density of n = 1014 cm-2) across a length of 2 nm. We present electrostatic simulations as well as electronic transport, photocurrent, cyclic voltammetry and optical spectroscopy measurements to characterize the performance of the gating technique applied to graphene devices. The high spatial resolution, high doping capacity, full tunability and self-aligned device geometry of the presented technique opens a new venue for nanoscale metamaterial engineering of 2D materials for complete optical absorption, nonlinear optics and sensing, among other applications.
Density functional theory for polymeric systems in 2D
NASA Astrophysics Data System (ADS)
Słyk, Edyta; Roth, Roland; Bryk, Paweł
2016-06-01
We propose density functional theory for polymeric fluids in two dimensions. The approach is based on Wertheim’s first order thermodynamic perturbation theory (TPT) and closely follows density functional theory for polymers proposed by Yu and Wu (2002 J. Chem. Phys. 117 2368). As a simple application we evaluate the density profiles of tangent hard-disk polymers at hard walls. The theoretical predictions are compared against the results of the Monte Carlo simulations. We find that for short chain lengths the theoretical density profiles are in an excellent agreement with the Monte Carlo data. The agreement is less satisfactory for longer chains. The performance of the theory can be improved by recasting the approach using the self-consistent field theory formalism. When the self-avoiding chain statistics is used, the theory yields a marked improvement in the low density limit. Further improvements for long chains could be reached by going beyond the first order of TPT.
Density functional theory for polymeric systems in 2D.
Słyk, Edyta; Roth, Roland; Bryk, Paweł
2016-06-22
We propose density functional theory for polymeric fluids in two dimensions. The approach is based on Wertheim's first order thermodynamic perturbation theory (TPT) and closely follows density functional theory for polymers proposed by Yu and Wu (2002 J. Chem. Phys. 117 2368). As a simple application we evaluate the density profiles of tangent hard-disk polymers at hard walls. The theoretical predictions are compared against the results of the Monte Carlo simulations. We find that for short chain lengths the theoretical density profiles are in an excellent agreement with the Monte Carlo data. The agreement is less satisfactory for longer chains. The performance of the theory can be improved by recasting the approach using the self-consistent field theory formalism. When the self-avoiding chain statistics is used, the theory yields a marked improvement in the low density limit. Further improvements for long chains could be reached by going beyond the first order of TPT. PMID:27115343
Experiments on 2D Vortex Patterns with a Photoinjected Pure Electron Plasma
NASA Astrophysics Data System (ADS)
Durkin, Daniel; Fajans, Joel
1998-11-01
The equations governing the evolution of a strongly magnetized pure electron plasma are analogous to those of an ideal 2D fluid; plasma density is analogous to fluid vorticity. Therefore, we can study vortex dynamics with pure electron plasmas. We generate our electron plasma with a photocathode electron source. The photocathode provides greater control over the initial profile than previous thermionic sources and allows us to create complicated initial density distributions, corresponding to complicated vorticity distributions in a fluid. Results on the stability of 2D vortex patterns will be presented: 1) The stability of N vortices arranged in a ring; 2) The stability of N vortices arranged in a ring with a central vortex; 3) The stability of more complicated vortex patterns.(http://socrates.berkeley.edu/ )fajans/
Force Density Function Relationships in 2-D Granular Media
NASA Technical Reports Server (NTRS)
Youngquist, Robert C.; Metzger, Philip T.; Kilts, Kelly N.
2004-01-01
An integral transform relationship is developed to convert between two important probability density functions (distributions) used in the study of contact forces in granular physics. Developing this transform has now made it possible to compare and relate various theoretical approaches with one another and with the experimental data despite the fact that one may predict the Cartesian probability density and another the force magnitude probability density. Also, the transforms identify which functional forms are relevant to describe the probability density observed in nature, and so the modified Bessel function of the second kind has been identified as the relevant form for the Cartesian probability density corresponding to exponential forms in the force magnitude distribution. Furthermore, it is shown that this transform pair supplies a sufficient mathematical framework to describe the evolution of the force magnitude distribution under shearing. Apart from the choice of several coefficients, whose evolution of values must be explained in the physics, this framework successfully reproduces the features of the distribution that are taken to be an indicator of jamming and unjamming in a granular packing. Key words. Granular Physics, Probability Density Functions, Fourier Transforms
Electron dynamics and valley relaxation in 2D semiconductors
NASA Astrophysics Data System (ADS)
Gundogdu, Kenan
2015-03-01
Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley dependent career spin polarization, which is the basis for recently proposed valleytronic applications. Since the durations of valley population provide the time window in which valley specific processes take place, it is an essential parameter for developing valleytronic devices. These systems also exhibit unusually strong many body affects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. But there is not much known about the impact of strong many particle correlations on spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS2 and WS2. We found that excitonic many body interactions significantly contribute to the relaxation process. Biexciton formation reveals hole valley spin relaxation time. Our results also suggest initial fast intervalley electron scattering and electron spin relaxation leads to loss of electron valley polarization, which then facilitates hole valley relaxation via excitonic spin exchange interaction.
Electronic structure study on 2D hydrogenated Icosagens nitride nanosheets
NASA Astrophysics Data System (ADS)
Ramesh, S.; Marutheeswaran, S.; Ramaclus, Jerald V.; Paul, Dolon Chapa
2014-12-01
Metal nitride nanosheets has attracted remarkable importance in surface catalysis due to its characteristic ionic nature. In this paper, using density functional theory, we investigate geometric stability and electronic properties of hydrogenated Icosagen nitride nanosheets. Binding energy of the sheets reveals hydrogenation is providing more stability. Band structure of the hydrogenated sheets is found to be n-type semiconductor. Partial density of states shows metals (B, Al, Ga and In) and its hydrogens dominating in the Fermi region. Mulliken charge analysis indications that hydrogenated nanosheets are partially hydridic surface nature except boron nitride.
Visualization of electronic density
Grosso, Bastien; Cooper, Valentino R.; Pine, Polina; Hashibon, Adham; Yaish, Yuval; Adler, Joan
2015-04-22
An atom’s volume depends on its electronic density. Although this density can only be evaluated exactly for hydrogen-like atoms, there are many excellent numerical algorithms and packages to calculate it for other materials. 3D visualization of charge density is challenging, especially when several molecular/atomic levels are intertwined in space. We explore several approaches to 3D charge density visualization, including the extension of an anaglyphic stereo visualization application based on the AViz package to larger structures such as nanotubes. We will describe motivations and potential applications of these tools for answering interesting questions about nanotube properties.
Visualization of electronic density
NASA Astrophysics Data System (ADS)
Grosso, Bastien; Cooper, Valentino R.; Pine, Polina; Hashibon, Adham; Yaish, Yuval; Adler, Joan
2015-10-01
The spatial volume occupied by an atom depends on its electronic density. Although this density can only be evaluated exactly for hydrogen-like atoms, there are many excellent algorithms and packages to calculate it numerically for other materials. Three-dimensional visualization of charge density is challenging, especially when several molecular/atomic levels are intertwined in space. In this paper, we explore several approaches to this, including the extension of an anaglyphic stereo visualization application based on the AViz package for hydrogen atoms and simple molecules to larger structures such as nanotubes. We will describe motivations and potential applications of these tools for answering interesting physical questions about nanotube properties.
Wavelet characterization of 2D turbulence and intermittency in magnetized electron plasmas
NASA Astrophysics Data System (ADS)
Romé, M.; Chen, S.; Maero, G.
2016-06-01
A study of the free relaxation of turbulence in a two-dimensional (2D) flow is presented, with a focus on the role of the initial vorticity conditions. Exploiting a well-known analogy with 2D inviscid incompressible fluids, the system investigated here is a magnetized pure electron plasma. The dynamics of this system are simulated by means of a 2D particle-in-cell code, starting from different spiral density (vorticity) distributions. A wavelet multiresolution analysis is adopted, which allows the coherent and incoherent parts of the flow to be separated. Comparison of the turbulent evolution in the different cases is based on the investigation of the time evolution of statistical properties, including the probability distribution functions and structure functions of the vorticity increments. It is also based on an analysis of the enstrophy evolution and its spectrum for the two components. In particular, while the statistical features assess the degree of flow intermittency, spectral analysis allows us not only to estimate the time required to reach a state of fully developed turbulence, but also estimate its dependence on the thickness of the initial spiral density distribution, accurately tracking the dynamics of both the coherent structures and the turbulent background. The results are compared with those relevant to annular initial vorticity distributions (Chen et al 2015 J. Plasma Phys. 81 495810511).
Local electronic structures and 2D topological phase transition of ultrathin Sb films
NASA Astrophysics Data System (ADS)
Kim, Sunghwan; Jin, Kyung-Hwan; Park, Joonbum; Kim, Jun Sung; Jhi, Seung-Hoon; Yeom, Han Woong
We investigate local electronic structures of ultrathin Sb islands and their edges grown on Bi2Te2Se by scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations. The Sb islands of various thickness are grown with atomically well ordered edge structure over the 3 bilayers (BL). On the surfaces and edges of these islands, we clearly resolve edge-localized electronic states by STS measurements, which depend on the thickness. The DFT calculations identify that the strongly localized edge states of 4 and 5 BL films correspond to a quantum spin Hall (QSH) states while the edge states of 3 BL are trivial. Our experimental and theoretical results confirm the 2D topological phase transition of the ultrathin Sb films from trivial to QSH phase. Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science and Department of Physics, Pohang University of Science and Technology, Korea.
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2
NASA Astrophysics Data System (ADS)
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M.; Tongay, Sefaattin
2016-02-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics.
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M.; Tongay, Sefaattin
2016-01-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics. PMID:26846617
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2.
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M; Tongay, Sefaattin
2016-01-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics. PMID:26846617
2D Carbon Nanotube Network: A New material for Electronics
NASA Astrophysics Data System (ADS)
Gruner, George
2006-03-01
This talk will focus on the electronic properties of two dimensional carbon nanotube networks, and on their application potential. Percolation issues, together with the frequency, and temperature dependent activity will be discussed. The network can be tuned from having semiconducting to metallic like behavior, and doping with electron withdrawing and donating species leads to networks with tailor-made electronic properties. The network is also highly transparent in the visible spectral range, this attribute -- together with simple room temperature fab processes -- opens up application opportunities in the area of electronics, opto-electronics, photovoltaics and sensors. Recent results on solar cells, OLEDs and smart windows will be reviewed. Field effect transistors that incorporate nanotube network conducting channels, together with complex functional devices that incorporate networks and functional molecules will also be discussed. Finally a comparison will be made with conventional and emerging materials that compete area of disposable, flexible and printable electronics.
2D density model of the Chinese continental lithosphere along a NW-SE transect
NASA Astrophysics Data System (ADS)
Šimonová, Barbora; Bielik, Miroslav; Dérerová, Jana
2015-06-01
This paper presents a 2D density model along a transect from NW to SE China. The model was first constructed by the transformation of seismic velocity to density, revealed by previous deep seismic soundings (DSS) investigations in China. Then, the 2D density model was updated using the GM-SYS software by fitting the computed to the observed gravity data. Based on the density distribution of anomalous layers we divided the Chinese continental crust along the transect into three regions: north-western, central and south-eastern. The first one includes the Junggar Basin, Tianshan and Tarim Basin. The second part consists of the Qilian Orogen, the Qaidam Basin and the Songpan Ganzi Basin. The third region is represented by the Yangtze and the Cathaysia blocks. The low velocity body (vp =5.2 - 6.2 km/s) at the junction of the North-western and Central parts at a depth between 21 - 31 km, which was discovered out by DSS, was also confirmed by our 2D density modelling.
Optical Signatures from Magnetic 2-D Electron Gases in High Magnetic Fields to 60 Tesla
Crooker, S.A.; Kikkawa, J.M.; Awschalom, D.D.; Smorchikova, I.P.; Samarth, N.
1998-11-08
We present experiments in the 60 Tesla Long-Pulse magnet at the Los Alamos National High Magnetic Field Lab (NHMFL) focusing on the high-field, low temperature photoluminescence (PL) from modulation-doped ZnSe/Zn(Cd,Mn)Se single quantum wells. High-speed charge-coupled array detectors and the long (2 second) duration of the magnet pulse permit continuous acquisition of optical spectra throughout a single magnet shot. High-field PL studies of the magnetic 2D electron gases at temperatures down to 350mK reveal clear intensity oscillations corresponding to integer quantum Hall filling factors, from which we determine the density of the electron gas. At very high magnetic fields, steps in the PL energy are observed which correspond to the partial unlocking of antiferromagnetically bound pairs of Mn^{2+} spins.
Corbino Disk Viscometer for 2D Quantum Electron Liquids
NASA Astrophysics Data System (ADS)
Tomadin, Andrea; Vignale, Giovanni; Polini, Marco
2014-12-01
The shear viscosity of a variety of strongly interacting quantum fluids, ranging from ultracold atomic Fermi gases to quark-gluon plasmas, can be accurately measured. On the contrary, no experimental data exist, to the best of our knowledge, on the shear viscosity of two-dimensional quantum electron liquids hosted in a solid-state matrix. In this work we propose a Corbino disk device, which allows a determination of the viscosity of a quantum electron liquid from the dc potential difference that arises between the inner and the outer edge of the disk in response to an oscillating magnetic flux.
Corbino disk viscometer for 2D quantum electron liquids.
Tomadin, Andrea; Vignale, Giovanni; Polini, Marco
2014-12-01
The shear viscosity of a variety of strongly interacting quantum fluids, ranging from ultracold atomic Fermi gases to quark-gluon plasmas, can be accurately measured. On the contrary, no experimental data exist, to the best of our knowledge, on the shear viscosity of two-dimensional quantum electron liquids hosted in a solid-state matrix. In this work we propose a Corbino disk device, which allows a determination of the viscosity of a quantum electron liquid from the dc potential difference that arises between the inner and the outer edge of the disk in response to an oscillating magnetic flux. PMID:25526137
Interstellar Electron Density Spectra
NASA Astrophysics Data System (ADS)
Lambert, Hendrick Clark
This study concerns the investigation of the form of the wavenumber spectrum of the Galactic electron density fluctuations through an examination of the scattering of the radio pulses emitted by pulsars as they propagate through the diffuse ionized interstellar gas. A widely used model for the electron density spectrum is based on the simple power-law: Pne(q)∝ q-β, where β = 11/3 is usually assumed, corresponding to Kolmogorov's turbulence spectrum. The simple Kolmogorov model provides satisfactory agreement for observations along many lines of sight; however, major inconsistencies remain. The inconsistencies suggest that an increase in the ratio of the power between the high (10-8[ m]-1≤ q<=10-7[ m]-1) and low (10-13[ m]-1≤ q<=10-12[ m]-1) wavenumbers is needed. This enhancement in the ratio can in turn be achieved by either including an inner scale, corresponding to a dissipation scale for the turbulent cascade, in the Kolmogorov spectrum or by considering steeper spectra. Spectra with spectral exponents β > 4 have been in general rejected based on observations of pulsar refractive scintillations. The special case of β = 4 has been given little attention and is analyzed in detail. Physically, this 'β = 4' model corresponds to the random distribution, both in location and orientation, of discrete objects with relatively sharp boundaries across the line of sight. An outer scale is included in the model to account for the average size of such objects. We compare the predictions of the inner-scale and β = 4 models both with published observations and observations we made as part of this investigation. We conclude that the form of the wavenumber spectrum is dependent on the line of sight. We propose a composite spectrum featuring a uniform background turbulence in presence of randomly distributed discrete objects, as modeled by the β = model.
2D quasi-ordered nitrogen-enriched porous carbon nanohybrids for high energy density supercapacitors
NASA Astrophysics Data System (ADS)
Kan, Kan; Wang, Lei; Yu, Peng; Jiang, Baojiang; Shi, Keying; Fu, Honggang
2016-05-01
Two-dimensional (2D) quasi-ordered nitrogen-enriched porous carbon (QNPC) nanohybrids, with the characteristics of an ultrathin graphite nanosheet framework and thick quasi-ordered nitrogen-doped carbon cladding with a porous texture, have been synthesized via an in situ polymerization assembly method. In the synthesis, the expandable graphite (EG) is enlarged by an intermittent microwave method, and then aniline monomers are intercalated into the interlayers of the expanded EG with the assistance of a vacuum. Subsequently, the intercalated aniline monomers could assemble on the interlayer surface of the expanded EG, accompanied by the in situ polymerization from aniline monomers to polyaniline. Meanwhile, the expanded EG could be exfoliated to graphite nanosheets. By subsequent pyrolysis and activation processes, the QNPC nanohybrids could be prepared. As supercapacitor electrodes, a typical QNPC12-700 sample derived from the precursor containing an EG content of 12%, with a high level of nitrogen doping of 5.22 at%, offers a high specific capacitance of 305.7 F g-1 (1 A g-1), excellent rate-capability and long-term stability. Notably, an extremely high energy density of 95.7 Wh kg-1 at a power density of 449.7 W kg-1 in an ionic liquid electrolyte can be achieved. The unique structural features and moderate heteroatom doping of the QNPC nanohybrids combines electrochemical double layer and faradaic capacitance contributions, which make these nanohybrids ideal candidates as electrode materials for high-performance energy storage devices.Two-dimensional (2D) quasi-ordered nitrogen-enriched porous carbon (QNPC) nanohybrids, with the characteristics of an ultrathin graphite nanosheet framework and thick quasi-ordered nitrogen-doped carbon cladding with a porous texture, have been synthesized via an in situ polymerization assembly method. In the synthesis, the expandable graphite (EG) is enlarged by an intermittent microwave method, and then aniline monomers are
NASA Astrophysics Data System (ADS)
Han, Tao; Lai, Chao-Jen; Chen, Lingyun; Liu, Xinming; Shen, Youtao; Zhong, Yuncheng; Ge, Shuaiping; Yi, Ying; Wang, Tianpeng; Yang, Wei T.; Shaw, Chris C.
2009-02-01
Breast density has been recognized as one of the major risk factors for breast cancer. However, breast density is currently estimated using mammograms which are intrinsically 2D in nature and cannot accurately represent the real breast anatomy. In this study, a novel technique for measuring breast density based on the segmentation of 3D cone beam CT (CBCT) images was developed and the results were compared to those obtained from 2D digital mammograms. 16 mastectomy breast specimens were imaged with a bench top flat-panel based CBCT system. The reconstructed 3D CT images were corrected for the cupping artifacts and then filtered to reduce the noise level, followed by using threshold-based segmentation to separate the dense tissue from the adipose tissue. For each breast specimen, volumes of the dense tissue structures and the entire breast were computed and used to calculate the volumetric breast density. BI-RADS categories were derived from the measured breast densities and compared with those estimated from conventional digital mammograms. The results show that in 10 of 16 cases the BI-RADS categories derived from the CBCT images were lower than those derived from the mammograms by one category. Thus, breasts considered as dense in mammographic examinations may not be considered as dense with the CBCT images. This result indicates that the relation between breast cancer risk and true (volumetric) breast density needs to be further investigated.
Dynamical symmetry breaking in a 2D electron gas with a spectral node
NASA Astrophysics Data System (ADS)
Ziegler, Klaus
2013-09-01
We study a disordered 2D electron gas with a spectral node in a vicinity of the node. After identifying the fundamental dynamical symmetries of this system, the spontaneous breaking of the latter by a Grassmann field is studied within a nonlinear sigma model approach. This allows us to reduce the average two-particle Green's function to a diffusion propagator with a random diffusion coefficient. The latter has non-degenerate saddle points and is treated by the conventional self-consistent Born approximation. This leads to a renormalized chemical potential and a renormalized diffusion coefficient, where the DC conductivity increases linearly with the density of quasiparticles. Applied to the special case of Dirac fermions, our approach provides a comprehensive description of the minimal conductivity at the Dirac node as well as for the V-shape conductivity inside the bands.
Electronic and geometrical properties of monoatomic and diatomic 2D honeycomb lattices. A DFT study
NASA Astrophysics Data System (ADS)
Rojas, Ángela; Rey, Rafael; Fonseca, Karen; Grupo de Óptica e Información Cuántica Team
Since the discovery of graphene by Geim and Novoselov at 2004, several analogous systems have been theoretically and experimentally studied, due to their technological interest. Both monoatomic lattices, such as silicine and germanene, and diatomic lattices (h-GaAs and h-GaN) have been studied. Using Density Functional Theory we obtain and confirm the chemical stability of these hexagonal 2D systems through the total energy curves as a function of interatomic distance. Unlike graphene, silicine and germanene, gapless materials, h-GaAs and h-GaN exhibit electronic gaps, different from that of the bulk, which could be interesting for the industry. On the other hand, the ab initio band structure calculations for graphene, silicene and germanene show a non-circular cross section around K points, at variance with the prediction of usual Tight-binding models. In fact, we have found that Dirac cones display a dihedral group symmetry. This implies that Fermi speed can change up to 30 % due to the orientation of the wave vector, for both electrons and holes. Traditional analytic studies use the Dirac equation for the electron dynamics at low energies. However, this equation assumes an isotropic, homogeneous and uniform space. Authors would like to thank the División de Investigación Sede Bogotá for their financial support at Universidad Nacional de Colombia. A. M. Rojas-Cuervo would also like to thank the Colciencias, Colombia.
Momentum density and 2D-ACAR experiments in YBa sub 2 Cu sub 3 O sub 7
Bansil, A. . Dept. of Physics); Smedskjaer, L.C. )
1991-12-01
We compare measured c-projected 2D-ACAR spectrum from an untwinned single crystal of YBa{sub 2}Cu{sub 3}O{sub 7-x} with the corresponding band theory predictions. Many different one-dimensional sections through the spectrum are considered, together with the characteristic amplitudes and shapes of the spectral anisotropies, with a focus on identifying and delineating Fermi surface signatures in the spectra. The positron data clearly show several distinct features of the ridge Fermi surface predicted by the band theory, and give an indication of the pillbox Fermi sheet. The good agreement between theory and experiment suggests that the band theory framework based on the local density approximation (LDA) is capable of providing a substantially correct description of the momentum density and Fermiology of the normal ground state electronic structure of YBa{sub 2}Cu{sub 3}O{sub 7}.
Analysis of 2-d ultrasound cardiac strain imaging using joint probability density functions.
Ma, Chi; Varghese, Tomy
2014-06-01
Ultrasound frame rates play a key role for accurate cardiac deformation tracking. Insufficient frame rates lead to an increase in signal de-correlation artifacts resulting in erroneous displacement and strain estimation. Joint probability density distributions generated from estimated axial strain and its associated signal-to-noise ratio provide a useful approach to assess the minimum frame rate requirements. Previous reports have demonstrated that bi-modal distributions in the joint probability density indicate inaccurate strain estimation over a cardiac cycle. In this study, we utilize similar analysis to evaluate a 2-D multi-level displacement tracking and strain estimation algorithm for cardiac strain imaging. The effect of different frame rates, final kernel dimensions and a comparison of radio frequency and envelope based processing are evaluated using echo signals derived from a 3-D finite element cardiac model and five healthy volunteers. Cardiac simulation model analysis demonstrates that the minimum frame rates required to obtain accurate joint probability distributions for the signal-to-noise ratio and strain, for a final kernel dimension of 1 λ by 3 A-lines, was around 42 Hz for radio frequency signals. On the other hand, even a frame rate of 250 Hz with envelope signals did not replicate the ideal joint probability distribution. For the volunteer study, clinical data was acquired only at a 34 Hz frame rate, which appears to be sufficient for radio frequency analysis. We also show that an increase in the final kernel dimensions significantly affect the strain probability distribution and joint probability density function generated, with a smaller effect on the variation in the accumulated mean strain estimated over a cardiac cycle. Our results demonstrate that radio frequency frame rates currently achievable on clinical cardiac ultrasound systems are sufficient for accurate analysis of the strain probability distribution, when a multi-level 2-D
NASA Astrophysics Data System (ADS)
Cambré, Sofie; Muyshondt, Pieter; Federicci, Remi; Wenseleers, Wim
2015-11-01
Density gradient ultracentrifugation (DGU) becomes increasingly important for the sorting of nanomaterials according to the particles' density, hence structure and dimensions, which determine their unique properties, but the further development of this separation technique is hindered by the limited precision with which the densities could be characterized. In this work, we determine these densities by position-dependent 2D wavelength-dependent IR fluorescence-excitation and resonant Raman spectroscopy measured directly in the density gradient after ultracentrifugation. We apply this method to study the diameter and chirality-dependent sorting of empty and water-filled single-walled carbon nanotubes coated with two different surfactants, sodium cholate (SC) and sodium deoxycholate (DOC). The results elucidate the long standing contradiction that SC would provide better diameter sorting, while DOC is the most efficient surfactant to solubilise the nanotubes. A more predictable separation is obtained for empty DOC-coated nanotubes since their density is found to vary very smoothly with diameter. The accurate and chirality-dependent densities furthermore provide information on the surfactant coating, which is also important for other separation techniques, and allow to determine the mass percentage of water encapsulated inside the nanotubes.Density gradient ultracentrifugation (DGU) becomes increasingly important for the sorting of nanomaterials according to the particles' density, hence structure and dimensions, which determine their unique properties, but the further development of this separation technique is hindered by the limited precision with which the densities could be characterized. In this work, we determine these densities by position-dependent 2D wavelength-dependent IR fluorescence-excitation and resonant Raman spectroscopy measured directly in the density gradient after ultracentrifugation. We apply this method to study the diameter and chirality
Kinetic electron bounce instability in a 2D current sheet - Implication for substorm dynamics
NASA Astrophysics Data System (ADS)
Fruit, G.; Tur, A.; Louarn, P.
2013-12-01
In the general context of understanding the possible destabilization of the magnetotail before a substorm, we propose a kinetic model for electromagnetic ballooning-type instabilities in resonant interaction with trapped bouncing electrons in a 2D current sheet. Tur et al. 2010 and Fruit et al. 2013 already used this model to investigate the possibilities of electrostatic instabilities. Here, we generalize the model for full electromagnetic perturbations. Starting with a modified Harris sheet as equilibrium state, the linearized gyrokinetic Vlasov equation is solved for electromagnetic fluctuations with period of the order of the electron bounce period. The particle motion is restricted to its first Fourier component along the magnetic field and this allows the complete time integration of the non local perturbed distribution functions. The dispersion relation for electromagnetic modes is finally obtained through the quasineutrality condition and the Ampere's law for the current density. It is found that for mildly stretched current sheet (Bz > 0.1 Blobes) undamped and stable modes oscillate at typical electron bounce frequency with wavelength (in y) of the order of the plasma sheet thickness. As the stretching of the plasma sheet becomes more intense, the frequency of these normal modes decreases and beyond a certain threshold in epsilon=Bz/Blobes < 0.05 typically, the mode becomes explosive (pure imaginary frequency) with typical growing rate of a few tens of seconds. The free energy contained in the electron bouncing motion could thus trigger and drive an electromagnetic instability able to disrupt the cross-tail current in a few seconds. The role of the temperature ratio Te/Ti is also evaluated.
Spin-Orbit Interaction and Related Transport Phenomena in 2d Electron and Hole Systems
NASA Astrophysics Data System (ADS)
Khaetskii, A.
