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Sample records for electronic energy band

  1. Determining the band gap and mean kinetic energy of atoms from reflection electron energy loss spectra

    SciTech Connect

    Vos, M.; Marmitt, G. G.; Finkelstein, Y.; Moreh, R.

    2015-09-14

    Reflection electron energy loss spectra from some insulating materials (CaCO{sub 3}, Li{sub 2}CO{sub 3}, and SiO{sub 2}) taken at relatively high incoming electron energies (5–40 keV) are analyzed. Here, one is bulk sensitive and a well-defined onset of inelastic excitations is observed from which one can infer the value of the band gap. An estimate of the band gap was obtained by fitting the spectra with a procedure that includes the recoil shift and recoil broadening affecting these measurements. The width of the elastic peak is directly connected to the mean kinetic energy of the atom in the material (Doppler broadening). The experimentally obtained mean kinetic energies of the O, C, Li, Ca, and Si atoms are compared with the calculated ones, and good agreement is found, especially if the effect of multiple scattering is taken into account. It is demonstrated experimentally that the onset of the inelastic excitation is also affected by Doppler broadening. Aided by this understanding, we can obtain a good fit of the elastic peak and the onset of inelastic excitations. For SiO{sub 2}, good agreement is obtained with the well-established value of the band gap (8.9 eV) only if it is assumed that the intensity near the edge scales as (E − E{sub gap}){sup 1.5}. For CaCO{sub 3}, the band gap obtained here (7 eV) is about 1 eV larger than the previous experimental value, whereas the value for Li{sub 2}CO{sub 3} (7.5 eV) is the first experimental estimate.

  2. Vanishing electronic energy loss of very slow light ions in insulators with large band gaps.

    PubMed

    Markin, S N; Primetzhofer, D; Bauer, P

    2009-09-11

    Electronic energy loss of light ions in nanometer films of materials with large band gaps has been studied for very low velocities. For LiF, a threshold velocity is observed at 0.1 a.u. (250 eV/u), below which the ions move without transferring energy to the electronic system. For KCl, a lower (extrapolated) threshold velocity is found, identical for H and He ions. For SiO2, no clear velocity threshold is observed for He particles. For protons and deuterons, electronic stopping is found to perfectly fulfill velocity scaling, as expected for binary ion-electron interaction. PMID:19792368

  3. Vanishing Electronic Energy Loss of Very Slow Light Ions in Insulators with Large Band Gaps

    SciTech Connect

    Markin, S. N.; Primetzhofer, D.; Bauer, P.

    2009-09-11

    Electronic energy loss of light ions in nanometer films of materials with large band gaps has been studied for very low velocities. For LiF, a threshold velocity is observed at 0.1 a.u. (250 eV/u), below which the ions move without transferring energy to the electronic system. For KCl, a lower (extrapolated) threshold velocity is found, identical for H and He ions. For SiO{sub 2}, no clear velocity threshold is observed for He particles. For protons and deuterons, electronic stopping is found to perfectly fulfill velocity scaling, as expected for binary ion-electron interaction.

  4. Tunable electronic band structures and zero-energy modes of heterosubstrate-induced graphene superlattices

    NASA Astrophysics Data System (ADS)

    Fan, Xiong; Huang, Wenjun; Ma, Tianxing; Wang, Li-Gang

    2016-04-01

    We propose a tunable electronic band gap and zero-energy modes in periodic heterosubstrate-induced graphene superlattices. Interestingly, there is an approximate linear relation between the band gap and the proportion of an inhomogeneous substrate (i.e., percentages of different components) in the proposed superlattice, and the effect of structural disorder on the relation is discussed. In an inhomogeneous substrate with equal widths, zero-energy states emerge in the form of Dirac points by using asymmetric potentials, and the positions of Dirac points are addressed analytically. Further, the Dirac point exists at k =0 only for specific potentials; every time it appears, the group velocity vanishes in the ky direction, and the resonance occurs. For general cases of an inhomogeneous substrate with unequal widths, part of the zero-energy states are described analytically, and differently, they are not always Dirac points. Our prediction may be realized on a heterosubstrate such as SiO2/BN .

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

    DOE PAGESBeta

    Bhattacharjee, Anirban; Chavarot-Kerlidou, Murielle; Dempsey, Jillian L.; Gray, Harry B.; Fujita, Etsuko; Muckerman, James T.; Fontecave, Marc; Artero, Vincent; Arantes, Guilherme M.; Field, Martin J.

    2014-08-11

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

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

    SciTech Connect

    Bhattacharjee, Anirban; Chavarot-Kerlidou, Murielle; Dempsey, Jillian L.; Gray, Harry B.; Fujita, Etsuko; Muckerman, James T.; Fontecave, Marc; Artero, Vincent; Arantes, Guilherme M.; Field, Martin J.

    2014-08-11

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

  7. Theoretical Modeling of Low Energy Electronic Absorption Bands in Reduced Cobaloximes

    PubMed Central

    Bhattacharjee, Anirban; Chavarot-Kerlidou, Murielle; Dempsey, Jillian L.; Gray, Harry B.; Fujita, Etsuko; Muckerman, James T.; Fontecave, Marc; Artero, Vincent; Arantes, Guilherme M.; Field, Martin J.

    2015-01-01

    The reduced Co(I) states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we have analyzed the low energy electronic absorption bands of two cobaloxime systems experimentally and using a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task. PMID:25113847

  8. Role of Electronic Structure In Ion Band State Theory of Low Energy Nuclear Reactions

    NASA Astrophysics Data System (ADS)

    Chubb, Scott

    2004-03-01

    The Nuts and Bolts of our Ion Band State (IBS) theory of low energy nuclear reactions (LENR's) in palladium-deuteride (PdD) and palladium-hydride (PdH) are the electrons that hold together or tear apart the bonds (or lack of bonds) between deuterons (d's) or protons (p's) and the host material. In PdDx and PdH_x, this bonding is strongly correlated with loading: in ambient loading conditions (x< 0. 6), the bonding in hibits IBS occupation. As x arrow 1, slight increases and decreases in loading can lead to vibrations (which have conventionally been thought to occur from phonons) that can induce potential losses or increases of p/d. Naive assumptions about phonons fail to include these losses and increases. These effects can occur because neither H or D has core electrons and because in either PdD or PdH, the electrons near the Fermi Energy have negligible overlap with the nucleus of either D or H. I use these ideas to develop a formal justification, based on a generalization of conventional band theory (Scott Chubb, "Semi-Classical Conduction of Charged and Neutral Particles in Finite Lattices," 2004 March Meeting."), for the idea that occupation of IBS's can occur and that this can lead to nuclear reactions.

  9. Energy band alignment and electronic states of amorphous carbon surfaces in vacuo and in aqueous environment

    SciTech Connect

    Caro, Miguel A.; Määttä, Jukka; Lopez-Acevedo, Olga; Laurila, Tomi

    2015-01-21

    In this paper, we obtain the energy band positions of amorphous carbon (a–C) surfaces in vacuum and in aqueous environment. The calculations are performed using a combination of (i) classical molecular dynamics (MD), (ii) Kohn-Sham density functional theory with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, and (iii) the screened-exchange hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). PBE allows an accurate generation of a-C and the evaluation of the local electrostatic potential in the a-C/water system, HSE yields an improved description of energetic positions which is critical in this case, and classical MD enables a computationally affordable description of water. Our explicit calculation shows that, both in vacuo and in aqueous environment, the a-C electronic states available in the region comprised between the H{sub 2}/H{sub 2}O and O{sub 2}/H{sub 2}O levels of water correspond to both occupied and unoccupied states within the a-C pseudogap region. These are localized states associated to sp{sup 2} sites in a-C. The band realignment induces a shift of approximately 300 meV of the a-C energy band positions with respect to the redox levels of water.

  10. Energy band alignment and electronic states of amorphous carbon surfaces in vacuo and in aqueous environment

    NASA Astrophysics Data System (ADS)

    Caro, Miguel A.; Määttä, Jukka; Lopez-Acevedo, Olga; Laurila, Tomi

    2015-01-01

    In this paper, we obtain the energy band positions of amorphous carbon (a-C) surfaces in vacuum and in aqueous environment. The calculations are performed using a combination of (i) classical molecular dynamics (MD), (ii) Kohn-Sham density functional theory with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, and (iii) the screened-exchange hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). PBE allows an accurate generation of a-C and the evaluation of the local electrostatic potential in the a-C/water system, HSE yields an improved description of energetic positions which is critical in this case, and classical MD enables a computationally affordable description of water. Our explicit calculation shows that, both in vacuo and in aqueous environment, the a-C electronic states available in the region comprised between the H2/H2O and O2/H2O levels of water correspond to both occupied and unoccupied states within the a-C pseudogap region. These are localized states associated to sp2 sites in a-C. The band realignment induces a shift of approximately 300 meV of the a-C energy band positions with respect to the redox levels of water.

  11. Band gap and defect states of MgO thin films investigated using reflection electron energy loss spectroscopy

    SciTech Connect

    Heo, Sung; Cho, Eunseog; Lee, Hyung-Ik; Park, Gyeong Su; Kang, Hee Jae; Nagatomi, T.; Choi, Pyungho; Choi, Byoung-Deog

    2015-07-15

    The band gap and defect states of MgO thin films were investigated by using reflection electron energy loss spectroscopy (REELS) and high-energy resolution REELS (HR-REELS). HR-REELS with a primary electron energy of 0.3 keV revealed that the surface F center (FS) energy was located at approximately 4.2 eV above the valence band maximum (VBM) and the surface band gap width (E{sub g}{sup S}) was approximately 6.3 eV. The bulk F center (F{sub B}) energy was located approximately 4.9 eV above the VBM and the bulk band gap width was about 7.8 eV, when measured by REELS with 3 keV primary electrons. From a first-principles calculation, we confirmed that the 4.2 eV and 4.9 eV peaks were F{sub S} and F{sub B}, induced by oxygen vacancies. We also experimentally demonstrated that the HR-REELS peak height increases with increasing number of oxygen vacancies. Finally, we calculated the secondary electron emission yields (γ) for various noble gases. He and Ne were not influenced by the defect states owing to their higher ionization energies, but Ar, Kr, and Xe exhibited a stronger dependence on the defect states owing to their small ionization energies.

  12. Quest for band renormalization and self-energy in correlated f-electron systems

    SciTech Connect

    Durakiewicx, Tomasz

    2009-01-01

    Coexisting energy scales are observed in f-electron materials. Information about some of the low-energy scales is imprinted in the electron self-energy which can be measured by angle-resolved photoemission (ARPES). Such measurements in d-electron materials over the last decade were based on high energy- and momentum-resolution ARPES techniques used to extract the self-energy information from measured spectra. Simultaneously, many-body theoretical approaches have been developed to find a link between self-energy and many-body interactions. Here we show the transcription of such methods from d-electrons to f-electrons by presenting the first example of low energy scales in f-electron material USb{sub 2}, measured with synchrotron-based ARPES. Proposed approach will help in answering the fundamental questions about the complex nature of the heavy fermion state.

  13. Complete description of ionization energy and electron affinity in organic solids: Determining contributions from electronic polarization, energy band dispersion, and molecular orientation

    NASA Astrophysics Data System (ADS)

    Yoshida, Hiroyuki; Yamada, Kazuto; Tsutsumi, Jun'ya; Sato, Naoki

    2015-08-01

    Ionization energy and electron affinity in organic solids are understood in terms of a single molecule perturbed by solid-state effects such as polarization energy, band dispersion, and molecular orientation as primary factors. However, no work has been done to determine the individual contributions experimentally. In this work, the electron affinities of thin films of pentacene and perfluoropentacene with different molecular orientations are determined to a precision of 0.1 eV using low-energy inverse photoemission spectroscopy. Based on the precisely determined electron affinities in the solid state together with the corresponding data of the ionization energies and other energy parameters, we quantitatively evaluate the contribution of these effects. It turns out that the bandwidth as well as the polarization energy contributes to the ionization energy and electron affinity in the solid state while the effect of the surface dipole is at most a few eV and does not vary with the molecular orientation. As a result, we conclude that the molecular orientation dependence of the ionization energy and electron affinity of organic solids originates from the orientation-dependent polarization energy in the film.

  14. Electron currents associated with an auroral band

    NASA Technical Reports Server (NTRS)

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

    1975-01-01

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

  15. Isolated energy level in the band gap of Yb2Si2O7 identified by electron energy-loss spectroscopy

    NASA Astrophysics Data System (ADS)

    Ogawa, Takafumi; Kobayashi, Shunsuke; Wada, Masashi; Fisher, Craig A. J.; Kuwabara, Akihide; Kato, Takeharu; Yoshiya, Masato; Kitaoka, Satoshi; Moriwake, Hiroki

    2016-05-01

    We report the detection of an isolated energy level in the band gap of crystalline Yb2Si2O7 in the low-energy-loss region of its electron energy-loss (EEL) spectrum, obtained using a monochromated scanning transmission electron microscope. The experimental results are corroborated by first-principles calculations of the theoretical EEL spectrum. The calculations reveal that unoccupied Yb 4 f orbitals constitute an isolated energy level about 1 eV below the conduction band minimum (CBM), resulting in a terrace about 1 eV wide at the band edge of the EEL spectrum. In the case of Yb2O3 , no band edge terrace is present because the unoccupied f level lies just below the CBM. We also examined optical absorption properties of Yb2Si2O7 using UV-vis diffuse reflectance spectroscopy, which shows that the isolated energy level could not be detected in the band edge of the obtained absorbance spectrum. These findings demonstrate the utility of low-loss EEL spectroscopy with high energy resolution for probing semilocalized electronic features.

  16. First-principles determination of band-to-band electronic transition energies in cubic and hexagonal AlGaInN alloys

    NASA Astrophysics Data System (ADS)

    Freitas, F. L.; Marques, M.; Teles, L. K.

    2016-08-01

    We provide approximate quasiparticle-corrected band gap energies for quaternary cubic and hexagonal AlxGayIn1-x-yN semiconductor alloys, employing a cluster expansion method to account for the inherent statistical disorder of the system. Calculated values are compared with photoluminescence measurements and discussed within the currently accepted model of emission in these materials by carrier localization. It is shown that bowing parameters are larger in the cubic phase, while the range of band gap variation is bigger in the hexagonal one. Experimentally determined transition energies are mostly consistent with band-to-band excitations.

  17. Cyclotron side band emissions from magnetospheric electrons

    NASA Technical Reports Server (NTRS)

    Maeda, K.

    1975-01-01

    Very low frequency emissions with subharmonic cyclotron frequency from magnetospheric electrons were detected by the S(3)-A satellite (Explorer 45) whose orbit is close to the magnetic equatorial plane where the wave-particle interaction is most efficient. These emissions were observed during the main phase of a geomagnetic storm in the nightside of the magnetosphere outside of the plasmasphere. During the event of these side-band emissions, the pitch angle distributions of high energy electrons (greater than 50 keV) and of energetic protons (greater than 100 keV) showed remarkable changes with time, whereas those of low energy electrons and protons remained approximately isotropic. In this type of event, emissions consist essentially of two bands, the one below the equatorial electron gyrofrequency, and the other above. The emissions below are whistler mode, and the emissions above are electrostatic mode.

  18. Calculation of the Energy-Band Structure of the Kronig-Penney Model Using the Nearly-Free and Tightly-Bound-Electron Approximations

    ERIC Educational Resources Information Center

    Wetsel, Grover C., Jr.

    1978-01-01

    Calculates the energy-band structure of noninteracting electrons in a one-dimensional crystal using exact and approximate methods for a rectangular-well atomic potential. A comparison of the two solutions as a function of potential-well depth and ratio of lattice spacing to well width is presented. (Author/GA)

  19. A shock-tube determination of the CN ground state dissociation energy and electronic transition moments for the CN violet and red band systems

    NASA Technical Reports Server (NTRS)

    Arnold, J. O.; Nicholls, R. W.

    1973-01-01

    The CN ground state dissociation energy and the sum of squares of the electronic transition moments of the CN violet bands have been simultaneously determined from spectral emission measurements behind incident shock waves. The unshocked test gases were composed of various CO2-CO-N2-Ar mixtures, and the temperatures behind the incident shocks ranged from 3500 to 8000 K. The variation of the electronic transition moment with internuclear separation was found to be small for both the CN violet and red band systems.

  20. Engineering the Electronic Band Structure for Multiband Solar Cells

    SciTech Connect

    Lopez, N.; Reichertz, L.A.; Yu, K.M.; Campman, K.; Walukiewicz, W.

    2010-07-12

    Using the unique features of the electronic band structure of GaNxAs1-x alloys, we have designed, fabricated and tested a multiband photovoltaic device. The device demonstrates an optical activity of three energy bands that absorb, and convert into electrical current, the crucial part of the solar spectrum. The performance of the device and measurements of electroluminescence, quantum efficiency and photomodulated reflectivity are analyzed in terms of the Band Anticrossing model of the electronic structure of highly mismatched alloys. The results demonstrate the feasibility of using highly mismatched alloys to engineer the semiconductor energy band structure for specific device applications.

  1. Banded Electron Structure Formation in the Inner Magnetosphere

    NASA Technical Reports Server (NTRS)

    Liemohn, M. W.; Khazanov, G. V.

    1997-01-01

    Banded electron structures in energy-time spectrograms have been observed in the inner magnetosphere concurrent with a sudden relaxation of geomagnetic activity. In this study, the formation of these banded structures is considered with a global, bounce-averaged model of electron transport, and it is concluded that this structure is a natural occurrence when plasma sheet electrons are captured on closed drift paths near the Earth. These bands do not appear unless there is capture of plasma sheet electrons; convection along open drift paths making open pass around the Earth do not have time to develop this feature. The separation of high-energy bands from the injection population due to the preferential advection of the gradient-curvature drift creates spikes in the energy distribution, which overlap to form a series of bands in the energy spectrograms. The lowest band is the bulk of the injected population in the sub-key energy range. Using the Kp history for an observed banded structure event, a cloud of plasma sheet electrons is captured and the development of their distribution function is examined and discussed.

  2. Self-consistent energy bands in aluminum and electronic surface states and resonances on the (001) surface

    NASA Astrophysics Data System (ADS)

    Seel, M.

    1983-07-01

    The band structure of aluminum has been calculated self-consistently with the use of the Kohn-Sham-Gaspar local exchange potential and the linear combination of Gaussian orbitals method. The resulting band structure, using a basis set of 28 contracted Gaussian-type orbitals, is in excellent agreement with the previous work of Singhal and Callaway in which a basis set of 60 uncontracted Gaussian functions was used. After projection of the bulk bands onto the two-dimensional (001) surface Brillouin zone, surface states and resonances have been calculated along the Δ¯ line with the use of the Green's-function formalism. At point Γ¯, the surface state is located 2.92 eV below EF, in excellent agreement with the experimental result, 2.8+/-0.2 eV below EF. In addition, a resonance is found 0.7 eV below EF with a half-width of 0.4 eV, hitherto interpreted only as Fermi edge intensity. From the midpoint of the Δ¯ axis moving onward up to the Fermi energy, the observed main peak is attributed to surface resonances.

  3. Optical constants and related electronic energy bands of lithium triborate crystal in the 6{endash}12-eV region

    SciTech Connect

    Chen, T.; Tao, R.; Rife, J.C.; Hunter, W.R.

    1998-01-01

    Reflectance of lithium triborate crystal in the 6{endash}12-eV region is measured with synchrotron radiation, and the principal values of optical constants are derived. Six absorption peaks are found in both X- and Y-polarized spectra. Among them the five transitions of the Y-polarized peaks and three of the X-polarized peaks are identified. The measurement suggests that the energy of the lowest conducting band 4A{sub 2} is 7.3 eV rather than 7.57 eV, as previously reported. {copyright} 1998 Optical Society of America

  4. Evidence for Temperature-Dependent Electron Band Dispersion in Pentacene

    SciTech Connect

    Koch,N.; Vollmer, A.; Salzmann, I.; Nickel, B.; Weiss, H.; Rabe, J.

    2006-01-01

    Evidence for temperature-dependent electron band dispersion in a pentacene thin film polymorph on graphite is provided by angle- and energy-dependent ultraviolet photoelectron spectroscopy. The bands derived from the highest occupied molecular orbital exhibit dispersion of {approx}190 meV at room temperature, and {approx}240 meV at 120 K. Intermolecular electronic coupling in pentacene thin films is thus confirmed to be dependent on temperature and possibly crystal structure, as suggested by additional infrared absorption measurements.

  5. Density dependence of the conduction-band energy {ital V}{sub 0} of excess electrons in fluid xenon

    SciTech Connect

    Frongillo, Y.; Plenkiewicz, B.; Jay-Gerin, J.

    1996-05-01

    The ground-state energy {ital V}{sub 0} (relative to vacuum) of quasifree excess electrons in fluid xenon is calculated as a function of fluid density {ital n}. The calculations are performed within the framework of the Wigner-Seitz mean-field approximation for nonpolar fluids, using an accurate atomic pseudopotential to model the excess electron-xenon interaction. The calculated values of {ital V}{sub 0}({ital n}) are compared to experimental data and with other theoretical results. {copyright} {ital 1996 The American Physical Society.}

  6. Probing optical band gaps at the nanoscale in NiFe₂O₄ and CoFe₂O₄ epitaxial films by high resolution electron energy loss spectroscopy

    SciTech Connect

    Dileep, K.; Loukya, B.; Datta, R.; Pachauri, N.; Gupta, A.

    2014-09-14

    Nanoscale optical band gap variations in epitaxial thin films of two different spinel ferrites, i.e., NiFe₂O₄ (NFO) and CoFe₂O₄ (CFO), have been investigated by spatially resolved high resolution electron energy loss spectroscopy. Experimentally, both NFO and CFO show indirect/direct band gaps around 1.52 eV/2.74 and 2.3 eV, and 1.3 eV/2.31 eV, respectively, for the ideal inverse spinel configuration with considerable standard deviation in the band gap values for CFO due to various levels of deviation from the ideal inverse spinel structure. Direct probing of the regions in both the systems with tetrahedral A site cation vacancy, which is distinct from the ideal inverse spinel configuration, shows significantly smaller band gap values. The experimental results are supported by the density functional theory based modified Becke-Johnson exchange correlation potential calculated band gap values for the different cation configurations.

  7. Theoretical study of energy states of two-dimensional electron gas in pseudomorphically strained InAs HEMTs taking into account the non-parabolicity of the conduction band

    SciTech Connect

    Nishio, Yui; Yamaguchi, Satoshi; Yamazaki, Youichi; Watanabe, Akira; Tange, Takahiro; Iida, Tsutomu; Takanashi, Yoshifumi

    2013-12-04

    We determined rigorously the energy states of a two-dimensional electron gas (2DEG) in high electron mobility transistors (HEMTs) with a pseudomorphically strained InAs channel (InAs PHEMTs) taking into account the non-parabolicity of the conduction band for InAs. The sheet carrier concentration of 2DEG for the non-parabolic energy band was about 50% larger than that for the parabolic energy band and most of the electrons are confined strongly in the InAs layer. In addition, the threshold voltage for InAs PHEMTs was about 0.21 V lower than that for conventional InGaAs HEMTs.

  8. Theoretical study on electronic structure of bathocuproine: Renormalization of the band gap in the crystalline state and the large exciton binding energy

    NASA Astrophysics Data System (ADS)

    Yanagisawa, Susumu; Hatada, Shin-No-Suke; Morikawa, Yoshitada

    Bathocuproine (BCP) is a promising organic material of a hole blocking layer in organic light-emitting diodes or an electron buffer layer in organic photovoltaic cells. The nature of the unoccupied electronic states is a key characteristic of the material, which play vital roles in the electron transport. To elucidate the electronic properties of the molecular or crystalline BCP, we use the GW approximation for calculation of the fundamental gap, and the long-range corrected density functional theory for the molecular optical absorption. It is found that the band gap of the BCP single crystal is 4.39 eV, and it is in agreement with the recent low-energy inverse photoemission spectroscopy measurement. The polarization energy is estimated to be larger than 1 eV, demonstrating the large polarization effects induced by the electronic clouds surrounding the injected charge. The theoretical optical absorption energy is 3.68 eV, and the exciton binding energy is estimated to be 0.71 eV, implying the large binding in the eletron-hole pair distributed around the small part of the molecular region. This work was supported by the Grants-in-Aid for Young Scientists (B) (No. 26810009), and for Scientific Research on Innovative Areas ``3D Active-Site Science'' (No. 26105011) from Japan Society for the Promotion of Science.

  9. Electron energies in metals

    SciTech Connect

    Mahan, G.D. Tennessee Univ., Knoxville, TN . Dept. of Physics and Astronomy)

    1991-07-10

    The modern era of electron-electron interactions began a decade ago. Plummer's group initiated a program of using angular resolved photoemission to examine the band structure of the simple metals. Beginning with aluminum, and carrying on to sodium and potassium, they always found that the occupied energy bands were much narrower than expected. For example, the compressed energy bands for metallic potassium suggest a band effective mass of m* = 1.33m{sub e}. This should be compared to the band mass found from optical conductivity m*/m{sub e} = 1.01 {plus minus} 0.01. The discrepancy between these results is startling. It was this great difference which started my group doing calculations. Our program was two-fold. On one hand, we reanalyzed the experimental data, in order to see if Plummer's result was an experimental artifact. On the other hand, we completely redid the electron-electron self-energy calculations for simple metals, using the most modern choices of local-field corrections and vertex corrections. Our results will be reported in these lectures. They can be summarized as following: Our calculations give the same effective masses as the older calculations, so the theory is relatively unchanged; Our analysis of the experiments suggests that the recent measurements of band narrowing are an experimental artifact. 38 refs., 9 figs.

  10. Energy ranges and pitch angles of outer radiation belt electrons depleted by an intense dayside hydrogen band EMIC wave event on February 23, 2014

    NASA Astrophysics Data System (ADS)

    Engebretson, M. J.; Posch, J. L.; Huang, C. L.; Kanekal, S. G.; Fok, M. C. H.; Rodger, C. J.; Smith, C. W.; Spence, H. E.; Baker, D. N.; Kletzing, C.; Wygant, J. R.

    2015-12-01

    Although most studies of the effect of EMIC waves on relativistic electrons have focused on wave events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of an intense, long-duration hydrogen band EMIC wave event on February 23, 2014 that was stimulated by a gradual 4-hour rise and subsequent sharp increases in solar wind pressure. Large-amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p-p) that included triggered emissions appeared for over 4 hours at both Van Allen Probes while these spacecraft were outside the plasmapause, in a region with densities ~5-20 cm-3, as they passed near apogee from late morning through local noon. Observations of radiation belt electrons by the REPT and MagEIS instruments on these spacecraft showed that these waves caused significant depletions of more field-aligned electrons at ultrarelativistic energies from 5.2 MeV down to ~2 MeV, and some depletions at energies down to below 1 MeV as well.

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

    PubMed

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

    2016-04-20

    Currently, most efficient perovskite solar cells (PVKSCs) with a p-i-n structure require simultaneously electron transport layers (ETLs) and hole transport layers (HTLs) to help collecting photogenerated electrons and holes for obtaining high performance. ETL free planar PVKSC is a relatively new and simple structured solar cell that gets rid of the complex and high temperature required ETL (such as compact and mesoporous TiO2). Here, we demonstrate the critical role of high coverage of perovskite in efficient ETL free PVKSCs from an energy band and equivalent circuit model perspective. From an electrical point of view, we confirmed that the low coverage of perovskite does cause localized short circuit of the device. With coverage optimization, a planar p-i-n(++) device with a power conversion efficiency of over 11% was achieved, implying that the ETL layer may not be necessary for an efficient device as long as the perovskite coverage is approaching 100%. PMID:27020395

  12. Energy bands in some transition metals

    NASA Astrophysics Data System (ADS)

    Laurent, D. G.

    1981-08-01

    Self consistent linear combination of Gaussian orbitals energy band calculations were performed for the two paramagnetic 3d transition metals, chromium and vanadium. The energy bands densities of states and Fermi surfaces were obtained using the two most popular local exchange correlation potentials (Kohn-Sham-Gaspar and von Barth-Hedin) for chromium and the Kohn-Sham-Gaspar potential alone for vanadium. A comparison was made with the available experimental data. New interpretations for some of the neutron scattering data are made in the chromium case. Results are also presented for the Compton profiles and optical conductivities. These correlate well with the experiments if appropriate angular averages (for the Compton profile) and lifetime effcts (for the optical conductivity) are included. The electron energy loss spectrum, computed over the range 0-6.5 eV agreed well with experiment.

  13. Electronic band structure of magnetic bilayer graphene superlattices

    SciTech Connect

    Pham, C. Huy; Nguyen, T. Thuong

    2014-09-28

    Electronic band structure of the bilayer graphene superlattices with δ-function magnetic barriers and zero average magnetic flux is studied within the four-band continuum model, using the transfer matrix method. The periodic magnetic potential effects on the zero-energy touching point between the lowest conduction and the highest valence minibands of pristine bilayer graphene are exactly analyzed. Magnetic potential is shown also to generate the finite-energy touching points between higher minibands at the edges of Brillouin zone. The positions of these points and the related dispersions are determined in the case of symmetric potentials.

  14. W-band free-electron masers

    SciTech Connect

    Freund, H. P.; Jackson, R. H.; Danly, B. G.; Levush, B.

    1999-05-07

    Theoretical analyses of high power W-band (i.e., {approx_equal}94 GHz) free-electron maser amplifiers are presented for a helical wiggler/cylindrical waveguide configuration using the three-dimensional slow-time-scale ARACHNE simulation code [9]. The geometry treated by ARACHNE is that of an electron beam propagating through the cylindrical waveguide subject to a helical wiggler and an axial guide magnetic field. Two configurations are discussed. The first is the case of a reversed-guide field geometry where the guide field is oriented antiparallel to the helicity of the wiggler field. Using a 330 kV/20 A electron beam, efficiencies of the order of 7% are calculated with a bandwidth (FWHM) of 5 GHz. The second example employs a strong guide field of 20 kG oriented parallel to the helicity of the wiggler. Here, efficiencies of greater than 8% are possible with a FWHM bandwidth of 4.5 GHz using a 300 kV/20 A electron beam. A normalized emittance of 95 mm-mrad is assumed in both cases, and no beam losses are observed for either case. Both cases assume interaction with the fundamental TE{sub 11} mode, which has acceptably low losses in the W-band.

  15. Band-Gap Widening at the Cu(In,Ga)(S,Se)2 Surface: A Novel Determination Approach Using Reflection Electron Energy Loss Spectroscopy.

    PubMed

    Hauschild, Dirk; Handick, Evelyn; Göhl-Gusenleitner, Sina; Meyer, Frank; Schwab, Holger; Benkert, Andreas; Pohlner, Stephan; Palm, Jörg; Tougaard, Sven; Heske, Clemens; Weinhardt, Lothar; Reinert, Friedrich

    2016-08-17

    Using reflection electron energy loss spectroscopy (REELS), we have investigated the optical properties at the surface of a chalcopyrite-based Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell absorber, as well as an indium sulfide (InxSy) buffer layer before and after annealing. By fitting the characteristic inelastic scattering cross-section λK(E) to cross sections evaluated by the QUEELS-ε(k,ω)-REELS software package, we determine the surface dielectric function and optical properties of these samples. A comparison of the optical values at the surface of the InxSy film with bulk ellipsometry measurements indicates a good agreement between bulk- and surface-related optical properties. In contrast, the properties of the CIGSSe surface differ significantly from the bulk. In particular, a larger (surface) band gap than for bulk-sensitive measurements is observed, providing a complementary and independent confirmation of earlier photoelectron spectroscopy results. Finally, we derive the inelastic mean free path λ for electrons in InxSy, annealed InxSy, and CIGSSe at a kinetic energy of 1000 eV. PMID:27463021

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

    SciTech Connect

    Heo, Sung; Lee, Hyung-Ik; Park, Jong-Bong; Ko, Dong-Su; Chung, JaeGwan; Kim, KiHong; Kim, Seong Heon; Yun, Dong-Jin; Ham, YongNam; Park, Gyeong Su; Song, Taewon; Lee, Dongho Nam, Junggyu; Kang, Hee Jae; Choi, Pyung-Ho; Choi, Byoung-Deog

    2015-06-29

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

  17. F-electron systems: Pushing band theory

    SciTech Connect

    Koelling, D.D.

