Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N27: Electronic Structure I |
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Sponsoring Units: DCOMP Chair: Michael Mehl, Naval Research Laboratory Room: Baltimore Convention Center 324 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N27.00001: Tight binding calculations of vibrational and thermal properties of amorphous silicon Michael Mehl, Joseph Feldman, Dimitris Papaconstantopoulos, Noam Bernstein By displacing atoms by different amounts and computing atomic forces within the NRL tight binding method we obtain all second order (harmonic) and some third order (anharmonic) coupling constants of a 1000 atom TB-relaxed Wooten CRN model of amorphous silicon. The harmonic force constant results allow us to study various properties including vibrational density of states, dynamic structure factors, specific heat and thermal conductivity within Kubo theory. We shall present results of these applications and compare to experiment and previous work based on the Stillinger Weber potential. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N27.00002: Manifestation of Negative Compressibility in Low-Density Electron Liquids: Anomaly in the Ion-Pair Distribution Function in Supercritical Fluid Rb Yasutami Takada It is a well-known fact that the electronic compressibility $\kappa$ diverges in the 3D electron gas as the density parameter $r_s$ approaches 5.25. A recent investigation clarifies that this divergence is due to the excitonic effect in the electron-hole pair excitation, in particular, to its zero-energy excitation [1]. For $r_s>5.25$, $\kappa$ becomes negative, leading to the negative static dielectric function $\varepsilon(q,0)$ for at least small $q$ owing to the compressibility sum rule. Then we can expect that two positive test charges do not repel but attract to each other in such a system. Keeping this situation in mind, we have calculated the ion-pair distribution function g(R) in the expanded Rb liquid metal by using the Monte Carlo method and found interesting features in g(R) characteristic to the negative $\varepsilon (q,0)$ [2]. Such features have been observed by the recent measurement of g(R) in the supercritical fluid Rb metal with continuously increasing $r_s$ from 5.25 [3]. This confirms the situation of $\kappa<0$ in the low-density 3D electron gas for the first time. [1] YT, J. Superconductivity {\bf 18}, No.3 (2005). [2] H. Maebashi and YT, to be submitted. [3] K. Matsuda and K. Tamura, private communication. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N27.00003: Finite temperature quasiparticle self-consistent GW approximation Sergey Faleev, Mark van Schilfgaarde, Takao Kotani, Francois Leonard, Michael Desjarlais We present a new ab initio method for electronic structure calculations of materials at finite temperature (FT) based on the all-electron quasiparticle self-consistent GW (QPscGW) approximation and Keldysh time-loop Green's function approach. We apply the method to Si, Ge, GaAs, InSb, and diamond and show that the band gaps of these materials universally decrease with temperature in contrast with the local density approximation (LDA) of density functional theory (DFT) where the band gaps universally increase. At temperatures of a few eV the difference between quasiparticle energies obtained in FT-QPscGW and FT-LDA approaches significantly reduces. This result suggests that existing simulations of very high temperature materials based on the FT-LDA are more justified then it might appear from well-known LDA band gap errors at zero-temperature. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N27.00004: Iterative minimization by Kohn--Sham inversion and potential mixing Roope Astala, Malcolm Stott Applications of Hohenberg--Kohn--Sham density functional theory to problems in materials physics are critically dependent on algorithms for iterating the Kohn--Sham equations to self-consistency. We present an approach for obtaining the self-consistent solution, which explores a connection between iterative minimization and Kohn--Sham inversion, \textit{i.e.} finding a self-consistent potential for a given density. The central idea is to perform the Kohn--Sham inversion using a position-dependent Lagrange multiplier and to construct a new trial potential from the result. The method is variational, in contrast to commonly-used density mixing approaches, and has excellent convergence. We demonstrate the method using a real-space pseudopotential implementation with applications to small molecules. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N27.00005: Atomic Forces and Geometry Optimisation with the {\sc onetep} Linear-Scaling DFT Method Arash Mostofi, Peter Haynes, Chris-Kriton Skylaris, Mike Payne {\sc onetep}[1] (Order-$N$ Electronic Total Energy Package), is a density functional method, based on plane-waves, whose computational cost scales only linearly with the number of atoms. \\ {\sc onetep} uses a localised yet orthogonal basis of periodic cardinal sine (psinc) functions[2], also known as Dirichlet or Fourier Lagrange-mesh functions, which are formed from a discrete sum of plane-waves. The localised non-orthogonal generalised Wannier functions (NGWFs) which span the occupied subspace are represented in terms of these psinc functions and are optimised during the calculation. \\ This choice of basis and optimisation of the NGWFs results in smooth potential energy surfaces and enables the use of the Hellmann-Feynman theorem for the calculation of atomic forces. These have been implemented within a quasi-Newton geometry optimisation scheme and preliminary results are presented. \\ \\ {[1]} {\it J.~Chem.~Phys.} {\bf 122}, 084119 (2005). \\ {[2]} {\it J.~Chem.~Phys.} {\bf 119}, 8842 (2003). [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N27.00006: Optimized norm-conserving Hartree-Fock pseudopotentials Eric J. Walter, Wissam A. Al-Saidi We report soft Hartree-Fock based pseudopotentials obtained using the optimized pseudopotential method \footnote{ A.~M.~Rappe, K.~M.~Rabe, E.~Kaxiras, and J.~D.~Joannopoulos, Phys. Rev. B {\bf 41}, 1227 (1990).}. The spurious long range tail due to the non locality of the exchange potential is removed using a self-consistent damping mechanism as employed in exact exchange \footnote{ E.~Engel, A.~H\"ock, R.~N.~Schmid, R.~M.~Dreizler, and N.~Chetty, Phys. Rev. B {\bf 64}, 125111 (2001).} and recent Hartree-Fock pseudopotentials\footnote{J. R. Trail and R. J. Needs, J. Chem. Phys. {\bf 122}, 014112 (2005).}. The binding energies of several dimers computed using these pseudopotentials within a planewave Hartree-Fock code show good agreement with all-electron results. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N27.00007: Effect of Ti and metal vacancies on the dehydrogenation of Na$_{3}$AlH$_{6}$ Sa Li, Puru Jena, Rajeev Ahuja Electronic and structural properties of sodium-aluminum hexahydride (Na$_{3}$AlH$_{6})$ formed during the decomposition reaction of sodium alanate (NaAlH$_{4})$ are calculated using density functional theory and generalized gradient approximation for exchange and correlation potential. The roles of Ti substitution at the Na and Al sites as well as that of Na and Al vacancies on the desorption of hydrogen have also been investigated. We show that the improvement in dehydrogenation of Na$_{3}$AlH$_{6 }$is due to the addition of Ti much the same way as that in NaAlH$_{4}$, namely through the weakening of the metal-hydrogen bond. However, as in the case of NaAlH$_{4}$, vacancies are found to be more effective in desorbing hydrogen at lower temperatures than Ti substitution at the Na or Al sites. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N27.00008: Excitation Energies from Time-Dependent Density Functional Theory within Modified Linear Response: Inclusion of the Electron-Hole Hartree-Fock Interaction Chunping Hu, Osamu Sugino, Yoshiyuki Miyamoto Time-dependent density functional theory (TD-DFT) within linear response (LR) has gained enormous popularity in the calculation of electronic excitations, whereas it is known to give considerably underestimated excitation energies for Rydberg and charge-transfer excitations. Although the incorrect long-range behavior of exchange-correlation (XC) potentials has been blamed for this problem, a different point of view on the LR scheme without any correction of XC potentials is presented here. Analyzing approximate excitation energies from LR within adiabatic local density approximation (ALDA) and the exact exchange (EXX) scheme, we propose a modified LR theory to strictly include the electron-hole Hartree-Fock interaction kernel, and to make excitation energy expression in ALDA explicitly similar to the EXX one. TD-LDA calculations within modified LR on typical diatomic molecules show that excitation energies of both Rydberg and charge-transfer excitations can be greatly improved to the EXX-level accuracy. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N27.00009: Quasiparticle Corrections to the Electronic Properties of Point Defects Arno Schindlmayr, Magnus Hedstr\"om, Philipp Eggert, Matthias Scheffler We present a quantitative ab initio method for calculating defect states and charge-transition levels of point defects in semiconductors. It relies on a separation into lattice and electronic energy contributions, which are treated within density-functional theory and many-body perturbation theory, respectively. We use the $GW$ approximation for the self-energy to determine the quasiparticle corrections to defect states in the band gap. As an example, we consider anion vacancies on the (110) surfaces of III-V semiconductors. The calculated charge-transition levels, in particular, show a clear improvement over the local-density approximation and are in close agreement with the available experimental data. As the surface is simulated by a slab within the supercell approximation, we place special emphasis on a convergence analysis of the quasiparticle properties in this approach. The dynamic polarization between the periodic images can be understood within a simple model, which also allows an a posteriori correction. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N27.00010: Probing the Ab-initio Fermi Surface Efficiently using Wannier Interpolation Jonathan Yates, Ivo Souza, Xinjie Wang, David Vanderbilt Modern {\it ab-initio} techniques are able to provide an accurate description of the electronic structure for a wide range of materials. However, evaluation of the transport properties of metals requires an extremely detailed, and hence computationally expensive, sampling of the Fermi surface. We show that the electron group velocity and effective mass can be obtained directly from the Wannier representation of a system. This leads to an efficient and precise method for the calculation of transport properties using Wannier interpolation. We will present calculations of the ordinary Hall coefficient and thermoelectric power for a variety of materials. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N27.00011: Nonlinear Optical Response of Polar Semiconductors in the Terahertz Range Eric Roman, Jonathan Yates, Marek Veithen, David Vanderbilt, Ivo Souza Using the Berry-phase finite-field method, we compute from first-principles the recently measured\footnote{ T. Dekorsy, V. A. Yakovlev, W. Seidel, M. Helm, and F. Keilmann, Phys. Rev. Lett. {\bfseries 90}, 055508 (2003).} infrared (IR) dispersion of the nonlinear susceptibility $\chi^{(2)}$ in III-V zincblende semiconductors. At far-IR (terahertz) frequencies, in addition to the purely electronic response $\chi^{(2)}_{\infty}$, the total $\chi^{(2)}$ depends on three other parameters, $C_1$, $C_2$, and $C_3$, describing the contributions from ionic motion. They relate to the TO Raman polarizability and the second-order displacement-induced dielectric polarization and forces, respectively. Contrary to a widely-accepted model,\footnote{C. Flytzanis, Phys. Rev. B {\bf 6}, 1264 (1972).} but in agreement with the recent experiments on GaAs,$^1$ we find that the contribution from mechanical anharmonicity dominates over electrical anharmonicity. By using Richardson extrapolation to evaluate the Berry's phase in $k$-space by finite differences, we are able to improve the convergence of the nonlinear susceptibility from the usual\footnote{P. Umari and A. Pasquarello, Phys. Rev. B {\bf 68}, 085114 (2003).} ${\cal O}[(\Delta k)^2]$ to ${\cal O}[(\Delta k)^4]$, dramatically reducing the computational cost. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N27.00012: Breathing oscillations accompanying Bloch oscillations of wavepackets in periodic potentials Miguel Pruneda, Ivo Souza Using a 1D tight-binding model, we study the evolution of a well-localized wavepacket of Bloch states under an applied electric field. We apply a novel algorithm~\footnote{ along the lines of I. Souza {\it et. al.}, Phys. Rev. B {\bf 69}, 085106 (2004)} for solving numerically the equations of motion which does not rely on the single-band approximation and can thus be used to explore interband Zener tunneling effects. In addition to the well-known Bloch oscillations of the center of the packet, we show that as the waveform moves in k-space, its real-space width varies in response to the change in the local quantum metric,~\footnote{N. Marzari and D. Vanderbilt, Phys. Rev. B {\bf 56}, 12\,847 (1997).} $g(k)$, of the underlying Bloch states. A generalized uncertainty relation is obtained between the spread in position and in {\it crystal} momentum of a wavepacket. It differs from the usual position/momentum uncertainty relation because of the interband matrix elements of the position operator in the crystal-momentum representation, which introduce a correction in terms of $g(k)$. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N27.00013: Nonadiabatic Transition in the Quantum Hall Effect Manabu Machida, Naomichi Hatano, Jun Goryo We analyze the nonadiabatic transition in a 2D electron system with a periodic potential in the quantum Hall regime. We obtain corrections to the Chern-number term of the Hall conductance and a non-vanishing diagonal conductance. We treat the electric field as a time-dependent vector potential in the Hamiltonian. We calculate the time evolution of the density operator taking account of the first order of the electric field, and thereby study the electric conduction when the system evolves nonadiabatically. We thus obtain analytical expressions of the diagonal and off-diagonal conductances and calculate them numerically. [Preview Abstract] |
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