Spin-orbit interaction is responsible for many physical phenomena which are under intensive study currently. Here we discuss several of them. The first phenomenon is the edge spin accumulation, which appears due to spin-orbit interaction in 2D mesoscopic structures in the presence of a charge current. We consider the case of a strong spin-orbit-related splitting of the electron spectrum, i.e. a spin precession length is small compared to the mean free path l. The structure can be either in a ballistic regime (when the mean free path is the largest scale in the problem) or quasi-ballistic regime (when l is much smaller than the sample size). We show how physics of edge spin accumulation in different situations should be understood from the point of view of unitarity of boundary scattering. Using transparent method of scattering states, we are able to explain some previous puzzling theoretical results. We clarify the important role of the form of the spin-orbit Hamiltonian, the role of the boundary conditions, etc., and reveal the wrong results obtained in the field by other researchers. The relation between the edge spin density and the bulk spin current in different regimes is discussed. The detailed comparison with the existing theoretical works is presented. Besides, we consider several new transport phenomena which appear in the presence of spin-orbit interaction, for example, magnetotransport phenomena in an external classical magnetic field. In particular, new mechanism of negative magneto-resistance appears which is due to destruction of spin fluxes by the magnetic field, and which can be really pronounced in 2D systems with strong scatterers.
Nano-scale electronic and optoelectronic devices based on 2D crystals
NASA Astrophysics Data System (ADS)
Zhu, Wenjuan
In the last few years, the research community has been rapidly growing interests in two-dimensional (2D) crystals and their applications. The properties of these 2D crystals are diverse -- ranging from semi-metal such as graphene, semiconductors such as MoS2, to insulator such as boron nitride. These 2D crystals have many unique properties as compared to their bulk counterparts due to their reduced dimensionality and symmetry. A key difference is the band structures, which lead to distinct electronic and photonic properties. The 2D nature of the material also plays an important role in defining their exceptional properties of mechanical strength, surface sensitivity, thermal conductivity, tunable band-gap and their interaction with light. These unique properties of 2D crystals open up a broad territory of applications in computing, communication, energy, and medicine. In this talk, I will present our work on understanding the electrical properties of graphene and MoS2, in particular current transport and band-gap engineering in graphene, interface between gate dielectrics and graphene, and gap states in MoS2. I will also present our work on the nano-scale electronic devices (RF and logic devices) and photonic devices (plasmonic devices and photo-detectors) based on these 2D crystals.
High-Density 2D Homo- and Hetero- Plasmonic Dimers with Universal Sub-10-nm Gaps.
Zhang, Mingliang; Large, Nicolas; Koh, Ai Leen; Cao, Yang; Manjavacas, Alejandro; Sinclair, Robert; Nordlander, Peter; Wang, Shan X
2015-09-22
Fabrication of high-density plasmonic dimers on a large (wafer) scale is crucial for applications in surface-enhanced spectroscopy, bio- and molecular sensing, and optoelectronics. Here, we present an experimental approach based on nanoimprint lithography and shadow evaporation that allows for the fabrication of high-density, large-scale homo- (Au-Au and Ag-Ag) and hetero- (Au-Ag) dimer substrates with precise and consistent sub-10-nm gaps. We performed scanning electron, scanning transmission electron, and atomic force microscopy studies along with a complete electron energy-loss spectroscopy (EELS) characterization. We observed distinct plasmonic modes on these dimers, which are well interpreted by finite-difference time-domain (FDTD) and plasmon hybridization calculations. PMID:26202803
Kan, Kan; Wang, Lei; Yu, Peng; Jiang, Baojiang; Shi, Keying; Fu, Honggang
2016-05-21
Two-dimensional (2D) quasi-ordered nitrogen-enriched porous carbon (QNPC) nanohybrids, with the characteristics of an ultrathin graphite nanosheet framework and thick quasi-ordered nitrogen-doped carbon cladding with a porous texture, have been synthesized via an in situ polymerization assembly method. In the synthesis, the expandable graphite (EG) is enlarged by an intermittent microwave method, and then aniline monomers are intercalated into the interlayers of the expanded EG with the assistance of a vacuum. Subsequently, the intercalated aniline monomers could assemble on the interlayer surface of the expanded EG, accompanied by the in situ polymerization from aniline monomers to polyaniline. Meanwhile, the expanded EG could be exfoliated to graphite nanosheets. By subsequent pyrolysis and activation processes, the QNPC nanohybrids could be prepared. As supercapacitor electrodes, a typical QNPC12-700 sample derived from the precursor containing an EG content of 12%, with a high level of nitrogen doping of 5.22 at%, offers a high specific capacitance of 305.7 F g(-1) (1 A g(-1)), excellent rate-capability and long-term stability. Notably, an extremely high energy density of 95.7 Wh kg(-1) at a power density of 449.7 W kg(-1) in an ionic liquid electrolyte can be achieved. The unique structural features and moderate heteroatom doping of the QNPC nanohybrids combines electrochemical double layer and faradaic capacitance contributions, which make these nanohybrids ideal candidates as electrode materials for high-performance energy storage devices. PMID:27122446
NASA Astrophysics Data System (ADS)
Ostroumov, Evgeny E.; Jumper, Chanelle C.; Mulvaney, Rachel M.; Cogdell, Richard J.; Scholes, Gregory D.
2013-03-01
The study of LH2 protein of purple bacteria by broadband 2D electronic spectroscopy is presented. The dark 1Bu- carotenoid state is directly observed in 2D spectra and its role in carotenoid-bacteriochlorophyll interaction is discussed.
Anasori, Babak; Shi, Chenyang; Moon, Eun Ju; Xie, Yu; Voigt, Cooper A.; Kent, Paul R. C.; May, Steven J.; Billinge, Simon J. L.; Barsoum, Michel W.; Gogotsi, Yury
2016-02-24
In this paper, a transition from metallic to semiconducting-like behavior has been demonstrated in two-dimensional (2D) transition metal carbides by replacing titanium with molybdenum in the outer transition metal (M) layers of M3C2 and M4C3 MXenes. The MXene structure consists of n + 1 layers of near-close packed M layers with C or N occupying the octahedral site between them in an [MX]nM arrangement. Recently, two new families of ordered 2D double transition metal carbides MXenes were discovered, M'2M"C2 and M'2M"2C3 – where M' and M" are two different early transition metals, such as Mo, Cr, Ta, Nb, V, andmore » Ti. The M' atoms only occupy the outer layers and the M" atoms fill the middle layers. In other words, M' atomic layers sandwich the middle M"–C layers. Using X-ray atomic pair distribution function (PDF) analysis on Mo2TiC2 and Mo2Ti2C3 MXenes, we present the first quantitative analysis of structures of these novel materials and experimentally confirm that Mo atoms are in the outer layers of the [MC]nM structures. The electronic properties of these Mo-containing MXenes are compared with their Ti3C2 counterparts, and are found to be no longer metallic-like conductors; instead the resistance increases mildly with decreasing temperatures. Density functional theory (DFT) calculations suggest that OH terminated Mo–Ti MXenes are semiconductors with narrow band gaps. Measurements of the temperature dependencies of conductivities and magnetoresistances have confirmed that Mo2TiC2Tx exhibits semiconductor-like transport behavior, while Ti3C2Tx is a metal. Finally, this finding opens new avenues for the control of the electronic and optical applications of MXenes and for exploring new applications, in which semiconducting properties are required.« less
Destabilization of 2D magnetic current sheets by resonance with bouncing electron - a new theory
NASA Astrophysics Data System (ADS)
Fruit, Gabriel; Louarn, Philippe; Tur, Anatoly
2016-07-01
In the general context of understanding the possible destabilization of the magnetotail before a substorm, we propose a kinetic model for electromagnetic instabilities in resonant interaction with trapped bouncing electrons. The geometry is clearly 2D and uses Harris sheet profile. Fruit et al. 2013 already used this model to investigate the possibilities of electrostatic instabilities. Tur et al. 2014 generalizes the model for full electromagnetic perturbations. Starting with a modified Harris sheet as equilibrium state, the linearized gyrokinetic Vlasov equation is solved for electromagnetic fluctuations with period of the order of the electron bounce period (a few seconds). The particle motion is restricted to its first Fourier component along the magnetic field and this allows the complete time integration of the non local perturbed distribution functions. The dispersion relation for electromagnetic modes is finally obtained through the quasi neutrality condition and the Ampere's law for the current density. The present talk will focus on the main results of this theory. The electrostatic version of the model may be applied to the near-Earth environment (8-12 R_{E}) where beta is rather low. It is showed that inclusion of bouncing electron motion may enhance strongly the growth rate of the classical drift wave instability. This model could thus explain the generation of strong parallel electric fields in the ionosphere and the formation of aurora beads with wavelength of a few hundreds of km. In the electromagnetic version, it is found that for mildly stretched current sheet (B_{z} > 0.1 B _{lobes}) undamped modes oscillate at typical electron bounce frequency with wavelength of the order of the plasma sheet thickness. As the stretching of the plasma sheet becomes more intense, the frequency of these normal modes decreases and beyond a certain threshold in B_{z}/B _{lobes}, the mode becomes explosive (pure imaginary frequency) with typical growing rate of a few
2-D simulation of a waveguide free electron laser having a helical undulator
Kim, S.K.; Lee, B.C.; Jeong, Y.U.
1995-12-31
We have developed a 2-D simulation code for the calculation of output power from an FEL oscillator having a helical undulator and a cylindrical waveguide. In the simulation, the current and the energy of the electron beam is 2 A and 400 keV, respectively. The parameters of the permanent-magnet helical undulator are : period = 32 mm, number of periods = 20, magnetic field = 1.3 kG. The gain per pass is 10 and the output power is calculated to be higher than 10 kW The results of the 2-D simulation are compared with those of 1-D simulation.
2D Spatial Frequency Considerations in Comparing 1D Power Spectral Density Measurements
Takacs, P.Z.; Barber, S.; Church, E.L.; Kaznatcheev, K.; McKinney, W.R.; Yashchuk, V.Y.
2010-06-14
The frequency footprint of ID and 2D profiling instruments needs to be carefully considered in comparing ID surface roughness spectrum measurements made by different instruments. Contributions from orthogonal direction frequency components can not be neglected. The use of optical profiling instruments is ubiquitous in the measurement of the roughness of optical surfaces. Their ease-of-use and non-contact measurement method found widespread use in the optics industry for measuring the quality of delicate optical surfaces. Computerized digital data acquisition with these instruments allowed for quick and easy calculation of surface roughness statistics, such as root-mean-square (RMS) roughness. The computing power of the desktop computer allowed for the rapid conversion of spatial domain data into the frequency domain, enabling the application of sophisticated signal processing techniques to be applied to the analysis of surface roughness, the most powerful of which is the power spectral density (PSP) function. Application of the PSD function to surface statistics introduced the concept of 'bandwidth-limited' roughness, where the value of the RMS roughness depends critically upon the spatial frequency response of the instrument. Different instruments with different spatial frequency response characteristics give different answers when measuring the same surface.
Image of electron densities from line and plane projections
NASA Astrophysics Data System (ADS)
Kontrym-Sznajd, G.; Samsel-Czekała, M.; Biasini, M.
2008-04-01
We compare Fourier transforms with orthogonal polynomials techniques applied in reconstructing three-dimensional electron-positron momentum densities from two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectra and electron momentum densities from one-dimensional Compton profiles (1D-CP). In the case of Fourier transforms, we show results for two different algorithms: filtered back projection and Fourier-Bessel method. These techniques are presented for 2D-ACAR spectra in Y, ErGa3 and model profiles.
NASA Astrophysics Data System (ADS)
McCold, Cliff E.; Fu, Qiang; Howe, Jane Y.; Hihath, Joshua
2015-09-01
Composite molecule-nanoparticle hybrid systems have recently emerged as important materials for applications ranging from chemical sensing to nanoscale electronics. However, creating reproducible and repeatable composite materials with precise properties has remained one of the primary challenges to the implementation of these technologies. Understanding the sources of variation that dominate the assembly and transport behavior is essential for the advancement of nanoparticle-array based devices. In this work, we use a combination of charge-transport measurements, electron microscopy, and optical characterization techniques to determine the role of morphology and structure on the charge transport properties of 2-dimensional monolayer arrays of molecularly-interlinked Au nanoparticles. Using these techniques we are able to determine the role of both assembly-dependent and particle-dependent defects on the conductivities of the films. These results demonstrate that assembly processes dominate the dispersion of conductance values, while nanoparticle and ligand features dictate the mean value of the conductance. By performing a systematic study of the conductance of these arrays as a function of nanoparticle size we are able to extract the carrier mobility for specific molecular ligands. We show that nanoparticle polydispersity correlates with the void density in the array, and that because of this correlation it is possible to accurately determine the void density within the array directly from conductance measurements. These results demonstrate that conductance-based measurements can be used to accurately and non-destructively determine the morphological and structural properties of these hybrid arrays, and thus provide a characterization platform that helps move 2-dimensional nanoparticle arrays toward robust and reproducible electronic systems.Composite molecule-nanoparticle hybrid systems have recently emerged as important materials for applications ranging from
Electron trapping and acceleration across a parabolic plasma density profile.
Kim, J U; Hafz, N; Suk, H
2004-02-01
It is known that as a laser wakefield passes through a downward density transition in a plasma some portion of the background electrons are trapped in the laser wakefield and the trapped electrons are accelerated to relativistic high energies over a very short distance. In this study, by using a two-dimensional (2D) particle-in-cell (PIC) simulation, we suggest an experimental scheme that can manipulate electron trapping and acceleration across a parabolic plasma density channel, which is easier to produce and more feasible to apply to the laser wakefield acceleration experiments. In this study, 2D PIC simulation results for the physical characteristics of the electron bunches that are emitted from the parabolic density plasma channel are reported in great detail. PMID:14995568
NASA Astrophysics Data System (ADS)
Panzer, Matthew; Frisbie, C. Daniel
2007-03-01
The traditional choice of SiO2 for the gate dielectric material in organic field-effect transistors (OFETs) limits the amount of charge that one can induce via the field effect due to its relatively weak dielectric strength. In fact, the maximum 2D charge density achievable (near SiO2 breakdown, typically >100 V applied) is only ˜10^13 charges/cm^2, while the 2D molecular packing density of many common organic semiconductors is on the order of 5 x 10^14 molecules/cm^2. In order to achieve a higher fraction of charged semiconductor molecules in the 2D OFET channel, a dielectric layer with a higher capacitance is required. We have used a solid polymer electrolyte as an OFET dielectric in order to obtain 2D charge densities exceeding 10^14 charges/cm2 at operating voltages under 3 V in a variety of organic semiconductors. We have observed metallic conductivity values (˜1000 S/cm) and nearly temperature-independent resistance ratios in poly(3-hexylthiophene) films using a polymer electrolyte-gated OFET. In addition, conductivity maxima at carrier densities approaching 1 charge/molecule were observed in oligomeric, polymeric, and single-crystal organic semiconductors alike. This phenomenon may be caused by carrier correlations or a complete emptying of the semiconductor transport band at very high charge densities.
Temperature-dependent quantum electron transport in 2D point contacts.
Krishtop, T V; Nagaev, K E
2013-02-01
We consider the transmission of electrons through a two-dimensional ballistic point contact in the low-conductance regime near the pinch-off region. The scattering of electrons by Friedel oscillations of charge density results in a contribution to the conductance proportional to the temperature. The sign of this linear term depends on the range of the electron-electron interaction and appears to be negative for the relevant experimental parameters. PMID:23288558
2D electron temperature diagnostic using soft x-ray imaging technique
Nishimura, K. Sanpei, A. Tanaka, H.; Ishii, G.; Kodera, R.; Ueba, R.; Himura, H.; Masamune, S.; Ohdachi, S.; Mizuguchi, N.
2014-03-15
We have developed a two-dimensional (2D) electron temperature (T{sub e}) diagnostic system for thermal structure studies in a low-aspect-ratio reversed field pinch (RFP). The system consists of a soft x-ray (SXR) camera with two pin holes for two-kinds of absorber foils, combined with a high-speed camera. Two SXR images with almost the same viewing area are formed through different absorber foils on a single micro-channel plate (MCP). A 2D T{sub e} image can then be obtained by calculating the intensity ratio for each element of the images. We have succeeded in distinguishing T{sub e} image in quasi-single helicity (QSH) from that in multi-helicity (MH) RFP states, where the former is characterized by concentrated magnetic fluctuation spectrum and the latter, by broad spectrum of edge magnetic fluctuations.
Effects of 2D and Finite Density Fluctuations on O-X Correlation Reflectometry
G.J. Kramer; R. Nazikian; E. Valeo
2001-07-05
The correlation between O-mode and X-mode reflectometer signals is studied with a 1D and 2D reflectometer model in order to explore its feasibilities as a q-profile diagnostic. It was found that 2D effects and finite fluctuation levels both decrease the O-X correlation. At very low fluctuation levels, which are usually present in the plasma core, there is good possibility to determine the local magnetic field strength and use that as a constraint for the equilibrium reconstruction.
Kaindl, Robert A.; Carnahan, Marc A.; Hagele, Daniel; Chemla, D.S.
2006-09-02
Excitons are of fundamental interest and of importance foropto-electronic applications of bulk and nano-structured semiconductors.This paper discusses the utilization of ultrafast terahertz (THz) pulsesfor the study of characteristic low-energy excitations of photoexcitedquasi 2D electron-hole (e-h) gases. Optical-pump THz-probe spectroscopyat 250-kHz repetition rate is employed to detect characteristic THzsignatures of excitons and unbound e-h pairs in GaAs quantum wells.Exciton and free-carrier densities are extracted from the data using atwo-component model. We report the detailed THz response and pairdensities for different photoexcitation energies resonant to heavy-holeexcitons, light-hole excitons, or the continuum of unbound pairs. Suchexperiments can provide quantitative insights into wavelength, time, andtemperature dependence of the low-energy response and composition ofoptically excited e-h gases in low-dimensionalsemiconductors.
Pair interaction energy for a 12-electron 2D square Quantum Dot.
NASA Astrophysics Data System (ADS)
Nissenbaum, Daniel; Barbiellini, Bernardo; Bansil, Arun
2004-03-01
We have investigated a system of 12 electrons enclosed in a 2D square well representing a quantum dot. We employ a Jastrow-type wavefunction with Slater determinants and optimize the Jastrow parameter using the variational Monte Carlo method. We use the Metropolis algorithm to select a large distribution of configuration points and to perform a relatively noiseless calculation of the radial distribution function and to obtain insight into the contrast between the Fermi hole for the same-spin electrons and the Coulomb hole for the opposite-spin electrons. The calculated pair interaction energy provides a handle for constructing a model Hamiltonian useful for the study of spontaneous spin magnetization of the system. Work supported in part by the USDOE.
Electron Microscopy: From 2D to 3D Images with Special Reference to Muscle
2015-01-01
This is a brief and necessarily very sketchy presentation of the evolution in electron microscopy (EM) imaging that was driven by the necessity of extracting 3-D views from the essentially 2-D images produced by the electron beam. The lens design of standard transmission electron microscope has not been greatly altered since its inception. However, technical advances in specimen preparation, image collection and analysis gradually induced an astounding progression over a period of about 50 years. From the early images that redefined tissues, cell and cell organelles at the sub-micron level, to the current nano-resolution reconstructions of organelles and proteins the step is very large. The review is written by an investigator who has followed the field for many years, but often from the sidelines, and with great wonder. Her interest in muscle ultrastructure colors the writing. More specific detailed reviews are presented in this issue. PMID:26913146
NASA Astrophysics Data System (ADS)
de La Barrera, Sergio; Mende, Patrick; Li, Jun; Feenstra, Randall; Lin, Yu-Chuan; Robinson, Joshua; Vishwanath, Suresh; Xing, Huili
Among the many properties that evolve as isolated 2D materials are brought together to form a heterostructure, rearrangement of charges between layers due to unintentional doping results in dipole fields at the interface, which critically affect the electronic properties of the structure. Here we report a method for directly measuring work function differences, and hence electrostatic potential variations, across the surface of 2D materials and heterostructures thereof using low energy electron microscopy (LEEM). Study of MoSe2 grown by molecular beam epitaxy on epitaxial graphene on SiC with LEEM reveals a large work function difference between the MoSe2 and the graphene, indicating charge transfer between the layers and a subsequent dipole layer. In addition to quantifying dipole effects between transition metal dichalcogenides and graphene, direct imaging of the surface, diffraction information, and the spectroscopic dependence of electron reflectivity will be discussed. This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of the six SRC STARnet Centers, sponsored by MARCO and DARPA.
Optical and Electronic Properties of 2D Graphitic Carbon-Nitride and Carbon Enriched Alloys
NASA Astrophysics Data System (ADS)
Therrien, Joel; Li, Yancen; Schmidt, Daniel; Masaki, Michael; Syed, Abdulmannan
The two-dimensional form of graphitic carbon-nitride (gCN) has been successfully synthesized using a simple CVD process. In it's pure form, the carbon to nitrogen ratio is 0.75. By adding a carbon bearing gas to the growth environment, the C/N ratio can be increased, ultimately reaching the pure carbon form: graphene. Unlike attempts at making a 2D alloy system out of BCN, the CN system does not suffer from phase segregation and thus forms a homogeneous alloy. The synthesis approach and electronic and optical properties will be presented for the pure gCN and a selection of alloy compositions.
Electron phase coherent effects in nanostructures and coupled 2D systems
Simmons, J.A.; Lyo, S.K.; Klem, J.F.; Sherwin, M.E.; Harff, N.E.; Eiles, T.M.; Wendt, J.R.
1995-05-01
This report describes the research accomplishments achieved under the LDRD Project ``Electron Phase Coherent Effects in Nanostructures and Coupled 2D Systems.`` The goal of this project was to discover and characterize novel quantum transport phenomena in small semiconductor structures at low temperatures. Included is a description of the purpose of the research, the various approaches used, and a detailed qualitative description of the numerous new results obtained. The first appendix gives a detailed listing of publications, presentations, patent applications, awards received, and various other measures of the LDRD project success. Subsequent appendices consist of reprinted versions of several specific,`` scientific journal publications resulting from this LDRD project.
Hall-Effect Thruster Simulations with 2-D Electron Transport and Hydrodynamic Ions
NASA Technical Reports Server (NTRS)
Mikellides, Ioannis G.; Katz, Ira; Hofer, Richard H.; Goebel, Dan M.
2009-01-01
A computational approach that has been used extensively in the last two decades for Hall thruster simulations is to solve a diffusion equation and energy conservation law for the electrons in a direction that is perpendicular to the magnetic field, and use discrete-particle methods for the heavy species. This "hybrid" approach has allowed for the capture of bulk plasma phenomena inside these thrusters within reasonable computational times. Regions of the thruster with complex magnetic field arrangements (such as those near eroded walls and magnets) and/or reduced Hall parameter (such as those near the anode and the cathode plume) challenge the validity of the quasi-one-dimensional assumption for the electrons. This paper reports on the development of a computer code that solves numerically the 2-D axisymmetric vector form of Ohm's law, with no assumptions regarding the rate of electron transport in the parallel and perpendicular directions. The numerical challenges related to the large disparity of the transport coefficients in the two directions are met by solving the equations in a computational mesh that is aligned with the magnetic field. The fully-2D approach allows for a large physical domain that extends more than five times the thruster channel length in the axial direction, and encompasses the cathode boundary. Ions are treated as an isothermal, cold (relative to the electrons) fluid, accounting for charge-exchange and multiple-ionization collisions in the momentum equations. A first series of simulations of two Hall thrusters, namely the BPT-4000 and a 6-kW laboratory thruster, quantifies the significance of ion diffusion in the anode region and the importance of the extended physical domain on studies related to the impact of the transport coefficients on the electron flow field.
Dosimetric verification of gated delivery of electron beams using a 2D ion chamber array.
Yoganathan, S A; Das, K J Maria; Raj, D Gowtham; Kumar, Shaleen
2015-01-01
The purpose of this study was to compare the dosimetric characteristics; such as beam output, symmetry and flatness between gated and non-gated electron beams. Dosimetric verification of gated delivery was carried for all electron beams available on Varian CL 2100CD medical linear accelerator. Measurements were conducted for three dose rates (100 MU/min, 300 MU/min and 600 MU/min) and two respiratory motions (breathing period of 4s and 8s). Real-time position management (RPM) system was used for the gated deliveries. Flatness and symmetry values were measured using Imatrixx 2D ion chamber array device and the beam output was measured using plane parallel ion chamber. These detector systems were placed over QUASAR motion platform which was programmed to simulate the respiratory motion of target. The dosimetric characteristics of gated deliveries were compared with non-gated deliveries. The flatness and symmetry of all the evaluated electron energies did not differ by more than 0.7 % with respect to corresponding non-gated deliveries. The beam output variation of gated electron beam was less than 0.6 % for all electron energies except for 16 MeV (1.4 %). Based on the results of this study, it can be concluded that Varian CL2100 CD is well suitable for gated delivery of non-dynamic electron beams. PMID:26170552
Dosimetric verification of gated delivery of electron beams using a 2D ion chamber array
Yoganathan, S. A.; Das, K. J. Maria; Raj, D. Gowtham; Kumar, Shaleen
2015-01-01
The purpose of this study was to compare the dosimetric characteristics; such as beam output, symmetry and flatness between gated and non-gated electron beams. Dosimetric verification of gated delivery was carried for all electron beams available on Varian CL 2100CD medical linear accelerator. Measurements were conducted for three dose rates (100 MU/min, 300 MU/min and 600 MU/min) and two respiratory motions (breathing period of 4s and 8s). Real-time position management (RPM) system was used for the gated deliveries. Flatness and symmetry values were measured using Imatrixx 2D ion chamber array device and the beam output was measured using plane parallel ion chamber. These detector systems were placed over QUASAR motion platform which was programmed to simulate the respiratory motion of target. The dosimetric characteristics of gated deliveries were compared with non-gated deliveries. The flatness and symmetry of all the evaluated electron energies did not differ by more than 0.7 % with respect to corresponding non-gated deliveries. The beam output variation of gated electron beam was less than 0.6 % for all electron energies except for 16 MeV (1.4 %). Based on the results of this study, it can be concluded that Varian CL2100 CD is well suitable for gated delivery of non-dynamic electron beams. PMID:26170552
Neutrino-electron Scattering in 2-D Models of Supernova Convection
NASA Astrophysics Data System (ADS)
DeNisco, K. R.; Swesty, F. D.; Myra, E. S.
2005-12-01
We present results from 2-D supernova simulations which include the effects of neutrino-electron scattering. The importance of neutrino-electron scattering in stellar collapse has been known for two decades. Yet it has often been neglected in multidimensional simulations due to the difficulty of implementing it consistently. The inclusion of this process is numerically challenging because of the extremely short scattering timescales involved. The stiffness resulting from this short timescale precludes an explicit numerical treatment of this phenomenon, such as those that have recently been utilized in some 2-D models. We describe our fully-implicit treatment of this process and present our initial results. This work was performed at the State University of New York at Stony Brook as part of the TeraScale Supernova Initiative, and is funded by SciDAC grant DE-FC02-01ER41185 from the U.S. Department of Energy, Office of Science High-Energy, Nuclear, and Advanced Scientific Computing Research Programs. We gratefully acknowledge support of the National Energy Research Scientific Computing Center (NERSC) for computational and consulting support.
Correlating Structural and Electronic Degrees of Freedom in 2D Transition Metal Dichalcogenides
NASA Astrophysics Data System (ADS)
Tung, I.-Cheng; Zhang, Z.; Seyler, K. L.; Jones, A. M.; Clark, G.; Xiao, D.; Laanait, N.; Xu, X.; Wen, H.