    1990-08-01

    The f-electron orbitals have always been the incomplete atomic shell acting as a local moment weakly interacting with the remaining electronic structure'' in the minds of most people. So examining them using a band theory where one views them as itinerant once was -- and to some extent even today still is -- considered with some skepticism. Nonetheless, a very significant community has successfully utilized band theory as a probe of the electronic structure of the appropriate actinides and rare earths. Those people actually using the approach would be the first to declare that it is not the whole solution. Instead, one is pushing and even exceeding its limits of applicability. However, the appropriate procedure is to push the model consistently to its limits, patch where possible, and then look to see where discrepancies remain. I propose to offer a selected review of past developments (emphasizing the career to date of A. J. Freeman in this area), offer a list of interesting puzzles for the future, and then make some guesses as to the techniques one might want to use. 27 refs.

  18. Engineering flat electronic bands in quasiperiodic and fractal loop geometries

    NASA Astrophysics Data System (ADS)

    Nandy, Atanu; Chakrabarti, Arunava

    2015-11-01

    Exact construction of one electron eigenstates with flat, non-dispersive bands, and localized over clusters of various sizes is reported for a class of quasi-one-dimensional looped networks. Quasiperiodic Fibonacci and Berker fractal geometries are embedded in the arms of the loop threaded by a uniform magnetic flux. We work out an analytical scheme to unravel the localized single particle states pinned at various atomic sites or over clusters of them. The magnetic field is varied to control, in a subtle way, the extent of localization and the location of the flat band states in energy space. In addition to this we show that an appropriate tuning of the field can lead to a re-entrant behavior of the effective mass of the electron in a band, with a periodic flip in its sign.

  19. Analytical band Monte Carlo analysis of electron transport in silicene

    NASA Astrophysics Data System (ADS)

    Yeoh, K. H.; Ong, D. S.; Ooi, C. H. Raymond; Yong, T. K.; Lim, S. K.

    2016-06-01

    An analytical band Monte Carlo (AMC) with linear energy band dispersion has been developed to study the electron transport in suspended silicene and silicene on aluminium oxide (Al2O3) substrate. We have calibrated our model against the full band Monte Carlo (FMC) results by matching the velocity-field curve. Using this model, we discover that the collective effects of charge impurity scattering and surface optical phonon scattering can degrade the electron mobility down to about 400 cm2 V‑1 s‑1 and thereafter it is less sensitive to the changes of charge impurity in the substrate and surface optical phonon. We also found that further reduction of mobility to ∼100 cm2 V‑1 s‑1 as experimentally demonstrated by Tao et al (2015 Nat. Nanotechnol. 10 227) can only be explained by the renormalization of Fermi velocity due to interaction with Al2O3 substrate.

  20. Shadow bands in models of correlated electrons

    SciTech Connect

    Moreo, A.; Haas, S.; Dagotto, E.

    1995-08-01

    A consequence of strong antiferromagnetic correlations in models of high-Tc cuprates is the appearance in photoemission (PES) calculations of considerable more weight above the Fermi momentum p{sub F} than expected for non-interacting electrons. This effect, qualitatively discussed by Kampf and Schrieffer under the name of {open_quotes}shadow bands{close_quotes}, is here quantitatively analyzed in the two dimensional Hubbard and t-J models using Monte Carlo and exact diagonalization techniques in the realistic strong coupling regime.

  1. Analysis of energy states of two-dimensional electron gas in pseudomorphically strained InSb high-electron-mobility transistors taking into account the nonparabolicity of the conduction band

    NASA Astrophysics Data System (ADS)

    Nishio, Yui; Sato, Takato; Hirayama, Naomi; Iida, Tsutomu; Takanashi, Yoshifumi

    2016-08-01

    We propose a high electron mobility transistor with a pseudomorphically strained InSb channel (InSb-PHEMT) having an InSb composite channel layer in which the Al y In1‑ y Sb sub-channel layer is inserted between the InSb channel and the Al x In1‑ x Sb barrier layers to increase the conduction-band offset (ΔE C) at the heterointerface between the InSb channel and the Al x In1‑ x Sb barrier layers. The energy states for the proposed InSb-PHEMTs are calculated using our analytical method, taking account of the nonparabolicity of the conduction band. For the proposed InSb-PHEMTs, putting the sub-channel layers into the channel is found to be effective for obtaining a sufficiently large ΔE C (∼0.563 eV) to restrain electrons in the channel and increase the sheet concentration of two-dimensional electron gas to as high as 2.5 × 1012 cm‑2, which is comparable to that of InAs-PHEMTs. This also leads to a large transconductance of PHEMTs. In the proposed InSb-PHEMTs, electrons are strongly bound to the channel layer compared with InAs-PHEMTs, despite the effective mass at the conduction band (0.0139 m 0) of InSb being smaller than that of InAs and ΔE C for the InSb-PHEMTs being 25% smaller than that for the InAs-PHEMTs. This is because the bandgap energy of InSb is about one-half that of InAs, and hence, the nonparabolicity parameter of InSb is about twice as large as that of InAs.

  2. Controlling the band gap energy of cluster-assembled materials.

    PubMed

    Mandal, Sukhendu; Reber, Arthur C; Qian, Meichun; Weiss, Paul S; Khanna, Shiv N; Sen, Ayusman

    2013-11-19

    Cluster-assembled materials combine the nanoscale size and composition-dependent properties of clusters, which have highly tunable magnetic and electronic properties useful for a great variety of potential technologies. To understand the emergent properties as clusters are assembled into hierarchical materials, we have synthesized 23 cluster-assembled materials composed of As7(3-)-based motifs and different countercations and measured their band gap energies. We found that the band gap energy varies from 1.09 to 2.21 eV. In addition, we have carried out first principles electronic structure studies to identify the physical mechanisms that enable control of the band gap edges of the cluster assemblies. The choice of counterion has a profound effect on the band gap energy in ionic cluster assemblies. The top of the valence band is localized on the arsenic cluster, while the conduction band edge is located on the alkali metal counterions. Changing the counterion changes the position of the conduction band edge, enabling control of the band gap energy. We can also vary the architecture of the ionic solid by incorporating cryptates as counterions, which provide charge but are separated from the clusters by bulky ligands. Higher dimensionality typically decreases the band gap energy through band broadening; however band gap energies increased upon moving from zero-dimensional (0D) to two-dimensional (2D) assemblies. This is because internal electric fields generated by the counterion preferentially stabilize the adjacent lone pair orbitals that mark the top of the valence band. Thus, the choice of the counterion can control the position of the conduction band edge of ionic cluster assemblies. In addition, the dimensionality of the solid via internal electric fields can control the valence band edge. Through covalently linking arsenic clusters into composite building blocks, we have also been able to tune the band gap energy. We used a theoretical description based on

  3. Role of interface band structure on hot electron transport

    NASA Astrophysics Data System (ADS)

    Garramone, John J.

    Knowledge of electron transport through materials and interfaces is fundamentally and technologically important. For example, metal interconnects within integrated circuits suffer increasingly from electromigration and signal delay due to an increase in resistance from grain boundary and sidewall scattering since their dimensions are becoming shorter than the electron mean free path. Additionally, all semiconductor based devices require the transport of electrons through materials and interfaces where scattering and parallel momentum conservation are important. In this thesis, the inelastic and elastic scattering of hot electrons are studied in nanometer thick copper, silver and gold films deposited on silicon substrates. Hot electrons are electron with energy greater than kBT above the Fermi level (EF). This work was performed utilizing ballistic electron emission microscopy (BEEM) which is a three terminal scanning tunneling microscopy (STM) technique that measures the percentage of hot electrons transmitted across a Schottky barrier interface. Hot electron attenuation lengths of the metals were extracted by measuring the BEEM current as a function of metal overlayer thickness for both hot electron and hot hole injection at 80 K and under ultra high vacuum. The inelastic and elastic scattering lengths were extracted by fitting the energetic dependence of the measured attenuation lengths to a Fermi liquid based model. A sharp increase in the attenuation length is observed at low injection energies, just above the Schottky barrier height, only for metals on Si(001) substrates. In contrast, the attenuation length measured on Si(111) substrates shows a sharp decrease. These results indicate that interface band structure and parallel momentum conservation have significant impact upon the transport of hot electrons across non epitaxial metal-semiconductor interfaces. In addition, they help to separate effects upon hot electron transport that are inherent to the metal

  4. Electron-Phonon Renormalization of Electronic Band Structures of C Allotropes and BN Polymorphs

    NASA Astrophysics Data System (ADS)

    Tutchton, Roxanne M.; Marchbanks, Christopher; Wu, Zhigang

    The effect of lattice vibration on electronic band structures has been mostly neglected in first-principles calculations because the electron-phonon (e-ph) renormalization of quasi-particle energies is often small (< 100 meV). However, in certain materials, such as diamond, the electron-phonon coupling reduces the band gap by nearly 0.5 eV, which is comparable to the many-body corrections of the electronic band structures calculated using the density functional theory (DFT). In this work, we compared two implementations of the Allen-Heine-Cardona theory in the EPW code and the ABINIT package respectively. Our computations of Si and diamond demonstrate that the ABINIT implementation converges much faster. Using this method, the e-ph renormalizations of electronic structures of three C allotropes (diamond, graphite, graphene) and four BN polymorphs (zincblend, wurtzite, mono-layer, and layered-hexagonal) were calculated. Our results suggest that (1) all of the zero-point renormalizations of band gaps in these materials, except for graphene, are larger than 100 meV, and (2) there are large variations in e-ph renormalization of band gaps due to differences in crystal structure. This work was supported by a U.S. DOE Early Career Award (Grant No. DE-SC0006433). Computations were carried out at the Golden Energy Computing Organization at CSM and the National Energy Research Scientific Computing Center (NERSC).

  5. Cyclotron side-band emissions from ring-current electrons

    NASA Technical Reports Server (NTRS)

    Maeda, K.

    1976-01-01

    The paper examines temporal variations in electron energy spectra and pitch-angle distributions during a VLF-emission event observed by Explorer 45 in the main phase of a magnetic storm. It is noted that the observed event occurred outside the plasmasphere on the night side of the magnetosphere and that the dusk-side plasmapause had a double structure during the event. It is found that the VLF emissions consisted of two frequency bands, corresponding to the whistler and electrostatic modes, and that there was a sharp band of 'missing emissions' along frequencies equal to half the equatorial electron gyrofrequency. A peculiar pitch-angle distribution for high-energy electrons (50 to 350 keV) is noted. It is concluded that the VLF-producing particles were enhanced low-energy (about 5 keV) ring-current electrons which penetrated into the night side of the magnetosphere from the magnetotail plasma sheet and which drifted eastward after encountering the steep gradient of the geomagnetic field.

  6. Electronic band gaps and transport in Cantor graphene superlattices

    NASA Astrophysics Data System (ADS)

    Xu, Yi; He, Ying; Yang, Yanfang; Zhang, Huifang

    2015-04-01

    The electronic band gap and transport in Cantor graphene superlattices are investigated theoretically. It is found that such fractal structure can possess an unusual Dirac point located at the energy corresponding to the zero-averaged wave number (zero- k ‾) . The location of the Dirac point shifts to lower energy with the increase of order number. The zero- k ‾ gap is robust against the lattice constants and less sensitive to the incidence angle. Moreover, multi-Dirac-points may appear by adjusting the lattice constants and the order, and an expression for their location is derived. The control of electron transport in such fractal structure may lead to potential applications in graphene-based electronic devices.

  7. Unusual Changes in Electronic Band-Edge Energies of the Nanostructured Transparent n-Type Semiconductor Zr-Doped Anatase TiO2 (Ti1-xZrxO2; x < 0.3).

    PubMed

    Mieritz, Daniel G; Renaud, Adèle; Seo, Dong-Kyun

    2016-07-01

    By the establishment of highly controllable synthetic routes, electronic band-edge energies of the n-type transparent semiconductor Zr-doped anatase TiO2 have been studied holistically for the first time up to 30 atom % Zr, employing powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen gas sorption measurements, UV/vis spectroscopies, and Mott-Schottky measurements. The materials were produced through a sol-gel synthetic procedure that ensures good compositional homogeneity of the materials, while introducing nanoporosity in the structure, by achieving a mild calcination condition. Vegard's law was discovered among the homogeneous samples, and correlations were established between the chemical compositions and optical and electronic properties of the materials. Up to 20% Zr doping, the optical energy gap increases to 3.29 eV (vs 3.19 eV for TiO2), and the absolute conduction band-edge energy increases to -3.90 eV (vs -4.14 eV). The energy changes of the conduction band edge are more drastic than what is expected from the average electronegativities of the compounds, which may be due to the unnatural coordination environment around Zr in the anatase phase. PMID:27332108

  8. Observation of mini-band formation in the ground and high-energy electronic states of super-lattice solar cells

    NASA Astrophysics Data System (ADS)

    Usuki, Takanori; Matsuochi, Kouki; Nakamura, Tsubasa; Toprasertpong, Kasidit; Fukuyama, Atsuhiko; Sugiyama, Masakazu; Nakano, Yoshiaki; Ikari, Tetsuo

    2016-03-01

    Multiple Quantum wells (MQWs) have been studied as one promising material for high-efficiency nextgeneration solar cells. However, a portion of photo-excited carriers recombine in MQWs, resulting in the degradation of cell performance. Super-lattice (SL) structures, where quantum states in neighboring quantum wells strongly couple with each other, have been proposed for the carrier collection improvement via the tunneling transport through mini-bands. Therefore, it is important to characterize mini-band formation in various types of SL structures. We examined p-i-n GaAs-based solar cells whose i layers contain 20 stacks of InGaAs/GaAsP MQW structures with 2.1-nm GaAsP barriers (thin-barrier cell), with 2.1-nm barriers and 3-nm GaAs interlayers in between GaAsP barriers and InGaAs wells (stepbarrier cell), and with 7.8-nm barriers (thick-barrier cell). We investigated the optical absorption spectra of the SL solar cells using piezoelectric photo-thermal (PPT) spectroscopy. In the thick-barrier cell, one exciton peak was observed near the absorption edge of MQWs. On the other hand, we confirmed a split of the exciton peak for the thin-barrier SL, suggesting the formation of mini-band. Moreover, in the step-barrier cell, the mini-band at the ground state disappears since thick GaAs interlayers isolate each quantum-well ground state and, instead, the mini-band formation of highenergy states could be observed. By estimating from the energy-level calculation, this is attributed to the mini-band formation of light-hole states. This can well explain the improvement of carrier collection efficiency (CCE) of the thinbarrier and the step-barrier cells compared with the thick-barrier cell.

  9. Density of States for Warped Energy Bands

    PubMed Central

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

    2016-01-01

    Warping of energy bands can affect the density of states (DOS) in ways that can be large or subtle. Despite their potential for significant practical impacts on materials properties, these effects have not been rigorously demonstrated previously. Here we rectify this using an angular effective mass formalism that we have developed. To clarify the often confusing terminology in this field, “band warping” is precisely defined as pertaining to any multivariate energy function E(k) that does not admit a second-order differential at an isolated critical point in k-space, which we clearly distinguish from band non-parabolicity. We further describe band “corrugation” as a qualitative form of band warping that increasingly deviates from being twice differentiable at an isolated critical point. These features affect the density-of-states and other parameters ascribed to band warping in various ways. We demonstrate these effects, providing explicit calculations of DOS and their effective masses for warped energy dispersions originally derived by Kittel and others. Other physical and mathematical examples are provided to demonstrate fundamental distinctions that must be drawn between DOS contributions that originate from band warping and contributions that derive from band non-parabolicity. For some non-degenerate bands in thermoelectric materials, this may have profound consequences of practical interest. PMID:26905029

  10. Density of States for Warped Energy Bands

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    Warping of energy bands can affect the density of states (DOS) in ways that can be large or subtle. Despite their potential for significant practical impacts on materials properties, these effects have not been rigorously demonstrated previously. Here we rectify this using an angular effective mass formalism that we have developed. To clarify the often confusing terminology in this field, “band warping” is precisely defined as pertaining to any multivariate energy function E(k) that does not admit a second-order differential at an isolated critical point in k-space, which we clearly distinguish from band non-parabolicity. We further describe band “corrugation” as a qualitative form of band warping that increasingly deviates from being twice differentiable at an isolated critical point. These features affect the density-of-states and other parameters ascribed to band warping in various ways. We demonstrate these effects, providing explicit calculations of DOS and their effective masses for warped energy dispersions originally derived by Kittel and others. Other physical and mathematical examples are provided to demonstrate fundamental distinctions that must be drawn between DOS contributions that originate from band warping and contributions that derive from band non-parabolicity. For some non-degenerate bands in thermoelectric materials, this may have profound consequences of practical interest.

  11. Tuning the electronic band gap of graphene by oxidation

    SciTech Connect

    Dabhi, Shweta D.; Jha, Prafulla K.

    2015-06-24

    Using plane wave pseudo potential density functional theory, we studied the electronic properties of graphene with different C:O ratio. In this work, we discussed the changes that occur in electronic band structure of graphene functionalized with different amount of epoxy group. Electronic band gap depends on C:O ratio in graphene oxide containing epoxy group. The present work will have its implication for making devices with tunable electronic properties by oxidizing graphene.

  12. Electronic Band Engineering of Epitaxial Graphene by Atomic Intercalation

    NASA Astrophysics Data System (ADS)

    Jayasekera, Thushari; Sandin, Andreas; Xu, Shu; Wheeler, Virginia; Gaskill, D. K.; Rowe, J. E.; Kim, K. W.; Dougherty, Daniel B.; Buongiorno Nardelli, M.

    2012-02-01

    Using calculations from first principles, we have investigated possible ways of engineering the electronic band structure of epitaxial graphene on SiC. In particular, intercalation of different atomic species, such as Hydrogen, Fluorine, Sodium, Germanium, Carbon and Silicon is shown to modify and tune the interface electronic properties and band alignments. Our results suggest that intercalation in graphene is quite different from that in graphite, and could provide a fundamentally new way to achieve electronic control in graphene electronics.

  13. Layer specific optical band gap measurement at nanoscale in MoS2 and ReS2 van der Waals compounds by high resolution electron energy loss spectroscopy

    NASA Astrophysics Data System (ADS)

    Dileep, K.; Sahu, R.; Sarkar, Sumanta; Peter, Sebastian C.; Datta, R.

    2016-03-01

    Layer specific direct measurement of optical band gaps of two important van der Waals compounds, MoS2 and ReS2, is performed at nanoscale by high resolution electron energy loss spectroscopy. For monolayer MoS2, the twin excitons (1.8 and 1.95 eV) originating at the K point of the Brillouin zone are observed. An indirect band gap of 1.27 eV is obtained from the multilayer regions. Indirect to direct band gap crossover is observed which is consistent with the previously reported strong photoluminescence from the monolayer MoS2. For ReS2, the band gap is direct, and a value of 1.52 and 1.42 eV is obtained for the monolayer and multilayer, respectively. The energy loss function is dominated by features due to high density of states at both the valence and conduction band edges, and the difference in analyzing band gap with respect to ZnO is highlighted. Crystalline 1T ReS2 forms two dimensional chains like superstructure due to the clustering between four Re atoms. The results demonstrate the power of HREELS technique as a nanoscale optical absorption spectroscopy tool.

  14. Electronic Band Gap of SrSe at High Pressure

    SciTech Connect

    Atkinson,T.; Chynoweth, K.; Cervantes, P.

    2006-01-01

    The electronic band gap of SrSe, in the CsCl-stuctured phase, was measured to 42 GPa via optical absorption studies. The indirect electronic band gap was found to close monotonically with pressure for the range of pressures studied. The change in band gap with respect to pressure, dE{sub gap}/dP, was determined to be -6.1(5)x10{sup -3} eV/GPa. By extrapolation of our line fit, we estimate band gap closure to occur at 180(20) GPa.

  15. The electronic structure of heavy fermions: Narrow temperature independent bands

    SciTech Connect

    Arko, A.J.; Joyce, J.J.; Smith, J.L.; Andrews, A.B.

    1996-08-01

    The electronic structure of both Ce and U heavy fermions appears to consist of extremely narrow temperature independent bands. There is no evidence from photoemission for a collective phenomenon normally referred to as the Kondo resonance. In uranium compounds a small dispersion of the bands is easily measurable.

  16. Global Kinetic Modeling of Banded Electron Structures in the Plasmasphere

    NASA Technical Reports Server (NTRS)

    Liemohn, M. W.; Khazanov, G. V.

    1997-01-01

    Significant fluxes of 10 eV to 30 keV electrons have been detected in the plasmasphere, appearing as banded structures in energy with broad spatial extents and slowly evolving over several days. It is thought that these populations are decaying plasma sheet electrons injected into the corotating region of near-Earth space. This capture can occur when the convective electric field drops rapidly and the Alfven boundary suddenly outward, trapping the inner edge of the plasma sheet along closed drift paths. Our bounce-averaged kinetic model of superthermal electron transport is able to simulate this capture and the subsequent drift, diffusion, and decay of the plasma cloud. Results of this simulation will be shown and discussed, from the initial injection during the elevated convection to the final loss of the particles. It is thought that not only Coulomb collisions but also wave-particle interactions play a significant role in altering the plasma cloud. Quasilinear diffusion is currently being incorporated into the model and the importance of this mechanism will be examined. Also, the high anisotropy of the trapped population could be unstable and generate plasma waves. These and other processes will be investigated to determine the final fate of the cloud and to quantify where, how, and when the energy of the plasma cloud is deposited. Comparisons with CRRES observations of these events are shown to verify the model and explain the data.

  17. High reliability K-band electron tubes for satellite communication

    NASA Astrophysics Data System (ADS)

    Hirata, H.; Kawashima, F.

    1980-09-01

    Design methods and performance results for long life and high reliability satellite TWT and high power klystron with wide band tunability are presented. Emphasis is placed on problems observed during early operation. Tuning instability is solved by analyzing possible spurious modes at the tuner and by designing a structure to suppress spurious modes. A feedback mechanism is also discussed to handle instability in preset-tuning. Cathode design, electron beam focusing, and electron gun structure at a high frequency band are examined.

  18. Experimental study of energy harvesting in UHF band

    NASA Astrophysics Data System (ADS)

    Bernacki, Ł.; Gozdur, R.; Salamon, N.

    2016-04-01

    A huge progress of down-sizing technology together with trend of decreasing power consumption and, on the other hand, increasing efficiency of electronics give the opportunity to design and to implement the energy harvesters as main power sources. This paper refers to the energy that can be harvested from electromagnetic field in the unlicensed frequency bands. The paper contains description of the most popular techniques and transducers that can be applied in energy harvesting domain. The overview of current research and commercial solutions was performed for bands in ultra-high frequency range, which are unlicensed and where transmission is not limited by administrative arrangements. During the experiments with Powercast’s receiver, the same bands as sources of electromagnetic field were taken into account. This power source is used for conducting radio-communication process and excess energy could be used for powering the extra electronic circuits. The paper presents elaborated prototype of energy harvesting system and the measurements of power harvested in ultra-high frequency range. The evaluation of RF energy harvesters for powering ultra-low power (ULP) electronic devices was performed based on survey and results of the experiments.

  19. Eastern Band of Cherokee Strategic Energy Plan

    SciTech Connect

    Souther Carolina Institute of energy Studies-Robert Leitner

    2009-01-30

    The Eastern Band of Cherokee Indians was awarded a grant under the U.S. Department of Energy Tribal Energy Program (TEP) to develop a Tribal Strategic Energy Plan (SEP). The grant, awarded under the “First Steps” phase of the TEP, supported the development of a SEP that integrates with the Tribe’s plans for economic development, preservation of natural resources and the environment, and perpetuation of Tribal heritage and culture. The Tribe formed an Energy Committee consisting of members from various departments within the Tribal government. This committee, together with its consultant, the South Carolina Institute for Energy Studies, performed the following activities: • Develop the Tribe’s energy goals and objectives • Establish the Tribe’s current energy usage • Identify available renewable energy and energy efficiency options • Assess the available options versus the goals and objectives • Create an action plan for the selected options

  20. Terra MODIS Band 27 Electronic Crosstalk Effect and Its Removal

    NASA Technical Reports Server (NTRS)

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

    2012-01-01

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

  1. Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon

    PubMed Central

    Zhu, Zhen; Shao, Hezhu; Dong, Xiao; Li, Ning; Ning, Bo-Yuan; Ning, Xi-Jing; Zhao, Li; Zhuang, Jun

    2015-01-01

    We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing. PMID:26012369

  2. Band gaps, ionization potentials, and electron affinities of periodic electron systems via the adiabatic-connection fluctuation-dissipation theorem

    NASA Astrophysics Data System (ADS)

    Trushin, Egor; Betzinger, Markus; Blügel, Stefan; Görling, Andreas

    2016-08-01

    An approach to calculate fundamental band gaps, ionization energies, and electron affinities of periodic electron systems is explored. Starting from total energies obtained with the help of the adiabatic-connection fluctuation-dissipation (ACFD) theorem, these physical observables are calculated according to their basic definition by differences of the total energies of the N -, (N -1 ) -, and (N +1 ) -electron system. The response functions entering the ACFD theorem are approximated here by the direct random phase approximation (dRPA). For a set of prototypical semiconductors and insulators it is shown that even with this quite drastic approximation the resulting band gaps are very close to experiment and of a similar quality to those from the computationally more involved G W approximation. By going beyond the dRPA in the future the accuracy of the calculated band gaps may be significantly improved further.

  3. Electronic band structure and photoemission: A review and projection

    SciTech Connect

    Falicov, L.M.

    1987-09-01

    A brief review of electronic-structure calculations in solids, as a means of interpreting photoemission spectra, is presented. The calculations are, in general, of three types: ordinary one-electron-like band structures, which apply to bulk solids and are the basis of all other calculations; surface modified calculations, which take into account, self-consistently if at all possible, the presence of a vacuum-solid interface and of the electronic modifications caused thereby; and many-body calculations, which go beyond average-field approximations and consider dynamic rearrangement effects caused by electron-electron correlations during the photoemission process. 44 refs.

  4. Electron emission from conduction band of heavily phosphorus doped diamond negative electron affinity surface

    NASA Astrophysics Data System (ADS)

    Yamada, Takatoshi; Masuzawa, Tomoaki; Mimura, Hidenori; Okano, Ken

    2016-02-01

    Hydrogen (H)-terminated surfaces of diamond have attracted significant attention due to their negative electron affinity (NEA), suggesting high-efficiency electron emitters. Combined with n-type doping technique using phosphorus (P) as donors, the unique NEA surface makes diamond a promising candidate for vacuum cold-cathode applications. However, high-electric fields are needed for the electron emission from the n-type doped diamond with NEA. Here we have clarified the electron emission mechanism of field emission from P-doped diamond having NEA utilizing combined ultraviolet photoelectron spectroscopy/field emission spectroscopy (UPS/FES). An UP spectrum has confirmed the NEA of H-terminated (1 1 1) surface of P-doped diamond. Despite the NEA, electron emission occurs only when electric field at the surface exceeds 4.2  ×  106 V cm-1. Further analysis by UPS/FES has revealed that the emitted energy level is shifted, indicating that the electron emission mechanism of n-type diamond having NEA surface does not follow a standard field emission theory, but is dominated by potential barrier formed within the diamond due to upward band bending. The reduction of internal barrier is the key to achieve high-efficiency electron emitters using P-doped diamond with NEA, of which application ranges from high-resolution electron spectroscopy to novel vacuum nanoelectronics devices.

  5. Electronic structure and band alignment at an epitaxial spinel/perovskite heterojunction.

    PubMed

    Qiao, Liang; Li, Wei; Xiao, Haiyan; Meyer, Harry M; Liang, Xuelei; Nguyen, N V; Weber, William J; Biegalski, Michael D

    2014-08-27

    The electronic properties of solid-solid interfaces play critical roles in a variety of technological applications. Recent advances of film epitaxy and characterization techniques have demonstrated a wealth of exotic phenomena at interfaces of oxide materials, which are critically dependent on the alignment of their energy bands across the interface. Here we report a combined photoemission and electrical investigation of the electronic structures across a prototypical spinel/perovskite heterojunction. Energy-level band alignment at an epitaxial Co3O4/SrTiO3(001) heterointerface indicates a chemically abrupt, type I heterojunction without detectable band bending at both the film and substrate. The unexpected band alignment for this typical p-type semiconductor on SrTiO3 is attributed to its intrinsic d-d interband excitation, which significantly narrows the fundamental band gap between the top of the valence band and the bottom of the conduction band. The formation of the type I heterojunction with a flat-band state results in a simultaneous confinement of both electrons and holes inside the Co3O4 layer, thus rendering the epitaxial Co3O4/SrTiO3(001) heterostructure to be a very promising material for high-efficiency luminescence and optoelectronic device applications. PMID:25075939

  6. Electronic correlation effects in multi-band systems

    NASA Astrophysics Data System (ADS)

    Song, Kok Wee

    The recent dominant trends in condensed matter physics research can be roughly summarized into three newly discovered materials: topological insulators, graphene, and iron-based superconductors. All these materials exhibit many intriguing properties which are fundamentally related to their electronic band structure. Therefore, this lead to many intense investigations on multi-band electronic system to explore new physics. The physics of multi-band electronic structure is fascinating in several aspects. Without many-body effects, because of the gauge freedom of Bloch states, topological insulators can give rise a robust metallic behavior at its boundaries. In graphene, the touching between conduction and valence band at Fermi level yields a new criticality class which exhibit many unconventional electronic properties, especially its quasi-relativistic behavior. Turning to the many-body effects, for instance, the iron-based superconductors can sustain an superconducting ground state despite of no attractive interactions in the system. Therefore, a deeper understanding for the conventional notions in condensed matter physics has put forward by many of these experimental observations. In this thesis, the many-body effects in multi-band systems are the main focus, especially the study of graphene and iron-based superconductors which can be compared to experiments. These theoretical studies intend to understand how the underlying electronic bands degree of freedom can give rise to Fermi-liquid instabilities, and how these effects can be related to intriguing physical properties. We first study the electrons correlation effects in bilayer graphene by a renormalization group technique. In this study, we build a microscopic model of bilayer graphene from a tight-binding approach. In our finding, the peculiar Fermi surface configuration leads to critical behavior which is beyond the Fermi-liquid paradigm. Furthermore, due to the electron-electron interactions between

  7. Analysis of electron emission from GaAs(Cs,O) by low energy electron microscopy

    NASA Astrophysics Data System (ADS)

    Jin, Xiuguang

    2015-10-01

    Low-energy electron microscopy was carried out to study the electron emission process from a GaAs photocathode with a negative electron affinity (NEA) surface. The relationship between emission current and electron affinity was investigated in detail to obtain information regarding the electron tunneling in the vacuum barrier and the electron distribution in the interior of GaAs, especially with respect to photoelectron capture in the band bending region. A comparison of the calculated quantized sub-band energies in the band bending region confirmed that the majority of photoelectrons fell within sub-bands, from where a large portion of the photoelectrons escape into the vacuum.

