Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V11: Electronic Structure: Theory and Spectra II |
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Sponsoring Units: FIAP Chair: Richard Martin, University of Illinois at Urbana-Champaign Room: D222 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V11.00001: First principles study of strained Si/Ge core-shell nanowires along [110] direction Xihong Peng, Paul Logan First principles density-functional calculations were performed to study the electronic properties of Si/Ge core-shell nanowires along the [110] direction with the diameter of the wires up to 5 nm. It was found the band gap of the core-shell wires is smaller than that of both pure Si and Ge wires, given the same diameter. This reduced band gap is ascribed to the intrinsic strain between Ge and Si layers, which partially counters the quantum confinement effect. External uniaxial strain is further applied to the Si/Ge core-shell nanowires for tuning the band structure. At the $\Gamma $ point, the energy levels of both conduction and valence bands are significantly altered by applied strain, which results in an evident change of the band gap. In contrast, for the K vectors far away from $\Gamma $, the variation of the conduction/valence band with strain is much reduced. In addition, with a sufficient tensile strain ($\sim $1{\%}), the valence band edge shifts away from $\Gamma $, which indicates that the band gap of the Si/Ge core-shell nanowires experiences a transition from direct to indirect. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V11.00002: Electronic structure of partially hydrogenated graphene superlattices Joo-Hyoung Lee, Jeffrey Grossman First-principles calculations based on density functional theory are performed to investigate the electronic structure of graphene-graphane superlattices (GSLs) by varying the widths of both the graphene and graphane regions. For the armchair-type interface between the graphene and graphane strips (AGSLs), the superlattices become semiconducting with a band gap exhibiting a similar dependence on the width of the graphene region as in armchair graphene nanoribbons. In contrast with the nanoribbons, however, the band gap of AGSLs shows both direct and indirect characteristics, depending on the graphane width. On the other hand, GSLs with a zigzag interface (ZGSLs) possess magnetic ground states except for those with a very narrow graphene strip. While an anti-ferromagnetic (AFM) phase is found to be energetically more stable than the ferromagnetic (FM) one, the energy difference between the two phases is so small ($< 10$~meV) that these two phases become nearly degenerate. These findings point toward an alternative route for graphene-based applications without requiring physical cutting as in graphene nanoribbons. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V11.00003: First-Principle Calculations of The Conductivity of Br-Doped Graphite Rashid Hamdan, Chao Cao, Alexander Kemper, Hai-Ping Cheng First-principles calculations are used to study the enhanced in- plane conductivity that was observed experimentally in Br-doped graphite systems.\footnote{Tongay et al. Phys. Rev. B 81, 115428 (2010)} The band structure near the Fermi surface of the doped systems with different bromine concentrations compared to that of pure graphite, and the charge transfer between carbon and bromine atoms are analyzed to understand the conductivity change in the different directions. In addition, we address the effect of the compression of graphite layers on the stability of the bromine molecule Br$_{2}$ between these layers and thus on the conductivity of the system. Our calculations show that for large separation between doped graphite layers bromine is more stable in the molecular form (Br$_{2}$) and has a negligible effect on in- plane conductivity. However, with increased compression (decreased layer-layer separations) Br$_{2}$ molecule tend to dissociate and exchange charge with the nearby graphite layers causing an increase in hole conductivity. