Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B39: First-principles modeling of excited-state phenomena in materials II: GW+BSE for Strong Correlation and Core LevelsFocus
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Sponsoring Units: DCOMP DMP DCP Chair: Serdar Ogut, Univ of Illinois - Chicago Room: 703 |
Monday, March 2, 2020 11:15AM - 11:51AM |
B39.00001: Many-body effective energy theory: photoemission at strong correlation Invited Speaker: Stefano Di Sabatino Photoemission is a powerful tool to obtain insight into the electronic structure and excitations |
Monday, March 2, 2020 11:51AM - 12:03PM |
B39.00002: Stochastic Many-Body Perturbation theory beyond the GW approximation Vojtech Vlcek I will present new stochastic approaches for the computation of electronic excitations within the many-body perturbation theory. The new methods go beyond the popular G0W0 approximation and include non-local vertex corrections in the screened Coulomb interaction (G0W0tc) as well as in the self-energy (G0W0tcΓX). I will discuss a stochastic implementation in real-time and space which scales linearly with the number of electrons. I will demonstrate that the vertex corrections predominantly affect unoccupied states and that they are crucial for predicting of the corresponding quasiparticle energies. |
Monday, March 2, 2020 12:03PM - 12:15PM |
B39.00003: Stochastic resolution of identity to second order Green’s function: ground-state and quasi-particle properties. Wenjie Dou, Tyler Takeshita, Ming Chen, Roi Baer, Daniel Neuhauser, Eran Rabani We develop a stochastic resolution of identity approach to the real-time second-order Green’s function (real-time sRI-GF2) theory, extending our recent work for imaginary-time Matsubara Green’s function (J. Chem. Phys.151, 044114 (2019)). The approach provides a framework to obtain the quasi-particle spectra across a wide range of frequencies as well as predict ionization potentials and electron affinities. To assess the accuracy of the real-time sRI-GF2, we study a series of molecules and compare our results to experiments and to a many-body perturbation approach based on the GW approximation, where we find that the real-time sRI-GF2 is as accurate as self-consistent GW. The stochastic formulation reduces the formal scaling toO(N^3), where N is the number of electrons. This is illustrated for a chain of hydrogen dimers, where we observe as lightly lower than cubic scaling for systems containing up to N≈1000. |
Monday, March 2, 2020 12:15PM - 12:27PM |
B39.00004: Effects of electron-hole interactions in single-particle excitations within the GW approach Meng Wu, Zhenglu Li, Steven Louie There are a number of schemes in the literature to do “self-consistent” GW calculations at different levels going beyond the G0W0 approximation. For single-particle excitations (e.g., the quasiparticle bandgap in semiconductors), a straightforward self-consistent update of both the single-particle Green’s function G and the screened Coulomb interaction W generally gives less satisfactory results than those from the G0W0 approach as compared to experiment, which is due to an under-screening introduced and accumulated in the treatment of dielectric screening at the random phase approximation (RPA) level, where electron-hole interactions are neglected. In this work, we investigate the importance of electron-hole interactions in modifying W and hence the GW self-energy, as well as in reshaping single-particle excitations at the GW level. We present our theoretical formalism, along with first-principles results for several conventional semiconductors. |
Monday, March 2, 2020 12:27PM - 12:39PM |
B39.00005: Core-Level Spectra for Disordered Systems from GW Dorothea Golze, Patrick Rinke We apply our recently developed GW core-level method to predict highly accurate X-ray photoelectron spectra (XPS) of disordered carbon-based materials, which require model sizes of more than 100 atoms. GW has become the method of choice for the computation of valence excitations [1]. We recently showed that GW can be also used for core excitations, even though it requires computationally more accurate techniques for the frequency integration of the self-energy than for valence states [2]. In addition, partial self-consistent schemes and relativistic corrections are crucial. For a benchmark set of small molecules, we find that GW-computed absolute and relative core-level binding energies deviate only 0.3 and 0.2 eV from experiment, respectively. Core-level spectroscopy is one of the few techniques that can be used to characterize disordered materials, such as functionalized amorphous carbon, which shows potential as coating and electrode material. However, the experimental spectra are difficult to interpret. We show that our method provides reliable computational references to support the peak assignment in experimental XPS spectra of amorphous carbon. |
Monday, March 2, 2020 12:39PM - 12:51PM |
B39.00006: Core and valence electron excitations in SrTiO3 and MgO: a first-principles study including many-body effects Vijaya Begum, Markus Ernst Gruner, Rossitza Pentcheva Using density functional theory calculations combined with many-body perturbation theory we investigate the optical and XAS spectra of two paradigmatic oxides, SrTiO3 and MgO. For both systems taking into account quasiparticle (GW) and in particular excitonic effects (Bethe-Salpeter equation) is decisive to obtain good agreement with experiment. For the cubic phase of SrTiO3 [1], the theoretical optical spectrum shows a prominent excitonic peak at 6.58 eV which involves interband transition between O-2p and Ti-eg states. The main effect of the tetragonal distortion below 105 K is observed around this peak due to splitting of the eg bands. The optical spectrum of MgO shows the best agreement with experiment using a hybrid exchange-correlation functional. Furthermore, the x-ray absorption spectra of the O and Mg K-edge are in good agreement with experiment. The analysis of the origin of the peaks in k-space indicates a strong hybridization of the respective unoccupied p and d-states, whereas the real space visualization of the exciton wavefunction illustrates its localization and bound nature. |