  8. Intermolecular energy-band dispersion in PTCDA multilayers

    NASA Astrophysics Data System (ADS)

    Yamane, Hiroyuki; Kera, Satoshi; Okudaira, Koji K.; Yoshimura, Daisuke; Seki, Kazuhiko; Ueno, Nobuo

    2003-07-01

    The electronic structure of a well-oriented perylene-3,4,9,10-tetracarboxylic acid-dianhydride multilayer prepared on MoS2 single crystal surface were studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. From the photon energy dependence of normal emission spectra, we observed an intermolecular energy-band dispersion of about 0.2 eV for the highest occupied molecular orbital (HOMO) band of single π character. The observed energy-band dispersion showed a cosine curve, which originates from the intermolecular π-π interaction. Analyses using the tight-binding model gave that the transfer integral of about 0.05 eV for the π-π interaction, the effective mass of HOMO hole m*h=5.28m0, and the hole mobility μh>3.8 cm2/V s. This is the first observation of the intermolecular energy-band dispersion of a conventional single-component organic semiconductor only with the weak intermolecular van der Waals interaction.

  9. Geometry-independent energy band simulator for radially symmetric diodes

    NASA Astrophysics Data System (ADS)

    Kirkpatrick, T.; Buonassisi, T.

    2016-07-01

    In this work, a geometrically independent method to calculate the energy band diagram of radially symmetric diodes is reported. For radially symmetric diodes, the calculation of electron (or hole) energies across the junction can be reduced to a singular spatially dependent variable. Because geometry is not incorporated into the calculation a priori, by reducing the physics to a single spatial variable, energy band calculations can be performed in multiple geometries, simultaneously, for direct comparison to each other. The calculation outlined herein is pseudo-analytical and does not utilize finite element and/or control volume methods. It is, therefore, capable of generating spatially analytic equations for analyzing limiting scenarios of the junction, beneficial for yielding insight into the physics and design criteria of depletion for non-planar semiconducting devices.

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

    PubMed

    Tablero, C

    2005-09-15

    A study using first principles of the electronic properties of S32Zn31Cr, a material derived from the SZn host semiconductor where a Cr atom has been substituted for each of the 32 Zn atoms, is presented. This material has an intermediate band sandwiched between the valence and conduction bands of the host semiconductor, which in a formal band-theoretic picture is metallic because the Fermi energy is located within the impurity band. The potential technological application of these materials is that when they are used to absorb photons in solar cells, the efficiency increases significantly with respect to the host semiconductor. An analysis of the gaps, bandwidths, density of states, total and orbital charges, and electronic density is carried out. The main effects of the local-density approximation with a Hubbard term corrections are an increase in the bandwidth, a modification of the relative composition of the five d and p transition-metal orbitals, and a splitting of the intermediate band. The results demonstrate that the main contribution to the intermediate band is the Cr atom. For values of U greater than 6 eV, where U is the empirical Hubbard term U parameter, this band is unfolded, thus creating two bands, a full one below the Fermi energy and an empty one above it, i.e., a metal-insulator transition. PMID:16392585

  11. Electronic structure of graphene on a reconstructed Pt(100) surface: Hydrogen adsorption, doping, and band gaps

    NASA Astrophysics Data System (ADS)

    Ulstrup, Søren; Nilsson, Louis; Miwa, Jill A.; Balog, Richard; Bianchi, Marco; Hornekær, Liv; Hofmann, Philip

    2013-09-01

    We probe the structure and electronic band structure of graphene grown on a Pt(100) substrate using scanning tunneling microscopy, low energy electron diffraction, and angle-resolved photoemission spectroscopy. It is found that the graphene layer lacks a well-defined azimuthal orientation with respect to the substrate, causing a circular smearing of the π band instead of a well-defined Dirac cone near the Fermi level. The graphene is found to be electron doped placing the Dirac point ˜0.45 eV below the Fermi level, and a gap of 0.15±0.03 eV is found at the Dirac point. We dose atomic hydrogen and monitor the coverage on the graphene by analyzing the impurity-induced broadening of the π-band width. Saturation of graphene on Pt(100) with hydrogen does not expand the band gap, but instead hydrogen-mediated broadening and rehybridization of the graphene sp2 bonds into sp3 leads to a complete disruption of the graphene π band, induces a lifting of the Pt(100) reconstruction, and introduces a dispersing Pt state near the Fermi level. Deposition of rubidium on graphene on Pt(100) leads to further electron doping, pushing the Dirac point to a binding energy of ˜1.35 eV, and increasing the band gap to 0.65±0.04 eV.

  12. High energy electron cooling

    SciTech Connect

    Parkhomchuk, V.

    1997-09-01

    High energy electron cooling requires a very cold electron beam. The questions of using electron cooling with and without a magnetic field are presented for discussion at this workshop. The electron cooling method was suggested by G. Budker in the middle sixties. The original idea of the electron cooling was published in 1966. The design activities for the NAP-M project was started in November 1971 and the first run using a proton beam occurred in September 1973. The first experiment with both electron and proton beams was started in May 1974. In this experiment good result was achieved very close to theoretical prediction for a usual two component plasma heat exchange.

  13. Measurements of the energy band gap and valence band structure of AgSbTe2

    NASA Astrophysics Data System (ADS)

    Jovovic, V.; Heremans, J. P.

    2008-06-01

    The de Haas-van Alphen effect, galvanomagnetic and thermomagnetic properties of high-quality crystals of AgSbTe2 are measured and analyzed. The transport properties reveal the material studied here to be a very narrow-gap semiconductor (Eg≈7.6±3meV) with ˜5×1019cm-3 holes in a valence band with a high density of states and thermally excited ˜1017cm-3 high-mobility (2200cm2/Vs) electrons at 300 K. The quantum oscillations are measured with the magnetic field oriented along the ⟨111⟩ axis. Taken together with the Fermi energy derived from the transport properties, the oscillations confirm the calculated valence band structure composed of 12 half-pockets located at the X -points of the Brillouin zone, six with a density-of-states effective mass mda∗≫0.21me and six with mdb∗≫0.55me , giving a total density-of-states effective mass, including Fermi pocket degeneracy, of md∗≈1.7±0.2me ( me is the free electron mass). The lattice term dominates the thermal conductivity, and the electronic contribution in samples with both electrons and holes present is in turn dominated by the ambipolar term. The low thermal conductivity and very large hole mass of AgSbTe2 make it a most promising p -type thermoelectric material.

  14. Band structure and Fermi surface of electron-doped C60 monolayers.

    PubMed

    Yang, W L; Brouet, V; Zhou, X J; Choi, Hyoung J; Louie, Steven G; Cohen, Marvin L; Kellar, S A; Bogdanov, P V; Lanzara, A; Goldoni, A; Parmigiani, F; Hussain, Z; Shen, Z-X

    2003-04-11

    C60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali-doped C60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions. PMID:12690192

  15. Band structure and fermi surface of Electron-Doped C{sub 60} Monolayers

    SciTech Connect

    Yang, W.L.; Brouet, V.; Zhou, X.J.; Choi, Hyoung J.; Louie, Steven G.; Cohen, Marvin L.; Kellar, S.A.; Bogdanov, P.V.; Lanzara, A.; Goldoni, A.; Parmigiani, F.; Hussain, Z.; Shen, Z-X.

    2003-11-06

    C60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali doped C60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions.

  16. Unoccupied-electronic-band structure of graphite studied by angle-resolved secondary-electron emission and inverse photoemission

    NASA Astrophysics Data System (ADS)

    Maeda, F.; Takahashi, T.; Ohsawa, H.; Suzuki, S.; Suematsu, H.

    1988-03-01

    Angle-resolved inverse photoemission spectroscopy (ARIPES) and angle-resolved secondary-electron emission spectroscopy (ARSEES) have been performed for graphite to establish experimentally the unoccupied-electronic-band structure as well as to study the difference between the two techniques. Remarkable differences have been found in the experimental two-dimensional band structures obtained by the two methods. The experimental results have been compared with the two different band calculations by R. C. Tatar and S. Rabii [Phys. Rev. B 25, 4126 (1982)] and by N. A. W. Holzwarth, S. G. Louie, and S. Rabii [Phys. Rev. B 26, 5382 (1982)] with special attention to the energy position of the three-dimensional interlayer band. The possible origin of the difference between ARIPES and ARSEES has also been discussed.

  17. Electronic Power Conditioner for Ku-band Travelling Wave Tube

    NASA Astrophysics Data System (ADS)

    Kowstubha, Palle; Krishnaveni, K.; Ramesh Reddy, K.

    2016-07-01

    A highly sophisticated regulated power supply is known as electronic power conditioner (EPC) is required to energise travelling wave tubes (TWTs), which are used as RF signal amplifiers in satellite payloads. The assembly consisting of TWT and EPC together is known as travelling wave tube amplifier (TWTA). EPC is used to provide isolated and conditioned voltage rails with tight regulation to various electrodes of TWT and makes its RF performance independent of solar bus variations which are caused due to varying conditions of eclipse and sunlit. The payload mass and their power consumption is mainly due to the existence of TWTAs that represent about 35 % of total mass and about 70-90 % (based on the type of satellite application) of overall dc power consumption. This situation ensures a continuous improvement in the design of TWTAs and their associated EPCs to realize more efficient and light products. Critical technologies involved in EPCs are design and configuration, closed loop regulation, component and material selection, energy limiting of high voltage (HV) outputs and potting of HV card etc. This work addresses some of these critical technologies evolved in realizing and testing the state of art of EPC and it focuses on the design of HV supply with a HV and high power capability, up to 6 kV and 170 WRF, respectively required for a space TWTA. Finally, an experimental prototype of EPC with a dc power of 320 W provides different voltages required by Ku-band TWT in open loop configuration.

  18. Electronic transitions in GdN band structure

    SciTech Connect

    Vidyasagar, R. Kita, T.; Sakurai, T.; Ohta, H.

    2014-05-28

    Using the near-infrared (NIR) absorbance spectroscopy, electronic transitions and spin polarization of the GdN epitaxial film have been investigated; and the GdN epitaxial film was grown by a reactive rf sputtering technique. The GdN film exhibited three broad bands in the NIR frequency regimes; and those bands are attributable primarily to the minority and majority spin transitions at the X-point and an indirect transition along the Γ-X symmetric direction of GdN Brillouin zone. We experimentally observe a pronounced red-shift of the indirect band gap when cooling down below the Curie temperature which is ascribed to the orbital-dependent coulomb interactions of Gd-5dxy electrons, which tend to push-up the N-2p bands. On the other hand, we have evaluated the spin polarization of 0.17 (±0.005), which indicates that the GdN epitaxial film has almost 100% spin-polarized carriers. Furthermore, the experimental result of GdN electronic transitions are consistent with the previous reports and are thus well-reproduced. The Arrott plots evidenced that the Curie temperature of GdN film is 36 K and the large spin moment is explained by the nitrogen vacancies and the intra-atomic exchange interaction.

  19. Ionization By Impact Electrons in Solids: Electron Mean Free Path Fitted Over A Wide Energy Range

    SciTech Connect

    Ziaja, B; London, R A; Hajdu, J

    2005-06-09

    We propose a simple formula for fitting the electron mean free paths in solids both at high and at low electron energies. The free-electron-gas approximation used for predicting electron mean free paths is no longer valid at low energies (E < 50 eV), as the band structure effects become significant at those energies. Therefore we include the results of the band structure calculations in our fit. Finally, we apply the fit to 9 elements and 2 compounds.

  20. Electronic energy states

    NASA Technical Reports Server (NTRS)

    1976-01-01

    One-electron wave functions are reviewed and approximate solutions of two-electron systems are given in terms of these one-electron functions. The symmetry effects associated with electron spin are reviewed and the effects of electron exchange on energy levels of the two-electron system are given. The coupling of electronic orbital and spin angular momentum is considered next and the Lande interval rule for Russell-Saunders or LS coupling is derived. The configurations possible for various multi-electron LS couplings are enumerated (examples from the first two rows of the periodic table are given), and the meaning of the spectroscopic nomenclature is discussed, particularly with respect to the degeneracies of the electron states involved. Next the nomenclature, symmetries, and degeneracies for electron states of diatomic molecules are discussed, and some examples for N2, O2, and NO are presented. The electronic partition functions and derivative thermodynamic properties are expressed in terms of these energies and degeneracies, and examples are given for some of the simple gas species encountered in the earth's atmosphere.

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

    PubMed

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

    2016-01-29

    We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap. PMID:26871327

  2. Surface-plasmon enhanced photodetection at communication band based on hot electrons

    NASA Astrophysics Data System (ADS)

    Wu, Kai; Zhan, Yaohui; Wu, Shaolong; Deng, Jiajia; Li, Xiaofeng

    2015-08-01

    Surface plasmons can squeeze light into a deep-subwavelength space and generate abundant hot electrons in the nearby metallic regions, enabling a new paradigm of photoconversion by the way of hot electron collection. Unlike the visible spectral range concerned in previous literatures, we focus on the communication band and design the infrared hot-electron photodetectors with plasmonic metal-insulator-metal configuration by using full-wave finite-element method. Titanium dioxide-silver Schottky interface is employed to boost the low-energy infrared photodetection. The photodetection sensitivity is strongly improved by enhancing the plasmonic excitation from a rationally engineered metallic grating, which enables a strong unidirectional photocurrent. With a five-step electrical simulation, the optimized device exhibits an unbiased responsivity of ˜0.1 mA/W and an ultra-narrow response band (FWHM = 4.66 meV), which promises to be a candidate as the compact photodetector operating in communication band.

  3. Electronic materials with a wide band gap: recent developments.

    PubMed

    Klimm, Detlef

    2014-09-01

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

  4. Electronic materials with a wide band gap: recent developments

    PubMed Central

    Klimm, Detlef

    2014-01-01

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

  5. Energy loss of ions at metal surfaces: Band-structure effects

    SciTech Connect

    Alducin, M.; Silkin, V.M.; Juaristi, J.I.; Chulkov, E.V.

    2003-03-01

    We study band-structure effects on the energy loss of protons scattered off the Cu (111) surface. The distance dependent stopping power for a projectile traveling parallel to the surface is calculated within the linear response theory. The self-consistent electronic response of the system is evaluated within the random-phase approximation. In order to characterize the surface band structure, the electronic single-particle wave functions and energies are obtained by solving the Schroedinger equation with a realistic one-dimensional model potential. This potential reproduces the main features of the Cu (111) surface: the energy band gap for electron motion along the surface normal, as well as the binding energy of the occupied surface state and the first image state. Comparison of our results with those obtained within the jellium model allows us to characterize the band-structure effects in the energy loss of protons interacting with the Cu (111) surface.

  6. Femtosecond electron injection from optically populated donor states into the conduction band of semiconductors

    NASA Astrophysics Data System (ADS)

    Ernstorfer, Ralph; Toeben, Lars; Gundlach, Lars; Felber, Silke; Galoppini, Elena; Wei, Qian; Eichberger, Rainer; Storck, Winfried; Zimmermann, Carsten; Willig, Frank

    2003-12-01

    Unoccupied donor states can be populated via light absorption at the surface of semiconductor in the range of the conduction band levels. Hot electrons are injected from such donor states into the conduction band of a semiconductor on a femtosecond time scale. Such donor states can have rather different physical properties, e.g. unoccupied surface bands formed via reconstruction of the clean surface of a semiconductor in contact with ultra high vacuum or chromophores in molecules that are anchored at the surface of the semiconductor. The energy levels of the donor states with respect to the bands in the semiconductor can be determined with UPS and fs-2PPE. Experimental data on the energetics and dynamics of electron injection are presented for the two different cases of donor states mentioned above. The influence of vibrational wavepackets on electron injection is discussed for the case of a molecular donor state. Energy loss of the hot electrons injected into the semiconductor is measured with energy and time resolution employing femtosecond two-photon-photoemission.

  7. Electronic band structure and optical properties of the cubic, Sc, Y and La hydride systems

    SciTech Connect

    Peterman, D.J.

    1980-01-01

    Electronic band structure calculations are used to interpret the optical spectra of the cubic Sc, Y and La hydride systems. Self-consistent band calculations of ScH/sub 2/ and YH/sub 2/ were carried out. The respective joint densities of states are computed and compared to the dielectric functions determined from the optical measurements. Additional calculations were performed in which the Fermi level or band gap energies are rigidly shifted by a small energy increment. These calculations are then used to simulate the derivative structure in thermomodulation spectra and relate the origin of experimental interband features to the calculated energy bands. While good systematic agreement is obtained for several spectral features, the origin of low-energy interband transitions in YH/sub 2/ cannot be explained by these calculated bands. A lattice-size-dependent premature occupation of octahedral sites by hydrogen atoms in the fcc metal lattice is suggested to account for this discrepancy. Various non-self-consistent calculations are used to examine the effect of such a premature occupation. Measurements of the optical absorptivity of LaH/sub x/ with 1.6 < x < 2.9 are presented which, as expected, indicate a more premature occupation of the octahedral sites in the larger LaH/sub 2/ lattice. These experimental results also suggest that, in contrast to recent calculations, LaH/sub 3/ is a small-band-gap semiconductor.

  8. Correlation between morphology, electron band structure, and resistivity of Pb atomic chains on the Si(5 5 3)-Au surface.

    PubMed

    Jałochowski, M; Kwapiński, T; Łukasik, P; Nita, P; Kopciuszyński, M

    2016-07-20

    Structural and electron transport properties of multiple Pb atomic chains fabricated on the Si(5 5 3)-Au surface are investigated using scanning tunneling spectroscopy, reflection high electron energy diffraction, angular resolved photoemission electron spectroscopy and in situ electrical resistance. The study shows that Pb atomic chains growth modulates the electron band structure of pristine Si(5 5 3)-Au surface and hence changes its sheet resistivity. Strong correlation between chains morphology, electron band structure and electron transport properties is found. To explain experimental findings a theoretical tight-binding model of multiple atomic chains interacting on effective substrate is proposed. PMID:27228462

  9. Correlation between morphology, electron band structure, and resistivity of Pb atomic chains on the Si(5 5 3)-Au surface

    NASA Astrophysics Data System (ADS)

    Jałochowski, M.; Kwapiński, T.; Łukasik, P.; Nita, P.; Kopciuszyński, M.

    2016-07-01

    Structural and electron transport properties of multiple Pb atomic chains fabricated on the Si(5 5 3)-Au surface are investigated using scanning tunneling spectroscopy, reflection high electron energy diffraction, angular resolved photoemission electron spectroscopy and in situ electrical resistance. The study shows that Pb atomic chains growth modulates the electron band structure of pristine Si(5 5 3)-Au surface and hence changes its sheet resistivity. Strong correlation between chains morphology, electron band structure and electron transport properties is found. To explain experimental findings a theoretical tight-binding model of multiple atomic chains interacting on effective substrate is proposed.

  10. Electronic- and band-structure evolution in low-doped (Ga,Mn)As

    SciTech Connect

    Yastrubchak, O.; Gluba, L.; Żuk, J.; Sadowski, J.; MAX-Lab, Lund University, 22100 Lund ; Krzyżanowska, H.; Department of Physics and Astronomy, Vanderbilt University, 6506 Stevenson Center, Nashville, Tennessee 37325 ; Domagala, J. Z.; Andrearczyk, T.; Wosinski, T.

    2013-08-07

    Modulation photoreflectance spectroscopy and Raman spectroscopy have been applied to study the electronic- and band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn doping in the range of low Mn content, up to 1.2%. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap-transition energy with increasing Mn content in very low-doped (Ga,Mn)As layers with n-type conductivity are interpreted as a result of merging the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy with increasing Mn content in (Ga,Mn)As layers with higher Mn content and p-type conductivity indicates the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in the (Ga,Mn)As diluted ferromagnetic semiconductor.

  11. Detecting band inversions by measuring the environment: fingerprints of electronic band topology in bulk phonon linewidths

    NASA Astrophysics Data System (ADS)

    Saha, Kush; Legare, Katherine; Garate, Ion

    The interplay between topological phases of matter and dissipative baths constitutes an emergent research topic with links to condensed matter, photonic crystals, cold atomic gases and quantum information. While recent studies suggest that dissipative baths can induce topological phases in intrinsically trivial quantum materials, the backaction of topological invariants on dissipative baths is overlooked. By exploring this back action for a centrosymmetric Dirac insulator coupled to phonons, we show that the linewidths of bulk optical phonons can reveal electronic band inversions. This result is the first known example where topological phases of an open quantum system may be detected by measuring the bulk properties of the surrounding environment.

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

    SciTech Connect

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

    2009-06-19

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

  13. Band gap and electronic structure of MgSiN{sub 2}

    SciTech Connect

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

    2014-09-15

    Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN{sub 2} is similar to that of wurtzite AlN. A band gap of 5.7 eV was calculated for both MgSiN{sub 2} (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN{sub 2} is 6.2 eV. MgSiN{sub 2} has an additional direct gap of 6.3 eV at the Γ point.

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

    DOE PAGESBeta

    Shi, Hongliang; Du, Mao-Hua

    2015-05-12

    Bulk semiconductors and insulators typically have continuous valence and conduction bands. In this paper, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., Cs2NaInBr6, Cs2NaBiCl6, and Tl2NaBiCl6. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use Cs2NaInBr6 as an example to show that the narrow bands can stabilize self-trappedmore » and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. Finally, these characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.« less

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

    SciTech Connect

    Shi, Hongliang; Du, Mao-Hua

    2015-05-12

    Bulk semiconductors and insulators typically have continuous valence and conduction bands. In this paper, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., Cs2NaInBr6, Cs2NaBiCl6, and Tl2NaBiCl6. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use Cs2NaInBr6 as an example to show that the narrow bands can stabilize self-trapped and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. Finally, these characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.

  16. Band gap engineering and \\vec{k}\\cdot \\vec{\\pi } electronic structure of lead and tin tellurides

    NASA Astrophysics Data System (ADS)

    Behera, S. S.; Tripathi, G. S.

    2016-06-01

    We study the effect of the variation of energy gap on the k\\cdot π electronic structure of PbTe and SnTe, using a six-level basis at the L point. The basis functions in both the systems have the same transformation properties. However, the basis functions of the band edge states in SnTe are reversed with respect to the same in PbTe. Band dispersions are obtained analytically for a two band model. As the band gap decreases, the bands become linear. Far bands are included in the electronic dispersion, using perturbation theory. Fermi energy and the Density of States at the Fermi energy, { D }({\\varepsilon }F), are calculated for different carrier concentrations and energy gaps through a self-consistent approach. Interesting results are seen when the energy gap is reduced from the respective equilibrium values. For both the systems, the Fermi energy increases as the gap is decreased. The behavior of { D }({\\varepsilon }F) is, however, different. It decreases with the gap. It is also on expected lines. Calculated values of the electronic effective mass, as a function of temperature, energy gap and carrier concentration, are compared with previously published data. As distinguished from a first principles calculation, the work has focused on the carrier dependent electronic parameters for use both by theorists and experimenters as well.

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

    SciTech Connect

    Li, Wei; Zhang, Qin; Kirillov, Oleg A.; Levin, Igor; Richter, Curt A.; Gundlach, David J.; Nguyen, N. V. E-mail: liangxl@pku.edu.cn; Bijesh, R.; Datta, S.; Liang, Yiran; Peng, Lian-Mao; Liang, Xuelei E-mail: liangxl@pku.edu.cn

    2014-11-24

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

  18. Wedge energy bands of monolayer black phosphorus: a first-principles study

    NASA Astrophysics Data System (ADS)

    Park, Minwoo; Bae, Hyeonhu; Lee, Seunghan; Yang, Li; Lee, Hoonkyung

    2016-08-01

    On the basis of first-principles calculations, we present intriguing electronic properties of halogen-striped functionalized monolayer black phosphorus. The halogen-striped monolayer black phosphorus is found to have a wedge energy band with the energy-momentum relation of E\\propto {{p}y} when the stripe–stripe distance is smaller than ~40 Å. Our tight-binding study shows that the wedge energy band occurs when 2-atom basis 1D lattices are periodically repeated aligned with each other in a 2D lattice. We also discuss the possible applications of this wedge energy band in electron supercollimation with high mobility or severely anisotropic electronic transport, which can be used for the development of optics-like nano-electronics.

  19. Wedge energy bands of monolayer black phosphorus: a first-principles study.

    PubMed

    Park, Minwoo; Bae, Hyeonhu; Lee, Seunghan; Yang, Li; Lee, Hoonkyung

    2016-08-01

    On the basis of first-principles calculations, we present intriguing electronic properties of halogen-striped functionalized monolayer black phosphorus. The halogen-striped monolayer black phosphorus is found to have a wedge energy band with the energy-momentum relation of [Formula: see text] when the stripe-stripe distance is smaller than ~40 Å. Our tight-binding study shows that the wedge energy band occurs when 2-atom basis 1D lattices are periodically repeated aligned with each other in a 2D lattice. We also discuss the possible applications of this wedge energy band in electron supercollimation with high mobility or severely anisotropic electronic transport, which can be used for the development of optics-like nano-electronics. PMID:27299467

  20. Tuning the electronic band-gap of fluorinated 3C-silicon carbide nanowires

    NASA Astrophysics Data System (ADS)

    Miranda Durán, Álvaro; Trejo Baños, Alejandro; Pérez, Luis Antonio; Cruz Irisson, Miguel

    The possibility of control and modulation of the electronic properties of silicon carbide nanowires (SiCNWs) by varying the wire diameter is well known. SiCNWs are particularly interesting and technologically important, due to its electrical and mechanical properties, allowing the development of materials with specific electronic features for the design of stable and robust electronic devices. Tuning the band gap by chemical surface passivation constitutes a way for the modification of the electronic band gap of these nanowires. We present, the structural and electronic properties of fluorinated SiCNWs, grown along the [111] crystallographic direction, which are investigated by first principles. We consider nanowires with six diameters, varying from 0.35 nm to 2.13 nm, and eight random covering schemes including fully hydrogen- and fluorine terminated ones. Gibbs free energy of formation and electronic properties were calculated for the different surface functionalization schemes and diameters considered. The results indicate that the stability and band gap of SiCNWs can be tuned by surface passivation with fluorine atoms This work was supported by CONACYT infrastructure project 252749 and UNAM-DGAPA-PAPIIT IN106714. A.M. would like to thank for financial support from CONACyT-Retención. Computing resources from proyect SC15-1-IR-27 of DGTIC-UNAM are acknowledged.

  1. Electron-impact excitation of the 31. 4-eV band in N sub 2

    SciTech Connect

    de Souza, G.G.B.; Bielschowsky, C.E.; Lucas, C.A.; Souza, A.C.A. )

    1990-08-01

    Generalized oscillator strengths (GOS) for the dipole-forbidden 31.4-eV band in N{sub 2} have been determined both experimentally and theoretically. The experimental values for the GOS were obtained using a crossed-beam electron spectrometer at 1-keV impact energy. The theoretical results were determined using the first Born approximation with {ital ab} {ital initio} configuration-interaction target wave functions.

  2. Achieving Higher Energies via Passively Driven X-band Structures

    NASA Astrophysics Data System (ADS)

    Sipahi, Taylan; Sipahi, Nihan; Milton, Stephen; Biedron, Sandra

    2014-03-01

    Due to their higher intrinsic shunt impedance X-band accelerating structures significant gradients with relatively modest input powers, and this can lead to more compact particle accelerators. At the Colorado State University Accelerator Laboratory (CSUAL) we would like to adapt this technology to our 1.3 GHz L-band accelerator system using a passively driven 11.7 GHz traveling wave X-band configuration that capitalizes on the high shunt impedances achievable in X-band accelerating structures in order to increase our overall beam energy in a manner that does not require investment in an expensive, custom, high-power X-band klystron system. Here we provide the design details of the X-band structures that will allow us to achieve our goal of reaching the maximum practical net potential across the X-band accelerating structure while driven solely by the beam from the L-band system.

  3. The electronic band structure of GaBiAs/GaAs layers: Influence of strain and band anti-crossing

    NASA Astrophysics Data System (ADS)

    Batool, Z.; Hild, K.; Hosea, T. J. C.; Lu, X.; Tiedje, T.; Sweeney, S. J.

    2012-06-01

    The GaBixAs1-x bismide III-V semiconductor system remains a relatively underexplored alloy particularly with regards to its detailed electronic band structure. Of particular importance to understanding the physics of this system is how the bandgap energy Eg and spin-orbit splitting energy Δo vary relative to one another as a function of Bi content, since in this alloy it becomes possible for Δo to exceed Eg for higher Bi fractions, which occurrence would have important implications for minimising non-radiative Auger recombination losses in such structures. However, this situation had not so far been realised in this system. Here, we study a set of epitaxial layers of GaBixAs1-x (2.3% ≤ x ≤ 10.4%), of thickness 30-40 nm, grown compressively strained onto GaAs (100) substrates. Using room temperature photomodulated reflectance, we observe a reduction in Eg, together with an increase in Δo, with increasing Bi content. In these strained samples, it is found that the transition energy between the conduction and heavy-hole valence band edges is equal with that between the heavy-hole and spin-orbit split-off valence band edges at ˜9.0 ± 0.2% Bi. Furthermore, we observe that the strained valence band heavy-hole/light-hole splitting increases with Bi fraction at a rate of ˜15 (±1) meV/Bi%, from which we are able to deduce the shear deformation potential. By application of an iterative strain theory, we decouple the strain effects from our experimental measurements and deduce Eg and Δo of free standing GaBiAs; we find that Δo indeed does come into resonance with Eg at ˜10.5 ± 0.2% Bi. We also conclude that the conduction/valence band alignment of dilute-Bi GaBiAs on GaAs is most likely to be type-I.

  4. Theoretical studies of electronic band-tail states, Anderson transition and surfaces of amorphous semiconductors

    NASA Astrophysics Data System (ADS)

    Dong, Jianjun

    In this dissertation, we study the Anderson transition within the electronic band tail states, and amorphous surfaces. The disorder induced band tail states is one of the unique character of amorphous semiconductors. Because of the proximity to the Fermi level, the nature of these band tail states is of obvious interest to theory of doping and transport. The study of amorphous solid surface is also an interesting area for theory. It is possible to have some major rearrangements near surfaces of amorphous solids (the amorphous analog of surface reconstruction), and the local bonding environment could be dramatically different from that of bulk. The study of the surfaces can also help people toward understanding the growth mechanism. First, electronic band tail states of amorphous silicon and amorphous diamond were studied based on the large (4096 atom) and realistic structural models. To solve the large tight-binding Hamiltonian matrices, we used two order N methods: the maximum entropy method for computing the total densities of states, and the modified Lanczos techniques for computing the individual energy eigenstates in the band gap regions. The DC conductivity was estimated with the Kubo formula. Next, the structural and electronic properties of the surfaces of tetrahedral amorphous carbon (ta-C) were also studied with a first-principles, local basis LDA technique. We reported two structural models made under different conditions, and examined the transition of the local bonding environment from the bulk to the surface. In the study of band tail states, we observe that Anderson (local-to-extended) transition within the band states proceeds by "cluster proliferation". We interpret the nature of band tail states in terms of a "resonant cluster model" through which one can qualitatively understand the evolution of the states from midgap toward the mobility edges. In the study of ta-C surfaces, we observe that nearly 50% surface atoms are threefold coordinated and

  5. Monoclinic Tungsten Oxide with {100} Facet Orientation and Tuned Electronic Band Structure for Enhanced Photocatalytic Oxidations.