We have conducted a microscopic study of the interplay between structural and electronic degrees of freedom in two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers, multilayers and heterostructures. Using the recently developed full field x-ray reflection interface microscopy with the photoluminescence microscopic probe capability at the Advanced Photon Source, we demonstrated the x-ray reflection imaging of a monolayer 2D material for the first time. The structural variation across an exfoliated WSe2 monolayer is quantified by interlayer spacing relative to the crystal substrate and the smoothness of the layer. This structural information is correlated with the electronic properties of TMDs characterized by the in-situ photoluminescence measurements. This work is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-SC0012509. The use of Advanced Photon Source is supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357.
Enhancement of low-energy electron emission in 2D radioactive films.
Pronschinske, Alex; Pedevilla, Philipp; Murphy, Colin J; Lewis, Emily A; Lucci, Felicia R; Brown, Garth; Pappas, George; Michaelides, Angelos; Sykes, E Charles H
2015-09-01
High-energy radiation has been used for decades; however, the role of low-energy electrons created during irradiation has only recently begun to be appreciated. Low-energy electrons are the most important component of radiation damage in biological environments because they have subcellular ranges, interact destructively with chemical bonds, and are the most abundant product of ionizing particles in tissue. However, methods for generating them locally without external stimulation do not exist. Here, we synthesize one-atom-thick films of the radioactive isotope (125)I on gold that are stable under ambient conditions. Scanning tunnelling microscopy, supported by electronic structure simulations, allows us to directly observe nuclear transmutation of individual (125)I atoms into (125)Te, and explain the surprising stability of the 2D film as it underwent radioactive decay. The metal interface geometry induces a 600% amplification of low-energy electron emission (<10 eV; ref. ) compared with atomic (125)I. This enhancement of biologically active low-energy electrons might offer a new direction for highly targeted nanoparticle therapies. PMID:26076306
Enhancement of low-energy electron emission in 2D radioactive films
NASA Astrophysics Data System (ADS)
Pronschinske, Alex; Pedevilla, Philipp; Murphy, Colin J.; Lewis, Emily A.; Lucci, Felicia R.; Brown, Garth; Pappas, George; Michaelides, Angelos; Sykes, E. Charles H.
2015-09-01
High-energy radiation has been used for decades; however, the role of low-energy electrons created during irradiation has only recently begun to be appreciated. Low-energy electrons are the most important component of radiation damage in biological environments because they have subcellular ranges, interact destructively with chemical bonds, and are the most abundant product of ionizing particles in tissue. However, methods for generating them locally without external stimulation do not exist. Here, we synthesize one-atom-thick films of the radioactive isotope 125I on gold that are stable under ambient conditions. Scanning tunnelling microscopy, supported by electronic structure simulations, allows us to directly observe nuclear transmutation of individual 125I atoms into 125Te, and explain the surprising stability of the 2D film as it underwent radioactive decay. The metal interface geometry induces a 600% amplification of low-energy electron emission (<10 eV; ref. ) compared with atomic 125I. This enhancement of biologically active low-energy electrons might offer a new direction for highly targeted nanoparticle therapies.
Phase Diagram of Bilayer 2D Electron Systems at νT = 1
NASA Astrophysics Data System (ADS)
Champagne, Alexandre
2009-03-01
Bilayer 2D electron systems at total filling fraction νT = 1 and small interlayer spacing can support a strongly correlated phase which exhibits spontaneous interlayer phase coherence and may be described as an excitonic Bose condensate. We use electron interlayer tunnelling and transport to explore the phase diagram of bilayer 2D electron systems at νT = 1, and find that phase transitions between the excitonic νT = 1 phase and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing, d/l, the temperature, T, or the charge imbalance, δν=ν1-ν2. First, for the balanced (δν = 0) system we find that the amplitude of the resonant tunneling in the coherent νT = 1 phase obeys an empirical power law scaling versus d/l at various T, and the layer separation where the tunneling disappears scales linearly with T. Our results [1] offer strong evidence that a finite temperature phase transition separates the balanced interlayer coherent phase from incoherent phases which lack strong interlayer correlations. Secondly, we observe [2] that close to the phase boundary the coherent νT = 1 phase can be absent at δν = 0, present at intermediate δν, and absent again at large δν, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. Lastly, at δν = 1/3 we report [2] the observation of a direct phase transition between the coherent νT = 1 bilayer integer quantum Hall phase and the pair of single layer fractional quantized Hall states at ν1 = 2/3 and ν2 = 1/3.[4pt] [1] A.R. Champagne, et al., Phys. Rev. Lett. 100, 096801 (2008).[0pt] [2] A.R. Champagne, et al, Phys. Rev. B 78, 205310 (2008)
2D array of cold-electron nanobolometers with double polarised cross-dipole antennas
2012-01-01
A novel concept of the two-dimensional (2D) array of cold-electron nanobolometers (CEB) with double polarised cross-dipole antennas is proposed for ultrasensitive multimode measurements. This concept provides a unique opportunity to simultaneously measure both components of an RF signal and to avoid complicated combinations of two schemes for each polarisation. The optimal concept of the CEB includes a superconductor-insulator-normal tunnel junction and an SN Andreev contact, which provides better performance. This concept allows for better matching with the junction gate field-effect transistor (JFET) readout, suppresses charging noise related to the Coulomb blockade due to the small area of tunnel junctions and decreases the volume of a normal absorber for further improvement of the noise performance. The reliability of a 2D array is considerably increased due to the parallel and series connections of many CEBs. Estimations of the CEB noise with JFET readout give an opportunity to realise a noise equivalent power (NEP) that is less than photon noise, specifically, NEP = 4 10−19 W/Hz1/2 at 7 THz for an optical power load of 0.02 fW. PMID:22512950
Study of the height and density distributions of the 2-D granular system under vertical vibration
NASA Astrophysics Data System (ADS)
Pak, Hyuk Kyu; Kim, Kipom; Jun, Yonggun
1998-03-01
Melecular dynamic simulations and experiments are used to investigate the pattern formation of the granular materials in a vertically vibrated rigid container. The height and density distributions of the peak of the patterns in two dimensional system are measured using the simulation. The height distribution agrees with the experimental observation. At the peak of height of the pattern the density is observed minimum. From the information of the vertical velocities of the particles, the momentum flux distributions are studied also.
Plenio, M. B.; Almeida, J.; Huelga, S. F.
2013-12-21
We demonstrate that the coupling of excitonic and vibrational motion in biological complexes can provide mechanisms to explain the long-lived oscillations that have been obtained in nonlinear spectroscopic signals of different photosynthetic pigment protein complexes and we discuss the contributions of excitonic versus purely vibrational components to these oscillatory features. Considering a dimer model coupled to a structured spectral density we exemplify the fundamental aspects of the electron-phonon dynamics, and by analyzing separately the different contributions to the nonlinear signal, we show that for realistic parameter regimes purely electronic coherence is of the same order as purely vibrational coherence in the electronic ground state. Moreover, we demonstrate how the latter relies upon the excitonic interaction to manifest. These results link recently proposed microscopic, non-equilibrium mechanisms to support long lived coherence at ambient temperatures with actual experimental observations of oscillatory behaviour using 2D photon echo techniques to corroborate the fundamental importance of the interplay of electronic and vibrational degrees of freedom in the dynamics of light harvesting aggregates.
Self-Energy Correction to Momentum-Density Distribution of Positron-Electron Pairs
NASA Astrophysics Data System (ADS)
Tang, Z.; Nagai, Y.; Inoue, K.; Toyama, T.; Chiba, T.; Saito, M.; Hasegawa, M.
2005-03-01
Positron two-dimensional angular correlation of annihilation radiation (2D ACAR), i.e., the 2D projection of the electron momentum densities sampled by positron, in Si is employed to verify the prediction of the density functional theory within the local-density approximation (LDA). Carefully conducted test shows that the LDA introduces small but definite discrepancies to the 2D-ACAR anisotropies. Self-energy calculation using the GW method indicates that density-fluctuation contributes anisotropic momentum-density correction and thus improves the agreement between theory and experiment. These results provide valuable annotations to the arguments concerning the accuracy and validity of the LDA and GW schemes.
Lin, L. Ding, W. X.; Brower, D. L.
2014-11-15
Combined polarimetry-interferometry capability permits simultaneous measurement of line-integrated density and Faraday effect with fast time response (∼1 μs) and high sensitivity. Faraday effect fluctuations with phase shift of order 0.05° associated with global tearing modes are resolved with an uncertainty ∼0.01°. For physics investigations, local density fluctuations are obtained by inverting the line-integrated interferometry data. The local magnetic and current density fluctuations are then reconstructed using a parameterized fit of the polarimetry data. Reconstructed 2D images of density and magnetic field fluctuations in a poloidal cross section exhibit significantly different spatial structure. Combined with their relative phase, the magnetic-fluctuation-induced particle transport flux and its spatial distribution are resolved.
2D MEMS scanning for LIDAR with sub-Nyquist sampling, electronics, and measurement procedure
NASA Astrophysics Data System (ADS)
Giese, Thorsten; Janes, Joachim
2015-05-01
Electrostatic driven 2D MEMS scanners resonantly oscillate in both axes leading to Lissajous trajectories of a digitally modulated laser beam reflected from the micro mirror. A solid angle of about 0.02 is scanned by a 658nm laser beam with a maximum repetition rate of 350MHz digital pulses. Reflected light is detected by an APD with a bandwidth of 80MHz. The phase difference between the scanned laser light and the light reflected from an obstacle is analyzed by sub-Nyquist sampling. The FPGA-based electronics and software for the evaluation of distance and velocity of objects within the scanning range are presented. Furthermore, the measures to optimize the Lidar accuracy of about 1mm and the dynamic range of up to 2m are examined. First measurements demonstrating the capability of the system and the evaluation algorithms are discussed.
Finite-size scaling in a 2D disordered electron gas with spectral nodes
NASA Astrophysics Data System (ADS)
Sinner, Andreas; Ziegler, Klaus
2016-08-01
We study the DC conductivity of a weakly disordered 2D electron gas with two bands and spectral nodes, employing the field theoretical version of the Kubo–Greenwood conductivity formula. Disorder scattering is treated within the standard perturbation theory by summing up ladder and maximally crossed diagrams. The emergent gapless (diffusion) modes determine the behavior of the conductivity on large scales. We find a finite conductivity with an intermediate logarithmic finite-size scaling towards smaller conductivities but do not obtain the logarithmic divergence of the weak-localization approach. Our results agree with the experimentally observed logarithmic scaling of the conductivity in graphene with the formation of a plateau near {{e}2}/π h .
Finite-size scaling in a 2D disordered electron gas with spectral nodes.
Sinner, Andreas; Ziegler, Klaus
2016-08-01
We study the DC conductivity of a weakly disordered 2D electron gas with two bands and spectral nodes, employing the field theoretical version of the Kubo-Greenwood conductivity formula. Disorder scattering is treated within the standard perturbation theory by summing up ladder and maximally crossed diagrams. The emergent gapless (diffusion) modes determine the behavior of the conductivity on large scales. We find a finite conductivity with an intermediate logarithmic finite-size scaling towards smaller conductivities but do not obtain the logarithmic divergence of the weak-localization approach. Our results agree with the experimentally observed logarithmic scaling of the conductivity in graphene with the formation of a plateau near [Formula: see text]. PMID:27270084
Effective Mass and g-factor of 2D Electrons in a HgTe Quantum Well from THz Photoresponse
NASA Astrophysics Data System (ADS)
Pakmehr, Mehdi; Stier, A. V.; Zhang, H. D.; Bruene, C.; Buhmann, H.; Molenkamp, L.; McCombe, B. D.
2013-03-01
There is current interest in HgTe because of its interesting ``inverted'' band structure and large spin-orbit interaction, and because it is a topological insulator under quantum confinement, Well-widths close to that at which the band structure goes from the ``inverted'' to the normal structure are of particular interest. We have used photoresponse excited by several lines from an optically pumped THz laser and magnetotransport measurements to determine the cyclotron effective mass and g-factor of 2D electrons in the gamma_6 conduction band of a high quality HgTe quantum well (ns = 1.55 x 1012 cm-2; 6 nm well) at low temperatures. One of the two samples studied was gated, which allowed density to be varied by over 30%. We find m* =0.039me and g = -18 at the highest density from fits to the PR with the field normal to the plane of the QW, and separately from CR transmission measurements and tilted field experiments. We will also discuss electron spin resonance measurements near filling factors 7 and 9. Supported in part by NSF DMR 1008138
Particle visualization in high-power impulse magnetron sputtering. I. 2D density mapping
Britun, Nikolay Palmucci, Maria; Konstantinidis, Stephanos; Snyders, Rony
2015-04-28
Time-resolved characterization of an Ar-Ti high-power impulse magnetron sputtering discharge has been performed. This paper deals with two-dimensional density mapping in the discharge volume obtained by laser-induced fluorescence imaging. The time-resolved density evolution of Ti neutrals, singly ionized Ti atoms (Ti{sup +}), and Ar metastable atoms (Ar{sup met}) in the area above the sputtered cathode is mapped for the first time in this type of discharges. The energetic characteristics of the discharge species are additionally studied by Doppler-shift laser-induced fluorescence imaging. The questions related to the propagation of both the neutral and ionized discharge particles, as well as to their spatial density distributions, are discussed.
2D Optical Streaking for Ultra-Short Electron Beam Diagnostics
Ding, Y.T.; Huang, Z.; Wang, L.; /SLAC
2011-12-14
field ionization, which occurs in plasma case, gases species with high field ionization threshold should be considered. For a linear polarized laser, the kick to the ionized electrons depends on the phase of the laser when the electrons are born and the unknown timing jitter between the electron beam and laser beam makes the data analysis very difficult. Here we propose to use a circular polarized laser to do a 2-dimensional (2D) streaking (both x and y) and measure the bunch length from the angular distribution on the screen, where the phase jitter causes only a rotation of the image on the screen without changing of the relative angular distribution. Also we only need to know the laser wavelength for calibration. A similar circular RF deflecting mode was used to measure long bunches. We developed a numerical particle-in-Cell (PIC) code to study the dynamics of ionization electrons with the high energy beam and the laser beam.
Influence of cell wall shape and density on the mechanical behaviour of 2D foam structures
Harders, Harald . E-mail: h.harders@tu-bs.de; Hupfer, Knut; Roesler, Joachim
2005-03-01
This article describes simulations on the influence of the cell wall shape as well as the density on the elastic stiffness of regular and stochastic honeycomb structures. Starting from an equation by Gibson and Ashby for regular honeycombs, an analytical model is developed that describes the influence of different cell wall shapes on the elastic response of these structures. In addition, this analytical model is modified in order to use free parameters that can be fitted to finite element simulation results of stochastic Voronoi honeycomb structures. The model describes the results well. Young's modulus depends strongly on the cell wall shape, achieving a maximum for slightly concave shapes.
Electron (charge) density studies of cellulose models
Technology Transfer Automated Retrieval System (TEKTRAN)
Introductory material first describes electron density approaches and demonstrates visualization of electron lone pairs and bonding as concentrations of electron density. Then it focuses on the application of Bader’s Quantum Theory of Atoms-in-Molecules (AIM) to cellulose models. The purpose of the ...
Strongly Metallic Electron and Hole 2D Transport in an Ambipolar Si-Vacuum Field Effect Transistor
NASA Astrophysics Data System (ADS)
Hu, Binhui; Yazdanpanah, M. M.; Kane, B. E.; Hwang, E. H.; Das Sarma, S.
2015-07-01
We report experiment and theory on an ambipolar gate-controlled Si(111)-vacuum field effect transistor where we study electron and hole (low-temperature 2D) transport in the same device simply by changing the external gate voltage to tune the system from being a 2D electron system at positive gate voltage to a 2D hole system at negative gate voltage. The electron (hole) conductivity manifests strong (moderate) metallic temperature dependence with the conductivity decreasing by a factor of 8 (2) between 0.3 K and 4.2 K with the peak electron mobility (˜18 m2/V s ) being roughly 20 times larger than the peak hole mobility (in the same sample). Our theory explains the data well using random phase approximation screening of background Coulomb disorder, establishing that the observed metallicity is a direct consequence of the strong temperature dependence of the effective screened disorder.
NASA Astrophysics Data System (ADS)
Apruzese, J. P.; Giuliani, J. L.; Hansen, S. B.
2012-09-01
In modeling optically thick, high energy density plasmas (HEDP), radiation transport is of comparable importance to atomic physics, hydrodynamics, and other transport processes. Radiation transport theory is the framework for calculating radiation output based on atomic kinetics, and is thus critical to designing and diagnosing experiments in which radiation production is significant. Starting in the 1960s, the astrophysics community established benchmarks for computational radiation transport, based on a parameterized two-level-atom, and mostly applicable to media with very high optical depths and low collisional quenching of radiative transitions. The purpose of the work reported here is to establish a similar computational radiation transport benchmark that is more relevant to a laboratory HEDP environment. Our model consists of 8 levels of mostly K-shell Al ions, ranging from the ground state of Li-like Al to its bare nucleus. Rates connecting the levels are given by well-known, readily reproducible analytic prescriptions. The results presented consist of level populations as a function of position within the cylindrical medium, and emitted line profiles. For the plasma conditions considered, the magnitudes and spatial variations of the populations are a sensitive indicator of the accuracy of the radiation transport model, and are critical in calculating experimentally relevant quantities such as radiative powers and line ratios.
Momentum density and 2D-ACAR experiments in YBa{sub 2}Cu{sub 3}O{sub 7}
Bansil, A.; Smedskjaer, L.C.
1991-12-01
We compare measured c-projected 2D-ACAR spectrum from an untwinned single crystal of YBa{sub 2}Cu{sub 3}O{sub 7-x} with the corresponding band theory predictions. Many different one-dimensional sections through the spectrum are considered, together with the characteristic amplitudes and shapes of the spectral anisotropies, with a focus on identifying and delineating Fermi surface signatures in the spectra. The positron data clearly show several distinct features of the ridge Fermi surface predicted by the band theory, and give an indication of the pillbox Fermi sheet. The good agreement between theory and experiment suggests that the band theory framework based on the local density approximation (LDA) is capable of providing a substantially correct description of the momentum density and Fermiology of the normal ground state electronic structure of YBa{sub 2}Cu{sub 3}O{sub 7}.
Güell, Aleix G; Ebejer, Neil; Snowden, Michael E; McKelvey, Kim; Macpherson, Julie V; Unwin, Patrick R
2012-07-17
Carbon nanotubes have attracted considerable interest for electrochemical, electrocatalytic, and sensing applications, yet there remains uncertainty concerning the intrinsic electrochemical (EC) activity. In this study, we use scanning electrochemical cell microscopy (SECCM) to determine local heterogeneous electron transfer (HET) kinetics in a random 2D network of single-walled carbon nanotubes (SWNTs) on an Si/SiO(2) substrate. The high spatial resolution of SECCM, which employs a mobile nanoscale EC cell as a probe for imaging, enables us to sample the responses of individual portions of a wide range of SWNTs within this complex arrangement. Using two redox processes, the oxidation of ferrocenylmethyl trimethylammonium and the reduction of ruthenium (III) hexaamine, we have obtained conclusive evidence for the high intrinsic EC activity of the sidewalls of the large majority of SWNTs in networks. Moreover, we show that the ends of SWNTs and the points where two SWNTs cross do not show appreciably different HET kinetics relative to the sidewall. Using finite element method modeling, we deduce standard rate constants for the two redox couples and demonstrate that HET based solely on characteristic defects in the SWNT side wall is highly unlikely. This is further confirmed by the analysis of individual line profiles taken as the SECCM probe scans over an SWNT. More generally, the studies herein demonstrate SECCM to be a powerful and versatile method for activity mapping of complex electrode materials under conditions of high mass transport, where kinetic assignments can be made with confidence. PMID:22635266
Electron-beam induced photoresist shrinkage influence on 2D profiles
NASA Astrophysics Data System (ADS)
Bunday, Benjamin; Cordes, Aaron; Allgair, John; Aguilar, Daniel Bellido; Tileli, Vasiliki; Thiel, Bradley; Avitan, Yohanan; Peltinov, Ram; Bar-Zvi, Mayaan; Adan, Ofer; Chirko, Konstantin
2010-03-01
For many years, lithographic resolution has been the main obstacle in keeping the pace of transistor densification to meet Moore's Law. For the 32 nm node and beyond, new lithography techniques will be used, including immersion ArF (iArF) lithography and extreme ultraviolet lithography (EUVL). As in the past, these techniques will use new types of photoresists with the capability to print smaller feature widths and pitches. Also, such smaller feature sizes will require thinner layers of photoresists, such as under 100 nm. In previous papers, we focused on ArF and iArF photoresist shrinkage. We evaluated the magnitude of shrinkage for both R&D and mature resists as a function of chemical formulation, lithographic sensitivity, scanning electron microscope (SEM) beam condition, and feature size. Shrinkage results were determined by the well accepted methodology described in ISMI's CD-SEM Unified Specification. A model for resist shrinkage, while derived elsewhere, was presented, that can be used to curve-fit to the shrinkage data resulting from multiple repeated measurements of resist features. Parameters in the curve-fit allow for metrics quantifying total shrinkage, shrinkage rate, and initial critical dimension (CD) before e-beam exposure. With these parameters and exhaustive measurements, a fundamental understanding of the phenomenology of the shrinkage trends was achieved, including how the shrinkage behaves differently for different sized features. This work was extended in yet another paper in which we presented a 1-D model for resist shrinkage that can be used to curve-fit to shrinkage curves. Calibration of parameters to describe the photoresist material and the electron beam were all that were required to fit the model to real shrinkage data, as long as the photoresist was thick enough that the beam could not penetrate the entire layer of resist. In this paper, we extend this work yet again to a 2-D model of a trapezoidal photoresist profile. This model thus
Collimated fast electron beam generation in critical density plasma
Iwawaki, T. Habara, H.; Morita, K.; Tanaka, K. A.; Baton, S.; Fuchs, J.; Chen, S.; Nakatsutsumi, M.; Rousseaux, C.; Filippi, F.; Nazarov, W.
2014-11-15
Significantly collimated fast electron beam with a divergence angle 10° (FWHM) is observed when an ultra-intense laser pulse (I = 10{sup 14 }W/cm{sup 2}, 300 fs) irradiates a uniform critical density plasma. The uniform plasma is created through the ionization of an ultra-low density (5 mg/c.c.) plastic foam by X-ray burst from the interaction of intense laser (I = 10{sup 14 }W/cm{sup 2}, 600 ps) with a thin Cu foil. 2D Particle-In-Cell (PIC) simulation well reproduces the collimated electron beam with a strong magnetic field in the region of the laser pulse propagation. To understand the physical mechanism of the collimation, we calculate energetic electron motion in the magnetic field obtained from the 2D PIC simulation. As the results, the strong magnetic field (300 MG) collimates electrons with energy over a few MeV. This collimation mechanism may attract attention in many applications such as electron acceleration, electron microscope and fast ignition of laser fusion.
X-ray study of femtosecond structural dynamics in the 2D charge density wave compound 1T-TaS2
NASA Astrophysics Data System (ADS)
Laulhé, C.; Cario, L.; Corraze, B.; Janod, E.; Huber, T.; Lantz, G.; Boulfaat, S.; Ferrer, A.; Mariager, S. O.; Johnson, J. A.; Grübel, S.; Lübcke, A.; Ingold, G.; Beaud, P.; Johnson, S. L.; Ravy, S.
2015-03-01
1T-TaS2 is a 2D metallic compound which undergoes a series of electronically driven phase transitions toward charge density wave and Mott phases. Its intricate electron-phonon interactions and electron-electron correlations have been promising peculiar out-of-equilibrium dynamics. In this paper, we provide the first direct information on the atomic structure response to an ultra-fast infrared laser pulse in the commensurate phase of 1T-TaS2, by using femtosecond time-resolved X-ray diffraction. We show that ultra-fast excitation with near-infrared photons drives a displacive excitation of the amplitude mode of the commensurate charge density wave. About 3 ps after laser excitation, the system reaches a new, photo-induced state that is maintained for at least 10 ps. We give evidence that this long-lived state exhibits the same structural modulation as in the thermodynamically stable commensurate phase, with a large correlation length. Only the average amplitude of the modulation is found to decrease. We propose that the long-lived state is formed from the commensurate phase by reducing the modulation amplitude on few superlattice nodes. The underlying mechanism proposed is the annihilation of self-trapped polarons.
Electron density studies of methyl cellobioside
Technology Transfer Automated Retrieval System (TEKTRAN)
Experimental X-ray diffraction crystallography determines the variations in electron density that result from the periodic array of atoms in a crystal. Normally, the positions and type of atom are determined from the electron density based on an approximation that the atoms are spherical. However, t...
Tupala, E; Hall, H; Bergström, K; Särkioja, T; Räsänen, P; Mantere, T; Callaway, J; Hiltunen, J; Tiihonen, J
2001-05-01
Alcohol acts through mechanisms involving the brain neurotransmitter dopamine (DA) with the nucleus accumbens as the key zone for mediating these effects. We evaluated the densities of DA D(2)/D(3) receptors and transporters in the nucleus accumbens and amygdala of post-mortem human brains by using [(125)l]epidepride and [(125)I]PE2I as radioligands in whole hemispheric autoradiography of Cloninger type 1 and 2 alcoholics and healthy controls. When compared with controls, the mean binding of [(125)I]epidepride to DA D(2)/D(3) receptors was 20% lower in the nucleus accumbens and 41% lower in the amygdala, and [(125)I]PE2I binding to DA transporters in the nucleus accumbens was 39% lower in type 1 alcoholics. These data indicate that dopaminergic functions in these limbic areas may be impaired among type 1 alcoholics, due to the substantially lower number of receptor sites. Our results suggest that such a reduction may result in the chronic overuse of alcohol as an attempt to stimulate DA function. PMID:11326293
Melting Temperatures of 2D Electron Solids in the Lowest Landau Level from Microwave Spectroscopy
NASA Astrophysics Data System (ADS)
Chen, Y. P.; Ganapathy, S.; Lewis, R. M.; Engel, L. W.; Tsui, D. C.; Wang, Z. H.; Ye, P. D.; Pfeiffer, L. N.; West, K. W.
2005-03-01
We studied the temperature(T) dependence of the microwave conductivity spectra of two dimensional electron systems in the high magnetic field (B) insulating phase (HBIP) for Landau filling factor ν<˜1/5. Such an insulating phase, believed to be a pinned electron solid, supports a characteristic pinning resonance in the conductivity spectrum. Two samples were studied. Sample 1 is a heterojunction with density n˜7x10^10 cm^ -2 and mobility μ˜5x10^6cm^2/Vs and has a single resonance in the HBIP. Sample 2 is a 65nm-wide QW with n˜6x10 ^10cm-2 and μ˜10x10^6cm^2/V and was recently found to have two resonances in the HBIP, interpreted as corresponding to two different solid phases, with one crossing over to the other as ν is reduced [1]. We studied the higher-T behavior of the resonances at many different combinations of n (through backgating) and B, and measured the characteristic temperatures Tc at which the resonances disappear. We foundTc is a non-increasing function of ν for either sample, although the function differs significantly for both samples. We interpret Tc as the melting temperature of the electron solid(s) to a quantum liquid, for which ν captures the importance of inter-electron quantum correlation. [1] Y.P. Chen et al., Phys.Rev.Lett. 93, 206805 (2004)
The mapping of electronic energy distributions using experimental electron density.