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V11.00004: Study of Bulk Modulus in Zincblende Nitrogen-doped Gallium Phosphide Alloys Using Density Functional Theory Brandon M. Butler, John A. Turner, Muhammad N. Huda The prospect of solar energy as a renewable resource is ever-increasing. Density functional theory (DFT) calculations can elicit reliable behavior predictions in energy conversion materials to achieve higher efficiencies. Chemical stability of the photo-catalysts in aqueous solution is of particular interest for its long term performances. The bulk modulus is a mechanical property that is a good indicator of material stability. GaP has a low band gap and is a good candidate for use as a photocatalyst for hydrogen evolution by splitting water. Unfortunately, it is not stable and highly susceptible to corrosion over a very short time period, making it unfeasible for long-term use. GaN has too high of a band gap but a good stability factor. While these materials both possess desirable qualities, they cannot be used solitarily. We will report electronic properties and bulk moduli from the total energy calculations of the zincblende and wurtzite species using DFT-GGA and DFT+U as a function of doping concentration $x$. We will also present the density of states and charge density distribution of the alloy materials to study the localization/delocalization effects of N defects levels and their impact on the alloys' stability. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V11.00005: Topological Insulators in Ternary Compounds with a Honeycomb Lattice Hai-Jun Zhang, Stanislav Chadov, Lukas Muchler, Binghai Yan, Xiao-Liang Qi, J\"urgen K\"ubler, Shou-Cheng Zhang, Claudia Felser One of the most exciting subjects in solid state physics is a single layer of graphite which exhibits a variety of unconventional novel properties. The key feature of its electronic structure are linear dispersive bands which cross in a single point at the Fermi energy. This is so-called Dirac cone. The ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. These materials exhibit the surface states formed by only a single Dirac cone at the G point together with the small direct band gap opened by a strong spin-orbit coupling (SOC) in the bulk. These materials are centro-symmetric, therefore, it is possible to determine the parity of their wave functions, and hence, their topological character. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V11.00006: The electronic structures of Cu delafossites nanocrystals for PEC hydrogen production: A density functional theory study Muhammad N. Huda, Yanfa Yan, Mowafak M. Al-Jassim Efficient photo-electrochemical (PEC) splitting of water to hydrogen by sun light usually requires that the semiconductor which will be used as a photoelectrodes will satisfy several electronic criteria. As naturally available semiconductors do not meet all these criteria, a thorough understanding of ``band-engineering'' for mixed alloys both at bulk and nano phases is necessary to successfully design these photoelectrodes. Recently Cu delafossites, Cu$M$O$_{2}$, have received much attentions as photo-catalysts for hydrogen production due to their unique properties such as stability in most aqueous solutions and excellent hole mobility. However, due to their large optical band gap they can absorb sun light only in the ultra-violet region. Hence, it is necessary to tailor their electronic properties to enhance their catalytic activities in the visible light regions. In this presentation density functional study of the Cu-delafossite nanocrystals will be presented. The stability of the nanocrystals will be discussed along with the reactivity of the different crystal faces. It will be shown that O-terminated$ M$-O octahedrons play a major role in the stability of these nanocrystals, which also makes these surfaces less reactive. We will discuss the charge (electron-hole) separation problems in these nanocrystals. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V11.00007: Lateral Quantum Well States on Pb Films Grown on Cu Step Surface Pingheng Zhou, Yang Liu, Tom Miller, Tai-Chang Chiang, Paolo Moras, Carlo Carbone The highly ordered Pb films were found to grow on Cu step surface as a ``magic'' heteroepitaxial grown model. The lateral quantum well states in these Pb have been investigated by angle-resolved photoemission. Across the step direction, the quantum well state display a dispersive character, with periodicity in reciprocal space defined by the step superlattice geometry. These observations are compared and analyzed with \textit{ab initio} calculations based on the full-potential linearized augmented plane wave method. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V11.00008: Influence of local environment on the characterization of the p-type TCO in silver vanadates Jino Im, Giancarlo Trimarchi, Haowei Peng, Kenneth Poeppelmeier, Arthur Freeman Cu and Ag oxides are often considered as possible candidates for p-type transparent conducting oxides (TCO's) because the d$^{10}$ valence structure usually gives rise to dispersive d-bands at the valence band maximum. Among them, multi-cation oxides of silver and vanadium show various atomic structures such as the $\alpha-$ and the $\beta-$phase of Ag$_{3}$VO$_{4}$ and KAg$_{11}$(VO$_{4}$)$_{4}$. Hence, these compounds, especially KAg$_{11}$(VO$_{4}$)$_{4}$, offer several local environments at Ag sites and it is interesting to assess how they influence the electronic structure. Based on first-principles density functional theory, we point out a relation between the local environment and d-s orbital mixing at the Ag site. In turn, this mixing determines the orbital composition of the band extrema and band gaps. The influence on band gaps of the substitution of Nb and Ta for V in Ag$_{3}$VO$_{4}$ and of the substitution of alkali metals for K in KAg$_{11}$(VO$_{4}$)$_{4}$ will also be discussed. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V11.00009: Electronic structures of Tl-based materials for $\gamma$-ray detectors; First-principles study Jung-Hwan Song, Hosub Jin, Arthur J. Freeman, Simon Johnsen, John Androulakis, Peter Sebastian, Zhifu Liu, John A. Peter, Nam-Ki Cho, Bruce Wessels, Mercouri G. Kanatzidis For Tl-based semiconductors, investigated to find good candidate materials for $\gamma$-ray detectors, we performed ab-initio calculations using the full-potential linearized augmented plane wave (FLAPW) method\footnote{Wimmer, Krakauer,Weinert, Freeman, Phys. Rev. B, {\bf 24}, 864 (1981)} to find their electronic structures and to estimate their physical properties such as band gaps, effective masses, absorption coefficients, dielectric constants, and work functions. Within the LDA scheme, the underestimation of the band gap is well-known and causes serious problems in obtaining optical properties. Therefore, we adopted the screened-exchange LDA (sX-LDA) scheme and acquired correct gap values close to experimental ones. With the sX-LDA, we found that Tl$_6$I$_4$S and Tl$_6$I$_4$Se have direct band gaps of 2.36 and 1.88 eV, respectively, and they exhibit dispersive bands near the band edges. Based on the calculated and experimental results, we discuss the relationship between atom species/crystal structure and electronic characteristics, and suggest several materials for $\gamma$-ray detectors. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V11.00010: Non-equilibrium Fermi-edge Singularity in Mesoscopic Devices Jin Zhang, Nicholas d'Ambrumenil, Boris Muzykantskii, David Cobden The non-equilibrium Fermi-edge singularity (NFES) was observed as a non-linearity in the random telegraph signal [1] in mesoscopic devices at low temperatures. Based on a modified NFES theory, we found that when a low-frequency ac signal is applied, the Fumi shift and the electron dephasing contribute in an opposite way to the violation of the detailed balance, which is measured in the logarithmic ratio of tunneling rates. In the case of large electron phase shift and weak dephasing, the usually-ignored Fumi shift plays an important role, and manifests itself as an ``S''- shape curve in the logarithmic ratio of rates. For stronger dephasing, near the transition threshold, the tunneling spectra are dominated by the increased effective temperature because of the bias voltage, while for energies larger than that of the probing signal, the thermal excitation restores the system to pseudo-equilibrium. The overall shape of the logarithmic ratio of rates shows a ``Z''-shape. \\[4pt] [1] D. H. Cobden, and B. A. Muzykantskii, Phys. Rev. Lett. 75, 4274 (1995) [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V11.00011: Dirac Point Degenerate with Massive Bands at a Topological Quantum Critical Point Swapnonil Banerjee, Warren Pickett, Justin Smith, Victor Pardo In the band structure of the Skutterudite, as the Sb sublattice in the unit cell is moved slightly retaining the crystal symmetry, the small gap at the Fermi energy closes due to a band crossing at Gamma. At this critical point a pair of linear (``Dirac'') bands are degenerate with two conventional bands. Because of the crystal symmetry three out of the four bands are degenerate even when one is away from the critical point. Insulators in 3D, as well as in 2D, can be characterized by topological invariants. When inversion symmetry is present (as in the space group 204 of Skutterudite), the Z2 invariant can be obtained from the parities of the occupied states at the invariant momenta, which in the bcc structure consist of Gamma, three H points, and the four P points. Here only the Gamma point requires consideration, since reoccupation occurs only there. The singlet has odd parity at Gamma while for the triplet it is even. As the critical point is crossed, the product of the parities of the occupied bands at Gamma, and hence the Z2 invariant, changes sign due to the reversal of the singlet- triplet position; the signal of a trivial to topological transition. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V11.00012: Sixteen-band atomic bond-orbital model for zinc-blende structures Hsiu-Fen Kao, Jih-Chen Chiang, W.T. Wang, Ikai Lo, Y.C. Hsu, C.L. Wu, D.J. Jang, Meng-En Lee, C.Y. Ren, Yen-Chih Tseng, Chun-Nan Chen We develop a sixteen-band atomic bond-orbital model (16ABOM) which is able to compute the spin splitting induced by bulk inversion asymmetry. This model is derived from the linear combination of atomic orbital (LCAO) scheme such that the characteristics of real atomic orbitals can be preserved for spin-splitting calculations. We derive the Hamiltonian of 16ABOM by performing a similarity transform on the nearest-neighbor LCAO Hamiltonian, followed by taking a second-order Taylor series expansion over $k$-vector at the $\Gamma $ point. The spin-splitting energies in bulk zinc-blendes are calculated using this model, and the results are in good agreement with LCAO and first-principles calculations. In addition, it is found the spin-orbit coupling between anti-bonding and bonding $p$-like states, which can be evaluated directly by this 16ABOM, dominates the magnitude of the spin splitting of the lowest conduction bands in middle-bandgap and wide-bandgap materials. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V11.00013: Real-space Green's Function Calculations including Hubbard Contributions Towfiq Ahmed, J. J. Kas, J. J. Rehr Hubbard model contributions are introduced into the real space Green's function formalism in terms of an effective self-energy, based on the LDA+$U$ method of Anisimov et al.\footnote{V. I. Anisimov, F. Aryasetiawan, and A. I. Lichtenstein, J. Phys.: Condens Matter 9, 767 (1997)} The effective self-energy is then applied to localized $d$-states in a material, e.g. at the metal sites of transition metal oxides. The approach is implemented in an extension of the FEFF9 spectroscopy code and leads to an efficient procedure for including strong correlation effects in the electronic structure and x-ray spectra of $d$-electron materials, such as transition metal oxides and high T$_c$ cuprates. Calculations are presented for the angular momentum projected density of states of MnO, NiO and La$_{(2-x)}$Sr$_x$CuO$_4$ and for the K-edge x-ray absorption and emission spectra of the O atoms in these materials, and the results are found to be in reasonable agreement with experiment. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V11.00014: A Plane-Wave Implementation of Quasiparticle Self-Consistent GW (QSGW) Derek Vigil Currey, Jack Deslippe, Steven G. Louie The use of GW techniques in calculating the quasiparticle properties of certain classes of materials, e.g. complex oxides, is sometimes hindered by the poor mean-field starting point that density functional theory (DFT) within standard Kohn-Sham implementations provides. There has been considerable effort in the community to improve upon the mean-field starting point for a broad range of materials. A recently proposed method, the quasiparticle self-consistent GW (QSGW) method, employs a process in which a mean-field exchange-correlation potential is approximated from and updated self-consistently using the self-energy operator from previous iteration GW calculations. We present an implementation of this method in a plane-wave basis, and discuss its accuracy, computational cost, and physical implications for a variety of semiconducting materials. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V11.00015: Assessment of dispersion-corrected density functional approaches for extended systems Wissam Al-Saidi, Vamsee Voora, Ken Jordan Standard density functional (DFT) methods are known to fail in describing the long range van der Waals interactions, and currently, there is a great interest in incorporating dispersion corrections in density functionals. Recently, Tkatchenko and Scheffler introduced a new scheme where dispersion corrections are included by a summation of damped interatomic C$_6$/R$^6$ terms. However, contrary to the DFT-D2 approach of Grimme, the C$_6$ coefficients depend on the electron density through a Hirshfeld atom-in-a-molecule decomposition scheme. We have implemented the vdW-TS approach in VASP and applied it to the study of a series of prototype dispersion-dominated systems including layered materials, noble-gas solids and molecular crystals. Full optimization of all degrees of freedom is possible in our implementation because dispersion corrections are computed for the forces acting on the atoms, and also the stresses on the unitcell. Our results show that the vdW-TS method yield good structure, bulk moduli, and cohesive energies of weakly bonded systems in much better agreement with experiment than those obtained with standard DFT approaches. [Preview Abstract] |
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