    PubMed

    Zhang, Ning; Chen, Chen; Mei, Zongwei; Liu, Xiaohe; Qu, Xiaolei; Li, Yunxiang; Li, Siqi; Qi, Weihong; Zhang, Yuanjian; Ye, Jinhua; Roy, Vellaisamy A L; Ma, Renzhi

    2016-04-27

    Exploring surface-exposed highly active crystal facets for photocatalytic oxidations is promising in utilizing monoclinic WO3 semiconductor. However, the previously reported highly active facets for monoclinic WO3 were mainly toward enhancing photocatalytic reductions. Here we report that the WO3 with {100} facet orientation and tuned surface electronic band structure can effectively enhance photocatalytic oxidation properties. The {100} faceted WO3 single crystals are synthesized via a facile hydrothermal method. The UV-visible diffuse reflectance, X-ray photoelectron spectroscopy valence band spectra, and photoelectrochemical measurements suggest that the {100} faceted WO3 has a much higher energy level of valence band maximum compared with the normal WO3 crystals without preferred orientation of the crystal face. The density functional theory calculations reveal that the shift of O 2p and W 5d states in {100} face induce a unique band structure. In comparison with the normal WO3, the {100} faceted WO3 exhibits an O2 evolution rate about 5.1 times in water splitting, and also shows an acetone evolution rate of 4.2 times as well as CO2 evolution rate of 3.8 times in gaseous degradation of 2-propanol. This study demonstrates an efficient crystal face engineering route to tune the surface electronic band structure for enhanced photocatalytic oxidations. PMID:27045790

  6. A class of monolayer metal halogenides MX2: Electronic structures and band alignments

    NASA Astrophysics Data System (ADS)

    Lu, Feng; Wang, Weichao; Luo, Xiaoguang; Xie, Xinjian; Cheng, Yahui; Dong, Hong; Liu, Hui; Wang, Wei-Hua

    2016-03-01

    With systematic first principles calculations, a class of monolayer metal halogenides MX2 (M = Mg, Ca, Zn, Cd, Ge, Pb; M = Cl, Br, I) has been proposed. Our study indicates that these monolayer materials are semiconductors with the band gaps ranging from 2.03 eV of ZnI2 to 6.08 eV of MgCl2. Overall, the band gap increases with the increase of the electronegativity of the X atom or the atomic number of the metal M. Meanwhile, the band gaps of monolayer MgX2 (X = Cl, Br) are direct while those of other monolayers are indirect. Based on the band edge curvatures, the derived electron (me) and hole (mh) effective masses of MX2 monolayers are close to their corresponding bulk values except that the me of CdI2 is three times larger and the mh for PbI2 is twice larger. Finally, the band alignments of all the studied MX2 monolayers are provided using the vacuum level as energy reference. These theoretical results may not only introduce the monolayer metal halogenides family MX2 into the emerging two-dimensional materials, but also provide insights into the applications of MX2 in future electronic, visible and ultraviolet optoelectronic devices.

  7. Doping and strain dependence of the electronic band structure in Ge and GeSn alloys

    NASA Astrophysics Data System (ADS)

    Xu, Chi; Gallagher, James; Senaratne, Charutha; Brown, Christopher; Fernando, Nalin; Zollner, Stefan; Kouvetakis, John; Menendez, Jose

    2015-03-01

    A systematic study of the effect of dopants and strain on the electronic structure of Ge and GeSn alloys is presented. Samples were grown by UHV-CVD on Ge-buffered Si using Ge3H8 and SnD4 as the sources of Ge and Sn, and B2H6/P(GeH3)3 as dopants. High-energy critical points in the joint-density of electronic states were studied using spectroscopic ellipsometry, which yields detailed information on the strain and doping dependence of the so-called E1, E1 +Δ1 , E0' and E2 transitions. The corresponding dependencies of the lowest direct band gap E0 and the fundamental indirect band gap Eindwere studied via room-T photoluminescence spectroscopy. Of particular interest for this work were the determination of deformation potentials, band gap renormalization effects, Burstein-Moss shifts due to the presence of carriers at band minima, and the dependence of other critical point parameters, such as amplitudes and phase angles, on the doping concentration. The selective blocking of transitions due to high doping makes it possible to investigate the precise k-space location of critical points. These studies are complemented with detailed band-structure calculations within a full-zone k-dot- p approach. Supported by AFOSR under DOD AFOSR FA9550-12-1-0208 and DOD AFOSR FA9550-13-1-0022.

  8. Band-gap measurements of direct and indirect semiconductors using monochromated electrons

    SciTech Connect

    Gu Lin; Srot, Vesna; Sigle, Wilfried; Koch, Christoph; Aken, Peter van; Ruehle, Manfred; Scholz, Ferdinand; Thapa, Sarad B.; Kirchner, Christoph; Jetter, Michael

    2007-05-15

    With the development of monochromators for transmission electron microscopes, valence electron-energy-loss spectroscopy (VEELS) has become a powerful technique to study the band structure of materials with high spatial resolution. However, artifacts such as Cerenkov radiation pose a limit for interpretation of the low-loss spectra. In order to reveal the exact band-gap onset using the VEELS method, semiconductors with direct and indirect band-gap transitions have to be treated differently. For direct semiconductors, spectra acquired at thin regions can efficiently minimize the Cerenkov effects. Examples of hexagonal GaN (h-GaN) spectra acquired at different thickness showed that a correct band-gap onset value can be obtained for sample thicknesses up to 0.5 t/{lambda}. In addition, {omega}-q maps acquired at different specimen thicknesses confirm the thickness dependency of Cerenkov losses. For indirect semiconductors, the correct band-gap onset can be obtained in the dark-field mode when the required momentum transfer for indirect transition is satisfied. Dark-field VEEL spectroscopy using a star-shaped entrance aperture provides a way of removing Cerenkov effects in diffraction mode. Examples of Si spectra acquired by displacing the objective aperture revealed the exact indirect transition gap E{sub g} of 1.1 eV.

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

    NASA Astrophysics Data System (ADS)

    Dolgashev, Valery A.; Bowden, Gordon; Ding, Yuantao; Emma, Paul; Krejcik, Patrick; Lewandowski, James; Limborg, Cecile; Litos, Michael; Wang, Juwen; Xiang, Dao

    2014-10-01

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

  10. Energy band diagram of device-grade silicon nanocrystals

    NASA Astrophysics Data System (ADS)

    Macias-Montero, M.; Askari, S.; Mitra, S.; Rocks, C.; Ni, C.; Svrcek, V.; Connor, P. A.; Maguire, P.; Irvine, J. T. S.; Mariotti, D.

    2016-03-01

    Device grade silicon nanocrystals (NCs) are synthesized using an atmospheric-pressure plasma technique. The Si NCs have a small and well defined size of about 2.3 nm. The synthesis system allows for the direct creation of thin films, enabling a range of measurements to be performed and easy implementation of this material in different devices. The chemical stability of the Si NCs is evaluated, showing relatively long-term durability thanks to hydrogen surface terminations. Optical and electrical characterization techniques, including Kelvin probe, ultraviolet photoemission spectroscopy and Mott-Schottky analysis, are employed to determine the energy band diagram of the Si NCs.Device grade silicon nanocrystals (NCs) are synthesized using an atmospheric-pressure plasma technique. The Si NCs have a small and well defined size of about 2.3 nm. The synthesis system allows for the direct creation of thin films, enabling a range of measurements to be performed and easy implementation of this material in different devices. The chemical stability of the Si NCs is evaluated, showing relatively long-term durability thanks to hydrogen surface terminations. Optical and electrical characterization techniques, including Kelvin probe, ultraviolet photoemission spectroscopy and Mott-Schottky analysis, are employed to determine the energy band diagram of the Si NCs. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr07705b

  11. Density of States for Warped or non-Warped Energy Bands

    NASA Astrophysics Data System (ADS)

    Mecholsky, Nicholas; Resca, Lorenzo; Pegg, Ian; Fornari, Marco

    The goal of this talk is to investigate when band warping affects density-of-states effective mass. Further, band ``corrugation,'' a form of band warping referring to energy dispersions that deviate ``more severely'' from being twice-differentiable at isolated critical points, may also correlate in different ways with density-of-states effective masses and other band warping parameters. In this talk, an angular effective mass formalism is developed and used to study the electronic density of states of warped and non-warped energy bands towards an application in thermoelectric transport design. We demonstrate effects of band warping and prove the superiority of the angular effective mass treatment for valence energy bands in cubic materials. We explore examples that can also be critical to precisely distinguish the contributions due to band warping and to band non-parabolicity in non-degenerate bands of thermoelectric materials that have a consequent practical interest. The presenter wished to thank the Vitreous State Laboratory.

  12. The Superatom States of Fullerenes and Their Hybridization into the Nearly Free Electron Bands of Fullerites

    SciTech Connect

    Zhao, Jin; Feng, Min; Yang, Jinlong; Petek, Hrvoje

    2009-04-07

    Motivated by the discovery of the superatom states of C₆₀ molecules, we investigate the factors that influence their energy and wave function hybridization into nearly free electron bands in molecular solids. As the n = 3 solutions of the radial Schro¨dinger equation of the central attractive potential consisting of the shortrange C atom core and the long-range collective screening potentials, respectively, located on the icosahedral C60 molecule shell and within its hollow core, superatom states are distinguished by their atom-like orbitals corresponding to different orbital angular momentum states (l = 0, 1, 2,...). Because they are less tightly bound than the π orbitals, that is, the n = 2 states, which are often exploited in the intermolecular electron transport in aromatic organic molecule semiconductors, superatom orbitals hybridize more extensively among aggregated molecules to form bands with nearly free electron dispersion. The prospect of exploiting the strong intermolecular coupling to achieve metal-like conduction in applications such as molecular electronics may be attained by lowering the energy of superatom states from 3.5 eV for single chemisorbed C₆₀ molecules to below the Fermi level; therefore, we study how the superatom state energies depend on factors such as their aggregation into 1D - 3D solids, cage size, and exo- and endohedral doping by metal atoms. We find, indeed, that if the ionization potential of endohedral atom, such as copper, is sufficiently large, superatom states can form the conduction band in the middle of the gap between the HOMO and LUMO of the parent C₆₀ molecule. Through a plane-wave density functional theory study, we provide insights for a new paradigm for intermolecular electronic interaction beyond the conventional one among the spn hybridized orbitals of the organic molecular solids that could lead to design of novel molecular materials and quantum structures with extraordinary optical and

  13. The superatom states of fullerenes and their hybridization into the nearly free electron bands of fullerites.

    PubMed

    Zhao, Jin; Feng, Min; Yang, Jinlong; Petek, Hrvoje

    2009-04-28

    Motivated by the discovery of the superatom states of C60 molecules, we investigate the factors that influence their energy and wave function hybridization into nearly free electron bands in molecular solids. As the n = 3 solutions of the radial Schrodinger equation of the central attractive potential consisting of the short-range C atom core and the long-range collective screening potentials, respectively, located on the icosahedral C60 molecule shell and within its hollow core, superatom states are distinguished by their atom-like orbitals corresponding to different orbital angular momentum states (l = 0, 1, 2,...). Because they are less tightly bound than the pi orbitals, that is, the n = 2 states, which are often exploited in the intermolecular electron transport in aromatic organic molecule semiconductors, superatom orbitals hybridize more extensively among aggregated molecules to form bands with nearly free electron dispersion. The prospect of exploiting the strong intermolecular coupling to achieve metal-like conduction in applications such as molecular electronics may be attained by lowering the energy of superatom states from 3.5 eV for single chemisorbed C60 molecules to below the Fermi level; therefore, we study how the superatom state energies depend on factors such as their aggregation into 1D-3D solids, cage size, and exo- and endohedral doping by metal atoms. We find, indeed, that if the ionization potential of endohedral atom, such as copper, is sufficiently large, superatom states can form the conduction band in the middle of the gap between the HOMO and LUMO of the parent C60 molecule. Through a plane-wave density functional theory study, we provide insights for a new paradigm for intermolecular electronic interaction beyond the conventional one among the sp(n) hybridized orbitals of the organic molecular solids that could lead to design of novel molecular materials and quantum structures with extraordinary optical and electronic properties. PMID

  14. Long-term drift induced by the electronic crosstalk in Terra MODIS Band 29

    NASA Astrophysics Data System (ADS)

    Sun, Junqiang; Madhavan, Sriharsha; Xiong, Xiaoxiong; Wang, Menghua

    2015-10-01

    Terra MODerate Resolution Imaging Spectroradiometer (MODIS) is one of the key sensors in the NASA's Earth Observing System, which has successfully completed 15 years of on-orbit operation. Terra MODIS continues to collect valuable information of the Earth's energy radiation from visible to thermal infrared wavelengths. The instrument has been well characterized over its lifetime using onboard calibrators whose calibration references are traceable to the National Institute of Standards and Technology standards. In this paper, we focus on the electronic crosstalk effect of Terra MODIS band 29, a thermal emissive band (TEB) whose center wavelength is 8.55 µm. Previous works have established the mechanism to describe the effect of the electronic crosstalk in the TEB channels of Terra MODIS. This work utilizes the established methodology to apply to band 29. The electronic crosstalk is identified and characterized using the regularly scheduled lunar observations. The moon being a near-pulse-like source allowed easy detection of extraneous signals around the actual Moon surface. First, the crosstalk-transmitting bands are identified along with their amplitudes. The crosstalk effect then is characterized using a moving average mechanism that allows a high fidelity of the magnitude to be corrected. The lunar-based analysis unambiguously shows that the crosstalk contamination is becoming more severe in recent years and should be corrected in order to maintain calibration quality for the affected spectral bands. Finally, two radiometrically well-characterized sites, Pacific Ocean and Libya 1 desert, are used to assess the impact of crosstalk effect. It is shown that the crosstalk contamination induces a long-term upward drift of 1.5 K in band 29 brightness temperature of MODIS Collection 6 L1B, which could significantly impact the science products. The crosstalk effect also induces strong detector-to-detector differences, which result in severe stripping in the Earth view

  15. Band-pass design optimization of piezoelectric cantilever bimorph energy harvester

    NASA Astrophysics Data System (ADS)

    Zhang, Long; Williams, Keith A.

    2011-03-01

    Piezoelectric energy harvesting has become a feasible method for powering micro portable electronics and wireless sensor networks by converting ambient vibration energy into electrical energy. As a thumb of rule, it is critical to tune the resonant frequency of the generator to the frequency of the environmental vibrations in order to induce the maximum structural deformation and then the maximum converted electrical energy through piezoelectric effect. However, it is well-known that the ambient vibrations are not usually fixed in only one single frequency and could span over a limited frequency band. In this paper, a band-pass design optimization of piezoelectric cantilever bimorph (PCB) energy harvester is presented based on the system transfer function of the PCB generator presented in a previous literature. For such an energy harvester, a group of PCB with dimensions appropriately selected can be integrated into a band-pass energy harvester working over a limited frequency band if the dimensions of piezoelectric bimorphs and proof masses are appropriately chosen. Further, the finite element analysis (FEA) of such a band-pass energy harvester is performed in ANSYS to validate the theoretical proposal. The result shows that the band-pass design optimization leads to a piezoelectric generator working over a certain frequency band while keeping outputting the relatively stable open-circuit voltage.

  16. Electronic d-band properties of gold nanoclusters grown on amorphous carbon

    NASA Astrophysics Data System (ADS)

    Visikovskiy, Anton; Matsumoto, Hisashi; Mitsuhara, Kei; Nakada, Toshitaka; Akita, Tomoki; Kido, Yoshiaki

    2011-04-01

    The electronic d-band properties are important factors for the emerging catalytic activity of Au nanoclusters of sub-5-nm size. We analyzed the d-band properties of Au nanoclusters grown on amorphous carbon supports by photoelectron spectroscopy using synchrotron-radiation light coupled with high-resolution ion scattering spectrometry which enables us to estimate the size and shape of Au nanoclusters. The d-band width (Wd), d-band center position (Ed), and apparent 5d3/2-d5/2 spin-orbit splitting (ESO) were determined as a function of a number of Au atoms per cluster (nA) and an average coordination number (nC) in a wide range (11band narrowing which is caused by hybridization of fewer wave functions of the valence electrons. However, Ed shifts to the higher binding energy side with decreasing cluster size. The rapid movement of Ed is attributed to the dynamic final-state effect, which results in higher binding energy shifts of core and valence states due to a positive hole created after photoelectron emission. We have estimated the contribution from the final-state effect and derived the approximated initial-state spectra. Modified data, however, still show a slight movement of the d-band center away from the Fermi level (EF) although the Ed values for Au nanoclusters are closer to EF compared to the bulk value. This behavior is ascribed to the contraction of average Au-Au bond length with decreasing cluster size.

  17. Electronic d-band properties of gold nanoclusters grown on amorphous carbon

    SciTech Connect

    Visikovskiy, Anton; Matsumoto, Hisashi; Mitsuhara, Kei; Nakada, Toshitaka; Kido, Yoshiaki; Akita, Tomoki

    2011-04-15

    The electronic d-band properties are important factors for the emerging catalytic activity of Au nanoclusters of sub-5-nm size. We analyzed the d-band properties of Au nanoclusters grown on amorphous carbon supports by photoelectron spectroscopy using synchrotron-radiation light coupled with high-resolution ion scattering spectrometry which enables us to estimate the size and shape of Au nanoclusters. The d-band width (W{sub d}), d-band center position (E{sub d}), and apparent 5d{sub 3/2}-d{sub 5/2} spin-orbit splitting (E{sub SO}) were determined as a function of a number of Au atoms per cluster (n{sub A}) and an average coordination number (n{sub C}) in a wide range (11band narrowing which is caused by hybridization of fewer wave functions of the valence electrons. However, E{sub d} shifts to the higher binding energy side with decreasing cluster size. The rapid movement of E{sub d} is attributed to the dynamic final-state effect, which results in higher binding energy shifts of core and valence states due to a positive hole created after photoelectron emission. We have estimated the contribution from the final-state effect and derived the approximated initial-state spectra. Modified data, however, still show a slight movement of the d-band center away from the Fermi level (E{sub F}) although the E{sub d} values for Au nanoclusters are closer to E{sub F} compared to the bulk value. This behavior is ascribed to the contraction of average Au-Au bond length with decreasing cluster size.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Electron pitch angle (Dαα) and momentum (Dpp) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L = 4.6 and 6.8 for electron energies ≤10 keV. Landau (n = 0) resonance and cyclotron harmonic resonances n = ±1, ±2, … ±5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (α) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n = +1 and n = +2. A major contribution to momentum diffusion coefficients appears from n = +2. However, the banded structures in Dαα and Dpp coefficients appear only in the profile of diffusion coefficients for n = +2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The Dpp diffusion coefficient for ECH waves is one to two orders smaller than Dαα coefficients. For chorus waves, Dpp coefficients are about an order of magnitude smaller than Dαα coefficients for the case n ≠ 0. In case of Landau resonance, the values of Dpp coefficient are generally larger than the values of Dαα coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89° and harmonic resonances n = +1, +2, and +3, whereas for whistler mode waves, the frequencies have been calculated for angle 10° and Landau

  19. The brightness ratio of H Lyman-α/H2 bands in FUV auroral emissions: A diagnosis for the energy of precipitating electrons and associated magnetospheric acceleration processes applied to Saturn

    NASA Astrophysics Data System (ADS)

    Tao, Chihiro; Lamy, Laurent; Prangé, Renée.

    2014-10-01

    We propose that the ratio of the auroral brightness of H Lyman-α to that of far ultraviolet H2 and the absolute value of the H2 brightness provide good indicators of the acceleration versus nonacceleration processes for field-aligned auroral electron precipitation in the Saturn magnetosphere-ionosphere coupled system. This finding is based on model results indicating that this ratio is a decreasing function of the auroral electron energy over the whole auroral energy range, as previously suggested by Cassini observations. For electron energies above 5 keV, the results agree with the Knight relation, as in the environments of the Earth and Jupiter. On the other hand, decreasing electron flux with increasing electron energies below a few keV is also found and alternately explained as a nonacceleration reflecting the magnetospheric plasma distribution and/or wave-particle interactions.

  20. Electronic band gaps and transport properties in periodically alternating mono- and bi-layer graphene superlattices

    NASA Astrophysics Data System (ADS)

    Fan, Xiong; Huang, Wenjun; Ma, Tianxing; Wang, Li-Gang; Lin, Hai-Qing

    2015-12-01

    We investigate the electronic band structure and transport properties of periodically alternating mono- and bi-layer graphene superlattices (MBLG SLs). In such MBLG SLs, there exists a zero-averaged wave vector (zero-\\overline{k} ) gap that is insensitive to the lattice constant. This zero-\\overline{k} gap can be controlled by changing both the ratio of the potential widths and the interlayer coupling coefficient of the bilayer graphene. We also show that there exist extra Dirac points; the conditions for these extra Dirac points are presented analytically. Lastly, we demonstrate that the electronic transport properties and the energy gap of the first two bands in MBLG SLs are tunable through adjustment of the interlayer coupling and the width ratio of the periodic mono- and bi-layer graphene.

  1. Electronic band gaps and transport properties in periodically alternating mono- and bi-layer graphene superlattices

    NASA Astrophysics Data System (ADS)

    Fan, Xiong; Huang, Wenjun; Ma, Tianxing; Wang, Li-Gang; Lin, Hai-Qing

    We investigated electronic band structure and transport properties of periodically alternating mono- and bi-layer graphene superlattices (MBLG SLs). In such MBLG SLs, there exists the zero-averaged wave vector (zero- k) gap, which is insensitive to the lattice constant, and this zero- k gap can be controlled via changing both the ratio of potentials' widths and the interlayer coupling coefficient of bilayer graphene. It is also found that there exist the extra Dirac points and their conditions are analytically presented. Lastly, it shows that the electronic transport properties and the energy gap (Eg) of the first two bands in MBLG SLs are tunable by the interlayer coupling and the widths' ratio of the periodic mono- and bi-layer graphene.

  2. Quantification of electronic band gap and surface states on FeS2(100)

    NASA Astrophysics Data System (ADS)

    Herbert, F. W.; Krishnamoorthy, A.; Van Vliet, K. J.; Yildiz, B.

    2013-12-01

    The interfacial electronic properties and charge transfer characteristics of pyrite, FeS2, are greatly influenced by the presence of electronic states at the crystal free surface. We investigate the surface electronic structure of FeS2 (100) using scanning tunneling spectroscopy (STS) and interpret the results using tunneling current simulations informed by density functional theory. Intrinsic, dangling bond surface states located at the band edges reduce the fundamental band gap Eg from 0.95 eV in bulk FeS2 to 0.4 ± 0.1 eV at the surface. Extrinsic surface states from sulfur and iron defects contribute to Fermi level pinning but, due to their relatively low density of states, no detectable tunneling current was measured at energies within the intrinsic surface Eg. These findings help elucidate the nature of energy alignment for electron transfer processes at pyrite surfaces, which are relevant to evaluation of electrochemical processes including corrosion and solar energy conversion.

  3. Wavefunction Properties and Electronic Band Structures of High-Mobility Semiconductor Nanosheet MoS2

    NASA Astrophysics Data System (ADS)

    Baik, Seung Su; Lee, Hee Sung; Im, Seongil; Choi, Hyoung Joon; Ccsaemp Team; Edl Team

    2014-03-01

    Molybdenum disulfide (MoS2) nanosheet is regarded as one of the most promising alternatives to the current semiconductors due to its significant band-gap and electron-mobility enhancement upon exfoliating. To elucidate such thickness-dependent properties, we have studied the electronic band structures of bulk and monolayer MoS2 by using the first-principles density-functional method as implemented in the SIESTA code. Based on the wavefunction analyses at the conduction band minimum (CBM) points, we have investigated possible origins of mobility difference between bulk and monolayer MoS2. We provide formation energies of substitutional impurities at the Mo and S sites, and discuss feasible electron sources which may induce a significant difference in the carrier lifetime. This work was supported by NRF of Korea (Grant Nos. 2009-0079462 and 2011-0018306), Nano-Material Technology Development Program (2012M3a7B4034985), and KISTI supercomputing center (Project No. KSC-2013-C3-008). Center for Computational Studies of Advanced Electronic Material Properties.

  4. Manifestation of Structure of Electron Bands in Double-Resonant Raman Spectra of Single-Walled Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Stubrov, Yurii; Nikolenko, Andrii; Gubanov, Viktor; Strelchuk, Viktor

    2016-01-01

    Micro-Raman spectra of single-walled carbon nanotubes in the range of two-phonon 2D bands are investigated in detail. The fine structure of two-phonon 2D bands in the low-temperature Raman spectra of the mixture and individual single-walled carbon nanotubes is considered as the reflection of structure of their π-electron zones. The dispersion behavior of 2D band fine structure components in the resonant Raman spectra of single-walled carbon nanotube mixture is studied depending on the energy of excitating photons. The role of incoming and outgoing electron-phonon resonances in the formation of 2D band fine structure in Raman spectra of single-walled carbon nanotubes is analyzed. The similarity of dispersion behavior of 2D phonon bands in single-walled carbon nanotubes, one-layer graphene, and bulk graphite is discussed.

  5. Manifestation of Structure of Electron Bands in Double-Resonant Raman Spectra of Single-Walled Carbon Nanotubes.

    PubMed

    Stubrov, Yurii; Nikolenko, Andrii; Gubanov, Viktor; Strelchuk, Viktor

    2016-12-01

    Micro-Raman spectra of single-walled carbon nanotubes in the range of two-phonon 2D bands are investigated in detail. The fine structure of two-phonon 2D bands in the low-temperature Raman spectra of the mixture and individual single-walled carbon nanotubes is considered as the reflection of structure of their π-electron zones. The dispersion behavior of 2D band fine structure components in the resonant Raman spectra of single-walled carbon nanotube mixture is studied depending on the energy of excitating photons. The role of incoming and outgoing electron-phonon resonances in the formation of 2D band fine structure in Raman spectra of single-walled carbon nanotubes is analyzed. The similarity of dispersion behavior of 2D phonon bands in single-walled carbon nanotubes, one-layer graphene, and bulk graphite is discussed. PMID:26729220

  6. Quantitative analysis on electric dipole energy in Rashba band splitting

    NASA Astrophysics Data System (ADS)

    Hong, Jisook; Rhim, Jun-Won; Kim, Changyoung; Ryong Park, Seung; Hoon Shim, Ji

    2015-09-01

    We report on quantitative comparison between the electric dipole energy and the Rashba band splitting in model systems of Bi and Sb triangular monolayers under a perpendicular electric field. We used both first-principles and tight binding calculations on p-orbitals with spin-orbit coupling. First-principles calculation shows Rashba band splitting in both systems. It also shows asymmetric charge distributions in the Rashba split bands which are induced by the orbital angular momentum. We calculated the electric dipole energies from coupling of the asymmetric charge distribution and external electric field, and compared it to the Rashba splitting. Remarkably, the total split energy is found to come mostly from the difference in the electric dipole energy for both Bi and Sb systems. A perturbative approach for long wave length limit starting from tight binding calculation also supports that the Rashba band splitting originates mostly from the electric dipole energy difference in the strong atomic spin-orbit coupling regime.

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

    PubMed

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

    2008-07-25

    Results of density-functional calculations for indium thiospinel semiconductors substituted at octahedral sites with isolated transition metals (M=Ti,V) show an isolated partially filled narrow band containing three t2g-type states per M atom inside the usual semiconductor band gap. Thanks to this electronic structure feature, these materials will allow the absorption of photons with energy below the band gap, in addition to the normal light absorption of a semiconductor. To our knowledge, we demonstrate for the first time the formation of an isolated intermediate electronic band structure through M substitution at octahedral sites in a semiconductor, leading to an enhancement of the absorption coefficient in both infrared and visible ranges of the solar spectrum. This electronic structure feature could be applied for developing a new third-generation photovoltaic cell. PMID:18764346

  8. Electronic crosstalk in Terra MODIS thermal emissive bands

    NASA Astrophysics Data System (ADS)

    Sun, Junqiang; Madhavan, Sriharsha; Xiong, Xiaoxiong; Wang, Menghua

    2015-09-01

    The MODerate-resolution Imaging Spectroradiometer (MODIS) is a legacy Earth remote sensing instrument in the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS). The first MODIS instrument was launched in December 1999 on board the Terra spacecraft. MODIS has 36 bands, among which bands 20-25 and bands 27-36 are thermal emissive bands covering a wavelength range from 3.7μm to 14.2μm. It has been found that there are severe contaminations in Terra bands 27-30 (6.7 μm - 9.73 μm) due to crosstalk of signals among themselves. The crosstalk effect induces strong striping artifacts in the Earth View (EV) images and causes large long-term drifts in the EV brightness temperature (BT) in these bands. An algorithm using a linear approximation derived from on-orbit lunar observations has been developed to correct the crosstalk effect for them. It was demonstrated that the crosstalk correction can substantially reduce the striping noise in the EV images and significantly remove the long-term drifts in the EV BT in the Long Wave InfraRed (LWIR) water vapor channels (bands 27-28). In this paper, the crosstalk correction algorithm previously developed is applied to correct the crosstalk effect in the remaining LWIR bands 29 and 30. The crosstalk correction successfully reduces the striping artifact in the EV images and removes long-term drifts in the EV BT in bands 29-30 as was done similarly for bands 27-28. The crosstalk correction algorithm can thus substantially improve both the image quality and the radiometric accuracy of the Level 1B (L1B) products of the LWIR PV bands, bands 27-30. From this study it is also understood that other Terra MODIS thermal emissive bands are contaminated by the crosstalk effect and that the algorithm can be applied to these bands for crosstalk correction.

  9. Electronic Band Structure and Optical Properties of Srn+1TinO3n+1 Ruddlesden-Popper Homologous Series

    NASA Astrophysics Data System (ADS)

    Reshak, Ali Hussain; Auluck, Sushil; Kityk, Ivan

    2008-07-01

    State-of-the-art calculations of electronic band structures, density of states and frequency-dependent optical properties have been reported for Srn+1TinO3n+1 (n=1, 2, 3, ∞) compounds. These materials possess indirect wide energy band gaps. The frequency dependent optical properties of n=1,2,3 compounds show considerable anisotropy and positive birefringence. The conduction band minimum is originates from Ti-d states, while the valence band maximum is governed by O-p states. The bandwidth of the Ti-d states is responsible for the decrease in the energy band gap as n changes from 1 to 2, 3, and ∞. We have analyzed the degree of hybridization on the basis of the ratio of the orbital overlapping within the muffin tin sphere.

  10. Evidence of Eu{sup 2+} 4f electrons in the valence band spectra of EuTiO{sub 3} and EuZrO{sub 3}

    SciTech Connect

    Kolodiazhnyi, T.; Valant, M.; Williams, J. R.; Bugnet, M.; Botton, G. A.; Ohashi, N.; Sakka, Y.

    2012-10-15

    We report on optical band gap and valence electronic structure of two Eu{sup 2+}-based perovskites, EuTiO{sub 3} and EuZrO{sub 3} as revealed by diffuse optical scattering, electron energy loss spectroscopy, and valence-band x-ray photoelectron spectroscopy. The data show good agreement with the first-principles studies in which the top of the valence band structure is formed by the narrow Eu 4f{sup 7} electron band. The O 2p band shows the features similar to those of the Ba(Sr)TiO{sub 3} perovskites except that it is shifted to higher binding energies. Appearance of the Eu{sup 2+} 4f{sup 7} band is a reason for narrowing of the optical band gap in the title compounds as compared to their Sr-based analogues.