Tsirelson, Vladimir G
2002-08-01
It is demonstrated that the approximate kinetic energy density calculated using the second-order gradient expansion with parameters of the multipole model fitted to experimental structure factors reproduces the main features of this quantity in a molecular or crystal position space. The use of the local virial theorem provides an appropriate derivation of approximate potential energy density and electronic energy density from the experimental (model) electron density and its derivatives. Consideration of these functions is not restricted by the critical points in the electron density and provides a comprehensive characterization of bonding in molecules and crystals. PMID:12149553
FMCW Reflectometry for Electron Density Measurements on LTX
NASA Astrophysics Data System (ADS)
Kubota, S.; Nguyen, X. V.; Peebles, W. A.; Majeski, R.; Kaita, R.
2012-10-01
An FMCW (frequency-modulated continuous-wave) reflectometer is being developed and installed on the Lithium Tokamak Experiment (LTX). The initial system will have two channels covering 13.5--33 GHz for (O-mode) electron density measurements in the range of 0.2-1.3x10^13 cm-3. The reflectometer is designed to provide electron density profile measurements for fueling studies using the molecular cluster injector (MCI), the supersonic gas injector (SGI), as well as external gas puffing. The ultrafast time resolution >=4 μs allows tracking of both the fast evolution of the density profile as well as fluctuations. A future third channel will extend the frequency range to 53 GHz for coverage up to 3.5x10^13 cm-3. The system design, along with simulations using ray tracing and 2-D full-wave codes showing the measurement capabilities and data as available, will be presented.
NASA Astrophysics Data System (ADS)
Fujihashi, Yuta; Fleming, Graham R.; Ishizaki, Akihito
2015-06-01
Recently, nuclear vibrational contribution signatures in two-dimensional (2D) electronic spectroscopy have attracted considerable interest, in particular as regards interpretation of the oscillatory transients observed in light-harvesting complexes. These transients have dephasing times that persist for much longer than theoretically predicted electronic coherence lifetime. As a plausible explanation for this long-lived spectral beating in 2D electronic spectra, quantum-mechanically mixed electronic and vibrational states (vibronic excitons) were proposed by Christensson et al. [J. Phys. Chem. B 116, 7449 (2012)] and have since been explored. In this work, we address a dimer which produces little beating of electronic origin in the absence of vibronic contributions, and examine the impact of protein-induced fluctuations upon electronic-vibrational quantum mixtures by calculating the electronic energy transfer dynamics and 2D electronic spectra in a numerically accurate manner. It is found that, at cryogenic temperatures, the electronic-vibrational quantum mixtures are rather robust, even under the influence of the fluctuations and despite the small Huang-Rhys factors of the Franck-Condon active vibrational modes. This results in long-lasting beating behavior of vibrational origin in the 2D electronic spectra. At physiological temperatures, however, the fluctuations eradicate the mixing, and hence, the beating in the 2D spectra disappears. Further, it is demonstrated that such electronic-vibrational quantum mixtures do not necessarily play a significant role in electronic energy transfer dynamics, despite contributing to the enhancement of long-lived quantum beating in 2D electronic spectra, contrary to speculations in recent publications.
Fujihashi, Yuta; Ishizaki, Akihito; Fleming, Graham R.
2015-06-07
Recently, nuclear vibrational contribution signatures in two-dimensional (2D) electronic spectroscopy have attracted considerable interest, in particular as regards interpretation of the oscillatory transients observed in light-harvesting complexes. These transients have dephasing times that persist for much longer than theoretically predicted electronic coherence lifetime. As a plausible explanation for this long-lived spectral beating in 2D electronic spectra, quantum-mechanically mixed electronic and vibrational states (vibronic excitons) were proposed by Christensson et al. [J. Phys. Chem. B 116, 7449 (2012)] and have since been explored. In this work, we address a dimer which produces little beating of electronic origin in the absence of vibronic contributions, and examine the impact of protein-induced fluctuations upon electronic-vibrational quantum mixtures by calculating the electronic energy transfer dynamics and 2D electronic spectra in a numerically accurate manner. It is found that, at cryogenic temperatures, the electronic-vibrational quantum mixtures are rather robust, even under the influence of the fluctuations and despite the small Huang-Rhys factors of the Franck-Condon active vibrational modes. This results in long-lasting beating behavior of vibrational origin in the 2D electronic spectra. At physiological temperatures, however, the fluctuations eradicate the mixing, and hence, the beating in the 2D spectra disappears. Further, it is demonstrated that such electronic-vibrational quantum mixtures do not necessarily play a significant role in electronic energy transfer dynamics, despite contributing to the enhancement of long-lived quantum beating in 2D electronic spectra, contrary to speculations in recent publications.
Xiao, Jin; Long, Mengqiu; Zhang, Xiaojiao; Ouyang, Jun; Xu, Hui; Gao, Yongli
2015-01-01
We have investigated the electronic structure and carrier mobility of four types of phosphorous monolayer sheet (α-P, β-P,γ-P and δ-P) using density functional theory combined with Boltzmann transport method and relaxation time approximation. It is shown that α-P, β-P and γ-P are indirect gap semiconductors, while δ-P is a direct one. All four sheets have ultrahigh carrier mobility and show anisotropy in-plane. The highest mobility value is ~3 × 105 cm2V−1s−1, which is comparable to that of graphene. Because of the huge difference between the hole and electron mobilities, α-P, γ-P and δ-P sheets can be considered as n-type semiconductors, and β-P sheet can be considered as a p-type semiconductor. Our results suggest that phosphorous monolayer sheets can be considered as a new type of two dimensional materials for applications in optoelectronics and nanoelectronic devices. PMID:26035176
Final LDRD report : the physics of 1D and 2D electron gases in III-nitride heterostructure NWs.
Armstrong, Andrew M.; Arslan, Ilke; Upadhya, Prashanth C.; Morales, Eugenia T.; Leonard, Francois Leonard; Li, Qiming; Wang, George T.; Talin, Albert Alec; Prasankumar, Rohit P.; Lin, Yong
2009-09-01
The proposed work seeks to demonstrate and understand new phenomena in novel, freestanding III-nitride core-shell nanowires, including 1D and 2D electron gas formation and properties, and to investigate the role of surfaces and heterointerfaces on the transport and optical properties of nanowires, using a combined experimental and theoretical approach. Obtaining an understanding of these phenomena will be a critical step that will allow development of novel, ultrafast and ultraefficient nanowire-based electronic and photonic devices.
Gold-induced nanowires on the Ge(100) surface yield a 2D and not a 1D electronic structure
NASA Astrophysics Data System (ADS)
de Jong, N.; Heimbuch, R.; Eliëns, S.; Smit, S.; Frantzeskakis, E.; Caux, J.-S.; Zandvliet, H. J. W.; Golden, M. S.
2016-06-01
Atomic nanowires on semiconductor surfaces induced by the adsorption of metallic atoms have attracted a lot of attention as possible hosts of the elusive, one-dimensional Tomonaga-Luttinger liquid. The Au/Ge(100) system in particular is the subject of controversy as to whether the Au-induced nanowires do indeed host exotic, 1D (one-dimensional) metallic states. In light of this debate, we report here a thorough study of the electronic properties of high quality nanowires formed at the Au/Ge(100) surface. The high-resolution ARPES data show the low-lying Au-induced electronic states to possess a dispersion relation that depends on two orthogonal directions in k space. Comparison of the E (kx,ky) surface measured using high-resolution ARPES to tight-binding calculations yields hopping parameters in the two different directions that differ by approximately factor of two. Additionally, by pinpointing the Au-induced surface states in the first, second, and third surface Brillouin zones and analyzing their periodicity in k||, the nanowire propagation direction seen clearly in STM can be imported into the ARPES data. We find that the larger of the two hopping parameters corresponds, in fact, to the direction perpendicular to the nanowires (tperp). This proves that the Au-induced electron pockets possess a two-dimensional, closed Fermi surface, and this firmly places the Au/Ge(100) nanowire system outside potential hosts of a Tomonaga-Luttinger liquid. We combine these ARPES data with scanning tunneling spectroscopic measurements of the spatially resolved electronic structure and find that the spatially straight—wirelike—conduction channels observed up to energies of order one electron volt below the Fermi level do not originate from the Au-induced states seen in the ARPES data. The former are rather more likely to be associated with bulk Ge states that are localized to the subsurface region. Despite our proof of the 2D (two-dimentional) nature of the Au
Two-dimensional B-C-O alloys: a promising class of 2D materials for electronic devices.
Zhou, Si; Zhao, Jijun
2016-04-21
Graphene, a superior 2D material with high carrier mobility, has limited application in electronic devices due to zero band gap. In this regard, boron and nitrogen atoms have been integrated into the graphene lattice to fabricate 2D semiconducting heterostructures. It is an intriguing question whether oxygen can, as a replacement of nitrogen, enter the sp(2) honeycomb lattice and form stable B-C-O monolayer structures. Here we explore the atomic structures, energetic and thermodynamic stability, and electronic properties of various 2D B-C-O alloys using first-principles calculations. Our results show that oxygen can be stably incorporated into the graphene lattice by bonding with boron. The B and O species favor forming alternate patterns into the chain- or ring-like structures embedded in the pristine graphene regions. These B-C-O hybrid sheets can be either metals or semiconductors depending on the B : O ratio. The semiconducting (B2O)nCm and (B6O3)nCm phases exist under the B- and O-rich conditions, and possess a tunable band gap of 1.0-3.8 eV and high carrier mobility, retaining ∼1000 cm(2) V(-1) s(-1) even for half coverage of B and O atoms. These B-C-O alloys form a new class of 2D materials that are promising candidates for high-speed electronic devices. PMID:27072060
Electronic structure of disordered CuPd alloys by positron-annihilation 2D-ACAR
Smedskjaer, L.C.; Benedek, R.; Siegel, R.W.; Legnini, D.G.; Stahulak, M.D.; Bansil, A.
1988-01-01
We report 2D-ACAR experiments and KKR CPA calculations on alpha-phase single-crystal Cu/sub 1-x/Pd/sub x/ in the range x less than or equal to 0.25. The flattening of the Fermi surface near (110) with increasing x predicted by theory is confirmed by our experimental results. 16 refs., 2 figs.
A Bioactive Carbon Nanotube-Based Ink for Printing 2D and 3D Flexible Electronics.
Shin, Su Ryon; Farzad, Raziyeh; Tamayol, Ali; Manoharan, Vijayan; Mostafalu, Pooria; Zhang, Yu Shrike; Akbari, Mohsen; Jung, Sung Mi; Kim, Duckjin; Comotto, Mattia; Annabi, Nasim; Al-Hazmi, Faten Ebrahim; Dokmeci, Mehmet R; Khademhosseini, Ali
2016-05-01
The development of electrically conductive carbon nanotube-based inks is reported. Using these inks, 2D and 3D structures are printed on various flexible substrates such as paper, hydrogels, and elastomers. The printed patterns have mechanical and electrical properties that make them beneficial for various biological applications. PMID:26915715
Ma, Xiaonan; Dostál, Jakub; Brixner, Tobias
2016-09-01
We demonstrate noncollinear coherent two-dimensional (2D) electronic spectroscopy for which broadband pulses are generated in an argon-filled hollow-core fiber pumped by a 1-kHz Ti:Sapphire laser. Compression is achieved to 7 fs duration (TG-FROG) using dispersive mirrors. The hollow fiber provides a clean spatial profile and smooth spectral shape in the 500-700 nm region. The diffractive-optic-based design of the 2D spectrometer avoids directional filtering distortions and temporal broadening from time smearing. For demonstration we record data of cresyl-violet perchlorate in ethanol and use phasing to obtain broadband absorptive 2D spectra. The resulting quantum beating as a function of population time is consistent with literature data. PMID:27607681
Electron density measurements in highly electronegative plasmas
NASA Astrophysics Data System (ADS)
Rafalskyi, D.; Lafleur, T.; Aanesland, A.
2016-08-01
In this paper we present experimental measurements of the electron density in very electronegative ‘ion–ion’ Ar–SF6 plasmas where previous investigations using Langmuir probes have observed electronegativities of up to 5000. The electron density is measured using a short matched dipole probe technique that provides a tolerance better than ±2 · 1013 m‑3. The results demonstrate that the electron density in the low pressure plasma source (which contains a magnetic filter) can be reduced to around 2.7 · 1013 m‑3 with a corresponding plasma electronegativity of about 4000; close to that from fluid simulation predictions. The highest electronegativity, and lowest electron density, is achieved with a pure SF6 plasma, while adding only 6% SF6 to Ar allows the electronegativity to be increased from 0 to a few hundred with a corresponding decrease in the electron density by more than a thousand. The impedance probe based on a short matched dipole appears to be a practical diagnostic that can be used for independent measurements of the electron density in very electronegative plasmas, and opens up the possibility to further investigate and optimize electronegative plasma sources.
Wavelet analysis of electron-density maps.
Main, P; Wilson, J
2000-05-01
The wavelet transform is a powerful technique in signal processing and image analysis and it is shown here that wavelet analysis of low-resolution electron-density maps has the potential to increase their resolution. Like Fourier analysis, wavelet analysis expresses the image (electron density) in terms of a set of orthogonal functions. In the case of the Fourier transform, these functions are sines and cosines and each one contributes to the whole of the image. In contrast, the wavelet functions (simply called wavelets) can be quite localized and may only contribute to a small part of the image. This gives control over the amount of detail added to the map as the resolution increases. The mathematical details are outlined and an algorithm which achieves a resolution increase from 10 to 7 A using a knowledge of the wavelet-coefficient histograms, electron-density histogram and the observed structure amplitudes is described. These histograms are calculated from the electron density of known structures, but it seems likely that the histograms can be predicted, just as electron-density histograms are at high resolution. The results show that the wavelet coefficients contain the information necessary to increase the resolution of electron-density maps. PMID:10771431
Spin-dependent momentum density distribution and Fermi surface of Ho via 2D-ACAR measurements
NASA Astrophysics Data System (ADS)
Hamid, A. S.; Uedono, A.
2004-03-01
The first direct measurements of the spin-dependent positron-electron momentum density and Fermi surface of Ho are presented. The measurements were performed using two-dimensional angular correlation of annihilation radiation (ACAR) experiments with reversal magnetic field directions parallel and anti-parallel to the polarization direction of the positron. The analysis confirmed that two hybrid bands influence the Fermi surface of Ho. They are 5d-6s conduction hybrid bands and partial hybridization of 4f-5d bands. In fact, the measured Fermi surface revealed the behavior of the magnetic electrons. Further, the reciprocal lattice points revealed the electronic spin density distribution behavior. The general layout of the Fermi surface of Ho showed a multiply connected surface as an open hole running along the A axis with minority spin distribution and two electron surfaces centered on K and H points, respectively. Furthermore, this Fermi surface showed anti-ferromagnetic character. The measured Fermi surface of Ho showed agreement with the results of a previous band structure calculation method. (
Nozaki, Hiroo; Fujii, Yosuke; Ichikawa, Kazuhide; Watanabe, Taku; Aihara, Yuichi; Tachibana, Akitomo
2016-07-01
We analyze the electronic structure of lithium ionic conductors, Li3PO4 and Li3PS4, using the electronic stress tensor density and kinetic energy density with special focus on the ionic bonds among them. We find that, as long as we examine the pattern of the eigenvalues of the electronic stress tensor density, we cannot distinguish between the ionic bonds and bonds among metalloid atoms. We then show that they can be distinguished by looking at the morphology of the electronic interface, the zero surface of the electronic kinetic energy density. © 2016 Wiley Periodicals, Inc. PMID:27232445
Absolute Measurement of Electron Cloud Density
Covo, M K; Molvik, A W; Cohen, R H; Friedman, A; Seidl, P A; Logan, G; Bieniosek, F; Baca, D; Vay, J; Orlando, E; Vujic, J L
2007-06-21
Beam interaction with background gas and walls produces ubiquitous clouds of stray electrons that frequently limit the performance of particle accelerator and storage rings. Counterintuitively we obtained the electron cloud accumulation by measuring the expelled ions that are originated from the beam-background gas interaction, rather than by measuring electrons that reach the walls. The kinetic ion energy measured with a retarding field analyzer (RFA) maps the depressed beam space-charge potential and provides the dynamic electron cloud density. Clearing electrode current measurements give the static electron cloud background that complements and corroborates with the RFA measurements, providing an absolute measurement of electron cloud density during a 5 {micro}s duration beam pulse in a drift region of the magnetic transport section of the High-Current Experiment (HCX) at LBNL.
Effect of the Nuclear Hyperfine Field on the 2D Electron Conductivity in the Quantum Hall Regime
VITKALOV,S.A.; BOWERS,C.R.; SIMMONS,JERRY A.; RENO,JOHN L.
2000-07-13
The effect of the nuclear hyperfine interaction on the dc conductivity of 2D electrons under quantum Hall effect conditions at filling factor v= 1 is observed for the first time. The local hyperfine field enhanced by dynamic nuclear polarization is monitored via the Overhauser shift of the 2D conduction electron spin resonance in AlGaAs/GaAs multiquantum-well samples. The experimentally observed change in the dc conductivity resulting from dynamic nuclear polarization is in agreement with a thermal activation model incorporating the Zeeman energy change due to the hyperfine interaction. The relaxation decay time of the dc conductivity is, within experimental error, the same as the relaxation time of the nuclear spin polarization determined from the Overhauser shift. These findings unequivocally establish the nuclear spin origins of the observed conductivity change.
Exact ground state for the four-electron problem in a 2D finite honeycomb lattice
NASA Astrophysics Data System (ADS)
Trencsényi, Réka; Glukhov, Konstantin; Gulácsi, Zsolt
2014-07-01
Working in a subspace with dimensionality much smaller than the dimension of the full Hilbert space, we deduce exact four-particle ground states in 2D samples containing hexagonal repeat units and described by Hubbard type of models. The procedure identifies first a small subspace ? in which the ground state ? is placed, than deduces ? by exact diagonalization in ?. The small subspace is obtained by the repeated application of the Hamiltonian ? on a carefully chosen starting wave vector describing the most interacting particle configuration, and the wave vectors resulting from the application of ?, till the obtained system of equations closes in itself. The procedure which can be applied in principle at fixed but arbitrary system size and number of particles is interesting on its own since it provides exact information for the numerical approximation techniques which use a similar strategy, but apply non-complete basis for ?. The diagonalization inside ? provides an incomplete image of the low lying part of the excitation spectrum, but provides the exact ?. Once the exact ground state is obtained, its properties can be easily analysed. The ? is found always as a singlet state whose energy, interestingly, saturates in the ? limit. The unapproximated results show that the emergence probabilities of different particle configurations in the ground state presents 'Zittern' (trembling) characteristics which are absent in 2D square Hubbard systems. Consequently, the manifestation of the local Coulomb repulsion in 2D square and honeycomb types of systems presents differences, which can be a real source in the differences in the many-body behaviour.
Integrating 2-D position sensitive X-ray detectors with low-density alkali halide storage targets
NASA Astrophysics Data System (ADS)
Haubold, H.-G.; Hoheisel, W.; Hiller, P.
1986-05-01
For the use in scattering experiments with synchrotron radiation, integrating position sensitive X-ray detectors are discussed. These detectors store the photon number equivalent charge (PNEC) in low-density alkali halide targets. Performance tests are given for a detector which uses a Gd 2O 2S fluorescence screen for X-ray detection and the low-density KCl storage target of a television SEC vidicon tube for photon integration. Rather than directly by X-rays, this target is charged by 6 keV electrons from the image intensifier section of the vidicon. Its excellent storage capability allows measurements of extremely high-contrast, high-flux X-ray patterns with the same accuracy as achieved with any single photon detection system if the discussed readout techniques are applied.
Layer-by-Layer Assembled 2D Montmorillonite Dielectrics for Solution-Processed Electronics.
Zhu, Jian; Liu, Xiaolong; Geier, Michael L; McMorrow, Julian J; Jariwala, Deep; Beck, Megan E; Huang, Wei; Marks, Tobin J; Hersam, Mark C
2016-01-01
Layer-by-layer assembled 2D montmorillonite nanosheets are shown to be high-performance, solution-processed dielectrics. These scalable and spatially uniform sub-10 nm thick dielectrics yield high areal capacitances of ≈600 nF cm(-2) and low leakage currents down to 6 × 10(-9) A cm(-2) that enable low voltage operation of p-type semiconducting single-walled carbon nanotube and n-type indium gallium zinc oxide field-effect transistors. PMID:26514248
Two-dimensional B-C-O alloys: a promising class of 2D materials for electronic devices
NASA Astrophysics Data System (ADS)
Zhou, Si; Zhao, Jijun
2016-04-01
Graphene, a superior 2D material with high carrier mobility, has limited application in electronic devices due to zero band gap. In this regard, boron and nitrogen atoms have been integrated into the graphene lattice to fabricate 2D semiconducting heterostructures. It is an intriguing question whether oxygen can, as a replacement of nitrogen, enter the sp2 honeycomb lattice and form stable B-C-O monolayer structures. Here we explore the atomic structures, energetic and thermodynamic stability, and electronic properties of various 2D B-C-O alloys using first-principles calculations. Our results show that oxygen can be stably incorporated into the graphene lattice by bonding with boron. The B and O species favor forming alternate patterns into the chain- or ring-like structures embedded in the pristine graphene regions. These B-C-O hybrid sheets can be either metals or semiconductors depending on the B : O ratio. The semiconducting (B2O)nCm and (B6O3)nCm phases exist under the B- and O-rich conditions, and possess a tunable band gap of 1.0-3.8 eV and high carrier mobility, retaining ~1000 cm2 V-1 s-1 even for half coverage of B and O atoms. These B-C-O alloys form a new class of 2D materials that are promising candidates for high-speed electronic devices.Graphene, a superior 2D material with high carrier mobility, has limited application in electronic devices due to zero band gap. In this regard, boron and nitrogen atoms have been integrated into the graphene lattice to fabricate 2D semiconducting heterostructures. It is an intriguing question whether oxygen can, as a replacement of nitrogen, enter the sp2 honeycomb lattice and form stable B-C-O monolayer structures. Here we explore the atomic structures, energetic and thermodynamic stability, and electronic properties of various 2D B-C-O alloys using first-principles calculations. Our results show that oxygen can be stably incorporated into the graphene lattice by bonding with boron. The B and O species favor
NASA Astrophysics Data System (ADS)
Gerzen, T.; Feltens, J.; Jakowski, N.; Galkin, I.; Reinisch, B.; Zandbergen, R.
2016-09-01
The electron density of the topside ionosphere and the plasmasphere contributes essentially to the overall Total Electron Content (TEC) budget affecting Global Navigation Satellite Systems (GNSS) signals. The plasmasphere can cause half or even more of the GNSS range error budget due to ionospheric propagation errors. This paper presents a comparative study of different plasmasphere and topside ionosphere data aiming at establishing an appropriate database for plasmasphere modelling. We analyze electron density profiles along the geomagnetic field lines derived from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite/Radio Plasma Imager (RPI) records of remote plasma sounding with radio waves. We compare these RPI profiles with 2D reconstructions of the topside ionosphere and plasmasphere electron density derived from GNSS based TEC measurements onboard the Challenging Minisatellite Payload (CHAMP) satellite. Most of the coincidences between IMAGE profiles and CHAMP reconstructions are detected in the region with L-shell between 2 and 5. In general the CHAMP reconstructed electron densities are below the IMAGE profile densities, with median of the CHAMP minus IMAGE residuals around -588 cm-3. Additionally, a comparison is made with electron densities derived from passive radio wave RPI measurements onboard the IMAGE satellite. Over the available 2001-2005 period of IMAGE measurements, the considered combined data from the active and passive RPI operations cover the region within a latitude range of ±60°N, all longitudes, and an L-shell ranging from 1.2 to 15. In the coincidence regions (mainly 2 ⩽ L ⩽ 4), we check the agreement between available active and passive RPI data. The comparison shows that the measurements are well correlated, with a median residual of ∼52 cm-3. The RMS and STD values of the relative residuals are around 22% and 21% respectively. In summary, the results encourage the application of IMAGE RPI data for
Beyond Graphene: Electronic and Mechanical Properties of Defective 2-D Materials
NASA Astrophysics Data System (ADS)
Terrones, Humberto
One of the challenges in the production of 2-D materials is the synthesis of defect free systems which can achieve the desired properties for novel applications. However, the reality so far indicates that we need to deal with defective systems and understand their main features in order to perform defect engineering in such a way that we can engineer a new material. In this talk I discuss first, the introduction of defects in a hierarchic way starting from 2-D graphene to form giant Schwarzites or graphene foams, which also can exhibit further defects, thus we can have several levels of defectiveness. In this context, it will be shown that giant Schwarzites, depending on their symmetry, can exhibit Dirac-Fermion behavior and further, possess protected topological states as shown by other authors. Regarding the mechanical properties of these systems, it is possible to tune the Poisson Ratio by the addition of defects, thus shedding light to the explanation of the almost zero Poisson ratios in experimentally obtained graphene foams. Second, the idea of Haeckelites, a planar sp2 graphene-like structure with heptagons and pentagons, can be extended to transition metal dichalcogenides (TMDs) with square and octagonal-like defects, finding semi-metallic behaviors with Dirac-Fermions, and even topological insulating properties. National Science Foundation (EFRI-1433311).
Binary and ternary recombination of H2D(+) and HD2(+) ions with electrons at 80 K.
Dohnal, Petr; Kálosi, Ábel; Plašil, Radek; Roučka, Štěpán; Kovalenko, Artem; Rednyk, Serhiy; Johnsen, Rainer; Glosík, Juraj
2016-08-24
The recombination of deuterated trihydrogen cations with electrons has been studied in afterglow plasmas containing mixtures of helium, argon, hydrogen and deuterium. By monitoring the fractional abundances of H3(+), H2D(+), HD2(+) and D3(+) as a function of the [D2]/[H2] ratio using infrared absorption observed in a cavity ring down absorption spectrometer (CRDS), it was possible to deduce effective recombination rate coefficients for H2D(+) and HD2(+) ions at a temperature of 80 K. From pressure dependences of the measured effective recombination rate coefficients the binary and the ternary recombination rate coefficients for both ions have been determined. The inferred binary and ternary recombination rate coefficients are: αbinH2D(80 K) = (7.1 ± 4.2) × 10(-8) cm(3) s(-1), αbinHD2(80 K) = (8.7 ± 2.5) × 10(-8) cm(3) s(-1), KH2D(80 K) = (1.1 ± 0.6) × 10(-25) cm(6) s(-1) and KHD2(80 K) = (1.5 ± 0.4) × 10(-25) cm(6) s(-1). PMID:27506912
Electron Momentum Distribution Mapping of Trans-Stilbene Projected to [101] by Positron 2D-ACAR
NASA Astrophysics Data System (ADS)
Selvakumar, S.; Sivaji, K.; Smith, S. V.
Electron momentum distribution (EMD) on trans-stilbene single crystal projected along [101] direction has been studied by using positron two dimensional -angular correlation of annihilation radiation (2D-ACAR). The projected EMD is explained with respect to the molecular arrangement in the plane. The EMD features reflected the delocalized electronic states in [101] direction. The results of EMD mapping did not show a characteristic ellipsoidal distribution at lower momentum region (LMR) as observed in trans-stilbene projected to [010] direction at room temperature. The LMR region exhibits a hexagonal contour projected to [101] direction.
NASA Astrophysics Data System (ADS)
Le Lay, Guy; Salomon, Eric; Angot, Thierry; Eugenia Dávila, Maria
2015-05-01
The realization of the first Field Effect Transistors operating at room temperature, based on a single layer silicene channel, open up highly promising perspectives, e.g., typically, for applications in digital electronics. Here, we describe recent results on the growth, characterization and electronic properties of novel synthetic two-dimensional materials beyond graphene, namely silicene and germanene, its silicon and germanium counterparts.
