Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U21: Focus Session: General Theory: Density Functional Theory and Beyond |
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Sponsoring Units: DCOMP Chair: Eric Shirley, National Institute of Standards and Technology Room: Colorado Convention Center 106 |
Thursday, March 8, 2007 8:00AM - 8:36AM |
U21.00001: Berry phase effects on electronic properties Invited Speaker: Geometric phase has found wide applications in physics, and has emerged as a unifying concept in describing and predicting electronic properties in solids. In this talk, I will review recent advances on Berry phase effects in insulators, metals, and semiconductors, with topics covering dielectric, magnetic, and transport properties. I will also discuss non-abealian generalization of the geometric phase concept and applications in spin transport. \newline \newline [1]. D. Xiao, J. Shi, and Q. Niu, Phys. Rev. Lett. \textbf{95}, 137204 (2005); \newline [2]. D. Xiao, Y. Yao, Z. Fang, and Q. Niu, Phys. Rev. Lett. \textbf{97}, 026603 (2006).; \newline [3]. D. Culcer, Y.G. Yao, and Q. Niu, Phys. Rev. B \textbf{72}, 085110 (2005). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U21.00002: Local and non-local vertex corrections in GW for extended and localized systems Martin Stankovski, Andrew Morris, Benjamin Robinson, Rex Godby, Kris Delaney, Patrick Rinke, Ulf von Barth, Carl-Olof Almbladh, Pablo Garc\'Ia-Gonz\'alez A non-local operator like the self-energy can be consistently calculated through many-body perturbation theory for systems of interacting electrons. This is usually done within the framework of Hedin's $GW$ approximation. If the initial Green's function is obtained within a local approximation like DFT-LDA, there is in principle a local vertex given by the static exchange-correlation kernel in the first iteration (Del Sole \emph{et al.} PRB {\bf 49}, 8024 (1994)). We present total energies and bandwidths for jellium and equivalent quantities for He, Be and Ne. We show that a local vertex implemented in both the screened interaction and the self-energy leads to unphysical results. A local vertex in the screened interaction only provides results on par with or slightly better than standard one-shot $G_ {0}W_{0}$. Finally, we obtain significant improvements by introducing non-local vertex corrections derived from non-local starting aproximations for the self-energy. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U21.00003: GW Calculations Starting from Generalized Kohn-Sham Schemes Frank Fuchs, J{\"u}rgen Furth{\"u}ller, Friedhelm Bechstedt, Maxim Shishkin, Georg Kresse The GW approximation of Hedin is arguably the most successful approach for the calculation of quasi-particle (QP) energies. Its accuracy has been proven for a variety of systems. Usually, the QP eigenvalues are calculated in a perturbative approach, starting from solutions of the Kohn-Sham equations with an exchange-correlation (XC) potential in local density or generalized gradient approximation (LDA/GGA). However, this standard approach fails for a number of systems such as InN with shallow 'semi-core' electrons and a 'negative gap' in LDA/GGA. Here we present $G_0W_0$ calculations for Si, ZnO and InN which start from solutions of the generalized Kohn-Sham (gKS) equations for the screened exchange (sX), HSE03, PBE0 and HF model of exchange and correlation. The calculations are performed in the all-electron wavefunction framework of the PAW method. Starting from a gKS solution is found to yield a positive gap and $d$-band positions close to the experimental values for all the functionals investigated here. Furthermore, with exception of HF the resulting gaps are almost the same for all the gKS functionals choosen as starting point. The results are analysed with respect to the wave-function character and localization. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U21.00004: The GW space-time formalism at finite temperatures Matthieu Verstraete, Christoph Freysoldt, Patrick Rinke, Rex Godby We present the generalization of the space-time formulation of the GW approximation in many-body perturbation theory. The main changes are introduced in the treatment of the imaginary time and frequency dependency of the polarizability, screening, and self-energy. The discrete Matsubara frequencies and finite imaginary time intervals for integration are taken into account. Efficient fitting and interpolation schemes are developed to avoid a large increase in the grid sizes when going to metallic and finite-T systems. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U21.00005: $GW$ study of f-shell systems Mark van Schilfgaarde, Athanasios N. Chantis, Takao Kotani, Robert C. Albers We have applied the recently developed Quasiparticle Self-Consistent GW (QS$GW$) method to rare-earth metals, rare-earth compounds, and actinide metals. Comparison with the available experimental data shows that this method can successfully describe the electronic structure of these materials within a unified ab-initio theoretical framework. Through this approach we can examine the approximations that enter into the LDA+U method and show ways of improving its implementation. Based on the QS$GW$ bands, a discussion of the role of electronic correlations on the band structure of uranium and plutonium will be presented. Finally, we uncover a completely new effect: a first-order metal-insulator transition in GdN, which is driven by dielectric function changes. This result may explain the ambiguities in the experimental data obtained for this material. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U21.00006: Exact-exchange based quasiparticle energy calculations applied to (transition) metal nitrides: ScN, InN and more P. Rinke, M. Scheffler, A. Qteish, J. Neugebauer The transition metal nitride ScN is emerging as a versatile material for promising technological applications, e.g. in spintronics and optoelectronics. Like for InN the electronic band structure of ScN -- a key quantity for devices -- has been difficult to access experimentally (due to growth related problems) and theoretically (due to strong self-interaction effects in the local-density approximation (LDA) to density functional theory (DFT)). Here we show that removing the self-interaction by applying DFT in the exact-exchange optimized effective potential approach (OEPx) correctly predicts ScN and InN to be semiconductors and not (semi)metals as found in the LDA. The OEPx ground state then provides a suitable starting point for quasiparticle energy calculations in the $G_0W_0$ approximation. Our OEPx+$G_0W_0$ gap supports recent experimental observations [1] that ScN has a much lower indirect band gap than previously thought [2]. We further show how a meaningful comparison to LDA based $G_0W_0$ calculations can be constructed that allows us to make contact with self-consistent $GW$ calculations.