  11. Electronic band structure calculations of bismuth-antimony nanowires

    NASA Astrophysics Data System (ADS)

    Levin, Andrei; Dresselhaus, Mildred

    2012-02-01

    Alloys of bismuth and antimony received initial interest due to their unmatched low-temperature thermoelectric performance, and have drawn more recent attention as the first 3D topological insulators. One-dimensional bismuth-antimony (BiSb) nanowires display interesting quantum confinement effects, and are expected to exhibit even better thermoelectric properties than bulk BiSb. Due to the small, anisotropic carrier effective masses, the electronic properties of BiSb nanowires show great sensitivity to nanowire diameter, crystalline orientation, and alloy composition. We develop a theoretical model for calculating the band structure of BiSb nanowires. For a given crystalline orientation, BiSb nanowires can be in the semimetallic, direct semiconducting, or indirect semiconducting phase, depending on nanowire diameter and alloy composition. These ``phase diagrams'' turn out to be remarkably similar among the different orientations, which is surprising in light of the anisotropy of the bulk BiSb Fermi surface. We predict a novel direct semiconducting phase for nanowires with diameter less than ˜15 nm, over a narrow composition range. We also find that, in contrast to the bulk and thin film BiSb cases, a gapless state with Dirac dispersion cannot be realized in BiSb nanowires.

  12. Band Alignment and Controllable Electron Migration between Rutile and Anatase TiO2

    PubMed Central

    Mi, Yang; Weng, Yuxiang

    2015-01-01

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

  13. Surface-plasmon enhanced photodetection at communication band based on hot electrons

    SciTech Connect

    Wu, Kai; Zhan, Yaohui E-mail: xfli@suda.edu.cn; Wu, Shaolong; Deng, Jiajia; Li, Xiaofeng E-mail: xfli@suda.edu.cn

    2015-08-14

    Surface plasmons can squeeze light into a deep-subwavelength space and generate abundant hot electrons in the nearby metallic regions, enabling a new paradigm of photoconversion by the way of hot electron collection. Unlike the visible spectral range concerned in previous literatures, we focus on the communication band and design the infrared hot-electron photodetectors with plasmonic metal-insulator-metal configuration by using full-wave finite-element method. Titanium dioxide-silver Schottky interface is employed to boost the low-energy infrared photodetection. The photodetection sensitivity is strongly improved by enhancing the plasmonic excitation from a rationally engineered metallic grating, which enables a strong unidirectional photocurrent. With a five-step electrical simulation, the optimized device exhibits an unbiased responsivity of ∼0.1 mA/W and an ultra-narrow response band (FWHM = 4.66 meV), which promises to be a candidate as the compact photodetector operating in communication band.

  14. A broad-band VLF-burst associated with ring-current electrons. [geomagnetic storms

    NASA Technical Reports Server (NTRS)

    Maeda, K.

    1982-01-01

    Frequency band broadening takes place just outside of the nighttime plasmasphere, where the density of cold plasma is known to be very low during the later phase of a geomagnetic storm. Instead of the gradual broadening of several hours duration, a burst type broadening of VLF emission lasting less than ten minutes was observed by Explorer 45 in a similar location. The magnetic field component of this emission is very weak and the frequency spreads below the local half electron cyclotron frequency. Corresponding enhancement of the anisotropic ring current electrons is also very sudden and limited below the order of 10 keV without significant velocity dispersion, in contrast to the gradual broadening events. The cause of this type of emission band spreading can be attributed to the generation of the quasielectrostatic whistler mode emission of short wavelength by hot bimaxwellian electrons surging into the domain of relatively low density magnetized cold plasma. The lack of energy dispersion in the enhanced electrons indicates that the inner edge of the plasma sheet, the source of these hot electrons, is not far from the location of this event.

  15. Band-to-band tunneling distance analysis in the heterogate electron-hole bilayer tunnel field-effect transistor

    NASA Astrophysics Data System (ADS)

    Padilla, J. L.; Palomares, A.; Alper, C.; Gámiz, F.; Ionescu, A. M.

    2016-01-01

    In this work, we analyze the behavior of the band-to-band tunneling distance between electron and hole subbands resulting from field-induced quantum confinement in the heterogate electron-hole bilayer tunnel field-effect transistor. We show that, analogously to the explicit formula for the tunneling distance that can be easily obtained in the semiclassical framework where the conduction and valence band edges are allowed states, an equivalent analytical expression can be derived in the presence of field-induced quantum confinement for describing the dependence of the tunneling distance on the body thickness and material properties of the channel. This explicit expression accounting for quantum confinement holds valid provided that the potential wells for electrons and holes at the top and bottom of the channel can be approximated by triangular profiles. Analytical predictions are compared to simulation results showing very accurate agreement.

  16. Band-structure-based collisional model for electronic excitations in ion-surface collisions

    SciTech Connect

    Faraggi, M.N.; Gravielle, M.S.; Alducin, M.; Silkin, V.M.; Juaristi, J.I.

    2005-07-15

    Energy loss per unit path in grazing collisions with metal surfaces is studied by using the collisional and dielectric formalisms. Within both theories we make use of the band-structure-based (BSB) model to represent the surface interaction. The BSB approach is based on a model potential and provides a precise description of the one-electron states and the surface-induced potential. The method is applied to evaluate the energy lost by 100 keV protons impinging on aluminum surfaces at glancing angles. We found that when the realistic BSB description of the surface is used, the energy loss obtained from the collisional formalism agrees with the dielectric one, which includes not only binary but also plasmon excitations. The distance-dependent stopping power derived from the BSB model is in good agreement with available experimental data. We have also investigated the influence of the surface band structure in collisions with the Al(100) surface. Surface-state contributions to the energy loss and electron emission probability are analyzed.

  17. Generation of Intense Narrow-Band Tunable Terahertz Radiation from Highly Bunched Electron Pulse Train

    NASA Astrophysics Data System (ADS)

    Li, Heting; Lu, Yalin; He, Zhigang; Jia, Qika; Wang, Lin

    2016-07-01

    We present the analysis and start-to-end simulation of an intense narrow-band terahertz (THz) source with a broad tuning range of radiation frequency, using a single-pass free electron laser (FEL) driven by a THz-pulse-train photoinjector. The fundamental radiation frequency, corresponding to the spacing between the electron microbunches, can be easily tuned by varying the spacing time between the laser micropulses. Since the prebunched electron beam is highly bunched at the first several harmonics, with the harmonic generation technique, the radiation frequency range can be further enlarged by several times. The start-to-end simulation results show that this FEL is capable of generating a few tens megawatts power, several tens micro-joules pulse energy, and a few percent bandwidth at the frequencies of 0.5-5 THz. In addition, several practical issues are considered.

  18. Narrow-band Electrostatic Noise generated by an electron velocity space hole

    NASA Technical Reports Server (NTRS)

    Richard, Robert L.; Ashour-Abdalla, Maha; Coroniti, Ferdinand V.

    1993-01-01

    Narrow-band Electrostatic Noise (NEN) is a common occurrence in the Earth's distant magnetotail. NEN is observed in a frequency range (100-316 Hz) that falls roughly between the electron and ion plasma frequencies. This mode may result from holes in the electron distribution function associated with slow shocks. An instability that is associated with this mode is studied using numerical simulations. The growth of the instability depends on the size and shape of the hole. The hole mode can also be driven unstable by either an anisotropy in the electron distribution function or an ion beam. In all these cases the instability saturates at a low level and only a fraction of the available free energy is released.

  19. Flat electronic bands in fractal-kagomé network and the effect of perturbation

    NASA Astrophysics Data System (ADS)

    Nandy, Atanu; Chakrabarti, Arunava

    2016-05-01

    We demonstrate an analytical prescription of demonstrating the flat band [FB] states in a fractal incorporated kagomé type network that can give rise to a countable infinity of flat non-dispersive eigenstates with a multitude of localization area. The onset of localization can, in principle, be delayed in space by an appropriate choice of energy regime. The length scale, at which the onset of localization for each mode occurs, can be tuned at will following the formalism developed within the framework of real space renormalization group. This scheme leads to an exact determination of energy eigenvalue for which one can have dispersionless flat electronic bands. Furthermore, we have shown the effect ofuniform magnetic field for the same non-translationally invariant network model that has ultimately led to an`apparent invisibility' of such staggered localized states and to generate absolutely continuous sub-bands in the energy spectrum and again an interesting re-entrant behavior of those FB states.

  20. Electronic band structure effects in monolayer, bilayer, and hybrid graphene structures

    NASA Astrophysics Data System (ADS)

    Puls, Conor

    Since its discovery in 2005, graphene has been the focus of intense theoretical and experimental study owing to its unique two-dimensional band structure and related electronic properties. In this thesis, we explore the electronic properties of graphene structures from several perspectives including the magnetoelectrical transport properties of monolayer graphene, gap engineering and measurements in bilayer graphene, and anomalous quantum oscillation in the monolayer-bilayer graphene hybrids. We also explored the device implications of our findings, and the application of some experimental techniques developed for the graphene work to the study of a complex oxide, Ca3Ru2O7, exhibiting properties of strongly correlated electrons. Graphene's high mobility and ballistic transport over device length scales, make it suitable for numerous applications. However, two big challenges remain in the way: maintaining high mobility in fabricated devices, and engineering a band gap to make graphene compatible with logical electronics and various optical devices. We address the first challenge by experimentally evaluating mobilities in scalable monolayer graphene-based field effect transistors (FETs) and dielectric-covered Hall bars. We find that the mobility is limited in these devices, and is roughly inversely proportional to doping. By considering interaction of graphene's Dirac fermions with local charged impurities at the interface between graphene and the top-gate dielectric, we find that Coulomb scattering is responsible for degraded mobility. Even in the cleanest devices, a band gap is still desirable for electronic applications of graphene. We address this challenge by probing the band structure of bilayer graphene, in which a field-tunable energy band gap has been theoretically proposed. We use planar tunneling spectroscopy of exfoliated bilayer graphene flakes demonstrate both measurement and control of the energy band gap. We find that both the Fermi level and

  1. Band structure and electron-phonon coupling in H3S : A tight-binding model

    NASA Astrophysics Data System (ADS)

    Ortenzi, L.; Cappelluti, E.; Pietronero, L.

    2016-08-01

    We present a robust tight-binding description, based on the Slater-Koster formalism, of the band structure of H3S in the Im3 ¯m structure, stable in the range of pressure P =180 -220 GPa. We show that the interatomic hopping between the 3 s and 3 p orbitals (and partially between the 3 p orbitals themselves) of sulfur is fundamental to capturing the relevant physics associated with the Van Hove singularities close to the Fermi level. Comparing the model so defined with density functional theory calculations we obtain a very good agreement not only of the overall band structure but also of the low-energy states and the Fermi surface properties. The description in terms of Slater-Koster parameters permits us also to evaluate at a microscopic level a hopping-resolved linear electron-lattice coupling which can be employed for further tight-binding analyses also at a local scale.

  2. Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands

    SciTech Connect

    Wickramaratne, Darshana E-mail: rlake@ece.ucr.edu; Lake, Roger K. E-mail: rlake@ece.ucr.edu; Zahid, Ferdows

    2015-08-21

    The valence band of a variety of few-layer, two-dimensional materials consist of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a “Mexican hat” or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III–VI materials GaS, GaSe, InS, InSe, for Bi{sub 2}Se{sub 3}, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III–VI materials and Bi{sub 2}Se{sub 3} is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.

  3. Analytic band Monte Carlo model for electron transport in Si including acoustic and optical phonon dispersion

    NASA Astrophysics Data System (ADS)

    Pop, Eric; Dutton, Robert W.; Goodson, Kenneth E.

    2004-11-01

    We describe the implementation of a Monte Carlo model for electron transport in silicon. The model uses analytic, nonparabolic electron energy bands, which are computationally efficient and sufficiently accurate for future low-voltage (<1V) nanoscale device applications. The electron-lattice scattering is incorporated using an isotropic, analytic phonon-dispersion model, which distinguishes between the optical/acoustic and the longitudinal/transverse phonon branches. We show that this approach avoids introducing unphysical thresholds in the electron distribution function, and that it has further applications in computing detailed phonon generation spectra from Joule heating. A set of deformation potentials for electron-phonon scattering is introduced and shown to yield accurate transport simulations in bulk silicon across a wide range of electric fields and temperatures. The shear deformation potential is empirically determined at Ξu=6.8eV, and consequently, the isotropically averaged scattering potentials with longitudinal and transverse acoustic phonons are DLA=6.39eV and DTA=3.01eV, respectively, in reasonable agreement with previous studies. The room-temperature electron mobility in strained silicon is also computed and shown to be in better agreement with the most recent phonon-limited data available. As a result, we find that electron coupling with g-type phonons is about 40% lower, and the coupling with f-type phonons is almost twice as strong as previously reported.

  4. Strain-Induced Energy Band Gap Opening in Two-Dimensional Bilayered Silicon Film

    NASA Astrophysics Data System (ADS)

    Ji, Z.; Zhou, R.; Lew Yan Voon, L. C.; Zhuang, Y.

    2016-06-01

    This work presents a theoretical study of the structural and electronic properties of bilayered silicon film (BiSF) under in-plane biaxial strain/stress using density functional theory (DFT). Atomic structures of the two-dimensional (2-D) silicon films are optimized by using both the local-density approximation (LDA) and generalized gradient approximation (GGA). In the absence of strain/stress, five buckled hexagonal honeycomb structures of the BiSF with triangular lattice have been obtained as local energy minima, and their structural stability has been verified. These structures present a Dirac-cone shaped energy band diagram with zero energy band gaps. Applying a tensile biaxial strain leads to a reduction of the buckling height. Atomically flat structures with zero buckling height have been observed when the AA-stacking structures are under a critical biaxial strain. Increase of the strain between 10.7% and 15.4% results in a band-gap opening with a maximum energy band gap opening of ˜0.17 eV, obtained when a 14.3% strain is applied. Energy band diagrams, electron transmission efficiency, and the charge transport property are calculated. Additionally, an asymmetric energetically favorable atomic structure of BiSF shows a non-zero band gap in the absence of strain/stress and a maximum band gap of 0.15 eV as a -1.71% compressive strain is applied. Both tensile and compressive strain/stress can lead to a band gap opening in the asymmetric structure.

  5. Photoconductivities from band states and a dissipative electron dynamics: Si(111) without and with adsorbed Ag clusters

    NASA Astrophysics Data System (ADS)

    Vazhappilly, Tijo; Hembree, Robert H.; Micha, David A.

    2016-01-01

    A new general computational procedure is presented to obtain photoconductivities starting from atomic structures, combining ab initio electronic energy band states with populations from density matrix theory, and implemented for a specific set of materials based on Si crystalline slabs and their nanostructured surfaces without and with adsorbed Ag clusters. The procedure accounts for charge mobility in semiconductors in photoexcited states, and specifically electron and hole photomobilities at Si(111) surfaces with and without adsorbed Ag clusters using ab initio energy bands and orbitals generated from a generalized gradient functional, however with excited energy levels modified to provide correct bandgaps. Photoexcited state populations for each band and carrier type were generated using steady state solution of a reduced density matrix which includes dissipative medium effects. The present calculations provide photoexcited electronic populations and photoinduced mobilities resulting from applied electric fields and obtained from the change of driven electron energies with their electronic momentum. Extensive results for Si slabs with 8 layers, without and with adsorbed Ag clusters, show that the metal adsorbates lead to substantial increases in the photomobility and photoconductivity of electrons and holes.

  6. Photoconductivities from band states and a dissipative electron dynamics: Si(111) without and with adsorbed Ag clusters.

    PubMed

    Vazhappilly, Tijo; Hembree, Robert H; Micha, David A

    2016-01-14

    A new general computational procedure is presented to obtain photoconductivities starting from atomic structures, combining ab initio electronic energy band states with populations from density matrix theory, and implemented for a specific set of materials based on Si crystalline slabs and their nanostructured surfaces without and with adsorbed Ag clusters. The procedure accounts for charge mobility in semiconductors in photoexcited states, and specifically electron and hole photomobilities at Si(111) surfaces with and without adsorbed Ag clusters using ab initio energy bands and orbitals generated from a generalized gradient functional, however with excited energy levels modified to provide correct bandgaps. Photoexcited state populations for each band and carrier type were generated using steady state solution of a reduced density matrix which includes dissipative medium effects. The present calculations provide photoexcited electronic populations and photoinduced mobilities resulting from applied electric fields and obtained from the change of driven electron energies with their electronic momentum. Extensive results for Si slabs with 8 layers, without and with adsorbed Ag clusters, show that the metal adsorbates lead to substantial increases in the photomobility and photoconductivity of electrons and holes. PMID:26772554

  7. Extreme sensitivity of the electric-field-induced band gap to the electronic topological transition in sliding bilayer graphene

    PubMed Central

    Lee, Kyu Won; Lee, Cheol Eui

    2015-01-01

    We have investigated the effect of electronic topological transition on the electric field-induced band gap in sliding bilayer graphene by using the density functional theory calculations. The electric field-induced band gap was found to be extremely sensitive to the electronic topological transition. At the electronic topological transition induced by layer sliding, four Dirac cones in the Bernal-stacked bilayer graphene reduces to two Dirac cones with equal or unequal Dirac energies depending on the sliding direction. While the critical electric field required for the band gap opening increases with increasing lateral shift for the two Dirac cones with unequal Dirac energies, the critical field is essentially zero with or without a lateral shift for the two Dirac cones with equal Dirac energies. The critical field is determined by the Dirac energy difference and the electronic screening effect. The electronic screening effect was also found to be enhanced with increasing lateral shift, apparently indicating that the massless helical and massive chiral fermions are responsible for the perfect and imperfect electronic screening, respectively. PMID:26635178

  8. Corrected electron inelastic mean free paths (IMFPs) for selected wide band semiconductors

    NASA Astrophysics Data System (ADS)

    Krawczyk, M.

    2008-03-01

    Elastic peak electron spectroscopy (EPES) has been widely used to determine the electron inelastic mean free paths (IMFPs) in solids. In this work, we investigated quantitatively the influence of surface excitations on electron IMFPs determined by EPES. We used IMFPs obtained from the early EPES measurements of the electron elastic backscattering probability from GaN and Cd0.88Mn0.12 Te wideband-gap semiconductors, and the Ni standard in the energy range 200-2000 eV. The total surface-excitation parameter (SEP) was evaluated using Chen and Werner approaches, and was applied for correcting the EPES IMFPs. These corrected values were then compared with those predicted by the TPP-2M formula. We found that implementation of the surface-excitation correction improved agreement between the resulting IMFPs for selected wide band semiconductors and the TPP-2M values at low-energy (E > 500 eV) electrons. The extent to which the IMFPs measured by EPES differ from the corresponding bulk values (on account of surface excitations) was found to depend on the semiconductor material with finite surface. Our results also clearly demonstrated the importance of accounting for surface excitations for accuracy of the IMFPs measured for GaN.

  9. Temperature dependence of band gaps in semiconductors: Electron-phonon interaction

    NASA Astrophysics Data System (ADS)

    Bhosale, J.; Ramdas, A. K.; Burger, A.; Muñoz, A.; Romero, A. H.; Cardona, M.; Lauck, R.; Kremer, R. K.

    2012-11-01

    We have theoretically investigated, by ab initio techniques, the phonon properties of several semiconductors with chalcopyrite structure. Comparison with experiments has led us to distinguish between materials with d electrons in the valence band (e.g., CuGaS2, AgGaS2) and those without d electrons (e.g., ZnSnAs2). The former exhibit a rather peculiar nonmonotonic temperature dependence of the energy gap which, so far, has resisted cogent theoretical description. We analyze this nonmonotonic temperature dependence by fitting two Bose-Einstein oscillators with weights of opposite sign leading to an increase at low temperatures and a decrease at higher temperatures and find that the energy of the former correlates well with characteristic peaks in the phonon density of states associated with low-energy vibrations of the d-electron elements. We hope that this work will encourage theoretical investigations of the electron-phonon interaction in this direction, especially of the current ab initio type.

  10. Physical properties and electronic band structure of noncentrosymmetric Th7Co3 superconductor.

    PubMed

    Sahakyan, M; Tran, V H

    2016-05-25

    The physical properties of the noncentrosymmetric superconductor Th7Co3 have been investigated by means of ac-magnetic susceptibility, magnetization, specific heat, electrical resistivity, magnetoresistance and Hall effect measurements. From these data it is established that Th7Co3 is a dirty type-II superconductor with [Formula: see text] K, [Formula: see text] and moderate electron-phonon coupling [Formula: see text]. Some evidences for anisotropic superconducting gap are found, including e.g. reduced specific heat jump ([Formula: see text]) at T c, diminished superconducting energy gap ([Formula: see text]) as compared to the BCS values, power law field dependence of the Sommerfeld coefficient at 0.4 K ([Formula: see text]), and a concave curvature of the [Formula: see text] line. The magnitudes of the thermodynamic critical field and the energy gap are consistent with mean-squared anisotropy parameter [Formula: see text]. The electronic specific heat in the superconducting state is reasonably fitted to an oblate spheroidal gap model. Calculations of scalar relativistic and fully relativistic electronic band structures reveal considerable differences in the degenerate structure, resulting from asymmetric spin-orbit coupling (ASOC). A large splitting energy of spin-up spin-down bands at the Fermi level E F, [Formula: see text] meV is observed and a sizeable ratio [Formula: see text] could classify the studied compound into the class of noncentrosymmetric superconductors with strong ASOC. The noncentrosymmetry of the crystal structure and the atomic relativistic effects are both responsible for an importance of ASOC in Th7Co3. The calculated results for the density of states show a Van Hove singularity just below E F and dominant role of the 6d electrons of Th to the superconductivity. PMID:27120582

  11. Electronic band structure calculation of GaNAsBi alloys and effective mass study

    NASA Astrophysics Data System (ADS)

    Habchi, M. M.; Ben Nasr, A.; Rebey, A.; El Jani, B.

    2013-11-01

    Electronic band structures of GaNxAs1-x-yBiy dilute nitrides-bismides have been determined theoretically within the framework of the band anticrossing (BAC) model and k ṡ p method. We have developed computer codes based on our extended BAC model, denoted (16 × 16), in which the dimension of the used states basis was equal to 16. We have investigated the band gap and the spin orbit splitting as a function of Bi composition for alloys lattice matched to GaAs. We have found that the substitution of As element by N and Bi impurities leads to a significant reduction of band gap energy by roughly 198 meV/%Bi. Meanwhile, spin orbit splitting increases by 56 meV/%Bi regardless N content. There is an excellent agreement between the model predictions and experiment reported in the literature. In addition, alloys compositions and oscillator strengths of transition energies have been calculated for GaNAsBi alloys which represent active zone of temperature insensitive (1.55 μm and 1.3 μm) wavelength laser diodes intended for optical fiber communications. A crossover at about 0.6 eV has occurred between Eg and Δso of GaN.039As.893Bi.068. When the quaternary is lattice mismatched to GaAs, resonance energy increases with Bi content if N content decreases. On the other hand, effective mass behavior of carriers at Γ point has been discussed with respect to alloy composition, k-directions and lattice mismatch.

  12. Finding the bare band: Electron coupling to two phonon modes in potassium-doped graphene on Ir(111)

    NASA Astrophysics Data System (ADS)

    Pletikosić, I.; Kralj, M.; Milun, M.; Pervan, P.

    2012-04-01

    We analyze renormalization of the π* band of n-doped epitaxial graphene on Ir(111) induced by electron-phonon coupling. Our procedure of extracting the bare band relies on recursive self-consistent refining of the functional form of the bare band until the convergence. We demonstrate that the components of the self-energy, as well as the spectral intensity obtained from angle-resolved photoelectron spectroscopy, show that the renormalization is due to the coupling to two distinct phonon excitations. From the velocity renormalization and an increase of the imaginary part of the self-energy we find the electron-phonon coupling constant to be ˜0.2, which is in fair agreement with a previous study of the same system, despite the notable difference in the width of spectroscopic curves. Our experimental results also suggest that potassium intercalated between graphene and Ir(111) does not introduce any additional increase of the quasiparticle scattering rate.

  13. Shallow electron traps in alkali halide crystals: Mollwo-Ivey relations of the optical absorption bands

    NASA Astrophysics Data System (ADS)

    Ziraps, Valters

    2001-03-01

    Evidences are given that two classes of the transient IR- absorption bands: (a) with max. at 0.27-0.36 eV in NaCl, KCl, KBr, KI and RbCl (due to shallow electron traps according G. Jacobs or due to bound polarons according E.V. Korovkin and T.A. Lebedkina) and (b) with max. at 0.15-0.36 eV in NaI, NaBr, NaCl:I, KCl:I, RbCl:I and RbBr:I (due to on-center STE localized at iodine dimer according M. Hirai and collaborators) are caused by the same defect- atomic alkali impurity center [M+]c0e- (electron e- trapped by a substitutional smaller size alkali cation impurity [M+]c0). The Mollwo-Ivey plots (for the transient IR-absorption bands) of the zero-phonon line energy E0 (for NaCl, KCl, KBr, RbCl and NaBr, KCl:I) and/or the low-energy edge valued E0 (for NaI, RbCl:I, RbBr:I) versus anion-cation distance (d) evidence that two types of the [M+]c0e- centers are predominant: (a) [Na+]c0e- in the KX and RbX host crystals with the relation E0approximately equals 6.15/d2.74, (b) [Li+]c03- in the NaX host crystals - E0approximately equals 29.4/d4.72. The Mollwo-Ivey relation E0approximately equals 18.36/d(superscript 2.70 is fulfilled as well for the F' band in NaCl, KCl, KBr, KI, RbCl, RbI if we use the F' center optical binding energy values E0.

  14. Theoretical and experimental differential cross sections for electron impact excitation of the electronic bands of furfural.

    PubMed

    Jones, D B; Neves, R F C; Lopes, M C A; da Costa, R F; do N Varella, M T; Bettega, M H F; Lima, M A P; García, G; Limão-Vieira, P; Brunger, M J

    2016-03-28

    We report results from a joint experimental and theoretical investigation into electron scattering from the important industrial species furfural (C5H4O2). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C5H4O2. The measurements were carried out at energies in the range 20-40 eV, and for scattered-electron angles between 10° and 90°. The energy resolution of those experiments was typically ∼80 meV. Corresponding Schwinger multichannel method with pseudo-potential calculations, for energies between 6-50 eV and with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were undertaken at the static exchange plus polarisation-level using a minimum orbital basis for single configuration interaction (MOB-SCI) approach. Agreement between the measured and calculated DCSs was qualitatively quite good, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOB-SCI. The role of multichannel coupling on the computed electronic-state DCSs is also explored in some detail. PMID:27036450

  15. Theoretical and experimental differential cross sections for electron impact excitation of the electronic bands of furfural

    NASA Astrophysics Data System (ADS)

    Jones, D. B.; Neves, R. F. C.; Lopes, M. C. A.; da Costa, R. F.; do N. Varella, M. T.; Bettega, M. H. F.; Lima, M. A. P.; García, G.; Limão-Vieira, P.; Brunger, M. J.

    2016-03-01

    We report results from a joint experimental and theoretical investigation into electron scattering from the important industrial species furfural (C5H4O2). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C5H4O2. The measurements were carried out at energies in the range 20-40 eV, and for scattered-electron angles between 10° and 90°. The energy resolution of those experiments was typically ˜80 meV. Corresponding Schwinger multichannel method with pseudo-potential calculations, for energies between 6-50 eV and with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were undertaken at the static exchange plus polarisation-level using a minimum orbital basis for single configuration interaction (MOB-SCI) approach. Agreement between the measured and calculated DCSs was qualitatively quite good, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOB-SCI. The role of multichannel coupling on the computed electronic-state DCSs is also explored in some detail.

  16. Quantitative analysis on electric dipole energy in Rashba band splitting.

    PubMed

    Hong, Jisook; Rhim, Jun-Won; Kim, Changyoung; Ryong Park, Seung; Hoon Shim, Ji

    2015-01-01

    We report on quantitative comparison between the electric dipole energy and the Rashba band splitting in model systems of Bi and Sb triangular monolayers under a perpendicular electric field. We used both first-principles and tight binding calculations on p-orbitals with spin-orbit coupling. First-principles calculation shows Rashba band splitting in both systems. It also shows asymmetric charge distributions in the Rashba split bands which are induced by the orbital angular momentum. We calculated the electric dipole energies from coupling of the asymmetric charge distribution and external electric field, and compared it to the Rashba splitting. Remarkably, the total split energy is found to come mostly from the difference in the electric dipole energy for both Bi and Sb systems. A perturbative approach for long wave length limit starting from tight binding calculation also supports that the Rashba band splitting originates mostly from the electric dipole energy difference in the strong atomic spin-orbit coupling regime. PMID:26323493

  17. Quantitative analysis on electric dipole energy in Rashba band splitting

    PubMed Central

    Hong, Jisook; Rhim, Jun-Won; Kim, Changyoung; Ryong Park, Seung; Hoon Shim, Ji

    2015-01-01

    We report on quantitative comparison between the electric dipole energy and the Rashba band splitting in model systems of Bi and Sb triangular monolayers under a perpendicular electric field. We used both first-principles and tight binding calculations on p-orbitals with spin-orbit coupling. First-principles calculation shows Rashba band splitting in both systems. It also shows asymmetric charge distributions in the Rashba split bands which are induced by the orbital angular momentum. We calculated the electric dipole energies from coupling of the asymmetric charge distribution and external electric field, and compared it to the Rashba splitting. Remarkably, the total split energy is found to come mostly from the difference in the electric dipole energy for both Bi and Sb systems. A perturbative approach for long wave length limit starting from tight binding calculation also supports that the Rashba band splitting originates mostly from the electric dipole energy difference in the strong atomic spin-orbit coupling regime. PMID:26323493

  18. 8-band and 14-band kp modeling of electronic band structure and material gain in Ga(In)AsBi quantum wells grown on GaAs and InP substrates

    SciTech Connect

    Gladysiewicz, M.; Wartak, M. S.; Kudrawiec, R.

    2015-08-07

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

  19. Dynamical and anharmonic effects on the electron-phonon coupling and the zero-point renormalization of the band structure

    NASA Astrophysics Data System (ADS)

    Antonius, Gabriel; Poncé, Samuel; Lantagne-Hurtubise, Étienne; Auclair, Gabriel; Côté, Michel; Gonze, Xavier

    2015-03-01

    The electron-phonon coupling in solids renormalizes the band structure, reducing the band gap by several tenths of an eV in light-atoms semiconductors. Using the Allen-Heine-Cardona theory (AHC), we compute the zero-point renormalization (ZPR) as well as the quasiparticle lifetimes of the full band structure in diamond, BN, LiF and MgO. We show how dynamical effects can be included in the AHC theory, and still allow for the use of a Sternheimer equation to avoid the summation over unoccupied bands. The convergence properties of the electron-phonon coupling self-energy with respect to the Brillouin zone sampling prove to be strongly affected by dynamical effects. We complement our study with a frozen-phonon approach, which reproduces the static AHC theory, but also allows to probe the phonon wavefunctions at finite displacements and include anharmonic effects in the self-energy. We show that these high-order components tend to reduce the strongest electron-phonon coupling elements, which affects significantly the band gap ZPR.

  20. High binding energy band structure of Bi-2212 as measured by ARPES

    NASA Astrophysics Data System (ADS)

    McElroy, K.; Graf, J.; Gweon, G.-H.; Zhou, S. Y.; Sahrakorpi, S.; Lindroos, M.; Markiewicz, R. S.; Bansil, A.; Eisaki, H.; Sasagawa, T.; Takagi, H.; Uchida, S.; Lanzara, A.

    2006-03-01

    The study of the electronic structure of high temperature superconductors by angle resolved photoemission spectroscopy (ARPES) has so far focused on the states near the Fermi level, believed to be fundamental for most of the properties of cuprates. However, it is well known that in doped Mott insulators the low and high energy physics are strongly coupled one to the other. Therefore, to gain insight on the real physics of cuprates a full characterization of the electronic band structure up to energies of the order of the lower Hubbard band and beyond is needed. Here we report a detailed, doping dependent study of the band structure of Bi2212 superconductors at energies of the order of 1-2 eV. The experimental results are interpreted in terms of local density approximation (LDA) based computations, where the presence of the ``spaghetti'' of Cu-O and O-bands is predicted. Comparison between computed and measured bands provides insight into many-body renormalization effects.