Electron density measurement by differential interferometry
Ding, W. X.; Brower, D. L.; Deng, B. H.; Yates, T.
2006-10-15
A novel differential interferometer is being developed to measure the electron density gradient and its fluctuations. Two separate laser beams with slight spatial offset and frequency difference are coupled into a single mixer making a heterodyne measurement of the phase difference which is <1% of the total phase change experienced by each beam separately. This measure of the differential phase is made at multiple spatial points and can be inverted directly to provide the local density distribution.
Kolbun, N; Adolfsson, E; Gustafsson, H; Lund, E
2014-06-01
Electron paramagnetic resonance imaging (EPRI) was performed to visualise 2D dose distributions of homogenously irradiated potassium dithionate tablets and to demonstrate determination of 1D dose profiles along the height of the tablets. Mathematical correction was applied for each relative dose profile in order to take into account the inhomogeneous response of the resonator using X-band EPRI. The dose profiles are presented with the spatial resolution of 0.6 mm from the acquired 2D images; this value is limited by pixel size, and 1D dose profiles from 1D imaging with spatial resolution of 0.3 mm limited by the intrinsic line-width of potassium dithionate. In this paper, dose profiles from 2D reconstructed electron paramagnetic resonance (EPR) images using the Xepr software package by Bruker are focussed. The conclusion is that using potassium dithionate, the resolution 0.3 mm is sufficient for mapping steep dose gradients if the dosemeters are covering only ±2 mm around the centre of the resonator. PMID:24748487
Charge density waves in strongly correlated electron systems.
Chen, Chih-Wei; Choe, Jesse; Morosan, E
2016-08-01
Strong electron correlations are at the heart of many physical phenomena of current interest to the condensed matter community. Here we present a survey of the mechanisms underlying such correlations in charge density wave (CDW) systems, including the current theoretical understanding and experimental evidence for CDW transitions. The focus is on emergent phenomena that result as CDWs interact with other charge or spin states, such as magnetism and superconductivity. In addition to reviewing the CDW mechanisms in 1D, 2D, and 3D systems, we pay particular attention to the prevalence of this state in two particular classes of compounds, the high temperature superconductors (cuprates) and the layered transition metal dichalcogenides. The possibilities for quantum criticality resulting from the competition between magnetic fluctuations and electronic instabilities (CDW, unconventional superconductivity) are also discussed. PMID:27376547
Charge density waves in strongly correlated electron systems
NASA Astrophysics Data System (ADS)
Chen, Chih-Wei; Choe, Jesse; Morosan, E.
2016-08-01
Strong electron correlations are at the heart of many physical phenomena of current interest to the condensed matter community. Here we present a survey of the mechanisms underlying such correlations in charge density wave (CDW) systems, including the current theoretical understanding and experimental evidence for CDW transitions. The focus is on emergent phenomena that result as CDWs interact with other charge or spin states, such as magnetism and superconductivity. In addition to reviewing the CDW mechanisms in 1D, 2D, and 3D systems, we pay particular attention to the prevalence of this state in two particular classes of compounds, the high temperature superconductors (cuprates) and the layered transition metal dichalcogenides. The possibilities for quantum criticality resulting from the competition between magnetic fluctuations and electronic instabilities (CDW, unconventional superconductivity) are also discussed.
Interlayer tunneling studies of highly imbalanced bilayer 2D electron systems at νT= 1
NASA Astrophysics Data System (ADS)
Champagne, A. R.; Eisenstein, J. P.; Pfeiffer, L. N.; West, K. W.
2007-03-01
When the separation between two parallel 2-dimensional electron systems (2DES) becomes comparable to the average distance between electrons within a single layer, the system can support a quantum Hall state with total filling factor νT=1. This state can be described as a Bose condensate of excitons. Previous studies [1] have shown that close to the νT=1 phase boundary, a small imbalance in the number of electrons in each layer can strengthen the condensate. We report on interlayer tunneling measurements of the effect of large imbalances as a function of the interlayer spacing. We explore the possibility of competing order between the excitonic state and the (1/3, 2/3) fractional states in the individual layers. This work was supported by the NSF and the DOE. [1] I. B. Spielman, et al., Phys. Rev. B 70, 081303 (2004).
A Robust High Current Density Electron Gun
NASA Astrophysics Data System (ADS)
Mako, F.; Peter, W.; Shiloh, J.; Len, L. K.
1996-11-01
Proof-of-principle experiments are proposed to validate a new concept for a robust, high-current density Pierce electron gun (RPG) for use in klystrons and high brightness electron sources for accelerators. This rugged, long-life electron gun avoids the difficulties associated with plasma cathodes, thermionic emitters, and field emission cathodes. The RPG concept employs the emission of secondary electrons in a transmission mode as opposed to the conventional mode of reflection, i.e., electrons exit from the back face of a thin negative electron affinity (NEA) material, and in the same direction as the incident beam. Current amplification through one stage of a NEA material could be over 50 times. The amplification is accomplished in one or more stages consisting of one primary emitter and one or more secondary emitters. The primary emitter is a low current density robust emitter (e.g., thoriated tungsten). The secondary emitters are thin NEA electrodes which emit secondary electrons in the same direction as the incident beam. Specific application is targeted for a klystron gun to be used by SLAC with a cold cathode at 30-40 amps/cm^2 output from the secondary emission stage, a ~2 μs pulse length, and ~200 pulses/second.
Anomalous giant piezoresistance in AlAs 2D electron systems with antidot lattices.
Gunawan, O; Gokmen, T; Shkolnikov, Y P; De Poortere, E P; Shayegan, M
2008-01-25
An AlAs two-dimensional electron system patterned with an antidot lattice exhibits a giant piezoresistance effect at low temperatures, with a sign opposite to the piezoresistance observed in the unpatterned region. We suggest that the origin of this anomalous giant piezoresistance is the nonuniform strain in the antidot lattice and the exclusion of electrons occupying the two conduction-band valleys from different regions of the sample. This is analogous to the well-known giant magnetoresistance effect, with valley playing the role of spin and strain the role of magnetic field. PMID:18233015
Simulations of Electron Density Perturbations in a Gas Discharge
NASA Astrophysics Data System (ADS)
Caplinger, James; Sotnikov, Vladimir; Main, Daniel
2015-11-01
Beginning with the idealized case of the Pierce diode, a series of particle-in-cell (PIC) simulations are conducted in order to characterize density perturbations in a laboratory gas discharge. This work is conducted to support future experimental investigations into electromagnetic scattering off of electron density perturbations excited by plasma flows. As a first step, 2D PIC simulations were conducted for the Pierce diode case, which is a simple model that exploits instabilities of a monochromatic electron beam between two grounded electrodes. These results were compared to the standard analytical solution. Departing from this idealized case we will include in the simulations electron-neutral collisions, particle creation from ionization, as well as an electric field generated by biased electrodes. A parameter study of electric field strength and collision frequency will be performed for values approaching the Pierce diode as well as extending to cases of expected laboratory parameters. If we can extract physical density spectra from simulations with parameters approaching experimental values, it may be possible to analyze electromagnetic scattering characteristics.
Lin, Lin; Yang, Chao; Lu, Jiangfeng; Ying, Lexing; E, Weinan
2009-09-25
We present an efficient parallel algorithm and its implementation for computing the diagonal of $H^-1$ where $H$ is a 2D Kohn-Sham Hamiltonian discretized on a rectangular domain using a standard second order finite difference scheme. This type of calculation can be used to obtain an accurate approximation to the diagonal of a Fermi-Dirac function of $H$ through a recently developed pole-expansion technique \\cite{LinLuYingE2009}. The diagonal elements are needed in electronic structure calculations for quantum mechanical systems \\citeHohenbergKohn1964, KohnSham 1965,DreizlerGross1990. We show how elimination tree is used to organize the parallel computation and how synchronization overhead is reduced by passing data level by level along this tree using the technique of local buffers and relative indices. We analyze the performance of our implementation by examining its load balance and communication overhead. We show that our implementation exhibits an excellent weak scaling on a large-scale high performance distributed parallel machine. When compared with standard approach for evaluating the diagonal a Fermi-Dirac function of a Kohn-Sham Hamiltonian associated a 2D electron quantum dot, the new pole-expansion technique that uses our algorithm to compute the diagonal of $(H-z_i I)^-1$ for a small number of poles $z_i$ is much faster, especially when the quantum dot contains many electrons.
Temperature and Pinning Effects on Driving a 2D Electron System on a Helium Film: A Numerical Study
NASA Astrophysics Data System (ADS)
Damasceno, Pablo F.; Dasilva, Cláudio José; Rino, José Pedro; Cândido, Ladir
2010-07-01
Using numerical simulations we investigated the dynamic response to an externally driven force of a classical two-dimensional (2D) electron system on a helium film at finite temperatures. A potential barrier located at the center of the system behaves as a pinning center that results in an insulator state below a threshold driving force. We have found that the current-voltage characteristic obeys the scaling relation I= f ξ , with ξ ranging from ˜(1.0-1.7) for different pinning strengths and temperatures. Our results may be used to understand the spread range of ξ in experiments with typical characteristic of plastic depinning.
NASA Astrophysics Data System (ADS)
Maiboroda, I. O.; Andreev, A. A.; Perminov, P. A.; Fedorov, Yu. V.; Zanaveskin, M. L.
2014-06-01
Specific features of how nonalloyed ohmic contacts to the 2D conducting channel of high-electron-mobility transistors based on AlGaN/(AlN)/GaN heterostructures are fabricated via deposition of heavily doped n +-GaN through a SiO2 mask by ammonia molecular-beam epitaxy have been studied. The technique developed makes it possible to obtain specific resistances of contacts to the 2D gas as low as 0.11 Ω mm on various types of Ga-face nitride heterostructures, which are several times lower than the resistance of conventional alloyed ohmic contacts.
Effective mass from microwave photoresistance in high-mobility 2D electron systems
NASA Astrophysics Data System (ADS)
Zudov, Michael; Hatke, Anthony; Watson, John; Manfra, Michael; Pfeiffer, Loren; West, Kenneth
2013-03-01
We have performed microwave photoresistance measurements in high mobility GaAs/AlGaAs quantum wells and investigated the value of the effective mass. Surprisingly, the effective mass, obtained from the period of microwave-induced resistance oscillations, is found to be considerably lower than the band mass in GaAs. This finding provides evidence for electron-electron interactions which can be probed by microwave photoresistance in very high Landau levels. In contrast, the measured magneto-plasmon dispersion revealed an effective mass which is close to the band mass, in accord with previous studies. The work at Minnesota and Purdue was supported by the DOE Grant Nos. DE-SC002567 and DE-SC0006671, respectively. The work at Princeton was partially funded by the Gordon and Betty Moore Foundation Foundation and the NSF MRSEC Program..
Parallel FE Electron-Photon Transport Analysis on 2-D Unstructured Mesh
Drumm, C.R.; Lorenz, J.
1999-03-02
A novel solution method has been developed to solve the coupled electron-photon transport problem on an unstructured triangular mesh. Instead of tackling the first-order form of the linear Boltzmann equation, this approach is based on the second-order form in conjunction with the conventional multi-group discrete-ordinates approximation. The highly forward-peaked electron scattering is modeled with a multigroup Legendre expansion derived from the Goudsmit-Saunderson theory. The finite element method is used to treat the spatial dependence. The solution method is unique in that the space-direction dependence is solved simultaneously, eliminating the need for the conventional inner iterations, a method that is well suited for massively parallel computers.
Teaching Chemistry with Electron Density Models
NASA Astrophysics Data System (ADS)
Shusterman, Gwendolyn P.; Shusterman, Alan J.
1997-07-01
Linus Pauling once said that a topic must satisfy two criteria before it can be taught to students. First, students must be able to assimilate the topic within a reasonable amount of time. Second, the topic must be relevant to the educational needs and interests of the students. Unfortunately, the standard general chemistry textbook presentation of "electronic structure theory", set as it is in the language of molecular orbitals, has a difficult time satisfying either criterion. Many of the quantum mechanical aspects of molecular orbitals are too difficult for most beginning students to appreciate, much less master, and the few applications that are presented in the typical textbook are too limited in scope to excite much student interest. This article describes a powerful new method for teaching students about electronic structure and its relevance to chemical phenomena. This method, which we have developed and used for several years in general chemistry (G.P.S.) and organic chemistry (A.J.S.) courses, relies on computer-generated three-dimensional models of electron density distributions, and largely satisfies Pauling's two criteria. Students find electron density models easy to understand and use, and because these models are easily applied to a broad range of topics, they successfully convey to students the importance of electronic structure. In addition, when students finally learn about orbital concepts they are better prepared because they already have a well-developed three-dimensional picture of electronic structure to fall back on. We note in this regard that the types of models we use have found widespread, rigorous application in chemical research (1, 2), so students who understand and use electron density models do not need to "unlearn" anything before progressing to more advanced theories.
Negative huge magnetoresistance in high-mobility 2D electron gases: DC-current dependence
NASA Astrophysics Data System (ADS)
Iñarrea, J.; Bockhorn, L.; Haug, R. J.
2016-07-01
Two-dimensional electron gases with very high mobility show a huge or giant negative magnetoresistance at low temperatures and low magnetic fields. We present an experimental and theoretical work on the influence of the applied current on the negative huge magnetoresistance of these systems. We obtain an unexpected and strong nonlinear behavior consisting in an increase of the negative huge magnetoresistance with increasing current, in other words, for increasing current the magnetoresistance collapses at small magnetic fields. This nonlinearity is explained by the subtle interplay of elastic scattering within Landau levels and between Landau levels.
Tunable Plasmonic Reflection by Bound 1D Electron States in a 2D Dirac Metal.
Jiang, B-Y; Ni, G X; Pan, C; Fei, Z; Cheng, B; Lau, C N; Bockrath, M; Basov, D N; Fogler, M M
2016-08-19
We show that the surface plasmons of a two-dimensional Dirac metal such as graphene can be reflected by linelike perturbations hosting one-dimensional electron states. The reflection originates from a strong enhancement of the local optical conductivity caused by optical transitions involving these bound states. We propose that the bound states can be systematically created, controlled, and liquidated by an ultranarrow electrostatic gate. Using infrared nanoimaging, we obtain experimental evidence for the locally enhanced conductivity of graphene induced by a carbon nanotube gate, which supports this theoretical concept. PMID:27588873
Tunable Plasmonic Reflection by Bound 1D Electron States in a 2D Dirac Metal
NASA Astrophysics Data System (ADS)
Jiang, B.-Y.; Ni, G. X.; Pan, C.; Fei, Z.; Cheng, B.; Lau, C. N.; Bockrath, M.; Basov, D. N.; Fogler, M. M.
2016-08-01
We show that the surface plasmons of a two-dimensional Dirac metal such as graphene can be reflected by linelike perturbations hosting one-dimensional electron states. The reflection originates from a strong enhancement of the local optical conductivity caused by optical transitions involving these bound states. We propose that the bound states can be systematically created, controlled, and liquidated by an ultranarrow electrostatic gate. Using infrared nanoimaging, we obtain experimental evidence for the locally enhanced conductivity of graphene induced by a carbon nanotube gate, which supports this theoretical concept.
2D transition-metal diselenides: phase segregation, electronic structure, and magnetism.
Manchanda, Priyanka; Skomski, Ralph
2016-02-17
Density-functional theory is used to investigate the phase-segregation behavior of two-dimensional transition-metal dichalcogenides, which are of current interest as beyond-graphene materials for optoelectronic and spintronic applications. Our focus is on the behavior of W1-x V x Se2 monolayers, whose end members are semiconducting WSe2 and ferromagnetic VSe2. The energetics favors phase segregation, but the spinodal decomposition temperature is rather low, about 420 K. The addition of V leads to a transition from a nonmagnetic semiconductor to a metallic ferromagnet, with a ferromagnetic moment of about 1.0 μ B per V atom. The transition is caused by a p-type doping mechanism, which shifts the Fermi level into the valence band. The finite-temperature structure and magnetism of the diselenide systems are discussed in terms of Onsager-type critical fluctuations and Bruggeman effective-medium behavior. PMID:26794410
2D transition-metal diselenides: phase segregation, electronic structure, and magnetism
NASA Astrophysics Data System (ADS)
Manchanda, Priyanka; Skomski, Ralph
2016-02-01
Density-functional theory is used to investigate the phase-segregation behavior of two-dimensional transition-metal dichalcogenides, which are of current interest as beyond-graphene materials for optoelectronic and spintronic applications. Our focus is on the behavior of W1-x V x Se2 monolayers, whose end members are semiconducting WSe2 and ferromagnetic VSe2. The energetics favors phase segregation, but the spinodal decomposition temperature is rather low, about 420 K. The addition of V leads to a transition from a nonmagnetic semiconductor to a metallic ferromagnet, with a ferromagnetic moment of about 1.0 μ B per V atom. The transition is caused by a p-type doping mechanism, which shifts the Fermi level into the valence band. The finite-temperature structure and magnetism of the diselenide systems are discussed in terms of Onsager-type critical fluctuations and Bruggeman effective-medium behavior.
Parallel Finite Element Electron-Photon Transport Analysis on 2-D Unstructured Mesh
Drumm, C.R.
1999-01-01
A computer code has been developed to solve the linear Boltzmann transport equation on an unstructured mesh of triangles, from a Pro/E model. An arbitriwy arrangement of distinct material regions is allowed. Energy dependence is handled by solving over an arbitrary number of discrete energy groups. Angular de- pendence is treated by Legendre-polynomial expansion of the particle cross sections and a discrete ordinates treatment of the particle fluence. The resulting linear system is solved in parallel with a preconditioned conjugate-gradients method. The solution method is unique, in that the space-angle dependence is solved si- multaneously, eliminating the need for the usual inner iterations. Electron cross sections are obtained from a Goudsrnit-Saunderson modifed version of the CEPXS code. A one-dimensional version of the code has also been develop@ for testing and development purposes.
Iterative Stable Alignment and Clustering of 2D Transmission Electron Microscope Images
Yang, Zhengfan; Fang, Jia; Chittuluru, Johnathan; Asturias, Francisco J.; Penczek, Pawel A.
2012-01-01
SUMMARY Identification of homogeneous subsets of images in a macromolecular electron microscopy (EM) image data set is a critical step in single-particle analysis. The task is handled by iterative algorithms, whose performance is compromised by the compounded limitations of image alignment and K-means clustering. Here we describe an approach, iterative stable alignment and clustering (ISAC) that, relying on a new clustering method and on the concepts of stability and reproducibility, can extract validated, homogeneous subsets of images. ISAC requires only a small number of simple parameters and, with minimal human intervention, can eliminate bias from two-dimensional image clustering and maximize the quality of group averages that can be used for ab initio three-dimensional structural determination and analysis of macromolecular conformational variability. Repeated testing of the stability and reproducibility of a solution within ISAC eliminates heterogeneous or incorrect classes and introduces critical validation to the process of EM image clustering. PMID:22325773
Orbital dependent Rashba splitting and electron-phonon coupling of 2D Bi phase on Cu(100) surface
Gargiani, Pierluigi; Lisi, Simone; Betti, Maria Grazia; Ibrahimi, Amina Taleb; Bertran, François; Le Fèvre, Patrick; Chiodo, Letizia
2013-11-14
A monolayer of bismuth deposited on the Cu(100) surface forms a highly ordered c(2×2) reconstructed phase. The low energy single particle excitations of the c(2×2) Bi/Cu(100) present Bi-induced states with a parabolic dispersion in the energy region close to the Fermi level, as observed by angle-resolved photoemission spectroscopy. The electronic state dispersion, the charge density localization, and the spin-orbit coupling have been investigated combining photoemission spectroscopy and density functional theory, unraveling a two-dimensional Bi phase with charge density well localized at the interface. The Bi-induced states present a Rashba splitting, when the charge density is strongly localized in the Bi plane. Furthermore, the temperature dependence of the spectral density close to the Fermi level has been evaluated. Dispersive electronic states offer a large number of decay channels for transitions coupled to phonons and the strength of the electron-phonon coupling for the Bi/Cu(100) system is shown to be stronger than for Bi surfaces and to depend on the electronic state symmetry and localization.
Electron Density Profiles of the Topside Ionosphere
NASA Technical Reports Server (NTRS)
Huang, Xue-Qin; Reinsch, Bodo W.; Bilitza, Dieter; Benson, Robert F.
2002-01-01
The existing uncertainties about the electron density profiles in the topside ionosphere, i.e., in the height region from h,F2 to - 2000 km, require the search for new data sources. The ISIS and Alouette topside sounder satellites from the sixties to the eighties recorded millions of ionograms but most were not analyzed in terms of electron density profiles. In recent years an effort started to digitize the analog recordings to prepare the ionograms for computerized analysis. As of November 2001 about 350000 ionograms have been digitized from the original 7-track analog tapes. These data are available in binary and CDF format from the anonymous ftp site of the National Space Science Data Center. A search site and browse capabilities on CDAWeb assist the scientific usage of these data. All information and access links can be found at http://nssdc.gsfc.nasa.gov/space/isis/isis- status.htm1. This paper describes the ISIS data restoration effort and shows how the digital ionograms are automatically processed into electron density profiles from satellite orbit altitude (1400 km for ISIS-2) down to the F peak. Because of the large volume of data an automated processing algorithm is imperative. The TOPside Ionogram Scaler with True height algorithm TOPIST software developed for this task is successfully scaling - 70% of the ionograms. An <
Monahan, Daniele M.; Whaley-Mayda, Lukas; Fleming, Graham R.; Ishizaki, Akihito
2015-08-14
Coherence oscillations measured in two-dimensional (2D) electronic spectra of pigment-protein complexes may have electronic, vibrational, or mixed-character vibronic origins, which depend on the degree of electronic-vibrational mixing. Oscillations from intrapigment vibrations can obscure the inter-site coherence lifetime of interest in elucidating the mechanisms of energy transfer in photosynthetic light-harvesting. Huang-Rhys factors (S) for low-frequency vibrations in Chlorophyll and Bacteriochlorophyll are quite small (S ≤ 0.05), so it is often assumed that these vibrations influence neither 2D spectra nor inter-site coherence dynamics. In this work, we explore the influence of S within this range on the oscillatory signatures in simulated 2D spectra of a pigment heterodimer. To visualize the inter-site coherence dynamics underlying the 2D spectra, we introduce a formalism which we call the “site-probe response.” By comparing the calculated 2D spectra with the site-probe response, we show that an on-resonance vibration with Huang-Rhys factor as small as S = 0.005 and the most strongly coupled off-resonance vibrations (S = 0.05) give rise to long-lived, purely vibrational coherences at 77 K. We moreover calculate the correlation between optical pump interactions and subsequent entanglement between sites, as measured by the concurrence. At 77 K, greater long-lived inter-site coherence and entanglement appear with increasing S. This dependence all but vanishes at physiological temperature, as environmentally induced fluctuations destroy the vibronic mixing.
A method for reconstructing the PDF of a 3D turbulent density field from 2D observations
NASA Astrophysics Data System (ADS)
Brunt, Christopher M.; Federrath, Christoph; Price, Daniel J.
2010-06-01
We introduce a method for calculating the probability density function (PDF) of a turbulent density field in three dimensions using only information contained in the projected two-dimensional column density field. We test the method by applying it to numerical simulations of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds. To a good approximation, the PDF of log(normalized column density) is a compressed, shifted version of the PDF of log(normalized density). The degree of compression can be determined observationally from the column density power spectrum, under the assumption of statistical isotropy of the turbulence.
NASA Astrophysics Data System (ADS)
Mirza, Babur M.
2016-05-01
A microscopic theory of integer and fractional quantum Hall effects is presented here. In quantum density wave representation of charged particles, it is shown that, in a two-dimensional electron gas coherent structures form under the low temperature and high density conditions. With a sufficiently high applied magnetic field, the combined N particle quantum density wave exhibits collective periodic oscillations. As a result the corresponding quantum Hall voltage function shows a step-wise change in multiples of the ratio h/e2. At lower temperatures further subdivisions emerge in the Hall resistance, exhibiting the fractional quantum Hall effect.
2D ACAR momentum density study of the nature of the positron surface state on Al(100)
Berko, S.; Canter, K.F.; Lynn, K.G.; Mills, A.P.; Roellig, L.O.; West, R.N.
1985-01-01
The two-dimensional angular correlation of the 2..gamma.. annihilation radiation (2D ACAR) has been measured from an Al(100) surface bombarded by 200-eV positrons. After removing the contribution of fast para-positronium annihilation, the spectrum from positrons annihilating at the surface exhibits a nearly isotropic conical shape with a (7.1 +- 0.5) mrad FWHM. 5 refs., 6 figs.
NASA Astrophysics Data System (ADS)
Cabo Montes de Oca, A.; March, N. H.; Cabo-Bizet, A.
2014-12-01
Former results for a tight-binding (TB) model of CuO planes in La2CuO4 are reinterpreted here to underline their wider implications. It is noted that physical systems being appropriately described by the TB model can exhibit the main strongly correlated electron system (SCES) properties, when they are solved in the HF approximation, by also allowing crystal symmetry breaking effects and noncollinear spin orientations of the HF orbitals. It is argued how a simple 2D square lattice system of Coulomb interacting electrons can exhibit insulator gaps and pseudogap states, and quantum phase transitions as illustrated by the mentioned former works. A discussion is also presented here indicating the possibility of attaining room temperature superconductivity, by means of a surface coating with water molecules of cleaved planes of graphite, being orthogonal to its c-axis. The possibility that 2D arrays of quantum dots can give rise to the same effect is also proposed to consideration. The analysis also furnishes theoretical insight to solve the Mott-Slater debate, at least for the La2CuO4 and TMO band structures. The idea is to apply a properly noncollinear GW scheme to the electronic structure calculation of these materials. The fact is that the GW approach can be viewed as a HF procedure in which the screening polarization is also determined. This directly indicates the possibility of predicting the assumed dielectric constant in the previous works. Thus, the results seem to identify that the main correlation properties in these materials are determined by screening. Finally, the conclusions also seem to be of help for the description of the experimental observations of metal-insulator transitions and Mott properties in atoms trapped in planar photonic lattices.
NASA Astrophysics Data System (ADS)
Kyrie, N. P.; Markov, V. S.; Frank, A. G.; Vasilkov, D. G.; Voronova, E. V.
2016-06-01
The distributions of the ion temperature, ion pressure, and electron density over the width (the major transverse dimension) of the current sheet have been studied for the first time. The current sheets were formed in discharges in argon and helium in 2D and 3D magnetic configurations. It is found that the temperature of argon ions in both 2D and 3D magnetic configurations is almost uniform over the sheet width and that argon ions are accelerated by the Ampère force. In contrast, the distributions of the electron density and the temperature of helium ions are found to be substantially nonuniform. As a result, in the 2D magnetic configuration, the ion pressure gradient across the sheet width makes a significant contribution (comparable with the Ampère force) to the acceleration of helium ions, whereas in the 3D magnetic configuration, the Ampère force is counterbalanced by the pressure gradient.
Electronic structural Moiré pattern effects on MoS2/MoSe2 2D heterostructures.