\\ $[$1$]$ A. A. Al-Brithen {\it et al.}, Phys.\ Rev.\ B {\bf 70}, 045303 (2004)\\ $[$2$]$ P. Rinke {\it et al.}, cond-mat/0611435 [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U21.00007: Electron Correlation Effects And The Electronic Structures of Perovskite Ruthenates SrTi1-xRuxO3 Pao-An Lin, T.Y. Chang, Horng-Tay Jeng, Chen-Shiung Hsue The behavior of the electronic structures of SrTi1-xRuxO3 crystals as the value of x is varied, were studied by ab initio LDA band structure calculation. The roles of on-site Coulomb interaction U were included by carrying out LDA+U calculations. It is found that electron correlation effects play an important role in the electronic structures involving the Ru4d orbitals in the Perovskite ternary Ruthenates SrTi1-xRuxO3. The on-site Coulomb interaction U is needed to describe correctly the metal- insulator transition. Comparing with LSDA calculation (including GGA(generalized gradient correction),the calculated spectrum from LDA+U band structure calculation are in much better agreement with published experimental results such as photoemission (PES) and oxygen 1s X-ray absorption (XAS) spectroscopy for clean surface. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U21.00008: \textit{Ab initio} calculations of intrinsic and extrinsic losses in x-ray spectra J. Kas, M. Prange, J. J. Rehr, L. W. Campbell, J. A. Soininen Typical calculations of x-ray absorption spectra (XAS) include inelastic losses within the quasi-particle approximation, which neglects satellite structure in the spectral function. While satellite effects are relatively small in the EXAFS region, they can be quite large for near edge spectra. Here we present an efficient \textit{ab initio} approach for calculating inelastic losses due to both intrinsic and extrinsic many body interactions, as well as interference between them. The method begins with a real-space multiple-scattering calculation of the dielectric function $\epsilon(\omega)$,\footnote{J.J. Rehr et al., cond-mat/0601241 (2006)} which is then fit to a many-pole model with approximately 10$^2$ poles. This yields a many-pole approximation to GW self-energies, as well as a many-pole spectral function, and estimates for the many-body amplitude factor in XAS. Results for the self-energy agree well with other calculations.\footnote{J.A. Soininen et al., J. Phys. Condens. Matter. 15 (2003)} The approach also gives improved agreement for core-level XAS, especially in the near edge region. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U21.00009: Reciprocal space calculations of EELS and XAS spectra without the supercell Kevin Jorissen, John Rehr Traditionally, one has two ways of calculating \textit{ab initio} x-ray and electron absorption spectra. Band structure calculations can treat periodic materials such as crystals, however, when a core hole is introduced and treated with a supercell approximation, unphysical interactions are often introduced. Real space calculations on the other hand, e.g. based on Green's function theory, can treat the core hole as an impurity, but have the disadvantage of finite cluster size effects. We present a hybrid method in which the perfect crystal is treated in reciprocal space, avoiding cluster size effects, and the inclusion of the core hole is done afterwards in real space, thus avoiding supercell effects. This strategy is implemented into the ab initio Green's function code FEFF8.6, which has recently been adapted for relativistic EELS-spectra. The method is illustrated, for example for the C K-edge of diamond. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U21.00010: Real space, real time approach for linear and non-linear optical response of nano-scale molecules Yoshinari Takimoto, Fernando Vila, John Rehr We present a real time, time-dependent density-functional theory approach for the calculation of the frequency-dependent optical responses, which is based on the approach of Tsolakidis et al. [1] This approach is extended for the calculation of non-linear response. Tensor components of linear polarizabilities and first order hyper-polarizabilities are extracted by fitting the net time dependent polarization with different electric field strengths. This real-space, real-time method is computationally efficient and generic in that it requires no symmetry assumptions. Results are presented for several organic molecules, e.g., FTC and PNA, including both linear and nonlinear response. [1] A. Tsolakidis, D. Sanchez-Portal and R.M. Martin, Phys. Rev. B \textbf{66}, 235416 (2002). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U21.00011: Assigning Spectral Features in Excitonic Spectra Eric L. Shirley Considerable progress has been made over the last several years in calculations of optical absorption spectra from first principles. This builds on the foundation of accurate electronic band structures, including many-body corrections to band energies, and realistic solution of the couple equation of motion for an electron-hole pair in the excited state. Omitting the electron-hole interaction (or excitonic effects) can severely hamper the accuracy of a spectrum. In narrow-gap semiconductors (Si, Ge, GaAs, etc.), this is a reasonably tolerable situation, in the sense that assigning spectral features can be (and has been) used to establish energies of interband transitions at critical points. In wider gap systems, such as LiF or MgO, the qualitative change in spectral features resulting from electron-hole interactions might call into question assignment of features to interband transitions. This talk presents an attempt to carry out such assignments, nonetheless. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U21.00012: A generalized Sham-Schl\"uter equation: the link between nonlocality and frequency dependence Matteo Gatti, Valerio Olevano, Lucia Reining, Ilya V. Tokatly We present an in principle exact approach to construct effective potentials and kernels for the calculation of electronic spectra. In particular, the potential that yields the spectral function needed to describe photoemission turns out to be dynamical but local and real. Using explicit examples we illustrate how the nonlocality of the physical self-energy is converted into a frequency dependence of the effective potential. We also show that our approach leads to a very short derivation of a kernel that gives a very good description of absorption or energy-loss spectra of a wide range of materials. [Preview Abstract] |
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