  1. Effect of low-temperature annealing on the electronic- and band-structures of (Ga,Mn)As epitaxial layers

    NASA Astrophysics Data System (ADS)

    Yastrubchak, O.; Wosinski, T.; Gluba, L.; Andrearczyk, T.; Domagala, J. Z.; Żuk, J.; Sadowski, J.

    2014-01-01

    The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial layers, caused by post-growth low-temperature annealing, on their electronic- and band-structure properties has been investigated by modulation photoreflectance (PR) spectroscopy. The annealing-induced changes in structural and magnetic properties of the layers were examined with high-resolution X-ray diffractometry and superconducting quantum interference device magnetometry, respectively. They confirmed an outdiffusion of Mn interstitials from the layers and an enhancement in their hole concentration, which were more efficient for the layer covered with a Sb cap acting as a sink for diffusing Mn interstitials. The PR results demonstrating a decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers, with respect to that in the reference GaAs one, are interpreted by assuming a merging of the Mn-related impurity band with the GaAs valence band. Whereas an increase in the band-gap-transition energy caused by the annealing treatment of the (Ga,Mn)As layers is interpreted as a result of annealing-induced enhancement of the free-hole concentration and the Fermi level location within the valence band. The experimental results are consistent with the valence-band origin of itinerant holes mediating ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for ferromagnetic semiconductors.

  2. Effect of low-temperature annealing on the electronic- and band-structures of (Ga,Mn)As epitaxial layers

    SciTech Connect

    Yastrubchak, O. Gluba, L.; Żuk, J.; Wosinski, T. Andrearczyk, T.; Domagala, J. Z.; Sadowski, J.

    2014-01-07

    The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial layers, caused by post-growth low-temperature annealing, on their electronic- and band-structure properties has been investigated by modulation photoreflectance (PR) spectroscopy. The annealing-induced changes in structural and magnetic properties of the layers were examined with high-resolution X-ray diffractometry and superconducting quantum interference device magnetometry, respectively. They confirmed an outdiffusion of Mn interstitials from the layers and an enhancement in their hole concentration, which were more efficient for the layer covered with a Sb cap acting as a sink for diffusing Mn interstitials. The PR results demonstrating a decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers, with respect to that in the reference GaAs one, are interpreted by assuming a merging of the Mn-related impurity band with the GaAs valence band. Whereas an increase in the band-gap-transition energy caused by the annealing treatment of the (Ga,Mn)As layers is interpreted as a result of annealing-induced enhancement of the free-hole concentration and the Fermi level location within the valence band. The experimental results are consistent with the valence-band origin of itinerant holes mediating ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for ferromagnetic semiconductors.

  3. Energy band gaps in graphene nanoribbons with corners

    NASA Astrophysics Data System (ADS)

    Szczȩśniak, Dominik; Durajski, Artur P.; Khater, Antoine; Ghader, Doried

    2016-05-01

    In the present paper, we study the relation between the band gap size and the corner-corner length in representative chevron-shaped graphene nanoribbons (CGNRs) with 120° and 150° corner edges. The direct physical insight into the electronic properties of CGNRs is provided within the tight-binding model with phenomenological edge parameters, developed against recent first-principle results. We show that the analyzed CGNRs exhibit inverse relation between their band gaps and corner-corner lengths, and that they do not present a metal-insulator transition when the chemical edge modifications are introduced. Our results also suggest that the band gap width for the CGNRs is predominantly governed by the armchair edge effects, and is tunable through edge modifications with foreign atoms dressing.

  4. Theoretical Analysis on the Band Structure Variance of the Electron Doped 1111 Iron-based Superconductors

    NASA Astrophysics Data System (ADS)

    Suzuki, K.; Usui, H.; Iimura, S.; Sato, Y.; Matsuishi, S.; Hosono, H.; Kuroki, K.

    We perform first principles band calculation of electron doped iron-based superconductors adopting the virtual crystal approximation. We find that when electrons are doped by element substitution in the blocking layer, the band structure near the Fermi level is affected due to the increase of the positive charge in the layer. On the other hand, when Fe in the conducting layer is substituted by Co, the band structure is barely affected. This difference should be a key factor in understanding the phase diagram of the heavily doped electron doped systems LnFeAsO1-xHx.

  5. All-electron GW quasiparticle band structures of group 14 nitride compounds

    NASA Astrophysics Data System (ADS)

    Chu, Iek-Heng; Kozhevnikov, Anton; Schulthess, Thomas C.; Cheng, Hai-Ping

    2014-07-01

    We have investigated the group 14 nitrides (M3N4) in the spinel phase (γ-M3N4 with M = C, Si, Ge, and Sn) and β phase (β-M3N4 with M = Si, Ge, and Sn) using density functional theory with the local density approximation and the GW approximation. The Kohn-Sham energies of these systems have been first calculated within the framework of full-potential linearized augmented plane waves (LAPW) and then corrected using single-shot G0W0 calculations, which we have implemented in the modified version of the Elk full-potential LAPW code. Direct band gaps at the Γ point have been found for spinel-type nitrides γ-M3N4 with M = Si, Ge, and Sn. The corresponding GW-corrected band gaps agree with experiment. We have also found that the GW calculations with and without the plasmon-pole approximation give very similar results, even when the system contains semi-core d electrons. These spinel-type nitrides are novel materials for potential optoelectronic applications because of their direct and tunable band gaps.

  6. All-electron GW quasiparticle band structures of group 14 nitride compounds

    SciTech Connect

    Chu, Iek-Heng; Cheng, Hai-Ping; Kozhevnikov, Anton; Schulthess, Thomas C.

    2014-07-28

    We have investigated the group 14 nitrides (M{sub 3}N{sub 4}) in the spinel phase (γ-M{sub 3}N{sub 4} with M = C, Si, Ge, and Sn) and β phase (β-M{sub 3}N{sub 4} with M = Si, Ge, and Sn) using density functional theory with the local density approximation and the GW approximation. The Kohn-Sham energies of these systems have been first calculated within the framework of full-potential linearized augmented plane waves (LAPW) and then corrected using single-shot G{sub 0}W{sub 0} calculations, which we have implemented in the modified version of the Elk full-potential LAPW code. Direct band gaps at the Γ point have been found for spinel-type nitrides γ-M{sub 3}N{sub 4} with M = Si, Ge, and Sn. The corresponding GW-corrected band gaps agree with experiment. We have also found that the GW calculations with and without the plasmon-pole approximation give very similar results, even when the system contains semi-core d electrons. These spinel-type nitrides are novel materials for potential optoelectronic applications because of their direct and tunable band gaps.

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

    NASA Astrophysics Data System (ADS)

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

    2013-10-01

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

  8. Peierls distortion and electronic bands in phosphorus allotropes

    NASA Astrophysics Data System (ADS)

    Falkovsky, L. A.

    2016-01-01

    A small difference between the rhombohedral phosphorus lattice (A-7 phase) and the simple cubic phase, as well as between phosphorene and the cubic structure, is used in order to construct their quasiparticle band dispersion. We exploit the Peierls idea of the Brillouin zone doubling/folding, which has been previously employed in consideration of semimetals of the V period and IV-VI semiconductors. In a common framework, individual properties of phosphorus allotropes are revealed.

  9. Electronic band structure of CaUO{sub 4} from first principles

    SciTech Connect

    Matar, S.F.; Demazeau, G.

    2009-10-15

    Band theoretical results are presented on calcium uranate, CaUO{sub 4}, based on computations within the density functional theory. From pseudo-potential calculations the equation of state is obtained with equilibrium lattice properties in agreement with experiment. For isotropic volume change the bulk modulus amounts to 180 GPa but a much higher value is found for anisotropic compression along the hexagonal c-axis. This is assigned to the short U-O distances in linear uranyl polycation. Scalar relativistic all-electron calculations point to a semiconductor with {approx}3eV band gap. From density of states, chemical bonding and electron localization function ELF, oxygen is found to behave both as ionic and covalent in the coordination sphere of uranium. The results provide an illustration of the peculiar role of uranyl cation UO{sub 2}{sup 2+} according to its chemical environment. - Energy versus volume variation for isotropic and anisotropic compressions within CaUO{sub 4}. Fit results with Birch EOS are given in inserts.

  10. Combined scattering loss of radiation belt relativistic electrons by simultaneous three-band EMIC waves: A case study

    NASA Astrophysics Data System (ADS)

    He, Fengming; Cao, Xing; Ni, Binbin; Xiang, Zheng; Zhou, Chen; Gu, Xudong; Zhao, Zhengyu; Shi, Run; Wang, Qi

    2016-05-01

    Multiband electromagnetic ion cyclotron (EMIC) waves can drive efficient scattering loss of radiation belt relativistic electrons. However, it is statistically uncommon to capture the three bands of EMIC waves concurrently. Utilizing data from the Electric and Magnetic Field Instrument Suite and Integrated Science magnetometer onboard Van Allen Probe A, we report the simultaneous presence of three (H+, He+, and O+) emission bands in an EMIC wave event, which provides an opportunity to look into the combined scattering effect of all EMIC emissions and the relative roles of each band in diffusing radiation belt relativistic electrons under realistic circumstances. Our quantitative results, obtained by quasi-linear diffusion rate computations and 1-D pure pitch angle diffusion simulations, demonstrate that the combined resonant scattering by the simultaneous three-band EMIC waves is overall dominated by He+ band wave diffusion, mainly due to its dominance over the wave power (the mean wave amplitudes are approximately 0.4 nT, 1.6 nT, and 0.15 nT for H+, He+, and O+ bands, respectively). Near the loss cone, while 2-3 MeV electrons undergo pitch angle scattering at a rate of the order of 10-6-10-5 s-1, 5-10 MeV electrons can be diffused more efficiently at a rate of the order of 10-3-10-2 s-1, which approaches the strong diffusion level and results in a moderately or heavily filled loss cone for the atmospheric loss. The corresponding electron loss timescales (i.e., lifetimes) vary from several days at the energies of ~2 MeV to less than 1 h at ~10 MeV. This case study indicates the leading contribution of He+ band waves to radiation belt relativistic electron losses during the coexistence of three EMIC wave bands and suggests that the roles of different EMIC wave bands in the relativistic electron dynamics should be carefully incorporated in future modeling efforts.

  11. Ionic metal K sub x C sub 60 : Cohesion and energy bands

    SciTech Connect

    Saito, S.; Oshiyama, A. )

    1991-11-15

    Microscopic total-energy electronic-structure calculations for K{sub {ital x}}C{sub 60} show that solid C{sub 60} weakly bonded via van der Waals forces is transformed upon potassium doping into a strongly condensed {ital ionic} {ital metal} in which both Madelung and kinetic energies contribute to its large cohesive energy and bulk modulus. We also find that K doping induces lattice contraction which results in nonrigid energy-band modification. The Fermi level for K{sub 3}C{sub 60} is found to be located close to a peak of the density of states.

  12. Differential cross sections for electron impact excitation of the electronic bands of phenol

    SciTech Connect

    Neves, R. F. C.; Jones, D. B.; Lopes, M. C. A.; Nixon, K. L.; Silva, G. B. da; Duque, H. V.; Oliveira, E. M. de; Lima, M. A. P.; Costa, R. F. da; Varella, M. T. do N.; Bettega, M. H. F.; and others

    2015-03-14

    We report results from a joint theoretical and experimental investigation into electron scattering from the important organic species phenol (C{sub 6}H{sub 5}OH). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C{sub 6}H{sub 5}OH. The measurements were carried out at energies in the range 15–40 eV, and for scattered-electron angles between 10{sup ∘} and 90{sup ∘}. The energy resolution of those experiments was typically ∼80 meV. Corresponding Schwinger multichannel method with pseudo-potentials calculations, with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were conducted at the static exchange plus polarisation (SEP)-level using a minimum orbital basis for single configuration interaction (MOBSCI) approach. Agreement between the measured and calculated DCSs was typically fair, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOBSCI.

  13. Characterization of a 2D soft x-ray tomography camera with discrimination in energy bands

    SciTech Connect

    Romano, A.; Pacella, D.; Gabellieri, L.; Tilia, B.; Piergotti, V.; Mazon, D.; Malard, P.

    2010-10-15

    A gas detector with a 2D pixel readout is proposed for a future soft x-ray (SXR) tomography with discrimination in energy bands separately per pixel. The detector has three gas electron multiplier foils for the electron amplification and it offers the advantage, compared with the single stage, to be less sensitive to neutrons and gammas. The energy resolution and the detection efficiency of the detector have been accurately studied in the laboratory with continuous SXR spectra produced by an electronic tube and line emissions produced by fluorescence (K, Fe, and Mo) in the range of 3-17 keV. The front-end electronics, working in photon counting mode with a selectable threshold for pulse discrimination, is optimized for high rates. The distribution of the pulse amplitude has been indirectly derived by means of scans of the threshold. Scans in detector gain have also been performed to assess the capability of selecting different energy ranges.

  14. High Resolution Emission Spectroscopy of the Alpha Pi-1 - Chi Sigma-1(+) Fourth Positive Band System of CO from Electron Impact

    NASA Technical Reports Server (NTRS)

    Beegle, Luther W.; Ajello, Joseph M.; James, Geoffrey K.; Alvarez, Marcos; Dziczek, Dariusz

    2000-01-01

    We report electron-impact induced fluorescence spectra [300 mA full width at half maximum (FWHM)] of CO for 20 and 100 eV impact energies of the spectral region of 1300 to 2050 A and high resolution spectra (FWHM) of the v'=5 to v"=l and the v'=3 to v"=O bands showing that the rotational structure of the band system are modeled accurately. The excitation function of the (0,1) band (1597 A) was measured from electron impact in the energy range from threshold to 750 eV and placed on an absolute scale from modem calibration standards.

  15. Electronic Band Structure, Optical, Thermal and Bonding Properties of XMg2O4(X = Si, Ge) Spinel Compounds

    NASA Astrophysics Data System (ADS)

    Semari, F.; Ouahrani, T.; Khachai, H.; Khenata, R.; Rabah, M.; Bouhemadou, A.; Murtaza, G.; Amin, B.; Rached, D.

    2013-07-01

    Bonding nature as well as structural, optoelectronic and thermal properties of the cubic XMg2O4(X = Si, Ge) spinel compounds have been calculated using a full-potential augmented plane-wave plus local orbitals (FP-APW+lo) method within the density functional theory. The exchange-correlation potential was treated with the PBE-GGA approximation to calculate the total energy. Moreover, the modified Becke-Johnson potential (TB-mBJ) was also applied to improve the electronic band structure calculations. The computed ground-state parameters (a, B, B‧ and u) are in excellent agreements with the available theoretical data. Calculations of the electronic band structure and bonding properties show that these compounds have a direct energy band gap (Γ-Γ) with a dominated ionic character and the TB-mBJ approximation yields larger fundamental band gaps compared to those obtained using the PBE-GGA. Optical properties such as the complex dielectric function ɛ(ω), reflectivity R(ω) and energy loss function L(ω), for incident photon energy up to 40 eV, have been predicted. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the effects of pressure P and temperature T on the thermal expansion coefficient, Debye temperature and heat capacity for the considered compounds are investigated for the first time.

  16. Micro-metric electronic patterning of a topological band structure using a photon beam.

    PubMed

    Frantzeskakis, E; De Jong, N; Zwartsenberg, B; Huang, Y K; Bay, T V; Pronk, P; Van Heumen, E; Wu, D; Pan, Y; Radovic, M; Plumb, N C; Xu, N; Shi, M; De Visser, A; Golden, M S

    2015-01-01

    In an ideal 3D topological insulator (TI), the bulk is insulating and the surface conducting due to the existence of metallic states that are localized on the surface; these are the topological surface states. Quaternary Bi-based compounds of Bi(2-x)Sb(x)Te(3-y)Se(y) with finely-tuned bulk stoichiometries are good candidates for realizing ideal 3D TI behavior due to their bulk insulating character. However, despite its insulating bulk in transport experiments, the surface region of Bi(2-x)Sb(x)Te(3-y)Se(y) crystals cleaved in ultrahigh vacuum also exhibits occupied states originating from the bulk conduction band. This is due to adsorbate-induced downward band-bending, a phenomenon known from other Bi-based 3D TIs. Here we show, using angle-resolved photoemission, how an EUV light beam of moderate flux can be used to exclude these topologically trivial states from the Fermi level of Bi1.46Sb0.54Te1.7Se1.3 single crystals, thereby re-establishing the purely topological character of the low lying electronic states of the system. We furthermore prove that this process is highly local in nature in this bulk-insulating TI, and are thus able to imprint structures in the spatial energy landscape at the surface. We illustrate this by 'writing' micron-sized letters in the Dirac point energy of the system. PMID:26543011

  17. Micro-metric electronic patterning of a topological band structure using a photon beam

    PubMed Central

    Frantzeskakis, E.; De Jong, N.; Zwartsenberg, B.; Huang, Y. K.; Bay, T. V.; Pronk, P.; Van Heumen, E.; Wu, D.; Pan, Y.; Radovic, M.; Plumb, N. C.; Xu, N.; Shi, M.; De Visser, A.; Golden, M. S.

    2015-01-01

    In an ideal 3D topological insulator (TI), the bulk is insulating and the surface conducting due to the existence of metallic states that are localized on the surface; these are the topological surface states. Quaternary Bi-based compounds of Bi2−xSbxTe3−ySey with finely-tuned bulk stoichiometries are good candidates for realizing ideal 3D TI behavior due to their bulk insulating character. However, despite its insulating bulk in transport experiments, the surface region of Bi2−xSbxTe3−ySey crystals cleaved in ultrahigh vacuum also exhibits occupied states originating from the bulk conduction band. This is due to adsorbate-induced downward band-bending, a phenomenon known from other Bi-based 3D TIs. Here we show, using angle-resolved photoemission, how an EUV light beam of moderate flux can be used to exclude these topologically trivial states from the Fermi level of Bi1.46Sb0.54Te1.7Se1.3 single crystals, thereby re-establishing the purely topological character of the low lying electronic states of the system. We furthermore prove that this process is highly local in nature in this bulk-insulating TI, and are thus able to imprint structures in the spatial energy landscape at the surface. We illustrate this by ‘writing’ micron-sized letters in the Dirac point energy of the system. PMID:26543011

  18. Micro-metric electronic patterning of a topological band structure using a photon beam

    NASA Astrophysics Data System (ADS)

    Frantzeskakis, E.; de Jong, N.; Zwartsenberg, B.; Huang, Y. K.; Bay, T. V.; Pronk, P.; van Heumen, E.; Wu, D.; Pan, Y.; Radovic, M.; Plumb, N. C.; Xu, N.; Shi, M.; de Visser, A.; Golden, M. S.

    2015-11-01

    In an ideal 3D topological insulator (TI), the bulk is insulating and the surface conducting due to the existence of metallic states that are localized on the surface; these are the topological surface states. Quaternary Bi-based compounds of Bi2-xSbxTe3-ySey with finely-tuned bulk stoichiometries are good candidates for realizing ideal 3D TI behavior due to their bulk insulating character. However, despite its insulating bulk in transport experiments, the surface region of Bi2-xSbxTe3-ySey crystals cleaved in ultrahigh vacuum also exhibits occupied states originating from the bulk conduction band. This is due to adsorbate-induced downward band-bending, a phenomenon known from other Bi-based 3D TIs. Here we show, using angle-resolved photoemission, how an EUV light beam of moderate flux can be used to exclude these topologically trivial states from the Fermi level of Bi1.46Sb0.54Te1.7Se1.3 single crystals, thereby re-establishing the purely topological character of the low lying electronic states of the system. We furthermore prove that this process is highly local in nature in this bulk-insulating TI, and are thus able to imprint structures in the spatial energy landscape at the surface. We illustrate this by ‘writing’ micron-sized letters in the Dirac point energy of the system.

  19. Transparent Conducting Oxides for Photovoltaics: Manipulation of Fermi Level, Work Function and Energy Band Alignment

    SciTech Connect

    Klein, A.; Körber, C.; Wachau, A.; Säuberlich, F.; Gassenbauer, Y.; Harvey, S.P.; Proffit, Diana E.; Mason, Thomas O.

    2010-11-02

    Doping limits, band gaps, work functions and energy band alignments of undoped and donor-doped transparent conducting oxides ZnO, In{sub 2}O{sub 3}, and SnO{sub 2} as accessed by X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) are summarized and compared. The presented collection provides an extensive data set of technologically relevant electronic properties of photovoltaic transparent electrode materials and illustrates how these relate to the underlying defect chemistry, the dependence of surface dipoles on crystallographic orientation and/or surface termination, and Fermi level pinning.

  20. Flat band analogues and flux driven extended electronic states in a class of geometrically frustrated fractal networks.

    PubMed

    Nandy, Atanu; Pal, Biplab; Chakrabarti, Arunava

    2015-04-01

    We demonstrate, by explicit construction, that a single band tight binding Hamiltonian defined on a class of deterministic fractals of the b = 3N Sierpinski type can give rise to an infinity of dispersionless, flat-band like states which can be worked out analytically using the scale invariance of the underlying lattice. The states are localized over clusters of increasing sizes, displaying the existence of a multitude of localization areas. The onset of localization can, in principle, be 'delayed' in space by an appropriate choice of the energy of the electron. A uniform magnetic field threading the elementary plaquettes of the network is shown to destroy this staggered localization and generate absolutely continuous sub-bands in the energy spectrum of these non-translationally invariant networks. PMID:25751071

  1. Electronic band structures of AV(2) (A = Ta, Ti, Hf and Nb) Laves phase compounds.

    PubMed

    Charifi, Z; Reshak, Ali Hussain; Baaziz, H

    2009-01-14

    First-principles density functional calculations, using the all-electron full potential linearized augmented plane wave method, have been performed in order to investigate the structural and electronic properties for Laves phase AV(2) (A = Ta, Ti, Hf and Nb) compounds. The generalized gradient approximation and the Engel-Vosko-generalized gradient approximation were used. Our calculations show that these compounds are metallic with more bands cutting the Fermi energy (E(F)) as we move from Nb to Ta, Hf and Ti, consistent with the increase in the values of the density of states at the Fermi level N(E(F)). N(E(F)) is controlled by the overlapping of V-p/d, A-d and A-p states around the Fermi energy. The ground state properties of these compounds, such as equilibrium lattice constant, are calculated and compared with the available literature. There is a strong/weak hybridization between the states, V-s states are strongly hybridized with A-s states below and above E(F). Around the Fermi energy we notice that V-p shows strong hybridization with A-p states. PMID:21813979

  2. Tunability of Band Gap in Multilayer Phosphorene by External Electric Fields and Electron Dopings

    NASA Astrophysics Data System (ADS)

    Baik, Seung Su; Choi, Hyoung Joon

    2015-03-01

    Black phosphorus (BP) and its two-dimensional derivative phosphorene are rapidly emerging nanoelectronic materials with potential applicability to field effect transistors and optoelectronic devices. Unlike the gapless semiconductor graphene, multilayer BP has a substantial band gap of ~ 0.2 eV and the band-gap size is reportedly varied by external electric fields. To explore the extensibility of such band-gap modulation, we have investigated electronic band structures of multilayer BP by using the first-principles density-functional method as implemented in the SIESTA code. By controlling the electron doping concentrations and the resultant electric fields therefrom, we examine the manageability of the band-gap size and the anisotropic carrier mobility. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2013-C3-062).

  3. Electronic band structure and effective mass parameters of Ge1-xSnx alloys

    NASA Astrophysics Data System (ADS)

    Lu Low, Kain; Yang, Yue; Han, Genquan; Fan, Weijun; Yeo, Yee-Chia

    2012-11-01

    This work investigates the electronic band structures of bulk Ge1-xSnx alloys using the empirical pseudopotential method (EPM) for Sn composition x varying from 0 to 0.2. The adjustable form factors of EPM were tuned in order to reproduce the band features that agree well with the reported experimental data. Based on the adjusted pseudopotential form factors, the band structures of Ge1-xSnx alloys were calculated along high symmetry lines in the Brillouin zone. The effective masses at the band edges were extracted by using a parabolic line fit. The bowing parameters of hole and electron effective masses were then derived by fitting the effective mass at different Sn compositions by a quadratic polynomial. The hole and electron effective mass were examined for bulk Ge1-xSnx alloys along specific directions or orientations on various crystal planes. In addition, employing the effective-mass Hamiltonian for diamond semiconductor, band edge dispersion at the Γ-point calculated by 8-band k.p. method was fitted to that obtained from EPM approach. The Luttinger-like parameters were also derived for Ge1-xSnx alloys. They were obtained by adjusting the effective-mass parameters of k.p method to fit the k.p band structure to that of the EPM. These effective masses and derived Luttinger parameters are useful for the design of optical and electronic devices based on Ge1-xSnx alloys.

  4. Study of energy band discontinuity in NiZnO/ZnO heterostructure using X-ray photoelectron spectroscopy

    NASA Astrophysics Data System (ADS)

    Dewan, Sheetal; Tomar, Monika; Goyal, Anshu; Kapoor, A. K.; Tandon, R. P.; Gupta, Vinay

    2016-05-01

    A heterostructure based on ZnO and Ni doped ZnO (NiZnO) thin films has been prepared on c-plane sapphire substrate by pulsed laser deposition technique. X-ray photo electron spectroscopy has been utilized to study the energy band discontinuities, i.e., valence band offset ( Δ E v ) and conduction band offset ( Δ E c ) at the interface of NiZnO and ZnO thin films. A type-II band alignment is identified at the interface of prepared heterostructure from the computed data, which is attractive for the realization of efficient optoelectronic devices.

  5. STABILITY IN BCC TRANSITION METALS: MADELUNG AND BAND-ENERGY EFFECTS DUE TO ALLOYING

    SciTech Connect

    Landa, A; Soderlind, P; Ruban, A; Peil, O; Vitos, L

    2009-08-28

    The phase stability of the bcc Group VB (V, Nb, and Ta) transition metals is explored by first-principles electronic-structure calculations. Alloying with a small amount of a neighboring metal can either stabilize or destabilize the bcc phase. This counterintuitive behavior is explained by competing mechanisms that dominate depending on particular dopand. We show that band-structure effects dictate stability when a particular Group VB metal is alloyed with its nearest neighbors within the same d-transition series. In this case, the neighbor with less (to the left) and more (to the right) d electrons, destabilize and stabilize bcc, respectively. When alloying with neighbors of different d-transition series, electrostatic Madelung energy dominates over the band energy and always stabilizes the bcc phase.

  6. Energy Band Gap Study of Semiconducting Single Walled Carbon Nanotube Bundle

    NASA Technical Reports Server (NTRS)

    Elkadi, Asmaa; Decrossas, Emmanuel; El-Ghazaly, Samir

    2013-01-01

    The electronic properties of multiple semiconducting single walled carbon nanotubes (s-SWCNTs) considering various distribution inside a bundle are studied. The model derived from the proposed analytical potential function of electron density for na individual s-SWCNT is general and can be easily applied to multiple nanotubes. This work demonstrates that regardless the number of carbon nanotubes, the strong coupling occurring between the closet neighbors reduces the energy band gap of the bundle by 10%. As expected, the coupling is strongly dependent on the distance separating the s-SWCNTs. In addition, based on the developed model, it is proposed to enhance this coupling effect by applying an electric field across the bundle to significantly reduce the energy band gap of the bundle by 20%.

  7. Energy Band Gap Study of Semiconducting Single Walled Carbon Nanotube Bundle

    NASA Technical Reports Server (NTRS)

    Elkadi, Asmaa; Decrossas, Emmanuel; El-Ghazaly, Samir

    2013-01-01

    The electronic properties of multiple semiconducting single walled carbon nanotubes (s-SWCNTs) considering various distribution inside a bundle are studied. The model derived from the proposed analytical potential function of the electron density for an individual s-SWCNT is general and can be easily applied to multiple nanotubes. This work demonstrates that regardless the number of carbon nanotubes, the strong coupling occurring between the closest neighbours reduces the energy band gap of the bundle by 10%. As expected, the coupling is strongly dependent on the distance separating the s-SWCNTs. In addition, based on the developed model, it is proposed to enhance this coupling effect by applying an electric field across the bundle to significantly reduce the energy band gap of the bundle by 20%.

  8. Photoemission and density functional theory study of Ir(111); energy band gap mapping

    NASA Astrophysics Data System (ADS)

    Pletikosić, I.; Kralj, M.; Šokčević, D.; Brako, R.; Lazić, P.; Pervan, P.

    2010-04-01

    We have performed combined angle-resolved photoemission spectroscopy (ARPES) experiments and density functional theory (DFT) calculations of the electronic structure of the Ir(111) surface, with the focus on the existence of energy band gaps. The investigation was motivated by the experimental results suggesting Ir(111) as an ideal support for the growth of weakly bonded graphene. Therefore, our prime interest was electronic structure around the \\bar {\\mathrm {K}} symmetry point. In accordance with DFT calculations, ARPES has shown a wide energy band gap with the shape of a parallelogram centred around the \\bar {\\mathrm {K}} point. Within the gap three surface states were identified; one just below the Fermi level and two spin-orbit split surface states at the bottom of the gap.

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

    SciTech Connect

    Engel, Edgar A. Needs, Richard J.; Monserrat, Bartomeu

    2015-12-28

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

  10. Prediction of electron energies in metal oxides.

    PubMed

    Walsh, Aron; Butler, Keith T

    2014-02-18

    The ability to predict energy levels in metal oxides is paramount to developinguseful materials, such as in the development of water photolysis catalysts and efficient photovoltaic cells. The binding energy of electrons in materials encompasses a wealth of information concerning their physicochemistry. The energies control the optical and electrical properties, dictating for which kinds of chemistry and physics a particular material is useful. Scientists have developed theories and models for electron energies in a variety of chemical systems over the past century. However, the prediction of quantitative energy levels in new materials remains a major challenge. This issue is of particular importance in metal oxide research, where novel chemistries have opened the possibility of a wide range of tailored systems with applications in important fields including light-emitting diodes, energy efficient glasses, and solar cells. In this Account, we discuss the application of atomistic modeling techniques, covering the spectrum from classical to quantum descriptions, to explore the alignment of electron energies between materials. We present a number of paradigmatic examples, including a series of oxides (ZnO, In2O3, and Cu2O). Such calculations allow the determination of a "band alignment diagram" between different materials and can facilitate the prediction of the optimal chemical composition of an oxide for use in a given application. Throughout this Account, we consider direct computational solutions in the context of heuristic models, which are used to relate the fundamental theory to experimental observations. We review a number of techniques that have been commonly applied in the study of electron energies in solids. These models have arisen from different answers to the same basic question, coming from solid-state chemistry and physics perspectives. We highlight common factors, as well as providing a critical appraisal of the strengths and weaknesses of each

  11. Photonic Band Gap resonators for high energy accelerators

    SciTech Connect

    Schultz, S.; Smith, D.R.; Kroll, N. |

    1993-12-31

    We have proposed that a new type of microwave resonator, based on Photonic Band Gap (PBG) structures, may be particularly useful for high energy accelerators. We provide an explanation of the PBG concept and present data which illustrate some of the special properties associated with such structures. Further evaluation of the utility of PBG resonators requires laboratory testing of model structures at cryogenic temperatures, and at high fields. We provide a brief discussion of our test program, which is currently in progress.

  12. Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS₂ and Metal/WSe₂ Heterojunctions.