Kang, Jun; Li, Jingbo; Li, Shu-Shen; Xia, Jian-Bai; Wang, Lin-Wang
2013-01-01
The structural and electronic properties of MoS2/MoSe2 bilayers are calculated using first-principles methods. It is found that the interlayer van der Waals interaction is not strong enough to form a lattice-matched coherent heterostructure. Instead, a nanometer-scale Moiré pattern structure will be formed. By analyzing the electronic structures of different stacking configurations, we predict that the valence-band maximum (VBM) state will come from the Γ point due to interlayer electronic coupling. This is confirmed by a direct calculation of a Moiré pattern supercell containing 6630 atoms using the linear scaling three-dimensional fragment method. The VBM state is found to be strongly localized, while the conduction band minimum (CBM) state is only weakly localized, and it comes from the MoS2 layer at the K point. We predict such wave function localization can be a general feature for many two-dimensional (2D) van der Waals heterostructures and can have major impacts on the carrier mobility and other electronic and optical properties. PMID:24079953
NASA Astrophysics Data System (ADS)
Mani, Ramesh; Kriisa, A.
2015-03-01
Negative diagonal magneto-conductivity/resistivity is a spectacular- and thought provoking- property of driven, far-from-equilibrium, low dimensional electronic systems. The physical response of this exotic electronic state is not yet fully understood since it is rarely encountered in experiment. The microwave-radiation-induced zero-resistance state in the high mobility GaAs/AlGaAs 2D electron system is believed to be an example where negative magneto-conductivity/resistivity is responsible for the observed phenomena. Here, we examine the magneto-transport characteristics of this negative conductivity/resistivity state in the microwave photo-excited two-dimensional electron system (2DES) through a numerical solution of the associated boundary value problem. The results suggest, surprisingly, that a bare negative diagonal conductivity/resistivity state in the 2DES under photo-excitation should yield a positive diagonal resistance with a concomitant sign reversal in the Hall voltage. Transport measurements are supported by the DOE, Office of Basic Energy Sciences, Material Sciences and Engineering Division under DE-SC0001762. Additional support by the ARO under W911NF-07-01-015.
Electronic structure and electron momentum density in TiSi
NASA Astrophysics Data System (ADS)
Ghaleb, A. M.; Mohammad, F. M.; Sahariya, Jagrati; Sharma, Mukesh; Ahuja, B. L.
2013-03-01
We report the electron momentum density in titanium monosilicide using 241Am Compton spectrometer. Experimental Compton profile has been compared with the theoretical profiles computed using linear combination of atomic orbitals (LCAO). The energy bands, density of states and Fermi surface structures of TiSi are reported using the LCAO and the full potential linearized augmented plane wave methods. Theoretical anisotropies in directional Compton profiles are interpreted in terms of energy bands. To confirm the conducting behavior, we also report the real space analysis of experimental Compton profile of TiSi.
Reboul, Cyril F; Bonnet, Frederic; Elmlund, Dominika; Elmlund, Hans
2016-06-01
A critical step in the analysis of novel cryogenic electron microscopy (cryo-EM) single-particle datasets is the identification of homogeneous subsets of images. Methods for solving this problem are important for data quality assessment, ab initio 3D reconstruction, and analysis of population diversity due to the heterogeneous nature of macromolecules. Here we formulate a stochastic algorithm for identification of homogeneous subsets of images. The purpose of the method is to generate improved 2D class averages that can be used to produce a reliable 3D starting model in a rapid and unbiased fashion. We show that our method overcomes inherent limitations of widely used clustering approaches and proceed to test the approach on six publicly available experimental cryo-EM datasets. We conclude that, in each instance, ab initio 3D reconstructions of quality suitable for initialization of high-resolution refinement are produced from the cluster centers. PMID:27184214
Correlated quantum transport of density wave electrons.
Miller, J H; Wijesinghe, A I; Tang, Z; Guloy, A M
2012-01-20
Recently observed Aharonov-Bohm quantum interference of the period h/2e in charge density wave rings strongly suggests that correlated density wave electron transport is a cooperative quantum phenomenon. The picture discussed here posits that quantum solitons nucleate and transport current above a Coulomb blockade threshold field. We propose a field-dependent tunneling matrix element and use the Schrödinger equation, viewed as an emergent classical equation as in Feynman's treatment of Josephson tunneling, to compute the evolving macrostate amplitudes, finding excellent quantitative agreement with voltage oscillations and current-voltage characteristics in NbSe(3). A proposed phase diagram shows the conditions favoring soliton nucleation versus classical depinning. PMID:22400766
Electron Density Calibration for Radiotherapy Treatment Planning
Herrera-Martinez, F.; Rodriguez-Villafuerte, M.; Martinez-Davalos, A.; Ruiz-Trejo, C.; Celis-Lopez, M. A.; Larraga-Gutierrez, J. M.; Garcia-Garduno, A.
2006-09-08
Computed tomography (CT) images are used as basic input data for most modern radiosurgery treatment planning systems (TPS). CT data not only provide anatomic information to delineate target volumes, but also allow the introduction of corrections for tissue inhomogeneities into dose calculations during the treatment planning procedure. These corrections involve the determination of a relationship between tissue electron density ({rho}e) and their corresponding Hounsfield Units (HU). In this work, an elemental analysis of different commercial tissue equivalent materials using Scanning Electron Microscopy was carried out to characterize their chemical composition. The tissue equivalent materials were chosen to ensure a large range of {rho}e to be included in the CT scanner calibration. A phantom was designed and constructed with these materials to simulate the size of a human head.
Symmetry measures of the electron density.
Casanova, David; Alemany, Pere; Alvarez, Santiago
2010-10-01
In this communication we define electronic symmetry operation and symmetry group measures, eSOM and eSGM, respectively, develop the basic algorithms to obtain them, and give some examples of the possible applications of these new computational tools. These new symmetry measures based on the electron density have been tested in an analysis of (a) the inversion symmetry for heteronuclear diatomic molecules, for the eclipsed and staggered conformations of ethane and tetrafluoroethane, and for a series of octahedral sulfur halides; (b) the reflection symmetry of three different conformers of tetrafluoroethene; and (c) the loss of C(6) symmetry along the B(2u) distortion mode of benzene and an analysis of rotational symmetry for different six-member ring heterocycles. PMID:20652983
Electron Density Calibration for Radiotherapy Treatment Planning
NASA Astrophysics Data System (ADS)
Herrera-Martínez, F.; Rodríguez-Villafuerte, M.; Martínez-Dávalos, A.; Ruiz-Trejo, C.; Celis-López, M. A.; Lárraga-Gutiérrez, J. M.; García-Garduño, A.
2006-09-01
Computed tomography (CT) images are used as basic input data for most modern radiosurgery treatment planning systems (TPS). CT data not only provide anatomic information to delineate target volumes, but also allow the introduction of corrections for tissue inhomogeneities into dose calculations during the treatment planning procedure. These corrections involve the determination of a relationship between tissue electron density (ρe) and their corresponding Hounsfield Units (HU). In this work, an elemental analysis of different commercial tissue equivalent materials using Scanning Electron Microscopy was carried out to characterize their chemical composition. The tissue equivalent materials were chosen to ensure a large range of ρe to be included in the CT scanner calibration. A phantom was designed and constructed with these materials to simulate the size of a human head.
Reanalysis of relativistic electron phase space density
NASA Astrophysics Data System (ADS)
Shprits, Yuri; Chen, Yue; Kondrashov, Dmitri
In this study we perform a reanalysis of the sparse relativistic electron data using a relatively simple one-dimensional radial diffusion model and a Kalman filtering approach. The results of the reanalysis clearly show pronounced peaks in the electron phase space density (PSD), which can not be explained by the variations in the outer boundary, and can only be produced by a local acceleration processes. The location of the innovation vector shows that local acceleration is most efficient at L* = 5.5. To verify that our results are not affected by the limitations of the satellite orbit and coverage, we performed an "identical twin" experiments with synthetic data specified only at the locations for which CRRES observations are available. Our results indicate that the model with data assimilation can accurately reproduce the underlying structure of the PSD even when data is sparse.
Sun, Jianjun; Xu, Jinbin; Cairns, Nigel J.; Perlmutter, Joel S.; Mach, Robert H.
2012-01-01
The dopamine D1, D2, D3 receptors, vesicular monoamine transporter type-2 (VMAT2), and dopamine transporter (DAT) densities were measured in 11 aged human brains (aged 77–107.8, mean: 91 years) by quantitative autoradiography. The density of D1 receptors, VMAT2, and DAT was measured using [3H]SCH23390, [3H]dihydrotetrabenazine, and [3H]WIN35428, respectively. The density of D2 and D3 receptors was calculated using the D3-preferring radioligand, [3H]WC-10 and the D2-preferring radioligand [3H]raclopride using a mathematical model developed previously by our group. Dopamine D1, D2, and D3 receptors are extensively distributed throughout striatum; the highest density of D3 receptors occurred in the nucleus accumbens (NAc). The density of the DAT is 10–20-fold lower than that of VMAT2 in striatal regions. Dopamine D3 receptor density exceeded D2 receptor densities in extrastriatal regions, and thalamus contained a high level of D3 receptors with negligible D2 receptors. The density of dopamine D1 linearly correlated with D3 receptor density in the thalamus. The density of the DAT was negligible in the extrastriatal regions whereas the VMAT2 was expressed in moderate density. D3 receptor and VMAT2 densities were in similar level between the aged human and aged rhesus brain samples, whereas aged human brain samples had lower range of densities of D1 and D2 receptors and DAT compared with the aged rhesus monkey brain. The differential density of D3 and D2 receptors in human brain will be useful in the interpretation of PET imaging studies in human subjects with existing radiotracers, and assist in the validation of newer PET radiotracers having a higher selectivity for dopamine D2 or D3 receptors. PMID:23185343
NASA Astrophysics Data System (ADS)
Hamid, A. S.
We measured the angular momentum density distribution of YNi2B2C to acquire information about its electronic structure. The measurements were performed using the full-scale utility of the two-dimensional angular correlation of annihilation radiation (2D-ACAR). The measured spectra clarified that Ni (3d) like state, predominantly, affected the Fermi surface of YNi2B2C. Further, s- and p-like-states enhanced its superconducting properties. The Fermi surface of YNi2B2C. was reconstructed using Fourier transformation followed by the LCW (Loucks, Crisp and West) folding procedure. It showed a large and complex surface similar to that of the high temperature superconductors HTS, with anisotropic properties. It also disclosed the effect of d-like state. Nevertheless, the current Fermi surface could deliver the needed topological information to isolate its features. The general layouts of this Fermi surface are; two large electron surfaces running along Γ-Z direction; as well as an additional large electron surface centered on X point; beside one hole surface centered on 100 point. This Fermi surface was interpreted in view of the earlier results.
Electron density measurements for plasma adaptive optics
NASA Astrophysics Data System (ADS)
Neiswander, Brian W.
Over the past 40 years, there has been growing interest in both laser communications and directed energy weapons that operate from moving aircraft. As a laser beam propagates from an aircraft in flight, it passes through boundary layers, turbulence, and shear layers in the near-region of the aircraft. These fluid instabilities cause strong density gradients which adversely affect the transmission of laser energy to a target. Adaptive optics provides corrective measures for this problem but current technology cannot respond quickly enough to be useful for high speed flight conditions. This research investigated the use of plasma as a medium for adaptive optics for aero-optics applications. When a laser beam passes through plasma, its phase is shifted proportionally to the electron density and gas heating within the plasma. As a result, plasma can be utilized as a dynamically controllable optical medium. Experiments were carried out using a cylindrical dielectric barrier discharge plasma chamber which generated a sub-atmospheric pressure, low-temperature plasma. An electrostatic model of this design was developed and revealed an important design constraint relating to the geometry of the chamber. Optical diagnostic techniques were used to characterize the plasma discharge. Single-wavelength interferometric experiments were performed and demonstrated up to 1.5 microns of optical path difference (OPD) in a 633 nm laser beam. Dual-wavelength interferometry was used to obtain time-resolved profiles of the plasma electron density and gas heating inside the plasma chamber. Furthermore, a new multi-wavelength infrared diagnostic technique was developed and proof-of-concept simulations were conducted to demonstrate the system's capabilities.
NASA Astrophysics Data System (ADS)
Guo, Shi; Zhu, Minyi; Hu, Shuming; Mitas, Lubos
2013-03-01
Very recently, a quantum Monte Carlo (QMC) method was proposed for Rashba spin-orbit operators which expands the applicability of QMC to systems with variable spins. It is based on incorporating the spin-orbit into the Green's function and thus samples (ie, rotates) the spinors in the antisymmetric part of the trial function [1]. Here we propose a new alternative for both variational and diffusion Monte Carlo algorithms for calculations of systems with variable spins. Specifically, we introduce a new spin representation which allows us to sample the spin configurations efficiently and without introducing additional fluctuations. We develop the corresponding Green's function which treats the electron spin as a dynamical variable and we use the fixed-phase approximation to eliminate the negative probabilities. The trial wave function is a Slater determinant of spinors and spin-indepedent Jastrow correlations. The method also has the zero variance property. We benchmark the method on the 2D electron gas with the Rashba interaction and we find very good overall agreement with previously obtained results. Research supported by NSF and ARO.
Enriquez, Miriam M.; Zhang, Cheng; Tan, Howe-Siang; Akhtar, Parveen; Garab, Győző; Lambrev, Petar H.
2015-06-07
The pathways and dynamics of excitation energy transfer between the chlorophyll (Chl) domains in solubilized trimeric and aggregated light-harvesting complex II (LHCII) are examined using two-dimensional electronic spectroscopy (2DES). The LHCII trimers and aggregates exhibit the unquenched and quenched excitonic states of Chl a, respectively. 2DES allows direct correlation of excitation and emission energies of coupled states over population time delays, hence enabling mapping of the energy flow between Chls. By the excitation of the entire Chl b Q{sub y} band, energy transfer from Chl b to Chl a states is monitored in the LHCII trimers and aggregates. Global analysis of the two-dimensional (2D) spectra reveals that energy transfer from Chl b to Chl a occurs on fast and slow time scales of 240–270 fs and 2.8 ps for both forms of LHCII. 2D decay-associated spectra resulting from the global analysis identify the correlation between Chl states involved in the energy transfer and decay at a given lifetime. The contribution of singlet–singlet annihilation on the kinetics of Chl energy transfer and decay is also modelled and discussed. The results show a marked change in the energy transfer kinetics in the time range of a few picoseconds. Owing to slow energy equilibration processes, long-lived intermediate Chl a states are present in solubilized trimers, while in aggregates, the population decay of these excited states is significantly accelerated, suggesting that, overall, the energy transfer within the LHCII complexes is faster in the aggregated state.
NASA Astrophysics Data System (ADS)
Zhou, Chen; Lei, Yong; Li, Bofeng; An, Jiachun; Zhu, Peng; Jiang, Chunhua; Zhao, Zhengyu; Zhang, Yuannong; Ni, Binbin; Wang, Zemin; Zhou, Xuhua
2015-12-01
Global Positioning System (GPS) computerized ionosphere tomography (CIT) and ionospheric sky wave ground backscatter radar are both capable of measuring the large-scale, two-dimensional (2-D) distributions of ionospheric electron density (IED). Here we report the spatial and temporal electron density results obtained by GPS CIT and backscatter ionogram (BSI) inversion for three individual experiments. Both the GPS CIT and BSI inversion techniques demonstrate the capability and the consistency of reconstructing large-scale IED distributions. To validate the results, electron density profiles obtained from GPS CIT and BSI inversion are quantitatively compared to the vertical ionosonde data, which clearly manifests that both methods output accurate information of ionopsheric electron density and thereby provide reliable approaches to ionospheric soundings. Our study can improve current understanding of the capability and insufficiency of these two methods on the large-scale IED reconstruction.
Imaginary time density-density correlations for two-dimensional electron gases at high density
Motta, M.; Galli, D. E.; Moroni, S.; Vitali, E.
2015-10-28
We evaluate imaginary time density-density correlation functions for two-dimensional homogeneous electron gases of up to 42 particles in the continuum using the phaseless auxiliary field quantum Monte Carlo method. We use periodic boundary conditions and up to 300 plane waves as basis set elements. We show that such methodology, once equipped with suitable numerical stabilization techniques necessary to deal with exponentials, products, and inversions of large matrices, gives access to the calculation of imaginary time correlation functions for medium-sized systems. We discuss the numerical stabilization techniques and the computational complexity of the methodology and we present the limitations related to the size of the systems on a quantitative basis. We perform the inverse Laplace transform of the obtained density-density correlation functions, assessing the ability of the phaseless auxiliary field quantum Monte Carlo method to evaluate dynamical properties of medium-sized homogeneous fermion systems.
Lan, Mu; Xiang, Gang Zhang, Xi
2014-08-28
We investigate the structural, electronic and magnetic properties of Mn atoms doped two-dimensional (2D) hexagonal GaAs nanosheets (GaAsNSs) using both first-principle calculations and Monte Carlo simulations. The first-principle molecular dynamics is first used to test the structural stability of Mn-doped GaAsNS ((Ga,Mn)AsNS). The analysis of spin-resolved electronic structures and determination of magnetic exchange interactions based on density functional theory (DFT) calculations reveals the existence of long-range exchange interaction in the system. Finally, Metropolis Monte Carlo simulation is employed to estimate Curie temperatures (T{sub C}s) of (Ga,Mn)AsNSs with different doping concentrations by different doping strategies. The results indicate that a T{sub C} up to 82 K can be obtained in regularly-doped (Ga,Mn)AsNSs and doping strategies have prominent impact on T{sub C}s of the systems, which emphasizes the importance of both long-range interactions and doping strategies in reduced dimensional diluted magnetic semiconductors (DMSs)
Increase of spin dephasing times in a 2D electron system with degree of initial spin polarization
NASA Astrophysics Data System (ADS)
Stich, D.; Korn, T.; Schulz, R.; Schuh, D.; Wegscheider, W.; Schüller, C.
2008-03-01
We report on time-resolved Faraday/Kerr rotation measurements on a high-mobility 2D electron system. A variable initial spin polarization is created in the sample by a circularly polarized pump pulse, and the spin polarization is tracked by measuring the Faraday/Kerr rotation of a time-delayed probe pulse. By varying the pump intensity, the initial spin polarization is changed from the low-polarization limit to a polarization degree of several percent. The observed spin dephasing time increases from less than 20 ps to more than 200 ps as the initial spin polarization is increased. To exclude sample heating effects, additional measurements with constant pump intensity and variable degree of circular polarization are performed. The results confirm the theoretical prediction by Weng and Wu [Phys. Rev. B 68 (2003) 075312] that the spin dephasing strongly depends on the initial spin polarization degree. The microscopic origin for this is the Hartree-Fock term in the Coulomb interaction, which acts as an effective out-of plane magnetic field.
Photoluminescence and the gallium problem for highest-mobility GaAs/AlGaAs-based 2d electron gases
NASA Astrophysics Data System (ADS)
Schläpfer, F.; Dietsche, W.; Reichl, C.; Faelt, S.; Wegscheider, W.
2016-05-01
The quest for extremely high mobilities of 2d electron gases in MBE-grown heterostructures is hampered by the available purity of the starting materials, particularly of the gallium. Here we compare the role of different Ga lots having nominally the highest possible quality on the mobility and the photoluminescence (PL) of modulation doped single interface structures and find significant differences. A weak exciton PL reveals that the purity of the Ga is insufficient. No high mobility can be reached with such a lot with a reasonable effort. On the other hand, a strong exciton PL indicates a high initial Ga purity, allowing to reach mobilities of 15 million (single interface) or 28 million cm2/V s (doped quantum wells) in our MBE systems. We discuss possible origins of the inconsistent Ga quality. Furthermore, we compare samples grown in different MBE systems over a period of several years and find that mobility and PL are correlated if similar structures and growth procedures are used.
Many-electron expansion: A density functional hierarchy for strongly correlated systems
NASA Astrophysics Data System (ADS)
Zhu, Tianyu; de Silva, Piotr; van Aggelen, Helen; Van Voorhis, Troy
2016-05-01
Density functional theory (DFT) is the de facto method for the electronic structure of weakly correlated systems. But for strongly correlated materials, common density functional approximations break down. Here, we derive a many-electron expansion (MEE) in DFT that accounts for successive one-, two-, three-, ... particle interactions within the system. To compute the correction terms, the density is first decomposed into a sum of localized, nodeless one-electron densities (ρi). These one-electron densities are used to construct relevant two- (ρi+ρj ), three- (ρi+ρj+ρk ), ... electron densities. Numerically exact results for these few-particle densities can then be used to correct an approximate density functional via any of several many-body expansions. We show that the resulting hierarchy gives accurate results for several important model systems: the Hubbard and Peierls-Hubbard models in 1D and the pure Hubbard model in 2D. We further show that the method is numerically convergent for strongly correlated systems: applying successively higher order corrections leads to systematic improvement of the results. MEE thus provides a hierarchy of density functional approximations that applies to both weakly and strongly correlated systems.
Hahn, Herwig Reuters, Benjamin; Geipel, Sascha; Schauerte, Meike; Kalisch, Holger; Vescan, Andrei; Benkhelifa, Fouad; Ambacher, Oliver
2015-03-14
GaN-based heterostructure FETs (HFETs) featuring a 2-D electron gas (2DEG) can offer very attractive device performance for power-switching applications. This performance can be assessed by evaluation of the dynamic on-resistance R{sub on,dyn} vs. the breakdown voltage V{sub bd}. In literature, it has been shown that with a high V{sub bd}, R{sub on,dyn} is deteriorated. The impairment of R{sub on,dyn} is mainly driven by electron injection into surface, barrier, and buffer traps. Electron injection itself depends on the electric field which typically peaks at the gate edge towards the drain. A concept suitable to circumvent this issue is the charge-balancing concept which employs a 2-D hole gas (2DHG) on top of the 2DEG allowing for the electric field peak to be suppressed. Furthermore, the 2DEG concentration in the active channel cannot decrease by a change of the surface potential. Hence, beside an improvement in breakdown voltage, also an improvement in dynamic behaviour can be expected. Whereas the first aspect has already been demonstrated, the second one has not been under investigation so far. Hence, in this report, the effect of charge-balancing is discussed and its impact on the dynamic characteristics of HFETs is evaluated. It will be shown that with appropriate device design, the dynamic behaviour of HFETs can be improved by inserting an additional 2DHG.
Laser-driven proton and deuteron acceleration from a pure solid-density H2/D2 cryogenic jet
NASA Astrophysics Data System (ADS)
Kim, Jongjin; Gauthier, Maxence; Aurand, Bastian; Curry, Chandra; Goede, Sebastian; Goyon, Clement; Williams, Jackson; Kerr, Shaun; Ruby, John; Propp, Adrienne; Ramakrishna, Bhuvanesh; Pak, Art; Hazi, Andy; Glenzer, Siegfried; Roedel, Christian
2015-11-01
Laser-driven proton acceleration has become of tremendous interest for the fundamental science and the potential applications in tumor therapy and proton radiography. We have developed a cryogenic liquid hydrogen jet, which can deliver a self-replenishing target of pure solid-density hydrogen or deuterium. This allows for a target compatible with high-repetition-rate experiments and results in a pure hydrogen plasma, facilitating comparison with simulations. A new modification has allowed for the formation of jets with rectangular profiles, facilitating comparison with foil targets. This jet was installed at the Titan laser and driven by laser pulses of 40-60 J of 527 nm laser light in 1 ps. The resulting proton and deuteron spectra were measured in multiple directions with Thomson parabola spectrometers and RCF stacks. The spectral and angular information suggest contribution from both the TNSA and RPA acceleration mechanisms.
DAMQT 2.0: A new version of the DAMQT package for the analysis of electron density in molecules
NASA Astrophysics Data System (ADS)
López, Rafael; Rico, Jaime Fernández; Ramírez, Guillermo; Ema, Ignacio; Zorrilla, David
2015-07-01
DAMQT 2.0 is a new version of the DAMQT package for the analysis of electron density in molecules and the fast computation of the density, density deformations, electrostatic potential and field, and Hellmann-Feynman forces. Algorithms for the partition of the electron density and the computation of related properties like density deformations, electrostatic potential and field and Hellmann-Feynman forces have been improved and their codes, fully rewritten. MPI versions of the most computational demanding modules are now included in the package for parallel computation. The Graphical User Interface has been also enhanced, with new features including a 2D plotter and significant improvements in the 3D viewer.
Bakulin, Artem A; Morgan, Sarah E; Kehoe, Tom B; Wilson, Mark W B; Chin, Alex W; Zigmantas, Donatas; Egorova, Dassia; Rao, Akshay
2016-01-01
Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems. PMID:26673260
NASA Astrophysics Data System (ADS)
Bakulin, Artem A.; Morgan, Sarah E.; Kehoe, Tom B.; Wilson, Mark W. B.; Chin, Alex W.; Zigmantas, Donatas; Egorova, Dassia; Rao, Akshay
2016-01-01
Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.
Metastable atom probe for measuring electron beam density profiles
NASA Technical Reports Server (NTRS)
Lockhart, J. M.; Zorn, J. C.
1972-01-01
Metastable atom probe was developed for measuring current density in electron beam as function of two arbitrary coordinates, with spatial resolution better than 0.5 mm. Probe shows effects of space charge, magnetic fields, and other factors which influence electron current density, but operates with such low beam densities that introduced perturbation is very small.
Electron density distributions in the high-latitude magnetosphere
NASA Technical Reports Server (NTRS)
Persoon, Ann M.
1988-01-01
Electron density profiles were constructed to study the plasma density depletions in the nightside auroral zone and the density variations with increasing altitude in the polar cap, using electric field spectrum measurements from the plasma wave instrument on DE-1. Sharply defined regions of depleted plasma densities were commonly observed on nightside auroral field lines, in which electron densities were strongly depleted in relation to the adjacent plasmaspheric and polar densities, forming a low-density cavity at about 70 deg invariant latitude. A correlation was found between low auroral plasma densities, upflowing ion distributions, and an energetic precipitating electron population, indicating that electron density depletions in the nightside auroral zone are directly associated with auroral acceleration processes.
Koo, Jaseung; Park, Jaehong; Tronin, Andrey; Zhang, Ruili; Krishnan, Venkata; Strzalka, Joseph; Kuzmenko, Ivan; Fry, H. Christopher; Therien, Michael J.; Blasie, J. Kent
2012-01-01
We show that simply designed amphiphilic 4-helix bundle peptides can be utilized to vectorially-orient a linearly-extended Donor-bridge-Acceptor (D-br-A) electron transfer (ET) chromophore within its core. The bundle’s interior is shown to provide a unique solvation environment for the D-br-A assembly not accessible in conventional solvents, and thereby control the magnitudes of both light-induced ET and thermal charge recombination rate constants. The amphiphilicity of the bundle’s exterior was employed to vectorially-orient the peptide-chromophore complex at a liquid-gas interface, and its ends tailored for subsequent covalent attachment to an inorganic surface, via a “directed assembly” approach. Structural data, combined with evaluation of the excited state dynamics exhibited by these peptide-chromophore complexes, demonstrates that densely-packed, acentrically ordered 2-D monolayer ensembles of such complexes at high in-plane chromophore densities approaching 1/200Å2 offer unique potential as active layers in binary heterojucntion photovoltaic devices. PMID:22242787
Kim, Young-Cheol; Jang, Sung-Ho; Oh, Se-Jin; Lee, Hyo-Chang; Chung, Chin-Wook
2013-05-15
A real-time measurement method for two-dimensional (2D) spatial distribution of the electron temperature and plasma density was developed. The method is based on the floating harmonic method and the real time measurement is achieved with little plasma perturbation. 2D arrays of the sensors on a 300 mm diameter wafer-shaped printed circuit board with a high speed multiplexer circuit were used. Experiments were performed in an inductive discharge under various external conditions, such as powers, gas pressures, and different gas mixing ratios. The results are consistent with theoretical prediction. Our method can measure the 2D spatial distribution of plasma parameters on a wafer-level in real-time. This method can be applied to plasma diagnostics to improve the plasma uniformity of plasma reactors for plasma processing.
Electronic Flux Density beyond the Born-Oppenheimer Approximation.
Schild, Axel; Agostini, Federica; Gross, E K U
2016-05-19
In the Born-Oppenheimer approximation, the electronic wave function is typically real-valued and hence the electronic flux density (current density) seems to vanish. This is unfortunate for chemistry, because it precludes the possibility to monitor the electronic motion associated with the nuclear motion during chemical rearrangements from a Born-Oppenheimer simulation of the process. We study an electronic flux density obtained from a correction to the electronic wave function. This correction is derived via nuclear velocity perturbation theory applied in the framework of the exact factorization of electrons and nuclei. To compute the correction, only the ground state potential energy surface and the electronic wave function are needed. For a model system, we demonstrate that this electronic flux density approximates the true one very well, for coherent tunneling dynamics as well as for over-the-barrier scattering, and already for mass ratios between electrons and nuclei that are much larger than the true mass ratios. PMID:26878256
Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals
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.
2016-01-01
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. PMID:27245646
Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals.
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
2016-01-01
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. PMID:27245646
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.
Temporal-spatial modeling of electron density enhancement due to successive lightning strokes
NASA Astrophysics Data System (ADS)
Lay, Erin H.; Rodger, Craig J.; Holzworth, Robert H.; Cho, Mengu; Thomas, Jeremy N.
2010-11-01
We report results on the temporal-spatial modeling of electron density enhancement due to successive lightning strokes. Stroke rates based on World-Wide Lightning Location Network measurements are used as input to an axisymmetric Finite Difference Time Domain model that describes the effect of lightning electromagnetic pulses (EMP) on the ionosphere. Each successive EMP pulse interacts with a modified background ionosphere due to the previous pulses, resulting in a nonlinear electron density perturbation over time that eventually reaches a limiting value. The qualitative ionospheric response to successive EMPs is presented in 2-D, axisymmetric space. Results from this study show that the nonlinear electron density perturbations due to successive lightning strokes must be taken into account and varies with altitude. The limiting maximum electron density is reached earlier in time for higher altitudes, and the most significant effect occurs at 88 km. The limiting modeled electron density profile in the 83-91 km altitude range does not depend on the initial electron density.
NASA Astrophysics Data System (ADS)
Ryu, K.; Lee, E.; Chae, J. S.; Parrot, M.; Pulinets, S.
2014-10-01
We report the processes and results of statistical analysis on the ionospheric electron density data measured by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite over a period of 6 years (2005-2010), in order to investigate the correlation between seismic activity and equatorial plasma density variations. To simplify the analysis, three equatorial regions with frequent earthquakes were selected and then one-dimensional time series analysis between the daily seismic activity indices and the equatorial ionization anomaly (EIA) intensity indices, which represent relative equatorial electron density increase, were performed for each region. The statistically significant values of the lagged cross-correlation function, particularly in the region with minimal effects of longitudinal asymmetry, indicate that some of the very large earthquakes with M > 5.0 in the low-latitude region can accompany observable precursory and concurrent EIA enhancements, even though the seismic activity is not the most significant driver of the equatorial ionospheric evolution. The physical mechanisms of the seismo-ionospheric coupling is consistent with our observation, and the possibility of earthquake prediction using the EIA intensity variation is discussed.
Ionospheric E-region electron density and neutral atmosphere variations
NASA Technical Reports Server (NTRS)
Stick, T. L.
1976-01-01
Electron density deviations from a basic variation with the solar zenith angle were investigated. A model study was conducted in which the effects of changes in neutral and relative densities of atomic and molecular oxygen on calculated electron densities were compared with incoherent scatter measurements in the height range 100-117 km at Arecibo, Puerto Rico. The feasibility of determining tides in the neutral atmosphere from electron density profiles was studied. It was determined that variations in phase between the density and temperature variation and the comparable magnitudes of their components make it appear improbable that the useful information on tidal modes can be obtained in this way.
Probing Electron Dynamics with the Laplacian of the Momentum Density
Sukumar, N.; MacDougall, Preston J.; Levit, M. Creon
2012-09-24
This chapter in the above-titled monograph presents topological analysis of the Laplacian of the electron momentum density in organic molecules. It relates topological features in this distribution to chemical and physical properties, particularly aromaticity and electron transport.
NASA Astrophysics Data System (ADS)
Kumar, Arbind; S. Roy, P. N.; Das, L. K.
2016-07-01
Power spectrum analysis of Complete Bouguer Anomaly (CBA) map of Eastern Ghat Mobile Belt (EGMB) and its surroundings in India through Two Dimensional (2D) spectral analysis has provided estimates of the ensemble average depths for the density discontinuities which represent crustal inhomogeneities. The spectral analysis method has helped to estimate the depths of a perturbing body sources which are obtained from the negative slopes of the linear relationship between the logarithmic power spectrum and the wave-numbers of the gravity field. The detailed analysis reveals three horizontal discontinuities (i) Phanerozoic sediment thickness (ii) Basement depth and (iii) Conrad discontinuity. The average thickness of Phanerozoic sediments is estimated to be 3 km whereas depth of basement and Conrad discontinuity are at 7 km and 14.5 km respectively. Additionally Mohorovicic discontinuity also estimated at a depth of 32.8 km in the study region.
NASA Astrophysics Data System (ADS)
Fullam, Jennifer; Boye, Carol; Standaert, Theodorus; Gaudiello, John; Tomlinson, Derek; Xiao, Hong; Fang, Wei; Zhang, Xu; Wang, Fei; Ma, Long; Zhao, Yan; Jau, Jack
2011-03-01
In this paper, we tested a novel methodology of measuring critical dimension (CD) uniformity, or CDU, with electron beam (e-beam) hotspot inspection and measurement systems developed by Hermes Microvision, Inc. (HMI). The systems were used to take images of two-dimensional (2D) array patterns and measure CDU values in a custom designated fashion. Because this methodology combined imaging of scanning micro scope (SEM) and CD value averaging over a large array pattern of optical CD, or OCD, it can measure CDU of 2D arrays with high accuracy, high repeatability and high throughput.
NASA Technical Reports Server (NTRS)
Huang, Gwo-Jong; Bringi, V. N.; Moisseev, Dmitri; Petersen, Walter A.; Bliven, Francis L.; Hudak, David
2014-01-01
The application of the 2D-video disdrometer to measure fall speed and snow size distribution and to derive liquid equivalent snow rate, mean density-size and reflectivity-snow rate power law is described. Inversion of the methodology proposed by Böhm provides the pathway to use measured fall speed, area ratio and '3D' size measurement to estimate the mass of each particle. Four snow cases from the Light Precipitation Validation Experiment are analyzed with supporting data from other instruments such as Precipitation Occurrence Sensor System (POSS), Snow Video Imager (SVI), a network of seven snow gauges and three scanning C9 band radars. The radar-based snow accumulations using the 2DVD-derived Ze-SR relation are in good agreement with a network of seven snow gauges and outperform the accumulations derived from a climatological Ze-SR relation used by the Finnish Meteorological Institute (FMI). The normalized bias between radar-derived and gauge accumulation is reduced from 96% when using the fixed FMI relation to 28% when using the Ze-SR relations based on 2DVD data. The normalized standard error is also reduced significantly from 66% to 31%. For two of the days with widely different coefficients of the Ze-SR power law, the reflectivity structure showed significant differences in spatial variability. Liquid water path estimates from radiometric data also showed significant differences between the two cases. Examination of SVI particle images at the measurement site corroborated these differences in terms of unrimed versus rimed snow particles. The findings reported herein support the application of Böhm's methodology for deriving the mean density-size and Ze-SR power laws using data from 2D-video disdrometer.
NASA Astrophysics Data System (ADS)
Huang, Gwo-Jong; Bringi, V. N.; Moisseev, Dmitri; Petersen, W. A.; Bliven, L.; Hudak, David
2015-02-01
The application of the 2D-video disdrometer to measure fall speed and snow size distribution and to derive liquid equivalent snow rate, mean density-size and reflectivity-snow rate power law is described. Inversion of the methodology proposed by Böhm provides the pathway to use measured fall speed, area ratio and '3D' size measurement to estimate the mass of each particle. Four snow cases from the Light Precipitation Validation Experiment are analyzed with supporting data from other instruments such as the Precipitation Occurrence Sensor System (POSS), Snow Video Imager (SVI), a network of seven snow gauges and three scanning C-band radars. The radar-based snow accumulations using the 2DVD-derived Ze-SR relation are in good agreement with a network of seven snow gauges and outperform the accumulations derived from a climatological Ze-SR relation used by the Finnish Meteorological Institute (FMI). The normalized bias between radar-derived and gauge accumulation is reduced from 96% when using the fixed FMI relation to 28% when using the Ze-SR relations based on 2DVD data. The normalized standard error is also reduced significantly from 66% to 31%. For two of the days with widely different coefficients of the Ze-SR power law, the reflectivity structure showed significant differences in spatial variability. Liquid water path estimates from radiometric data also showed significant differences between the two cases. Examination of SVI particle images at the measurement site corroborated these differences in terms of unrimed versus rimed snow particles. The findings reported herein support the application of Böhm's methodology for deriving the mean density-size and Ze-SR power laws using data from 2D-video disdrometer.
Momentum-space properties from coordinate-space electron density
Harbola, Manoj K.; Zope, Rajendra R.; Kshirsagar, Anjali; Pathak, Rajeev K.
2005-05-22
Electron density and electron momentum density, while independently tractable experimentally, bear no direct connection without going through the many-electron wave function. However, invoking a variant of the constrained-search formulation of density-functional theory, we develop a general scheme (valid for arbitrary external potentials) yielding decent momentum-space properties, starting exclusively from the coordinate-space electron density. A numerical illustration of the scheme is provided for the closed-shell atomic systems He, Be, and Ne in their ground state and for 1s{sup 1} 2s{sup 1} singlet electronic excited state for helium by calculating the Compton profiles and the
expectation values derived from given coordinate-space electron densities.
Electronic Transport Properties of New 2-D Materials GeH and NaSn2As2
NASA Astrophysics Data System (ADS)
He, Bin; Cultrara, Nicholas; Arguilla, Maxx; Goldberger, Joshua; Heremans, Joseph
2-D materials potentially have superior thermoelectric properties compared to traditional 3-D materials due to their layered structure. Here we present electrical and thermoelectric transport properties of 2 types of 2-D materials, GeH and NaSn2As2. GeH is a graphane analog which is prepared using chemical exfoliation of CaGe2 crystals. Intrinsic GeH is proven to be a highly resistive material at room temperature. Resistance and Seebeck coefficient of Ga doped GeH are measured in a cryostat with a gating voltage varying from -100V to 100V. NaSn2As2 is another 2-D system, with Na atom embedded between nearly-2D Sn-As layers. Unlike GeH, NaSn2As2 is a metal based of Hall measurements, with p-type behavior, and with van der Pauw resistances on the order of 5m Ω/square. Thermoelectric transport properties of NaSn2As2 will be reported. This work is support by the NSF EFRI-2DARE project EFRI-1433467.
Recent electron temperature and density results from the ATF Thomson scattering system
Rasmussen, D.A.; England, A.C.; Murakami, M.; Howe, H.C.; Clark, T.L.; Kindsfather, R.R.; Rayburn, T.M.; Stewart, K.A.; Rogers, P.S.; Bell, G.L.
1989-01-01
A spatial multipoint Thomson scattering system has been developed for the Advanced Toroidal Facility (ATF) torsatron. The system measures temperature and density at 15 vertical locations on a vertical chord for each laser shot (one per plasma discharge). By remotely relocating the laser beam and reconfiguring the viewing optics during a series of ATF discharges, a two-dimensional (2-D) electron temperature and density map of the plasma cross section can be obtained. Results obtained with this system during ATF operation in 1988 and early 1989 are presented. 7 refs., 6 figs.
NASA Astrophysics Data System (ADS)
Payan, A. P.; Rajendar, A.; Paty, C. S.; Bonfond, B.; Crary, F.
2012-12-01
Io is the primary source of plasma in the Jovian magnetosphere, continuously releasing approximately 1 ton/s of SO2 from volcanic eruptions. The interaction of Io with Jupiter's magnetosphere is strongly influenced by the density structure of the resulting plasma torus and the position of Io relative to the center of the torus [Bonfond et al. 2008]. This unusual interaction produces a complex auroral feature on Jupiter's ionosphere known as the Io footprint. Hubble Space Telescope (HST) observations of Jupiter's far-UV aurora during spring 2007 showed an increased number of isolated auroral blobs along with a continuous expansion of Jupiter's main auroral oval over a few months. These blobs were associated with several large injections of hot plasma between 9 and 27 Jovian radii. These events coincided with a large volcanic eruption of the Tvashtar Paterae on Io, as observed by the New Horizons spacecraft [Spencer et al., 2007]. This, in turn, may have resulted in a significant increase in the plasma torus density. Besides, on June 7th, 2007, the Io footprint momentarily became so faint that it disappeared under a diffuse patch of emission remaining from an injection blob [Bonfond et al., 2012]. The goal of the present study is to examine the relationship between the increased density of the plasma torus and the dimming of the Io footprint. We implement a 2D model of the Io plasma torus that treats the variable-density torus as being composed of discrete layers of uniform density. As the co-rotating plasma in the plasma torus impinges on Io, Alfvén waves are launched at a pushback angle obtained from Gurnett and Goertz [1981]. The waves propagate inside the plasma torus through reflection and refraction at density discontinuities where they lose some of their initial energy. Using the above model, we can track the Alfvén wave fronts in the plasma torus and determine the longitude at which they exit the torus along with the corresponding remaining energy. Since
Ligand identification using electron-density mapcorrelations
Terwilliger, Thomas C.; Adams, Paul D.; Moriarty, Nigel W.; Cohn,Judith D.
2006-12-01
A procedure for the identification of ligands bound incrystal structuresof macromolecules is described. Two characteristics ofthe density corresponding to a ligand are used in the identificationprocedure. One is the correlation of the ligand density with each of aset of test ligands after optimization of the fit of that ligand to thedensity. The other is the correlation of a fingerprint of the densitywith the fingerprint of model density for each possible ligand. Thefingerprints consist of an ordered list of correlations of each the testligands with the density. The two characteristics are scored using aZ-score approach in which the correlations are normalized to the mean andstandard deviation of correlations found for a variety of mismatchedligand-density pairs, so that the Z scores are related to the probabilityof observing a particular value of the correlation by chance. Theprocedure was tested with a set of 200 of the most commonly found ligandsin the Protein Data Bank, collectively representing 57 percent of allligands in the Protein Data Bank. Using a combination of these twocharacteristics of ligand density, ranked lists of ligand identificationswere made for representative (F-o-F-c) exp(i phi(c)) difference densityfrom entries in the Protein Data Bank. In 48 percent of the 200 cases,the correct ligand was at the top of the ranked list of ligands. Thisapproach may be useful in identification of unknown ligands in newmacromolecular structures as well as in the identification of whichligands in a mixture have bound to a macromolecule.
Entin, M. V.; Magarill, L. I.; Olshanetsky, E. B. Kvon, Z. D.; Mikhailov, N. N.; Dvoretsky, S. A.
2013-11-15
The influence of e-h scattering on the conductivity and magnetotransport of 2D semimetallic HgTe is studied both theoretically and experimentally. The presence of e-h scattering leads to the friction between electrons and holes resulting in a large temperature-dependent contribution to the transport coefficients. The coefficient of friction between electrons and holes is determined. The comparison of experimental data with the theory shows that the interaction between electrons and holes based on the long-range Coulomb potential strongly underestimates the e-h friction. The experimental results are in agreement with the model of strong short-range e-h interaction.
Litvinenko, I. A.; Lykov, V. A.
1997-04-15
The results of numerical simulation of fast electrons motion and generated electro-magnetic fields at the picosecond pulse laser interaction with flat target are presented. The calculations were performed with PM2D code, where relativistic equation of electron motion joint with Maxwell equations is solved by particle method in cells. The efficiency of fast electrons energy conversion to the transverse electromagnetic wave of picosecond duration can reach the value 10{sup -4} for the intensity of ultrashort laser pulse at the target 10{sup 16}-10{sup 17} W/cm{sup 2}.
NASA Astrophysics Data System (ADS)
Kontrym-Sznajd, G.; Samsel-Czekała, M.; Biasini, M.; Kubo, Y.
2004-09-01
A new method for filtering three-dimensional reconstructed densities is proposed. The algorithm is tested with simulated spectra and employed to study the electronic structure of the rare-earth compound LaB6 . For this system, momentum densities are reconstructed from theoretical and experimental two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) spectra. The experimental results are in good agreement with the band structure calculated with the full-potential linearized augmented-plane-wave (FLAPW) method within the local-density approximation (LDA), apart from the detection of small electron pockets in the 15th band. It is also shown that, unlike the electron-positron enhancement, the electron-electron correlations affect noticeably the momentum density.
Electron density depletions in the nightside auroral zone
NASA Technical Reports Server (NTRS)
Persoon, A. M.; Gurnett, D. A.; Peterson, W. K.; Waite, J. H., Jr.; Burch, J. L.; Green, J. L.
1988-01-01
Dynamics Explorer 1 measurements are used to investigate regions of low electron density in the nightside auroral zone. Sharply defined regions of low electron density are found in auroral zone crossings from the predusk hours until the early morning hours at all radial distances up to at least 4.6 earth radii. Densities in the auroral cavity are shown to fall to values below 0.3/cu cm. Within the auroral cavity, electron-density-profile variations of a factor of 2 or more on spatial scales of tens of kilometers are found, and the electron plasma frequency to electron cyclotron frequency ratios are 0.02-0.4. The results suggest associations between the density depletions in the nightside auroral zone and auroral acceleration processes.
NASA Technical Reports Server (NTRS)
Considine, David B.; Douglass, Anne R.
1994-01-01
A parameterization of NAT (nitric acid trihydrate) clouds is developed for use in 2D models of the stratosphere. The parameterization uses model distributions of HNO3 and H2O to determine critical temperatures for NAT formation as a function of latitude and pressure. National Meteorological Center temperature fields are then used to determine monthly temperature frequency distributions, also as a function of latitude and pressure. The fractions of these distributions which fall below the critical temperatures for NAT formation are then used to determine the NAT cloud surface area density for each location in the model grid. By specifying heterogeneous reaction rates as functions of the surface area density, it is then possible to assess the effects of the NAT clouds on model constituent distributions. We also consider the increase in the NAT cloud formation in the presence of a fleet of stratospheric aircraft. The stratospheric aircraft NO(x) and H2O perturbations result in increased HNO3 as well as H2O. This increases the probability of NAT formation substantially, especially if it is assumed that the aircraft perturbations are confined to a corridor region.
Electron and ion densities in interstellar clouds
NASA Technical Reports Server (NTRS)
Glassgold, A. E.; Langer, W. D.
1974-01-01
A quantitative theory of ionization in diffuse clouds is developed which includes H(+) charge exchange with O. Dissociative charge exchange of He(+) with H2 plays an important role in the densities of H(+) and He(+). The abundance of HD is also discussed.
NASA Astrophysics Data System (ADS)
Wang, Z.; McKeown Walker, S.; Tamai, A.; Wang, Y.; Ristic, Z.; Bruno, F. Y.; de la Torre, A.; Riccò, S.; Plumb, N. C.; Shi, M.; Hlawenka, P.; Sánchez-Barriga, J.; Varykhalov, A.; Kim, T. K.; Hoesch, M.; King, P. D. C.; Meevasana, W.; Diebold, U.; Mesot, J.; Moritz, B.; Devereaux, T. P.; Radovic, M.; Baumberger, F.
2016-08-01
Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3 interface, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs.
Wang, Z; McKeown Walker, S; Tamai, A; Wang, Y; Ristic, Z; Bruno, F Y; de la Torre, A; Riccò, S; Plumb, N C; Shi, M; Hlawenka, P; Sánchez-Barriga, J; Varykhalov, A; Kim, T K; Hoesch, M; King, P D C; Meevasana, W; Diebold, U; Mesot, J; Moritz, B; Devereaux, T P; Radovic, M; Baumberger, F
2016-08-01
Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3 interface, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs. PMID:27064529
Sokolowsky, Kathleen P; Bailey, Heather E; Hoffman, David J; Andersen, Hans C; Fayer, Michael D
2016-07-21
Two-dimensional infrared (2D IR) data are presented for a vibrational probe in three nematogens: 4-cyano-4'-pentylbiphenyl, 4-cyano-4'-octylbiphenyl, and 4-(trans-4-amylcyclohexyl)-benzonitrile. The spectral diffusion time constants in all three liquids in the isotropic phase are proportional to [T*/(T - T*)](1/2), where T* is 0.5-1 K below the isotropic-nematic phase transition temperature (TNI). Rescaling to a reduced temperature shows that the decays of the frequency-frequency correlation function (FFCF) for all three nematogens fall on the same curve, suggesting a universal dynamic behavior of nematogens above TNI. Spectral diffusion is complete before significant orientational relaxation in the liquid, as measured by optically heterodyne detected-optical Kerr effect (OHD-OKE) spectroscopy, and before any significant orientational randomization of the probe measured by polarization selective IR pump-probe experiments. To interpret the OHD-OKE and FFCF data, we constructed a mode coupling theory (MCT) schematic model for the relationships among three correlation functions: ϕ1, a correlator for large wave vector density fluctuations; ϕ2, the orientational correlation function whose time derivative is the observable in the OHD-OKE experiment; and ϕ3, the FFCF for the 2D IR experiment. The equations for ϕ1 and ϕ2 match those in the previous MCT schematic model for nematogens, and ϕ3 is coupled to the first two correlators in a straightforward manner. Resulting models fit the data very well. Across liquid crystals, the temperature dependences of the coupling constants show consistent, nonmonotonic behavior. A remarkable change in coupling occurs at ∼5 K above TNI, precisely where the rate of spectral diffusion in 5CB was observed to deviate from that of a similar nonmesogenic liquid. PMID:27363680
Gutzwiller density functional theory for correlated electron systems
Ho, K. M.; Schmalian, J.; Wang, C. Z.
2008-02-04
We develop a density functional theory (DFT) and formalism for correlated electron systems by taking as reference an interacting electron system that has a ground state wave function which exactly obeys the Gutzwiller approximation for all one-particle operators. The solution of the many-electron problem is mapped onto the self-consistent solution of a set of single-particle Schroedinger equations, analogously to standard DFT-local density approximation calculations.
Electron-positron momentum density in TTF-TCNQ
NASA Astrophysics Data System (ADS)
Ishibashi, Shoji; Manuel, Alfred A.; Hoffmann, Ludger; Bechgaard, Klaus
1997-01-01
We present measurements of the positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) in TTF-TCNQ. We report also theoretical simulations of the 2D-ACAR in which the electron wave functions were expressed as TTF or TCNQ molecular orbitals obtained from self-consistent quantum chemical calculations. The positron wave function was calculated taking the charge transfer from TTF to TCNQ as a parameter. The best agreement with the experiment is obtained for a charge transfer of 0.7 electrons from the TTF to the TCNQ molecules. This is larger than the value of 0.55 obtained from a study of the Kohn anomaly. We investigate also the shape and position of the Fermi surface and conclude that a simple planar Fermi surface is consistent with our measurements.
Measurement of electron density using reactance cutoff probe
NASA Astrophysics Data System (ADS)
You, K. H.; You, S. J.; Kim, D. W.; Na, B. K.; Seo, B. H.; Kim, J. H.; Seong, D. J.; Chang, H. Y.
2016-05-01
This paper proposes a new measurement method of electron density using the reactance spectrum of the plasma in the cutoff probe system instead of the transmission spectrum. The highly accurate reactance spectrum of the plasma-cutoff probe system, as expected from previous circuit simulations [Kim et al., Appl. Phys. Lett. 99, 131502 (2011)], was measured using the full two-port error correction and automatic port extension methods of the network analyzer. The electron density can be obtained from the analysis of the measured reactance spectrum, based on circuit modeling. According to the circuit simulation results, the reactance cutoff probe can measure the electron density more precisely than the previous cutoff probe at low densities or at higher pressure. The obtained results for the electron density are presented and discussed for a wide range of experimental conditions, and this method is compared with previous methods (a cutoff probe using the transmission spectrum and a single Langmuir probe).
Ionospheric Electron Density during Magnetically Active Times over Istanbul
NASA Astrophysics Data System (ADS)
Naz Erbaş, Bute; Kaymaz, Zerefsan; Ceren Moral, Aysegul; Emine Ceren Kalafatoglu Eyiguler, R. A..
2016-07-01
In this study, we analyze electron density variations over Istanbul using Dynasonde observations during the magnetically active times. In order to perform statistical analyses, we first determined magnetic storms and magnetospheric substorm intervals from October 2012 to October 2015 using Kyoto's magnetic index data. Corresponding ionospheric parameters, such as critical frequency of F2 region (foF2), maximum electron density height (hmF2), total electron density (TEC) etc. were retrieved from Dynasonde data base at Istanbul Technical University's Space Weather Laboratory. To understand the behavior of electron density during the magnetically active times, we remove the background quiet time variations first and then quantify the anomalies. In this presentation, we will report results from our preliminary analyses from the selected cases corresponding to the strong magnetic storms. Initial results show lower electron densities at noon times and higher electron densities in the late afternoon toward sunset times when compared to the electron densities of magnetically quiet times. We also compare the results with IRI and TIEGCM ionospheric models in order to understand the physical and dynamical causes of these variations. During the presentation we will also discuss the role of these changes during the magnetically active times on the GPS communications through ionosphere.
NASA Astrophysics Data System (ADS)
Alizadeh, Mohamad Mahdi; Schuh, Harald; Schmidt, Michael
2015-06-01
For space geodetic techniques, operating in microwave band, ionosphere is a dispersive medium; thus, signals traveling through this medium are in the first approximation, affected proportional to the inverse of the square of their frequencies. This effect allows gaining information about the parameters of the ionosphere in terms of total electron content (TEC) or the electron density (Ne). Making use of this phenomenon, space geodetic techniques have turned into a capable tool for studying the ionosphere in the last decades. Up to now, two-dimensional (2-D) models of Vertical TEC (VTEC) have been widely developed and used by different communities; however, due to the fact that these models provide information about the integral of the whole electron content along the vertical or slant raypath, these maps are not useful when information about the ionosphere at different altitude is required. This paper presents a recent study which aims at developing a global 3-D model of the electron density, using measurements from Global Navigation Satellite Systems and by applying the ray tracing technique to the upper atmosphere. The developed modeling approach represents the horizontal variations of the electron density, with two sets of spherical harmonic expansions of degree and order 15. The height dependency of the electron density is represented by a multilayered Chapman profile function for the bottomside and topside ionosphere, and an appropriate model for the plasmasphere. In addition to the geodetic applications of the developed models, within this study, the 3-D models of electron density can include geophysical parameters like maximum electron density and its corresponding height. High-resolution modeling of these parameters allows an improved geophysical interpretation, which is essential in all studies of the upper atmosphere, space weather, and for the solar-terrestrial environment.
NASA Astrophysics Data System (ADS)
Ernst, D.