    PubMed

    Li, Zhen; Ezhilarasu, Goutham; Chatzakis, Ioannis; Dhall, Rohan; Chen, Chun-Chung; Cronin, Stephen B

    2015-06-10

    Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, are free of dangling bonds and therefore make more "ideal" Schottky junctions than bulk semiconductors, which produce Fermi energy pinning and recombination centers at the interface with bulk metals, inhibiting charge transfer. Here, we observe a more than 10× enhancement in the indirect band gap photoluminescence of transition metal dichalcogenides (TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS2 and WSe2 and subsequently recombine radiatively with minority holes in the TMDC. Since the conduction band at the K-point is 0.5 eV higher than at the Σ-point, a lower Schottky barrier exists for the Σ-point band, making electron injection more favorable. Also, the Σ band consists of the sulfur pz orbital, which overlaps more significantly with the electron wave functions in the metal. This enhancement in the indirect emission only occurs for thick flakes of MoS2 and WSe2 (≥100 nm) and is completely absent in monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures, which is consistent with the hot electron injection model. PMID:25993397

  13. Molecular Electronic Angular Motion Transducer Broad Band Self-Noise.

    PubMed

    Zaitsev, Dmitry; Agafonov, Vadim; Egorov, Egor; Antonov, Alexander; Shabalina, Anna

    2015-01-01

    Modern molecular electronic transfer (MET) angular motion sensors combine high technical characteristics with low cost. Self-noise is one of the key characteristics which determine applications for MET sensors. However, until the present there has not been a model describing the sensor noise in the complete operating frequency range. The present work reports the results of an experimental study of the self-noise level of such sensors in the frequency range of 0.01-200 Hz. Based on the experimental data, a theoretical model is developed. According to the model, self-noise is conditioned by thermal hydrodynamic fluctuations of the operating fluid flow in the frequency range of 0.01-2 Hz. At the frequency range of 2-100 Hz, the noise power spectral density has a specific inversely proportional dependence of the power spectral density on the frequency that could be attributed to convective processes. In the high frequency range of 100-200 Hz, the noise is conditioned by the voltage noise of the electronics module input stage operational amplifiers and is heavily reliant to the sensor electrical impedance. The presented results allow a deeper understanding of the molecular electronic sensor noise nature to suggest the ways to reduce it. PMID:26610502

  14. Molecular Electronic Angular Motion Transducer Broad Band Self-Noise

    PubMed Central

    Zaitsev, Dmitry; Agafonov, Vadim; Egorov, Egor; Antonov, Alexander; Shabalina, Anna

    2015-01-01

    Modern molecular electronic transfer (MET) angular motion sensors combine high technical characteristics with low cost. Self-noise is one of the key characteristics which determine applications for MET sensors. However, until the present there has not been a model describing the sensor noise in the complete operating frequency range. The present work reports the results of an experimental study of the self-noise level of such sensors in the frequency range of 0.01–200 Hz. Based on the experimental data, a theoretical model is developed. According to the model, self-noise is conditioned by thermal hydrodynamic fluctuations of the operating fluid flow in the frequency range of 0.01–2 Hz. At the frequency range of 2–100 Hz, the noise power spectral density has a specific inversely proportional dependence of the power spectral density on the frequency that could be attributed to convective processes. In the high frequency range of 100–200 Hz, the noise is conditioned by the voltage noise of the electronics module input stage operational amplifiers and is heavily reliant to the sensor electrical impedance. The presented results allow a deeper understanding of the molecular electronic sensor noise nature to suggest the ways to reduce it. PMID:26610502

  15. Tuning semiconductor band edge energies for solar photocatalysis via surface ligand passivation.

    PubMed

    Yang, Shenyuan; Prendergast, David; Neaton, Jeffrey B

    2012-01-11

    Semiconductor photocatalysts capable of broadband solar photon absorption may be nonetheless precluded from use in driving water splitting and other solar-to-fuel related reactions due to unfavorable band edge energy alignment. Using first-principles density functional theory and beyond, we calculate the electronic structure of passivated CdSe surfaces and explore the opportunity to tune band edge energies of this and related semiconductors via electrostatic dipoles associated with chemisorbed ligands. We predict substantial shifts in band edge energies originating from both the induced dipole at the ligand/CdSe interface and the intrinsic dipole of the ligand. Building on important induced dipole contributions, we further show that, by changing the size and orientation of the ligand's intrinsic dipole moment via functionalization, we can control the direction and magnitude of the shifts of CdSe electronic levels. Our calculations suggest a general strategy for enabling new active semiconductor photocatalysts with both optimal opto-electronic, and photo- and electrochemical properties. PMID:22192078

  16. Topological band order, structural, electronic and optical properties of XPdBi (X = Lu, Sc) compounds

    NASA Astrophysics Data System (ADS)

    Narimani, M.; Nourbakhsh, Z.

    2016-05-01

    In this paper, the structural, electronic and optical properties of LuPdBi and ScPdBi compounds are investigated using the density functional theory by WIEN2K package within the generalized gradient approximation, local density approximation, Engel-Vosco generalized gradient approximations and modified Becke-Johnson potential approaches. The topological phases and band orders of these compounds are studied. The effect of pressure on band inversion strength, electron density of states and the linear coefficient of the electronic specific heat of these compounds is investigated. Furthermore, the effect of pressure on real and imaginary parts of dielectric function, absorption and reflectivity coefficients of these compounds is studied.

  17. Correlating structure and electronic band-edge properties in organolead halide perovskites nanoparticles.

    PubMed

    Zhu, Qiushi; Zheng, Kaibo; Abdellah, Mohamed; Generalov, Alexander; Haase, Dörthe; Carlson, Stefan; Niu, Yuran; Heimdal, Jimmy; Engdahl, Anders; Messing, Maria E; Pullerits, Tonu; Canton, Sophie E

    2016-06-01

    After having emerged as primary contenders in the race for highly efficient optoelectronics materials, organolead halide perovskites (OHLP) are now being investigated in the nanoscale regime as promising building blocks with unique properties. For example, unlike their bulk counterpart, quantum dots of OHLP are brightly luminescent, owing to large exciton binding energies that cannot be rationalized solely on the basis of quantum confinement. Here, we establish the direct correlation between the structure and the electronic band-edge properties of CH3NH3PbBr3 nanoparticles. Complementary structural and spectroscopic measurements probing long-range and local order reveal that lattice strain influences the nature of the valence band and modifies the subtle stereochemical activity of the Pb(2+) lone-pair. More generally, this work demonstrates that the stereochemical activity of the lone-pair at the metal site is a specific physicochemical parameter coupled to composition, size and strain, which can be employed to engineer novel functionalities in OHLP nanomaterials. PMID:27189431

  18. All-electron GW quasiparticle band structures of group 14 nitride compounds

    NASA Astrophysics Data System (ADS)

    Chu, Iek-Heng; Kozhenikov, Anton; Schulthess, Thomas; Cheng, Hai-Ping

    2014-03-01

    We have investigated the group 14 nitrides (M3N4) in both the spinel phase (with M =C, Si, Ge and Sn) and the beta phase (with M =Si, Ge and Sn) using density functional theory (DFT) with the local density approximation (LDA). The Kohn-Sham energies of these systems are first calculated within the framework of full-potential LAPW and then corrected using single-shot G0W0 calculations, which we have implemented in the Exciting-Plus code. Direct bands gap at the Γ point are found for all spinel-type nitrides. The calculated band gaps of Si3N4, Ge3N4 and Sn3N4 agree with experiment. We also find that for all systems studied, our GW calculations with and without the plasmon-pole approximation give very similar results, even when the system contains semi-core 3d electrons. These spinel-type nitrides are novel materials for potential optoelectronic applications. This work is supported by NSF/DMR-0804407 and DOE/BES-DE-FG02-02ER45995. Computations are performed using facilities at NERSC.

  19. Exploring the Electronic Band Structure of Organometal Halide Perovskite via Photoluminescence Anisotropy of Individual Nanocrystals.

    PubMed

    Täuber, Daniela; Dobrovolsky, Alexander; Camacho, Rafael; Scheblykin, Ivan G

    2016-08-10

    Understanding electronic processes in organometal halide perovskites, flourishing photovoltaic, and emitting materials requires unraveling the origin of their electronic transitions. Light polarization studies can provide important information regarding transition dipole moment orientations. Investigating individual methylammonium lead triiodide perovskite nanocrystals enabled us to detect the polarization of photoluminescence intensity and photoluminescence excitation, hidden in bulk samples by ensemble averaging. Polarization properties of the crystals were correlated with their photoluminescence spectra and electron microscopy images. We propose that distortion of PbI6 octahedra leads to peculiarities of the electronic band structure close to the band-edge. Namely, the lowest band transition possesses a transition dipole moment along the apical Pb-I-Pb bond resulting in polarized photoluminescence. Excitation of photoluminescence above the bandgap is unpolarized because it involves molecular orbitals delocalized both in the apical and equatorial directions of the perovskite octahedron. Trap-assisted emission at 77 K, rather surprisingly, was polarized similar to the bandgap emission. PMID:27462927

  20. Analysis of low energy electrons

    NASA Technical Reports Server (NTRS)

    Sharp, R. D.

    1973-01-01

    Simultaneous observations of low energy electrons in the plasma sheet and in the auroral zone were analyzed. Data from the MIT plasma experiment on the OGO-3 satellite and from the Lockheed experiment on the OV1-18 satellite were processed and compared. The OV1-18 carried thirteen magnetic electron spectrometers designed to measure the intensity, angular, and energy distributions of the auroral electrons and protons in the energy range below 50 keV. Two computer programs were developed for reduction of the OV1-18 data. One program computed the various plasma properties at one second intervals as a function of Universal Time and pitch angle; the other program produced survey plots showing the outputs of the various detectors on the satellite as a function of time on a scale of approximately 100 seconds per cm. The OV1-18 data exhibit the high degree of variability associated with substorm controlled phenomena.

  1. Exploring the electronic band structure of individual carbon nanotubes under 60 T

    NASA Astrophysics Data System (ADS)

    Nanot, Sébastien; Escoffier, Walter; Lassagne, Benjamin; Broto, Jean-Marc; Raquet, Bertrand

    2009-05-01

    Nano-sciences, and in particular nano-physics, constitute a fascinating world of investigations where the experimental challenges are to synthesize, to address (for instance optically or electrically) to explore and promote the remarkable physical properties of new nano-materials. Somehow, one of the most promising realization of nano-sciences lies in carbon-based nano-materials with sp covalent bonds. In particular, carbon nanotubes, graphene and more recently ultra-narrow graphene nano-ribbons are envisioned as elementary bricks of the future of nano-electronics. However, prior to such an achievement, the first steps consist in understanding their fundamental electronic properties when they constitute the drain-source channel of a gated device or inter-connexion elements. In this article, we present the richness of challenging experiments combining single-object measurements with an extreme magnetic environment. We demonstrate that an applied magnetic field ( B), along with a control of the electrostatic doping, drastically modifies the electronic band structure of a carbon nanotube based transistor. Several examples will be addressed in this presentation. When B is applied parallel to the tube axis, a quantum flux threading the tube induces a giant Aharonov-Bohm conductance modulation mediated by Schottky barriers whose profile is magnetic field dependent. In the perpendicular configuration, the applied magnetic field breaks the revolution symmetry along the circumference and non-conventional Landau states develop in the high field regime. By playing with a carbon nanotube based electronic Fabry-Perot resonator, the field dependence of the resonant states of the cavity reveals the onset of the first Landau state at zero energy. These experiments enlighten the outstanding efficiency of magneto-conductance experiments to probe the electronic properties of carbon based nano-materials. To cite this article: S. Nanot et al., C. R. Physique 10 (2009).

  2. Laboratory studies of UV emissions of H2 by electron impact - The Werner- and Lyman-band systems

    NASA Technical Reports Server (NTRS)

    Ajello, J. M.; Srivastava, S. K.; Yung, Y. L.

    1982-01-01

    The vacuum ultraviolet electron-impact-induced fluorescence emissions of H2 were studied for the Lyman and Werner band systems in the range of 120-170 nm, using an optical system containing a photomultiplier and a spectrometer, over an energy range from threshold to 400 eV. The emission cross sections for the Lyman and Werner transitions at 100 eV are determined. The cross-section ratio is in excellent agreement with theoretical calculations and experimental data for the optical oscillator strengths. The cross-section for cascading to the B state is stated as a percentage of the total emission cross section at both 100 and 300 eV, increasing substantially at 20 eV. The vibrational population distribution of the B state is found to be a function of electron-impact energy as the importance of cascading relative to direct excitation changes with electron-impact energy.

  3. Physical properties and electronic band structure of noncentrosymmetric Th7Co3 superconductor

    NASA Astrophysics Data System (ADS)

    Sahakyan, M.; Tran, V. H.

    2016-05-01

    The physical properties of the noncentrosymmetric superconductor Th7Co3 have been investigated by means of ac-magnetic susceptibility, magnetization, specific heat, electrical resistivity, magnetoresistance and Hall effect measurements. From these data it is established that Th7Co3 is a dirty type-II superconductor with {{T}\\text{c}}=1.8+/- 0.02 K, Hc2\\text{orb}<{{H}c2}(0)∼ 10~\\text{kOe}c2p and moderate electron–phonon coupling {λ\\text{el-\\text{ph}}}=0.56 . Some evidences for anisotropic superconducting gap are found, including e.g. reduced specific heat jump (Δ {{C}p}/γ {{T}\\text{c}}=1.01 ) at T c, diminished superconducting energy gap ({{Δ }0}/{{k}\\text{B}}{{T}\\text{c}}=2.17 ) as compared to the BCS values, power law field dependence of the Sommerfeld coefficient at 0.4 K ({{C}p}/T\\propto {{H}0.6} ), and a concave curvature of the {{H}c2}≤ft({{T}\\text{c}}\\right) line. The magnitudes of the thermodynamic critical field and the energy gap are consistent with mean-squared anisotropy parameter < {{a}2}> ∼ 0.23 . The electronic specific heat in the superconducting state is reasonably fitted to an oblate spheroidal gap model. Calculations of scalar relativistic and fully relativistic electronic band structures reveal considerable differences in the degenerate structure, resulting from asymmetric spin–orbit coupling (ASOC). A large splitting energy of spin-up spin-down bands at the Fermi level E F, Δ {{E}\\text{ASOC}}∼ 100 meV is observed and a sizeable ratio Δ {{E}\\text{ASOC}}/{{k}\\text{B}}{{T}\\text{c}}∼ 640 could classify the studied compound into the class of noncentrosymmetric superconductors with strong ASOC. The noncentrosymmetry of the crystal structure and the atomic relativistic effects are both responsible for an importance of ASOC in Th7Co3. The calculated results for the density of states show a Van Hove singularity just below E F and dominant role of the 6d electrons of Th to the superconductivity.

  4. Exact two-component relativistic energy band theory and application.

    PubMed

    Zhao, Rundong; Zhang, Yong; Xiao, Yunlong; Liu, Wenjian

    2016-01-28

    An exact two-component (X2C) relativistic density functional theory in terms of atom-centered basis functions is proposed for relativistic calculations of band structures and structural properties of periodic systems containing heavy elements. Due to finite radial extensions of the local basis functions, the periodic calculation is very much the same as a molecular calculation, except only for an Ewald summation for the Coulomb potential of fluctuating periodic monopoles. For comparison, the nonrelativistic and spin-free X2C counterparts are also implemented in parallel. As a first and pilot application, the band gaps, lattice constants, cohesive energies, and bulk moduli of AgX (X = Cl, Br, I) are calculated to compare with other theoretical results. PMID:26827200

  5. Determination of the quantum dot band gap dependence on particle size from optical absorbance and transmission electron microscopy measurements.

    PubMed

    Segets, Doris; Lucas, J Matthew; Klupp Taylor, Robin N; Scheele, Marcus; Zheng, Haimei; Alivisatos, A Paul; Peukert, Wolfgang

    2012-10-23

    This work addresses the determination of arbitrarily shaped particle size distributions (PSDs) from PbS and PbSe quantum dot (QD) optical absorbance spectra in order to arrive at a relationship between band gap energy and particle size over a large size range. Using a modified algorithm which was previously developed for ZnO, we take only bulk absorption data from the literature and match the PSDs derived from QD absorbance spectra with those from transmission electron microscopical (TEM) image analysis in order to arrive at the functional dependence of the band gap on particle size. Additional samples sized solely from their absorbance spectra with our algorithm show excellent agreement with TEM results. We investigate the influence of parameters of the TEM image analysis such as threshold value on the final result. The band gap versus size relationship developed from analysis of just two samples lies well within the bounds of a number of published data sets. We believe that our methodology provides an attractive shortcut for the study of various novel quantum-confined direct band gap semiconductor systems as it permits the band gap energies of a broad size range of QDs to be probed with relatively few synthetic experiments and without quantum mechanical simulations. PMID:22984808

  6. Generation and application of a subpicosecond high brightness electron single bunch at the S-band linear accelerator

    NASA Astrophysics Data System (ADS)

    Uesaka, Mitsuru; Kozawa, Takahiro; Kobayashi, Toshiaki; Ueda, Toru; Miya, Kenzo

    1996-04-01

    A subpicosecond 37-MeV electron single bunch was generated at the S-band linear accelerator of the Nuclear Engineering Research Laboratory, University of Tokyo. An original single bunch with a pulse width (FWHM) of less than 10 picoseconds was successfully compressed to a subpicosecond time domain by achromatic magnetic pulse compression. The energy modulation was optimally matched to the magnetic optics to achieve the most effective compression by tuning RF power and phase of the microwave. A femtosecond streak camera with a time resolution of 200 fs was used to measure the pulse shape of electron bunches by one shot via Cherenkov radiation emitted by the electrons in air. The specification of optical components was also optimized to avoid pulse broadening due to optical dispersion. Finally, the shortest pulse width in FWHM is 0.7ps in the best operating mode. The compressed bunch has an electric charge of 1 nC (6.2×109 electrons) on average. The subpicosecond electron single bunch will be utilized for exploration of ultrafast and fundamental radiation physics and chemistry. As the next project, we propose a femtosecond ultrafast quantum phenomena research facility where both 100-fs electron and laser pulses will be available. Especially, an X-band (11.424-GHz) femtosecond electron linac is under design.

  7. Band parameters of InGaAs/GaAs quantum dots: electronic properties study

    NASA Astrophysics Data System (ADS)

    Yahyaoui, M.; Sellami, K.; Boujdaria, K.; Chamarro, M.; Testelin, C.

    2013-12-01

    We have made a systematic investigation of the band diagram calculation of strained and unstrained InxGa1 - xAs alloys in order to extract accurate and adapted parameters which are useful to the electronic properties of InxGa1 - xAs/GaAs quantum dots. As an application, the 40-band k.p model is used to describe the band offsets as well as the band parameters in the strained InxGa1 - xAs/GaAs system. The κ valence band parameter as well as g* Landé factor depending of the indium concentration were estimated. These results are analyzed and compared with experiment.

  8. Nitrogen defects in wide band gap oxides: defect equilibria and electronic structure from first principles calculations.

    PubMed

    Polfus, Jonathan M; Bjørheim, Tor S; Norby, Truls; Haugsrud, Reidar

    2012-09-01

    The nitrogen related defect chemistry and electronic structure of wide band gap oxides are investigated by density functional theory defect calculations of N(O)(q), NH(O)(×), and (NH2)(O)(·) as well as V(O)(··) and OH(O)(·) in MgO, CaO, SrO, Al(2)O(3), In(2)O(3), Sc(2)O(3), Y(2)O(3), La(2)O(3), TiO(2), SnO(2), ZrO(2), BaZrO(3), and SrZrO(3). The N(O)(q) acceptor level is found to be deep and the binding energy of NH(O)(×) with respect to N(O)' and (OH(O)(·) is found to be significantly negative, i.e. binding, in all of the investigated oxides. The defect structure of the oxides was found to be remarkably similar under reducing and nitriding conditions (1 bar N(2), 1 bar H(2) and 1 × 10(-7) bar H(2)O): NH(O)(×) predominates at low temperatures and [N(O)'] = 2[V(O)(··) predominates at higher temperatures (>900 K for most of the oxides). Furthermore, we evaluate how the defect structure is affected by non-equilibrium conditions such as doping and quenching. In terms of electronic structure, N(O)' is found to introduce isolated N-2p states within the band gap, while the N-2p states of NH(O)(×) are shifted towards, or overlap with the VBM. Finally, we assess the effect of nitrogen incorporation on the proton conducting properties of oxides and comment on their corrosion resistance in nitriding atmospheres in light of the calculated defect structures. PMID:22828729

  9. Meissner Effect of Dirac Electrons in Superconducting State Due to Inter-Band Effect

    NASA Astrophysics Data System (ADS)

    Mizoguchi, Tomonari; Ogata, Masao

    2015-08-01

    Dirac electrons in solids show characteristic physical properties due to their linear dispersion relation and two-band nature. Although the transport phenomena of Dirac electrons in a normal state have intensively been studied, the transport phenomena in a superconducting state have not been fully understood. In particular, it is not clear whether Dirac electrons in a superconducting state show Meissner effect (ME), since a diamagnetic term of a current operator is absent as a result of the linear dispersion. We investigate the ME of three dimensional massive Dirac electrons in a superconducting state on the basis of Kubo formula, and clarify that Meissner kernel becomes finite by use of the inter-band contribution. This mechanism of the ME for Dirac electrons is completely different from that for the electrons in usual metals. Our result shows that the Meissner kernel remains finite even when the superconducting gap vanishes. This is an unavoidable problem in the Dirac electron system as reported in the previous works. Thus, we use a prescription in which we subtract the normal state contribution. In order to justify this prescription, we develop a specific model where the Meissner kernel is obtained by the prescription. We also derive the result for the electron gas by taking the non-relativistic limit of Dirac Hamiltonian, and clarify that the diamagnetic term of the Meissner kernel can be regarded as the inter-band contribution between electrons and positrons in terms of the Dirac model.

  10. Tuning two-dimensional band structure of Cu(111) surface-state electrons that interplay with artificial supramolecular architectures

    NASA Astrophysics Data System (ADS)

    Wang, Shiyong; Wang, Weihua; Tan, Liang Z.; Li, Xing Guang; Shi, Zilang; Kuang, Guowen; Liu, Pei Nian; Louie, Steven G.; Lin, Nian

    2013-12-01

    We report on the modulation of two-dimensional (2D) bands of Cu(111) surface-state electrons by three isostructural supramolecular honeycomb architectures with different periodicity or constituent molecules. Using Fourier-transformed scanning tunneling spectroscopy and model calculations, we resolved the 2D band structures and found that the intrinsic surface-state band is split into discrete bands. The band characteristics including band gap, band bottom, and bandwidth are controlled by the network unit cell size and the nature of the molecule-surface interaction. In particular, Dirac cones emerge where the second and third bands meet at the K points of the Brillouin zone of the supramolecular lattice.

  11. Engineering the electronic structure and band gap of boron nitride nanoribbon via external electric field

    NASA Astrophysics Data System (ADS)

    Chegel, Raad

    2016-06-01

    By using the third nearest neighbor modified tight binding (3NN-TB) method, the electronic structure and band gap of BNNRs under transverse electric fields are explored. The band gap of the BNNRs has a decreasing with increasing the intensity of the applied electric field, independent on the ribbon edge types. Furthermore, an analytic model for the dependence of the band gap in armchair and zigzag BNNRs on the electric field is proposed. The reduction of E g is similar for some N a armchair and N z zigzag BNNRs independent of their edges.

  12. Application of Superconducting Hot-Electron Bolometer Mixers for Terahertz-Band Astronomy

    NASA Astrophysics Data System (ADS)

    Maezawa, Hiroyuki

    2015-03-01

    Recently, a next-generation heterodyne mixer detector - a hot electron bolometer (HEB) mixer employing a superconducting microbridge - has gradually opened up terahertz-band astronomy. The surrounding state-of-the-art technologies including fabrication processes, 4 K cryostats, cryogenic low-noise amplifiers, local oscillator sources, micromachining techniques, and spectrometers, as well as the HEB mixers, have played a valuable role in the development of super-low-noise heterodyne spectroscopy systems for the terahertz band. The current developmental status of terahertz-band HEB mixer receivers and their applications for spectroscopy and astronomy with ground-based, airborne, and satellite telescopes are presented.

  13. The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

    NASA Astrophysics Data System (ADS)

    Dolgos, Michelle R.; Paraskos, Alexandra M.; Stoltzfus, Matthew W.; Yarnell, Samantha C.; Woodward, Patrick M.

    2009-07-01

    The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba 3(VO 4) 2, Pb 3(VO 4) 2, YVO 4, BiVO 4, CeVO 4 and Ag 3VO 4 were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba 3(VO 4) 2 and YVO 4 have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb 3(VO 4) 2 and BiVO 4 the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6 s orbitals with nonbonding O 2 p states at the top of the valence band, and (b) overlap of empty 6 p orbitals with antibonding V 3 d-O 2 p states at the bottom of the conduction band. In Ag 3VO 4 mixing between filled Ag 4 d and O 2 p states destabilizes states at the top of the valence band leading to a large decrease in the band gap ( Eg=2.2 eV). In CeVO 4 excitations from partially filled 4 f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce 1-xBi xVO 4 (0≤ x≤0.5) and Ce 1-xY xVO 4 ( x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi 3+ or Y 3+ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4 f orbitals.

  14. Coexisting Honeycomb and Kagome Characteristics in the Electronic Band Structure of Molecular Graphene.

    PubMed

    Paavilainen, Sami; Ropo, Matti; Nieminen, Jouko; Akola, Jaakko; Räsänen, Esa

    2016-06-01

    We uncover the electronic structure of molecular graphene produced by adsorbed CO molecules on a copper (111) surface by means of first-principles calculations. Our results show that the band structure is fundamentally different from that of conventional graphene, and the unique features of the electronic states arise from coexisting honeycomb and Kagome symmetries. Furthermore, the Dirac cone does not appear at the K-point but at the Γ-point in the reciprocal space and is accompanied by a third, almost flat band. Calculations of the surface structure with Kekulé distortion show a gap opening at the Dirac point in agreement with experiments. Simple tight-binding models are used to support the first-principles results and to explain the physical characteristics behind the electronic band structures. PMID:27176628

  15. Spatially resolved band alignments at Au-hexadecanethiol monolayer-GaAs(001) interfaces by ballistic electron emission microscopy

    SciTech Connect

    Junay, A.; Guézo, S. Turban, P.; Delhaye, G.; Lépine, B.; Tricot, S.; Ababou-Girard, S.; Solal, F.

    2015-08-28

    We study structural and electronic inhomogeneities in Metal—Organic Molecular monoLayer (OML)—semiconductor interfaces at the sub-nanometer scale by means of in situ Ballistic Electron Emission Microscopy (BEEM). BEEM imaging of Au/1-hexadecanethiols/GaAs(001) heterostructures reveals the evolution of pinholes density as a function of the thickness of the metallic top-contact. Using BEEM in spectroscopic mode in non-short-circuited areas, local electronic fingerprints (barrier height values and corresponding spectral weights) reveal a low-energy tunneling regime through the insulating organic monolayer. At higher energies, BEEM evidences new conduction channels, associated with hot-electron injection in the empty molecular orbitals of the OML. Corresponding band diagrams at buried interfaces can be thus locally described. The energy position of GaAs conduction band minimum in the heterostructure is observed to evolve as a function of the thickness of the deposited metal, and coherently with size-dependent electrostatic effects under the molecular patches. Such BEEM analysis provides a quantitative diagnosis on metallic top-contact formation on organic molecular monolayer and appears as a relevant characterization for its optimization.

  16. The electronic band structures of gadolinium chalcogenides: a first-principles prediction for neutron detecting

    NASA Astrophysics Data System (ADS)

    Li, Kexue; Liu, Lei; Yu, Peter Y.; Chen, Xiaobo; Shen, D. Z.

    2016-05-01

    By converting the energy of nuclear radiation to excited electrons and holes, semiconductor detectors have provided a highly efficient way for detecting them, such as photons or charged particles. However, for detecting the radiated neutrons, those conventional semiconductors hardly behave well, as few of them possess enough capability for capturing these neutral particles. While the element Gd has the highest nuclear cross section, here for searching proper neutron-detecting semiconductors, we investigate theoretically the Gd chalcogenides whose electronic band structures have never been characterized clearly. Among them, we identify that γ-phase Gd2Se3 should be the best candidate for neutron detecting since it possesses not only the right bandgap of 1.76 eV for devices working under room temperature but also the desired indirect gap nature for charge carriers surviving longer. We propose further that semiconductor neutron detectors with single-neutron sensitivity can be realized with such a Gd-chalcogenide on the condition that their crystals can be grown with good quality.

  17. The electronic band structures of gadolinium chalcogenides: a first-principles prediction for neutron detecting.

    PubMed

    Li, Kexue; Liu, Lei; Yu, Peter Y; Chen, Xiaobo; Shen, D Z

    2016-05-11

    By converting the energy of nuclear radiation to excited electrons and holes, semiconductor detectors have provided a highly efficient way for detecting them, such as photons or charged particles. However, for detecting the radiated neutrons, those conventional semiconductors hardly behave well, as few of them possess enough capability for capturing these neutral particles. While the element Gd has the highest nuclear cross section, here for searching proper neutron-detecting semiconductors, we investigate theoretically the Gd chalcogenides whose electronic band structures have never been characterized clearly. Among them, we identify that γ-phase Gd2Se3 should be the best candidate for neutron detecting since it possesses not only the right bandgap of 1.76 eV for devices working under room temperature but also the desired indirect gap nature for charge carriers surviving longer. We propose further that semiconductor neutron detectors with single-neutron sensitivity can be realized with such a Gd-chalcogenide on the condition that their crystals can be grown with good quality. PMID:27049355

  18. Energy band diagram of device-grade silicon nanocrystals.

    PubMed

    Macias-Montero, M; Askari, S; Mitra, S; Rocks, C; Ni, C; Svrcek, V; Connor, P A; Maguire, P; Irvine, J T S; Mariotti, D

    2016-03-17

    Device grade silicon nanocrystals (NCs) are synthesized using an atmospheric-pressure plasma technique. The Si NCs have a small and well defined size of about 2.3 nm. The synthesis system allows for the direct creation of thin films, enabling a range of measurements to be performed and easy implementation of this material in different devices. The chemical stability of the Si NCs is evaluated, showing relatively long-term durability thanks to hydrogen surface terminations. Optical and electrical characterization techniques, including Kelvin probe, ultraviolet photoemission spectroscopy and Mott-Schottky analysis, are employed to determine the energy band diagram of the Si NCs. PMID:26939617

  19. A microscopic two-band model for the electron-hole asymmetry in high-Tc superconductors and reentering behavior

    NASA Astrophysics Data System (ADS)

    Bru, J.-B.; Pedra, W. de Siqueira; Dömel, A.-S.

    2011-07-01

    To our knowledge there is no rigorously analyzed microscopic model explaining the electron-hole asymmetry of the critical temperature seen in high-Tc cuprate superconductors - at least no model not breaking artificially this symmetry. We present here a microscopic two-band model based on the structure of energetic levels of holes in CuO2 conducting layers of cuprates. In particular, our Hamiltonian does not contain ad hoc terms implying - explicitly - different masses for electrons and holes. We prove that two energetically near-lying interacting bands can explain the electron-hole asymmetry. Indeed, we rigorously analyze the phase diagram of the model and show that the critical temperatures for fermion densities below half-filling can manifest a very different behavior as compared to the case of densities above half-filling. This fact results from the inter-band interaction and intra-band Coulomb repulsion in interplay with thermal fluctuations between two energetic levels. So, if the energy difference between bands is too big (as compared to the energy scale defined by the critical temperatures of superconductivity) then the asymmetry disappears. Moreover, the critical temperature turns out to be a non-monotonic function of the fermion density and the phase diagram of our model shows "superconducting domes" as in high-Tc cuprate superconductors. This explains why the maximal critical temperature is attained at donor densities away from the maximal one. Outside the superconducting phase and for fermion densities near half-filling the thermodynamics governed by our Hamiltonian corresponds, as in real high-Tc materials, to a Mott-insulating phase. The nature of the inter-band interaction can be electrostatic (screened Coulomb interaction), magnetic (for instance, some Heisenberg-type one-site spin-spin interaction), or a mixture of both. If the inter-band interaction is predominately magnetic then - additionally to the electron-hole asymmetry - we observe a

  20. Structural analysis, electronic and optical properties of the synthesized Sb2S3 nanowires with small band gap

    NASA Astrophysics Data System (ADS)

    Validžić, I. Lj; Mitrić, M.; Abazović, N. D.; Jokić, B. M.; Milošević, A. S.; Popović, Z. S.; Vukajlović, F. R.

    2014-03-01

    We report a simple colloidal synthesis of two types of Sb2S3 nanowires with small band gap and high aspect ratio. Field-emission scanning electron and transmission electron microscopies confirmed formation of high aspect ratio Sb2S3 nanowires, separated in the form of bundles and coalesced with each other in long bars. Diffuse reflectance and absorption spectroscopies revealed that the optical band-gap energies of the synthesized nanowires separated in the form of bundles are 1.56 and 1.59 eV, and coalesced with each other in long bars are 1.36 and 1.28 eV, respectively. The structure refinement showed that Sb2S3 powders belong to the orthorhombic structure with space group Pnma (no. 62). It was found that Sb2S3 nanowires separated in the form of bundles predominantly grow along the [0 1 0] direction being in the needle-like shape. The nanowires coalesced with each other in long bars rise in the form of long bars, are ribbon-like in shape and have expressed {1 0 1} facets which grow along the [0 1 0] direction. No peaks in photoluminescence spectra were observed in the spectral range from 250 to 600 nm. In order to shed more light on the experimental results concerning the band-gap energies and, in the literature generally poorly investigated electronic properties of the synthesized material, we performed theoretical calculations of the electronic structure and optical properties of the Sb2S3 samples synthesized here. This was done on the basis of density functional theory with the generalized gradient approximation, and also with an improved version of the exchange potential suggested recently by Tran and Blaha. The main characteristic is the significant improvement of the band gap value.

  1. Measurements of the electronic transition moments of C2-band systems

    NASA Technical Reports Server (NTRS)

    Cooper, D. M.; Nicholls, R. W.

    1975-01-01

    Electronic transition moments of seven C2 singlet and triplet band systems, which are in the 0.2 to 1.2 micron spectral region, have been measured. The measurements were made in emission behind incident shock waves in C2H2-argon mixtures. Narrow band-pass radiometers were used to obtain absolute measurements of shock-excited C2 radiation from which absolute electronic transition moments are derived by a synthetic spectrum analysis. New results are reported for the Ballik-Ramsay, Phillips, Swan, Deslandres-d'Azambuja, Fox-Herzberg, Mulliken, and Freymark systems.

  2. Electron pairing in the presence of incipient bands in iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Chen, Xiao; Maiti, S.; Linscheid, A.; Hirschfeld, P. J.

    2015-12-01

    Recent experiments on certain Fe-based superconductors have hinted at a role for paired electrons in "incipient" bands that are close to, but do not cross, the Fermi level. Related theoretical works disagree on whether or not strong-coupling superconductivity is required to explain such effects, and whether a critical interaction strength exists. In this work, we consider various versions of the model problem of pairing of electrons in the presence of an incipient band, within a simple multiband weak-coupling BCS approximation. We categorize the problem into two cases: case (i), where superconductivity arises from the "incipient band pairing" alone, and case (ii), where it is induced on an incipient band by pairing due to Fermi-surface-based interactions. Negative conclusions regarding the importance of incipient bands have been drawn so far largely based on case (i), but we show explicitly that models under case (ii) are qualitatively different, and can explain the nonexponential suppression of Tc, as well as robust large gaps on an incipient band. In the latter situation, large gaps on the incipient band do not require a critical interaction strength. We also model the interplay between phonon and spin fluctuation driven superconductivity and describe situations in which they can enhance each other rather than compete. Finally, we discuss the effect of the dimensionality of the incipient band on our results. We argue that pairing on incipient bands may be significant and important in several Fe-based materials, including LiFeAs, FeSe intercalates, and FeSe monolayers on strontium titanate, and indeed may contribute to high critical temperatures in some cases.

  3. Vibrational effects on surface energies and band gaps in hexagonal and cubic ice

    NASA Astrophysics Data System (ADS)

    Engel, Edgar A.; Monserrat, Bartomeu; Needs, Richard J.

    2016-07-01

    Surface energies of hexagonal and cubic water ice are calculated using first-principles quantum mechanical methods, including an accurate description of anharmonic nuclear vibrations. We consider two proton-orderings of the hexagonal and cubic ice basal surfaces and three proton-orderings of hexagonal ice prism surfaces, finding that vibrations reduce the surface energies by more than 10%. We compare our vibrational densities of states to recent sum frequency generation absorption measurements and identify surface proton-orderings of experimental ice samples and the origins of characteristic absorption peaks. We also calculate zero point quantum vibrational corrections to the surface electronic band gaps, which range from -1.2 eV for the cubic ice basal surface up to -1.4 eV for the hexagonal ice prism surface. The vibrational corrections to the surface band gaps are up to 12% smaller than for bulk ice.

  4. Electronic structure of reconstructed InAs(001) surfaces - identification of bulk and surface bands based on their symmetries

    NASA Astrophysics Data System (ADS)

    Olszowska, Natalia; Kolodziej, Jacek J.

    2016-02-01

    Using angle-resolved photoelectron spectroscopy (ARPES) band structures of indium- and arsenic-terminated InAs(001) surfaces are investigated. These surfaces are highly reconstructed, elementary cells of their lattices contain many atoms in different chemical configurations, and moreover, they are composed of domains having related but different reconstructions. These domain-type surface reconstructions result in the reciprocal spaces containing regions with well-defined k→∥-vector and regions with not-well-defined one. In the ARPES spectra most of the surface related features appear as straight lines in the indeterminate k→∥-vector space. It is shown that, thanks to differences in crystal and surface symmetries, the single photon energy ARPES may be successfully used for classification of surface and bulk bands of electronic states on complex, highly reconstructed surfaces instead of the most often used variable photon energy studies.

  5. Secondary electron emission from lunar soil: Yields, energy distributions, and charging effects

    NASA Astrophysics Data System (ADS)

    Dukes, Catherine A.; Baragiola, Raúl A.

    2013-12-01

    We report the electron emission and charging of sub-mature lunar highland soil 61241 by electron impact under ultra-high vacuum for 40-2000 eV electrons. The energy distribution of emitted secondary electrons was measured as a function of primary electron energy under neutral charging conditions, and electron energy loss spectroscopy was used to determine the ˜8 eV band gap. Total electron yields were obtained with low electron fluxes. Imaging the soil with a Scanning Auger Microprobe using 10 keV electrons revealed differential grain motion induced by charging in ultra-high vacuum.

  6. Electronic band structures of Ge1-xSnx semiconductors: A first-principles density functional theory study

    NASA Astrophysics Data System (ADS)

    Lee, Ming-Hsien; Liu, Po-Liang; Hong, Yung-An; Chou, Yen-Ting; Hong, Jia-Yang; Siao, Yu-Jin

    2013-02-01

    We conduct first-principles total-energy density functional calculations to study the band structures in Ge1-xSnx infrared semiconductor alloys. The norm-conserving optimized pseudopotentials of Ge and Sn have been constructed for electronic structure calculations. The composition-bandgap relationships in Ge1-xSnx lattices are evaluated by a detailed comparison of structural models and their electronic band structures. The critical Sn composition related to the transition from indirect- to direct-gap in Ge1-xSnx alloys is estimated to be as low as x ˜ 0.016 determined from the parametric fit. Our results show that the crossover Sn concentration occurs at a lower critical Sn concentration than the values predicted from the absorption measurements. However, early results indicate that the reliability of the critical Sn concentration from such measurements is hard to establish, since the indirect gap absorption is much weaker than the direct gap absorption. We find that the direct band gap decreases exponentially with the Sn composition over the range 0 0.375, in very good agreement with the theoretical observed behavior [D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994, 1987]. For homonuclear and heteronuclear complexes of Ge1-xSnx alloys, the indirect band gap at L-pointis is found to decrease homonuclear Ge-Ge bonds or increase homonuclear Sn-Sn bonds as a result of the reduced L valley. All findings agree with previously reported experimental and theoretical results. The analysis suggests that the top of valence band exhibits the localization of bond charge and the bottom of the conduction band is composed of the Ge 4s4p and/or Sn 5s5p atomic orbits.

  7. Complete radiative terms for the electron/electronic energy equation

    SciTech Connect

    Stanley, S.A.; Carlson, L.A.

    1994-10-01

    A derivation of the radiative terms in the electron/electronic energy equation is presented, properly accounting for the effects of absorption and emission of radiation on the individual energy modes of the gas. This electron/electronic energy equation with the complete radiative terms has successfully been used to model the radiation-dominated precursor ahead of the bow shock of a hypersonic vehicle entering the Earth`s atmosphere. 8 refs.

  8. Broad band spectral energy distribution studies of Fermi bright blazars

    NASA Astrophysics Data System (ADS)

    Monte, C.; Giommi, P.; Cavazzuti, E.; Gasparrini, D.; Rainò, S.; Fuhrmann, L.; Angelakis, E.; Villata, M.; Raiteri, C. M.; Perri, M.; Richards, J.

    2011-02-01

    The Fermi Gamma-ray Space Telescope was successfully launched on June 11, 2008 and has already opened a new era for gamma-ray astronomy. The Large Area Telescope (LAT), the main instrument on board Fermi, presents a significant improvement in sensitivity over its predecessor EGRET, due to its large field of view and effective area, combined with its excellent timing capabilities. The preliminary results of the Spectral Energy Distribution Analysis performed on a sample of bright blazars are presented. For this study, the data from the first three months of data collection of Fermi have been used. The analysis is extended down to radio, mm, near-IR, optical, UV and X-ray bands and up to TeV energies based on unprecedented sample of simultaneous multi-wavelength observations by GASP-WEBT.

  9. Temperature Dependence of Band Gaps in Semiconductors: Electron-Phonon Interaction

    NASA Astrophysics Data System (ADS)

    Bhosale, J. S.; Ramdas, A. K.; Burger, A.; Muñoz, A.; Romero, A. H.; Cardona, M.; Lauck, R.; Kremer, R. K.

    2013-03-01

    A theoretical investigation with ab initio techniques of the electron-phonon interaction of semiconductors with chalcopyrite structure and its comparison with modulated reflectivity experiments yield a striking difference between those with (AgGaS2) and without (ZnSnAs2) d electrons in their valence bands. The former exhibit a non-monotonic temperature dependence of the band gaps whose origin is not yet fully understood. The analysis of this temperature dependence with the Bose-Einstein oscillator model[1] involving two oscillator terms having weights of opposite signs, provides an excellent agreement with the experimental data and correlates well with the characteristic peaks in the phonon density of states associated with the acoustical phonon modes. This work underscores the need for theoretical understanding of the electron-phonon interaction involving d electrons, particularly in ab initio investigations.

  10. Band gap bowing and electron localization of (GaxIn1-x)N

    SciTech Connect

    Lee, Byounghak; Wang, Lin-Wang

    2006-05-09

    The band gap bowing and the electron localization ofGaxIn1-xN are calculated using both the local density approximation (LDA)and screened-exchange local density functional (sX-LDA) methods. Thecalculated sX-LDA band gaps are in good agreement with the experimentallyobserved values, with errors of -0.26 and 0.09 eV for bulk GaN and InN,respectively. The LDA band gap errors are 1.33 and 0.81 eV for GaN andInN, in order. In contrast to the gap itself, the band gap bowingparameter is found to be very similar in sX-LDA and LDA. We identify thelocalization of hole states in GaxIn1-xN alloys along In-N-In chains. Thepredicted localizationis stronger in sX-LDA.

  11. Electron microscopy and x-ray diffraction evidence for two Z-band structural states.

    PubMed

    Perz-Edwards, Robert J; Reedy, Michael K

    2011-08-01

    In vertebrate muscles, Z-bands connect adjacent sarcomeres, incorporate several cell signaling proteins, and may act as strain sensors. Previous electron microscopy (EM) showed Z-bands reversibly switch between a relaxed, "small-square" structure, and an active, "basketweave" structure, but the mechanism of this transition is unknown. Here, we found the ratio of small-square to basketweave in relaxed rabbit psoas muscle varied with temperature, osmotic pressure, or ionic strength, independent of activation. By EM, the A-band and both Z-band lattice spacings varied with temperature and pressure, not ionic strength; however, the basketweave spacing was consistently 10% larger than small-square. We next sought evidence for the two Z-band structures in unfixed muscles using x-ray diffraction, which indicated two Z-reflections whose intensity ratios and spacings correspond closely to the EM measurements for small-square and basketweave if the EM spacings are adjusted for 20% shrinkage due to EM processing. We conclude that the two Z-reflections arise from the small-square and basketweave forms of the Z-band as seen by EM. Regarding the mechanism of transition during activation, the effects of Ca(2+) in the presence of force inhibitors suggested that the interconversion of Z-band forms was correlated with tropomyosin movement on actin. PMID:21806939

  12. Thermo electronic laser energy conversion

    NASA Technical Reports Server (NTRS)

    Hansen, L. K.; Rasor, N. S.

    1976-01-01

    The thermo electronic laser energy converter (TELEC) is described and compared to the Waymouth converter and the conventional thermionic converter. The electrical output characteristics and efficiency of TELEC operation are calculated for a variety of design variables. Calculations and results are briefly outlined. It is shown that the TELEC concept can potentially convert 25 to 50 percent of incident laser radiation into electric power at high power densities and high waste heat rejection temperatures.

  13. Electron concentrations calculated from the lower hybrid resonance noise band observed by Ogo 3.

    NASA Technical Reports Server (NTRS)

    Burtis, W. J.

    1973-01-01

    A noise band at the lower hybrid resonance (LHR) is often detected by the VLF and ELF receivers on Ogo 3, using the electric antenna. In some cases the noise band is at the geometric mean gyrofrequency as measured by the Goddard Space Flight Center (GSFC) magnetometer, and local LHR in a dense H(+) plasma is indicated; in such cases, electron concentration can be calculated, if it is assumed that heavy ions are negligible. Observations at midlatitudes and altitudes of a few earth radii show local concentrations as low as 1.4 electrons/cu cm. In one case the concentrations obtained from the LHR noise band agree with those measured simultaneously by the GSFC ion mass spectrometer within a factor of 2. In another case the concentration is observed to fall by a factor of 2 in 150 km and then to decrease roughly as R to the minus fourth power, in agreement with whistler measurements outside the plasmapause.

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

    DOE PAGESBeta

    Yamaguchi, Hisato; Ogawa, Shuichi; Watanabe, Daiki; Hozumi, Hideaki; Gao, Yongqian; Eda, Goki; Mattevi, Cecilia; Fujita, Takeshi; Yoshigoe, Akitaka; Ishizuka, Shinji; et al

    2016-04-08

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

  15. Electronic structure of transition metal dichalcogenides monolayers 1H-MX2 (M = Mo, W; X = S, Se, Te) from ab-initio theory: new direct band gap semiconductors

    NASA Astrophysics Data System (ADS)

    Kumar, A.; Ahluwalia, P. K.

    2012-06-01

    We report first principles calculations of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical atomic orbital basis sets based methods within the local density approximation. Electronic band structure and density of states calculations found that the states around the Fermi energy are mainly due to metal d states. From partial density of states we find a strong hybridisation between metal d and chalcogen p states below the Fermi energy. All studied compounds in this work have emerged as new direct band gap semiconductors. The electronic band gap is found to decrease as one goes from sulphides to the tellurides of both Mo and W. Reducing the slab thickness systematically from bulk to monolayers causes a blue shift in the band gap energies, resulting in tunability of the electronic band gap. The magnitudes of the blue shift in the band gap energies are found to be 1.14 eV, 1.16 eV, 0.78 eV, 0.64, 0.57 eV and 0.37 eV for MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, respectively, as we go from bulk phase (indirect band gap) to monolayer limit (direct band gap). This tunability in the electronic band gap and transitions from indirect to direct band make these materials potential candidates for the fabrication of optoelectronic devices.

  16. Electron pairing in the presence of incipient bands in iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Linscheid, Andy; Chen, Xiao; Maiti, Saurabh; Hirschfeld, Peter

    Recent experiments on certain Fe-based superconductors (SC) have hinted at a role for paired electrons in ``incipient'' bands that are close to, but do not cross the Fermi level. Within a simple multiband weak-coupling BCS approximation, we categorize the problem into two cases: case(I) where SC arises from the incipient band pairing alone, and case(II) where it is induced on an incipient band by pairing due to Fermi-surface based interactions. Negative conclusions regarding the importance of incipient bands are largely based on case(I). However, we show explicitly that models under case(II) can explain the mild suppression of Tc, as well as robust large gaps on an incipient band. We also model the interplay between phonon and spin fluctuation (SF) driven SC and describe the bootstrap of electron-phonon SC by SF coupling the incipient and the regular bands. We argue that pairing on incipient bands may be important in several Fe-based materials, including LiFeAs, FeSe intercalates and FeSe monolayers on SrTiO3, and indeed may contribute to high Tc in some cases. In addition, we address the question whether this conclusion holds if the SF interaction is derived explicitly in the incipient band scenario and retardation effects are included on the level of the Eliashberg equations. SM was supported by NHMFL through NSF-DMR-1157490, AL and PJH were supported in part by DOE DE-FG02-05ER46236.

  17. TiS3 nanoribbons: Width-independent band gap and strain-tunable electronic properties

    NASA Astrophysics Data System (ADS)

    Kang, Jun; Sahin, Hasan; Ozaydin, H. Duygu; Senger, R. Tugrul; Peeters, François M.

    2015-08-01

    The electronic properties, carrier mobility, and strain response of TiS3 nanoribbons (TiS3 NRs) are investigated by first-principles calculations. We found that the electronic properties of TiS3 NRs strongly depend on the edge type (a or b). All a-TiS3 NRs are metallic with a magnetic ground state, while b-TiS3 NRs are direct band gap semiconductors. Interestingly, the size of the band gap and the band edge position are almost independent of the ribbon width. This feature promises a constant band gap in a b-TiS3 NR with rough edges, where the ribbon width differs in different regions. The maximum carrier mobility of b-TiS3 NRs is calculated by using the deformation potential theory combined with the effective mass approximation and is found to be of the order 103cm2V-1s-1 . The hole mobility of the b-TiS3 NRs is one order of magnitude lower, but it is enhanced compared to the monolayer case due to the reduction in hole effective mass. The band gap and the band edge position of b-TiS3 NRs are quite sensitive to applied strain. In addition we investigate the termination of ribbon edges by hydrogen atoms. Upon edge passivation, the metallic and magnetic features of a-TiS3 NRs remain unchanged, while the band gap of b-TiS3 NRs is increased significantly. The robust metallic and ferromagnetic nature of a-TiS3 NRs is an essential feature for spintronic device applications. The direct, width-independent, and strain-tunable band gap, as well as the high carrier mobility, of b-TiS3 NRs is of potential importance in many fields of nanoelectronics, such as field-effect devices, optoelectronic applications, and strain sensors.

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

    NASA Astrophysics Data System (ADS)

    Yater, J. E.; Shaw, J. L.; Pate, B. B.; Feygelson, T. I.

    2016-02-01

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

  19. Quasiparticle band gap of organic-inorganic hybrid perovskites: Crystal structure, spin-orbit coupling, and self-energy effects

    NASA Astrophysics Data System (ADS)

    Gao, Weiwei; Gao, Xiang; Abtew, Tesfaye; Sun, Yiyang; Zhang, Shengbai; Zhang, Peihong

    The quasiparticle band gaps of organic-inorganic hybrid perovskites are often determined (and can be controlled) by various factors, complicating predictive materials optimization. Here we report a comprehensive investigation on the band gap formation mechanism in CH3NH3PbI3 by decoupling various contributing factors which ultimately determine their electronic structure and quasiparticle band gap. Four major factors, namely, quasiparticle self-energy, spin-orbit coupling, volume (lattice constant) effects, and structural distortions due to the presence of organic molecules, and their influences on the quasiparticle band structure of organometal hybrid perovskites are illustrated. We find that although methylammonium cations do not contribute directly to the electronic states near band edges, they play an important role in defining the band gap through a lattice distortion mechanism and by controlling the overall lattice constants (thus the chemical bonding of the optically active PbI3-). The spin-orbit coupling effects drastically reduce the electron and hole effective masses in these systems, which is beneficial for high carrier mobilities and small exciton binding energies. This work is supported by the National Natural Science Foundation of China (Grant No. 11328401), NSF (Grant No. DMR-0946404 and DMR-1506669), and the SUNY Networks of Excellence.

  20. Electronic band structure trends of perovskite halides: Beyond Pb and Sn to Ge and Si

    NASA Astrophysics Data System (ADS)

    Huang, Ling-yi; Lambrecht, Walter R. L.

    2016-05-01

    The trends in electronic band structure are studied in the cubic A B X3 halide perovskites for A =Cs ; B =Pb , Sn, Ge, Si; and X =I , Br, Cl. The gaps are found to decrease from Pb to Sn and from Ge to Si, but increase from Sn to Ge. The trend is explained in terms of the atom s levels of the group-IV element and the atomic sizes which changes the amount of hybridization with X -p and hence the valence bandwidth. Along the same series spin-orbit coupling also decreases and this tends to increase the gap because of the smaller splitting of the conduction band minimum. Both effects compensate each other to a certain degree. The trend with halogens is to reduce the gap from Cl to I, i.e., with decreasing electronegativity. The role of the tolerance factor in avoiding octahedron rotations and octahedron edge sharing is discussed. The Ge containing compounds have tolerance factor t >1 and hence do not show the series of octahedral rotation distortions and the existence of edge-sharing octahedral phases known for Pb and Sn-based compounds, but rather a rhombohedral distortion. CsGeI3 is found to have a suitable gap for photovoltaics both in its cubic (high-temperature) and rhombohedral (low-temperature) phases. The structural stability of the materials in the different phases is also discussed. We find the rhombohedral phase to have lower total energy and slightly larger gaps but to present a less significant distortion of the band structure than the edge-sharing octahedral phases, such as the yellow phase in CsSnI3. The corresponding silicon based compounds have not yet been synthesized and therefore our estimates are less certain but indicate a small gap for cubic CsSiI3 and CsSiBr3 of about 0.2 ±0.2 eV and 0.8 ±0.6 eV for CsSiCl3. The intrinsic stability of the Si compounds is discussed.

  1. Understanding the electronic band structure of Pt-alloys for surface reactivity

    NASA Astrophysics Data System (ADS)

    Jung, Jongkeun; Kim, Beomyoung; Hong, Ji Sook; Jin, Tae Won; Shim, Ji Hoon; Nemsak, Slavomir; Denlinger, Jonathan D.; Masashi, Arita; Kenya, Shimada; Kim, Changyoung; Mun, Bongjin Simon

    In polymer exchange membrane fuel cell (PEMFC), the oxygen reduction reaction (ORR) at cathode side has been continuously investigated due to its critical importance in performance of fuel cell. So far, even with best industrial catalyst made with Pt, the performance of ORR is too far below from the commercial purpose. In 2007, Stamenkovic et al. showed that Pt alloys with 3- dtransition metal exhibited significantly improved ORR performance and pointed out the altered electronic structure of surface as the major contributing factor for enhanced ORR. Since 1990, with the advance of DFT calculation, the trend of surface chemical reactivity is explained with the analysis of d-band structures, known as d-band model. While d-band provides valid insight on surface chemical reactivity based on the valence band DOS, the relation between surface work function and DOS has not been well understood. The element-specific local electronic band structure of Pt alloys are identified by ARPES measurement, and the correlation between surface work function and local charge density is investigated.

  2. Housing Electrons: Relating Quantum Numbers, Energy Levels, and Electron Configurations.

    ERIC Educational Resources Information Center

    Garofalo, Anthony

    1997-01-01

    Presents an activity that combines the concepts of quantum numbers and probability locations, energy levels, and electron configurations in a concrete, hands-on way. Uses model houses constructed out of foam board and colored beads to represent electrons. (JRH)

  3. Excitation of the ligand-to-metal charge transfer band induces electron tunnelling in azurin

    SciTech Connect

    Baldacchini, Chiara; Bizzarri, Anna Rita; Cannistraro, Salvatore

    2014-03-03

    Optical excitation of azurin blue copper protein immobilized on indium-tin oxide, in resonance with its ligand-to-metal charge transfer absorption band, resulted in a light-induced current tunnelling within the protein milieu. The related electron transport rate is estimated to be about 10{sup 5} s{sup −1}. A model based on resonant tunnelling through an azurin excited molecular state is proposed. The capability of controlling electron transfer processes through light pulses opens interesting perspectives for implementation of azurin in bio-nano-opto-electronic devices.

  4. Synthesis, structure and band gap energy of covalently linked cluster-assembled materials.

    PubMed

    Mandal, Sukhendu; Reber, Arthur C; Qian, Meichun; Liu, Ran; Saavedra, Hector M; Sen, Saikat; Weiss, Paul S; Khanna, Shiv N; Sen, Ayusman

    2012-10-28

    We have synthesized a series of cluster assembled materials in which the building blocks are As(7)(3-) clusters linked by group 12 metals, Zn, Cd and Hg, to investigate the effect of covalent linkers on the band gap energy. The synthesized assemblies include zero dimensional assemblies of [Zn(As(7))(2)](4-), [Cd(As(7))(2)](4-), [Hg(2)(As(7))(2)](4-), and [HgAsAs(14)](3-) in which the clusters are separated by cryptated counterions, and assemblies in which [Zn(As(7))(2)](4-), [Cd(As(7))(2)](4-) are linked by free alkali atoms into unusual three-dimensional structures. These covalently linked cluster-assembled materials have been characterized by elemental analysis, EDX and single-crystal X-ray diffraction. The crystal structure analysis revealed that in the case of Zn and Cd, the two As(7)(3-) units are linked by the metal ion, while in the case of Hg, two As(7)(3-) units are linked by either Hg-Hg or Hg-As dimers. Optical measurements indicate that the band gap energy ranges from 1.62 eV to 2.21 eV. A theoretical description based on cluster orbital theory is used to provide a microscopic understanding of the electronic character of the composite building blocks and the observed variations in the band gap energy. PMID:22940817

  5. ADVANCED X-BAND TEST ACCELERATOR FOR HIGH BRIGHTNESS ELECTRON AND GAMMA RAY BEAMS

    SciTech Connect

    Marsh, R A; Anderson, S G; Barty, C P; Chu, T S; Ebbers, C A; Gibson, D J; Hartemann, F V; Adolphsen, C; Jongewaard, E N; Raubenheimer, T; Tantawi, S G; Vlieks, A E; Wang, J W

    2010-05-12

    In support of Compton scattering gamma-ray source efforts at LLNL, a multi-bunch test stand is being developed to investigate accelerator optimization for future upgrades. This test stand will enable work to explore the science and technology paths required to boost the current 10 Hz monoenergetic gamma-ray (MEGa-Ray) technology to an effective repetition rate exceeding 1 kHz, potentially increasing the average gamma-ray brightness by two orders of magnitude. Multiple bunches must be of exceedingly high quality to produce narrow-bandwidth gamma-rays. Modeling efforts will be presented, along with plans for a multi-bunch test stand at LLNL. The test stand will consist of a 5.5 cell X-band rf photoinjector, single accelerator section, and beam diagnostics. The photoinjector will be a high gradient standing wave structure, featuring a dual feed racetrack coupler. The accelerator will increase the electron energy so that the emittance can be measured using quadrupole scanning techniques. Multi-bunch diagnostics will be developed so that the beam quality can be measured and compared with theory. Design will be presented with modeling simulations, and layout plans.

  6. Advanced X-Band Test Accelerator for High Brightness Electron and Gamma Ray Beams

    SciTech Connect

    Marsh, Roark; Anderson, Scott; Barty, Christopher; Chu, Tak Sum; Ebbers, Chris; Gibson, David; Hartemann, Fred; Adolphsen, Chris; Jongewaard, Erik; Raubenheimer, Tor; Tantawi, Sami; Vlieks, Arnold; Wang, Juwen; /SLAC

    2012-07-03

    In support of Compton scattering gamma-ray source efforts at LLNL, a multi-bunch test stand is being developed to investigate accelerator optimization for future upgrades. This test stand will enable work to explore the science and technology paths required to boost the current 10 Hz monoenergetic gamma-ray (MEGa-Ray) technology to an effective repetition rate exceeding 1 kHz, potentially increasing the average gamma-ray brightness by two orders of magnitude. Multiple bunches must be of exceedingly high quality to produce narrow-bandwidth gamma-rays. Modeling efforts will be presented, along with plans for a multi-bunch test stand at LLNL. The test stand will consist of a 5.5 cell X-band rf photoinjector, single accelerator section, and beam diagnostics. The photoinjector will be a high gradient standing wave structure, featuring a dual feed racetrack coupler. The accelerator will increase the electron energy so that the emittance can be measured using quadrupole scanning techniques. Multi-bunch diagnostics will be developed so that the beam quality can be measured and compared with theory. Design will be presented with modeling simulations, and layout plans.

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

    DOE PAGESBeta

    Wang, Zhenbin; Chu, Iek -Heng; Zhou, Fei; Ong, Shyue Ping

    2016-05-09

    Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu2+-activated emission identified through a comparison of the electronic structures of known narrow-band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu2+ 4f7 bands. By incorporating this descriptor in a high-throughput first-principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu2+-activated red-emitting phosphors that are predicted to exhibit goodmore » chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Lastly, our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth.« less

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

    NASA Astrophysics Data System (ADS)

    Kishi, Ayaka; Oda, Masato; Shinozuka, Yuzo

    2016-05-01

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

  9. Observation of strong electron pairing on bands without Fermi surfaces in LiFe1-xCoxAs

    NASA Astrophysics Data System (ADS)

    Miao, H.; Qian, T.; Shi, X.; Richard, P.; Kim, T. K.; Hoesch, M.; Xing, L. Y.; Wang, X.-C.; Jin, C.-Q.; Hu, J.-P.; Ding, H.

    2015-01-01

    In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally different picture of the electron pairs, which are believed to be formed locally by repulsive interactions, may prevail. Here we report a high-resolution angle-resolved photoemission spectroscopy study on LiFe1-xCoxAs. We reveal a large and robust superconducting gap on a band sinking below the Fermi level on Co substitution. The observed Fermi-surface-free superconducting order is also the largest over the momentum space, which rules out a proximity effect origin and indicates that the order parameter is not tied to the Fermi surface as a result of a surface instability.

  10. Enlarged band gap and electron switch in graphene-based step-barrier structure

    SciTech Connect

    Lu, Wei-Tao Ye, Cheng-Zhi; Li, Wen

    2013-11-04

    We study the transmission through a step-barrier in gapped graphene and propose a method to enlarge the band gap. The step-barrier structure consists of two or more barriers with different strengths. It is found that the band gap could be effectively enlarged and controlled by adjusting the barrier strengths in the light of the mass term. Klein tunneling at oblique incidence is suppressed due to the asymmetry of step-barrier, contrary to the cases in single-barrier and superlattices. Furthermore, a tunable conductance channel could be opened up in the conductance gap, suggesting an application of the structure as an electron switch.