2015-11-01
We present new experiments and nonlinear gyrokinetic simulations showing that density gradient driven TEM (DGTEM) turbulence dominates the inner core of H-Mode plasmas during strong electron heating. Thus α-heating may degrade inner core confinement in H-Mode plasmas with moderate density peaking. These DIII-D low torque quiescent H-mode experiments were designed to study DGTEM turbulence. Gyrokinetic simulations using GYRO (and GENE) closely match not only particle, energy, and momentum fluxes, but also density fluctuation spectra, with and without ECH. Adding 3.4 MW ECH doubles Te /Ti from 0.5 to 1.0, which halves the linear TEM critical density gradient, locally flattening the density profile. Density fluctuations from Doppler backscattering (DBS) intensify near ρ = 0.3 during ECH, displaying a band of coherent fluctuations with adjacent toroidal mode numbers. GYRO closely reproduces the DBS spectrum and its change in shape and intensity with ECH, identifying these as coherent TEMs. Prior to ECH, parallel flow shear lowers the effective nonlinear DGTEM critical density gradient 50%, but is negligible during ECH, when transport displays extreme stiffness in the density gradient. GS2 predictions show the DGTEM can be suppressed, to avoid degradation with electron heating, by broadening the current density profile to attain q0 >qmin > 1 . A related experiment in the same regime varied the electron temperature gradient in the outer half-radius (ρ ~ 0 . 65) using ECH, revealing spatially coherent 2D mode structures in the Te fluctuations measured by ECE imaging. Fourier analysis with modulated ECH finds a threshold in Te profile stiffness. Supported by the US DOE under DE-FC02-08ER54966 and DE-FC02-04ER54698.
Modification of ionospheric electron density by dust suspension
NASA Astrophysics Data System (ADS)
Srivastava, Sweta; Mishra, Rashmi; Singh Sodha, Mahendra
2016-05-01
On the basis of a dynamic analysis the effectiveness of dust suspension for the reduction and enhancement of electron density in the E-layer of the ionosphere has been investigated in this paper. The analysis is based on the modelling of the E-layer as the Chapman α layer (validated earlier); the electron/ion production function, arrived at by Chapman and effective electron temperature-dependent electron–ion recombination coefficients in agreement with observations have been used. The balance of the charge on the particles and the number/energy balance of the constituents have been taken into account. The following is the physics of the change in electron density in the ionosphere by the suspension of dust. First, the dust provides a source (emission) and sink (accretion) of electrons. Second, the dust emits photoelectrons with energies much higher than those of ambient electrons, which enhances the electron temperature, leading to a reduced electron–ion recombination coefficient, and thus to a higher electron density. An interplay of these processes and the natural processes of electron production/annihilation determines the electron density and temperature in the dust suspension in the ionosphere. The numerical results, corresponding to suspension of dust of silicate (high work function) and Cs coated bronze (low work function) in the E-layer at 105 \\text{km} are presented and discussed.
Southworth, S. H.; Kobrin, P. H.; Truesdale, C. M.; Lindle, D.; Owaki, S.; Shirley, D. A.
1980-12-01
Angular distributions of photoelectrons from the Xe 4d subshell, and N{sub 4,5}oo Auger electrons, have been measured using synchrotron radiation. The 4d asymmetry parameter exhibits strong oscillations with energy, in agreement with several theoretical calculations. The Auger electrons show large asymmetries due to alignment of Xe{sup +} by photoionization.
Picosecond imaging of low-density plasmas by electron deflectometry.
Centurion, M; Reckenthaeler, P; Krausz, F; Fill, E E
2009-02-15
We have imaged optical-field ionized plasmas with electron densities as low as 10(13) cm(-3) on a picosecond timescale using ultrashort electron pulses. Electric fields generated by the separation of charges are imprinted on a 20 keV probe electron pulse and reveal a cloud of electrons expanding away from a positively charged plasma core. Our method allows for a direct measurement of the electron energy required to escape the plasma and the total charge. Simulations reproduce the main features of the experiment and allow determination of the energy of the electrons. PMID:19373367
A determination of the current density in electron beams
NASA Technical Reports Server (NTRS)
Beil, R. J.
1982-01-01
Current gathering rotating probe techniques were used to examine the envelope shape and power density profile of electron beams used in electron beam welding devices. The electron power density contours which determine the shape of the weld vapor cavity, penetration, and local heat distribution were considered. A mathematical analysis consistent with a rotating probe technique necessary to determine the current density distribution (assumed symmetrically radial) in a cross-section of the beam is provided. An explanation of the experimental technique for obtaining data, a BASIC language computer program to determine the current density from the data, and a study indicating the level of confidence to be associated with results obtained are also provided. An example of the application of the analysis to some experimental electron beam data is included.
Wang, Tuo; Yang, Hui; Kubicki, James D; Hong, Mei
2016-06-13
The native cellulose of bacterial, algal, and animal origins has been well studied structurally using X-ray and neutron diffraction and solid-state NMR spectroscopy, and is known to consist of varying proportions of two allomorphs, Iα and Iβ, which differ in hydrogen bonding, chain packing, and local conformation. In comparison, cellulose structure in plant primary cell walls is much less understood because plant cellulose has lower crystallinity and extensive interactions with matrix polysaccharides. Here we have combined two-dimensional magic-angle-spinning (MAS) solid-state nuclear magnetic resonance (solid-state NMR) spectroscopy at high magnetic fields with density functional theory (DFT) calculations to obtain detailed information about the structural polymorphism and spatial distributions of plant primary-wall cellulose. 2D (13)C-(13)C correlation spectra of uniformly (13)C-labeled cell walls of several model plants resolved seven sets of cellulose chemical shifts. Among these, five sets (denoted a-e) belong to cellulose in the interior of the microfibril while two sets (f and g) can be assigned to surface cellulose. Importantly, most of the interior cellulose (13)C chemical shifts differ significantly from the (13)C chemical shifts of the Iα and Iβ allomorphs, indicating that plant primary-wall cellulose has different conformations, packing, and hydrogen bonding from celluloses of other organisms. 2D (13)C-(13)C correlation experiments with long mixing times and with water polarization transfer revealed the spatial distributions and matrix-polysaccharide interactions of these cellulose structures. Celluloses f and g are well mixed chains on the microfibril surface, celluloses a and b are interior chains that are in molecular contact with the surface chains, while cellulose c resides in the core of the microfibril, outside spin diffusion contact with the surface. Interestingly, cellulose d, whose chemical shifts differ most significantly from those of
NASA Astrophysics Data System (ADS)
Tierz, Pablo; Ramona Stefanescu, Elena; Sandri, Laura; Patra, Abani; Marzocchi, Warner; Sulpizio, Roberto
2014-05-01
Probabilistic hazard assessments of Pyroclastic Density Currents (PDCs) are of great interest for decision-making purposes. However, there is a limited number of published works available on this topic. Recent advances in computation and statistical methods are offering new opportunities beyond the classical Monte Carlo (MC) sampling which is known as a simple and robust method but it usually turns out to be slow and computationally intractable. In this work, Titan2D numerical simulator has been coupled to Polynomial Chaos Quadrature (PCQ) to propagate the simulator parametric uncertainty and compute VEI-based probabilistic hazard maps of dense PDCs formed as a result of column collapse at Vesuvius volcano, Italy. Due to the lack of knowledge about the exact conditions under which these PDCs will form, Probability Distribution Functions (PDFs) are assigned to the simulator input parameters (Bed Friction Angle and Volume) according to three VEI sizes. Uniform distributions were used for both parameters since there is insufficient information to assume that any value in the range is more likely that any other value. Reasonable (and compatible) ranges for both variables were constrained according to past eruptions at Vesuvius volcanic system. On the basis of reasoning above a number of quadrature points were taken within those ranges, which resulted in one execution of the TITAN2D code at each quadrature point. With a computational cost several orders of magnitude smaller than MC, exceedance probabilities for a given threshold of flow depth (and conditional to the occurrence of VEI3, VEI4 and VEI5 eruptions) were calculated using PCQ. Moreover, PCQ can be run at different threshold values of the same output variable (flow depth, speed, kinetic energy, …) and, therefore, it can serve to compute Exceedance Probability curves (aka hazard curves) at singular points inside the hazard domain, representing the most important and useful scientific input to quantitative risk
Electron Densities Near Io from Galileo Plasma Wave Observations
NASA Technical Reports Server (NTRS)
Gurnett, D. A.; Persoon, A. M.; Kurth, W. S.; Roux, A.; Bolton, S. J.
2001-01-01
This paper presents an overview of electron densities obtained near Io from the Galileo plasma wave instrument during the first four flybys of Io. These flybys were Io, which was a downstream wake pass that occurred on December 7, 1995; I24, which was an upstream pass that occurred on October 11, 1999; I25, which was a south polar pass that occurred on November 26, 1999; and I27, which was an upstream pass that occurred on February 22, 2000. Two methods were used to measure the electron density. The first was based on the frequency of upper hybrid resonance emissions, and the second was based on the low-frequency cutoff of electromagnetic radiation at the electron plasma frequency. For three of the flybys, Io, I25, and I27, large density enhancements were observed near the closest approach to Io. The peak electron densities ranged from 2.1 to 6.8 x 10(exp 4) per cubic centimeters. These densities are consistent with previous radio occultation measurements of Io's ionosphere. No density enhancement was observed during the I24 flyby, most likely because the spacecraft trajectory passed too far upstream to penetrate Io's ionosphere. During two of the flybys, I25 and I27, abrupt step-like changes were observed at the outer boundaries of the region of enhanced electron density. Comparisons with magnetic field models and energetic particle measurements show that the abrupt density steps occur as the spacecraft penetrated the boundary of the Io flux tube, with the region of high plasma density on the inside of the flux tube. Most likely the enhanced electron density within the Io flux tube is associated with magnetic field lines that are frozen to Io by the high conductivity of Io's atmosphere, thereby enhancing the escape of plasma along the magnetic field lines that pass through Io's ionosphere.
Electron density profile description in the international reference ionosphere
NASA Technical Reports Server (NTRS)
Rawer, K.; Bilitza, D.
1989-01-01
Problems encountered during efforts to reformulate the IRI description of the electron density profile are examined. Consideration is given to Booker's (1979) proposal that the unique, analytic profile functions should cover the entire ionospheric height range. The IRI topside model is reviewed and the electron density profile of the middle and lower ionosphere are discussed. Rawer's (1983) procedure for combining the topside, middle, and lower ionospheric profiles into one analytic profile is reviewed.
Electron density profile description in the international reference ionosphere
NASA Astrophysics Data System (ADS)
Rawer, K.; Bilitza, D.
1989-10-01
Problems encountered during efforts to reformulate the IRI description of the electron density profile are examined. Consideration is given to Booker's (1979) proposal that the unique, analytic profile functions should cover the entire ionospheric height range. The IRI topside model is reviewed and the electron density profile of the middle and lower ionosphere are discussed. Rawer's (1983) procedure for combining the topside, middle, and lower ionospheric profiles into one analytic profile is reviewed.
Large Area Synthesis of 2D Materials
NASA Astrophysics Data System (ADS)
Vogel, Eric
Transition metal dichalcogenides (TMDs) have generated significant interest for numerous applications including sensors, flexible electronics, heterostructures and optoelectronics due to their interesting, thickness-dependent properties. Despite recent progress, the synthesis of high-quality and highly uniform TMDs on a large scale is still a challenge. In this talk, synthesis routes for WSe2 and MoS2 that achieve monolayer thickness uniformity across large area substrates with electrical properties equivalent to geological crystals will be described. Controlled doping of 2D semiconductors is also critically required. However, methods established for conventional semiconductors, such as ion implantation, are not easily applicable to 2D materials because of their atomically thin structure. Redox-active molecular dopants will be demonstrated which provide large changes in carrier density and workfunction through the choice of dopant, treatment time, and the solution concentration. Finally, several applications of these large-area, uniform 2D materials will be described including heterostructures, biosensors and strain sensors.
NASA Astrophysics Data System (ADS)
Cappelli, Mark; Young, Chris; Cha, Eusnun; Fernandez, Eduardo; Stanford Plasma Physics Laboratory Collaboration; Eckerd College Collaboration
2015-09-01
We present a simple, zero-equation turbulence model for electron transport across the magnetic field of a plasma Hall thruster and integrate this model into 2-D hybrid particle-in-cell simulations of a 72 mm diameter laboratory thruster operating at 400 W. The turbulent transport model is based on the assumption that the primary means of electron energy dissipation is the turbulent eddy cascade in the electron fluid to smaller scales. Implementing the model into 2-D hybrid simulations is relatively straightforward and leverages the existing framework for solving the electron fluid equations. We find that the model captures the strong axial variation in the mobility seen in experiments. In particular, it predicts the existence of a strong transport barrier which anchors the region of plasma acceleration. The model also captures the time-averaged experimental discharge current and its fluctuations due to ionization instabilities. We observe quantitative agreement with recent laser induced fluorescence measurements of time-averaged xenon ion and neutral velocities along the channel centerline. This work was supported by the Air Force Office of Scientific Research.
Titanium trisulfide (TiS3): a 2D semiconductor with quasi-1D optical and electronic properties.
Island, Joshua O; Biele, Robert; Barawi, Mariam; Clamagirand, José M; Ares, José R; Sánchez, Carlos; van der Zant, Herre S J; Ferrer, Isabel J; D'Agosta, Roberto; Castellanos-Gomez, Andres
2016-01-01
We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance. PMID:26931161
Titanium trisulfide (TiS3): a 2D semiconductor with quasi-1D optical and electronic properties
Island, Joshua O.; Biele, Robert; Barawi, Mariam; Clamagirand, José M.; Ares, José R.; Sánchez, Carlos; van der Zant, Herre S. J.; Ferrer, Isabel J.; D’Agosta, Roberto; Castellanos-Gomez, Andres
2016-01-01
We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance. PMID:26931161
Titanium trisulfide (TiS3): a 2D semiconductor with quasi-1D optical and electronic properties
NASA Astrophysics Data System (ADS)
Island, Joshua O.; Biele, Robert; Barawi, Mariam; Clamagirand, José M.; Ares, José R.; Sánchez, Carlos; van der Zant, Herre S. J.; Ferrer, Isabel J.; D'Agosta, Roberto; Castellanos-Gomez, Andres
2016-03-01
We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance.
NASA Astrophysics Data System (ADS)
Valbuena, M. A.; Avila, J.; Pantin, V.; Drouard, S.; Guyot, H.; Asensio, M. C.
2006-05-01
Low dimensional (LD) metallic oxides have been a subject of continuous interest in the last two decades, mainly due to the electronic instabilities that they present at low temperatures. In particular, charge density waves (CDW) instabilities associated with a strong electron-phonon interaction have been found in Molybdenum metallic oxides such as KMo 6O 17 purple bronze. We report an angle resolved photoemission (ARPES) study from room temperature (RT) to T ˜40 K well below the Peierls transition temperature for this material, with CDW transition temperature TCDW ˜120 K. We have focused on photoemission spectra along ΓM high symmetry direction as well as photoemission measurements were taken as a function of temperature at one representative kF point in the Brillouin zone in order to look for the characteristic gap opening after the phase transition. We found out a pseudogap opening and a decrease in the density of states near the Fermi energy, EF, consistent with the partial removal of the nested portions of the Fermi surface (FS) at temperature below the CDW transition. In order to elucidate possible Fermi liquid (FL) or non-Fermi liquid (NFL) behaviour we have compared the ARPES data with that one reported on quasi-1D K 0.3MoO 3 blue bronze.
Fitting of Diverging Thermoelectric Power in a Strongly Interacting 2D Electron System of Si-MOSFETs
NASA Astrophysics Data System (ADS)
Kim, Hyun-Tak
2013-03-01
The diverging-effective mass (DEM) in a metallic system is evidence of strong correlation between fermions in strongly correlated systems. The identification of the DEM still remains to be revealed The effective mass, m* =mo/(1-ρ4) where ρ is band filling helps clarify the diverging thermoelectric power, S, measured in inhomogeneous Si-MOSFET systems. As a carrier density ns decreases, S increases rapidly This is regarded as the metal-insulator transition (MIT) near nc ~ 79x10-1cm-2, where nc is about 0.02% to nSi ~ 3.4x10-14cm-2 in Si. This can be solved in assuming that ρ = nc/ns increases as ns decreases. nc is an excited(doped) carrier density in the semiconductor induced by gate and can be also regarded as a metallic carrier density, that is, nc ≡ nseminon = nmetal. ns is given as ntot ≡ ns = nc + nseminon where nseminon is a carrier density in a nonmetallic phase. The carrier density measured by Hall effect is the sum of carriers both induced by gate field and generated by MIT. Moreover, a larger metallic phase is not made due to a conducting path in the field-effect structure after a metallic phase is formed. Thus, increasing ns indicates increasing nnon; this corresponds to an over-doping to increase inhomogeneity. It's fitting is given from S = (απ3 kB2T/3e)(1/EF)= (α 8π3kB2T/3h2)(m*/e*nc) =So(1/ ρ) (1/(1-ρ4)) , where e* = ρ e, ρ = nc/ns, T =0.8K, m* =mo/(1-ρ4), α = 0.6, and So = (α 8π3kB2T/3h2)(mo/enc) ~12.36 are used. The data S are closely fitted by m*
Evans, C. M. Krynski, Kamil; Streeter, Zachary; Findley, G. L.
2015-12-14
We present for the first time the quasi-free electron energy V{sub 0}(ρ) for H{sub 2}, D{sub 2}, and O{sub 2} from gas to liquid densities, on noncritical isotherms and on a near critical isotherm in each fluid. These data illustrate the ability of field enhanced photoemission (FEP) to determine V{sub 0}(ρ) accurately in strongly absorbing fluids (e.g., O{sub 2}) and fluids with extremely low critical temperatures (e.g., H{sub 2} and D{sub 2}). We also show that the isotropic local Wigner-Seitz model for V{sub 0}(ρ) — when coupled with thermodynamic data for the fluid — can yield optimized parameters for intermolecular potentials, as well as zero kinetic energy electron scattering lengths.
Voss, Clifford I.; Provost, A.M.
2002-01-01
SUTRA (Saturated-Unsaturated Transport) is a computer program that simulates fluid movement and the transport of either energy or dissolved substances in a subsurface environment. This upgraded version of SUTRA adds the capability for three-dimensional simulation to the former code (Voss, 1984), which allowed only two-dimensional simulation. The code employs a two- or three-dimensional finite-element and finite-difference method to approximate the governing equations that describe the two interdependent processes that are simulated: 1) fluid density-dependent saturated or unsaturated ground-water flow; and 2) either (a) transport of a solute in the ground water, in which the solute may be subject to: equilibrium adsorption on the porous matrix, and both first-order and zero-order production or decay; or (b) transport of thermal energy in the ground water and solid matrix of the aquifer. SUTRA may also be used to simulate simpler subsets of the above processes. A flow-direction-dependent dispersion process for anisotropic media is also provided by the code and is introduced in this report. As the primary calculated result, SUTRA provides fluid pressures and either solute concentrations or temperatures, as they vary with time, everywhere in the simulated subsurface system. SUTRA flow simulation may be employed for two-dimensional (2D) areal, cross sectional and three-dimensional (3D) modeling of saturated ground-water flow systems, and for cross sectional and 3D modeling of unsaturated zone flow. Solute-transport simulation using SUTRA may be employed to model natural or man-induced chemical-species transport including processes of solute sorption, production, and decay. For example, it may be applied to analyze ground-water contaminant transport problems and aquifer restoration designs. In addition, solute-transport simulation with SUTRA may be used for modeling of variable-density leachate movement, and for cross sectional modeling of saltwater intrusion in
Time resolved, 2-D hard X-ray imaging of relativistic electron-beam target interactions on ETA-II
Crist, C.E.; Sampayan, S.; Westenskow, G.; Caporaso, G.; Houck, T.; Weir, J.; Trimble, D.; Krogh, M.
1998-11-01
Advanced radiographic applications require a constant source size less than 1 mm. To study the time history of a relativistic electron beam as it interacts with a bremsstrahlung converter, one of the diagnostics they use is a multi-frame time-resolved hard x-ray camera. They are performing experiments on the ETA-II accelerator at Lawrence Livermore National Laboratory to investigate details of the electron beam/converter interactions. The camera they are using contains 6 time-resolved images, each image is a 5 ns frame. By starting each successive frame 10 ns after the previous frame, they create a 6-frame movie from the hard x-rays produced from the interaction of the 50-ns electron beam pulse.
Electron-positron momentum density in (TMTSF)2ClO4
NASA Astrophysics Data System (ADS)
Ishibashi, Shoji; Manuel, Alfred A.; Kohyama, Masanori; Tokumoto, Madoka; Anzai, Hiroyuki
1999-08-01
We have measured electron-positron momentum density in (TMTSF)2ClO4 by the positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique. Significant anisotropies are found in the momentum density reflecting the low dimensionality of the material. Ab initio plane-wave pseudopotential calculations have been also made to simulate the 2D-ACAR spectra. The calculated 2D-ACAR spectra are in good agreement with the experimental ones. In order to investigate the Fermi-surface structure, we constructed Lock-Crisp-West (LCW) remapped spectra. The resultant spectra show small steps at the expected Fermi-surface position for both the experiment and the calculation. The magnitude of the steps in the experimental LCW spectrum is one third of the calculated one but still one order of magnitude higher than the experimental detection limit. Although we cannot conclude the existence of an ideal Fermi surface due to the finite experimental resolution, we have confirmed, in the experiment, a structure corresponding to the calculated Fermi break.
Evidence of Electron Density Enhancements at Enceladus' Apoapsis
NASA Astrophysics Data System (ADS)
Persoon, A. M.; Gurnett, D. A.; Kurth, W. S.; Hospodarsky, G. B.; Groene, J. B.
2015-12-01
Enceladus' plumes are the dominant source of plasma in Saturn's magnetosphere. Icy particles and water vapor are vented into the inner magnetosphere through fissures in Enceladus' southern polar region. These fissures are subjected to tidal stresses that vary as Enceladus moves in a slightly eccentric orbit around Saturn. Plume activity is greatest when tidal stress is minimal. This occurs when Enceladus is farthest away from Saturn in its orbit (the Enceladus apoapsis). This study will show temporal variations in the electron density distribution that correlate with the position of Enceladus in its orbit around Saturn, with strong density enhancements in the vicinity of Enceladus when the moon is near apoapsis. Equatorial electron density measurements derived from the upper hybrid resonance frequency from the Cassini Radio and Plasma Wave Science (RPWS) experiment are used to illustrate these electron density enhancements.
Tomography of the ionospheric electron density with geostatistical inversion
NASA Astrophysics Data System (ADS)
Minkwitz, D.; van den Boogaart, K. G.; Gerzen, T.; Hoque, M.
2015-08-01
In relation to satellite applications like global navigation satellite systems (GNSS) and remote sensing, the electron density distribution of the ionosphere has significant influence on trans-ionospheric radio signal propagation. In this paper, we develop a novel ionospheric tomography approach providing the estimation of the electron density's spatial covariance and based on a best linear unbiased estimator of the 3-D electron density. Therefore a non-stationary and anisotropic covariance model is set up and its parameters are determined within a maximum-likelihood approach incorporating GNSS total electron content measurements and the NeQuick model as background. As a first assessment this 3-D simple kriging approach is applied to a part of Europe. We illustrate the estimated covariance model revealing the different correlation lengths in latitude and longitude direction and its non-stationarity. Furthermore, we show promising improvements of the reconstructed electron densities compared to the background model through the validation of the ionosondes Rome, Italy (RO041), and Dourbes, Belgium (DB049), with electron density profiles for 1 day.
Ao, L; Pham, A; Xiao, H Y; Zu, X T; Li, S
2016-03-14
We have systematically investigated the effects of different vacancy defects in 2D d(0) materials SnS2 and ZrS2 using first principles calculations. The theoretical results show that the single cation vacancy and the vacancy complex like V-SnS6 can induce large magnetic moments (3-4 μB) in these single layer materials. Other defects, such as V-SnS3, V-S, V-ZrS3 and V-ZrS6, can result in n-type conductivity. In addition, the ab initio studies also reveal that the magnetic and conductive properties from the cation vacancy and the defect complex V-SnS6 can be modified using the compressive/tensile strain of the in-plane lattices. Specifically, the V-Zr doped ZrS2 monolayer can be tuned from a ferromagnetic semiconductor to a metallic/half-metallic material with decreasing/increasing magnetic moments depending on the external compressive/tensile strains. On the other hand, the semiconducting and magnetic properties of V-Sn doped SnS2 is preserved under different lattice compression and tension. For the defect complex like V-SnS6, only the lattice compression can tune the magnetic moments in SnS2. As a result, by manipulating the fabrication parameters, the magnetic and conductive properties of SnS2 and ZrS2 can be tuned without the need for chemical doping. PMID:26888010
NASA Astrophysics Data System (ADS)
Samaraweera, Rasanga; Liu, Han-Chun; Wegscheider, Werner; Mani, Ramesh
Recent advancements in the growth techniques of the GaAs/AlGaAs two dimensional electron system (2DES) routinely yield high quality heterostructures with enhanced physical and electrical properties, including devices with 2D electron mobilities well above 107 cm2/Vs. These improvements have opened new pathways to study interesting physical phenomena associated with the 2D electron system. Negative giant-magnetoresistance (GMR) is one such phenomenon which can observed in the high mobility 2DES. However, the negative GMR in the GaAs/AlGaAs 2DES is still not fully understood. In this contribution, we present an experimental study of the bell-shape negative GMR in high mobility GaAs/AlGaAs devices and quantitatively analyze the results utilizing the multi-conduction model. The multi-conduction model includes interesting physical characteristics such as negative diagonal conductivity, non-vanishing off-diagonal conductivity, etc. The aim of the study is to examine GMR over a wider experimental parameter space and determine whether the multi-conduction model serves to describe the experimental results.
NASA Astrophysics Data System (ADS)
Kuzubov, A. A.; Eliseeva, N. S.; Krasnov, P. O.; Tomilin, F. N.; Fedorov, A. S.; Tolstaya, A. V.
2014-08-01
Silicon carbide is among the most common materials used in semiconductor engineering. Silicon carbide thin films are attractive from the standpoint of designing devices based on heterojunctions. This is due to a characteristic feature of this compound, such as polytypism, leading to the difference in the physical properties and also hampering the preparation of high-quality material samples. In this work, the thermodynamic stability and electronic structure of thin films based on the polytypes 3 C, 2 H, and 2 D with a thickness of a few nanometers have been studied.
Electron density and gas density measurements in a millimeter-wave discharge
NASA Astrophysics Data System (ADS)
Schaub, S. C.; Hummelt, J. S.; Guss, W. C.; Shapiro, M. A.; Temkin, R. J.
2016-08-01
Electron density and neutral gas density have been measured in a non-equilibrium air breakdown plasma using optical emission spectroscopy and two-dimensional laser interferometry, respectively. A plasma was created with a focused high frequency microwave beam in air. Experiments were run with 110 GHz and 124.5 GHz microwaves at powers up to 1.2 MW. Microwave pulses were 3 μs long at 110 GHz and 2.2 μs long at 124.5 GHz. Electron density was measured over a pressure range of 25 to 700 Torr as the input microwave power was varied. Electron density was found to be close to the critical density, where the collisional plasma frequency is equal to the microwave frequency, over the pressure range studied and to vary weakly with input power. Neutral gas density was measured over a pressure range from 150 to 750 Torr at power levels high above the threshold for initiating breakdown. The two-dimensional structure of the neutral gas density was resolved. Intense, localized heating was found to occur hundreds of nanoseconds after visible plasma formed. This heating led to neutral gas density reductions of greater than 80% where peak plasma densities occurred. Spatial structure and temporal dynamics of gas heating at atmospheric pressure were found to agree well with published numerical simulations.