Monday, March 2, 2020 12:51PM - 1:03PM |
B39.00007: Cumulant expansion of the exciton Green's function: A unified approach for many-body intrinsic, extrinsic, and interference effects in XAS Joshua Kas, John Rehr, John Vinson Recently, a real-time cumulant approach has been successful in describing many-body effects in x-ray photo-emission and absorption spectra in a variety of systems.1 While the results for XAS are promising, intrinsic and extrinsic effects are not treated on the same footing, with the intrinsic excitations calculated via real-time TDDFT, while extrinsic/interference effects are calculated in a frequency space approach. In addition, extrinsic and interference effects are based on a model of free electrons interacting with plasmons, most appropriate for the alkali metals. Here we present a unified approach which treats satellites in terms of the real-time density response to the sudden appearance of an exciton, which is in turn described by solutions of the Bethe-Salpeter equation at a specific energy. We apply the method to the K-edge XAS of SrTiO3 and analyze the energy dependence of extrinsic and interference effects. |
Monday, March 2, 2020 1:03PM - 1:15PM |
B39.00008: Computational characterization of the RIXS Raman-to-fluorescence crossover in BaFe2As2 Keith Gilmore, Jonathan Pelliciari, Thorsten Schmitt Resonant inelastic X-ray scattering (RIXS) studies have significantly enhanced our understanding of correlated materials. However, experimental and computational efforts have largely focused on insulating materials. We recently collected RIXS data on metallic BaFe2As2 at the Fe L3 edge, which exhibits a Raman-to-fluorescence crossover as the absorption threshold is traversed. By combining core and valence level Bethe-Salpeter solvers, we evaluate the RIXS cross section of BaFe2As2 from first principles. Our calculations capture the Raman-to-fluorescence crossover as well as the main loss features observed in the experiment. By i) considering additionally the absorption and non-resonant emission spectra, ii) invoking the threshold singularity theory of Nozières and Abrahams [1], iii) recognizing the role of the intermediate state lifetime, and iv) decomposing the orbital character of the intermediate and final excitonic states, we are able to quantitatively and qualitatively separate simpler band structure contributions from more complex many-body effects in the RIXS spectra of BaFe2As2. This analysis is applicable to other strongly correlated metals. |
Monday, March 2, 2020 1:15PM - 1:27PM |
B39.00009: Electronic structure of 3d-transition metal dioxide clusters from GW calculations Meisam Rezaei, Serdar Ogut Transition metal oxide clusters are not only scientifically interesting, but they are also challenging systems to model using first principles approaches due to strong electron correlations and their open-shell character. These challenges are particularly noteworthy when modeling their excited state properties. The GW approximation using atom-centered localized basis sets has recently emerged as a reliable method for studying excited state properties of confined systems. Here, we investigate various flavors of the GW method (one-shot, eigenvalue self-consistent) with different starting points when applied to quasiparticle spectra of 3d transition metal dioxide cluster anions ScO2-, TiO2-, VO2-, CrO2-, MnO2-, FeO2-, NiO2-, and CuO2-. We compare predictions from different levels of theory with each other and with experimental photoelectron spectra. |
Monday, March 2, 2020 1:27PM - 1:39PM |
B39.00010: Importance of long-range correlations in transition metal compounds: First-principle studies using the the multitier GW+EDMFT approach Fredrik Nilsson, Francesco Petocchi, Philipp Werner, Ferdi Aryasetiawan Transition metal compounds exhibit a wide range of intriguing properties, such as high temperature superconductivity and colossal magnetoresistance. The standard method to describe these materials is density functional theory + dynamical mean-field theory (DFT+DMFT), which can treat the strong onsite correlations between the 3d electrons to all orders but omits the long-range intersite correlations. In this talk I discuss the recently developed multitier combination of the GW-approximation and dynamical mean-field theory [1], a parameter-free ab-initio method which yields a fully self-consistent description of both long- and short-range correlations. A systematic study of the cubic perovskites Sr(V,Mo,Mn)O3 reveal that the long-range correlations, which are typically ignored for this class of materials, can have a profound influence on the interpretation of the spectra. Specifically spectral features previously interpreted as Hubbard bands are reinterpreted as plasmon satellites originating from long-range charge fluctuations. |
Monday, March 2, 2020 1:39PM - 1:51PM |
B39.00011: Dynamically Screened Excitons in Heteropolar Semiconductors: The Case of Halide
Perovskites Marina Filip, Jonah Haber, Jeffrey B Neaton The interaction between photogenerated electron-hole pairs in semiconductors is screened by their dielectric environment. In heteropolar semiconductors, dielectric screening originates with both electrons and polar phonons. State of the art GW/BSE methods for prediction of excitons typically only include the static electronic contribution to the screening. However, prior studies report on the importance of dynamical lattice contributions to screening from polar phonons for excitons in halide perovskites [1,2]. Here, we develop an extension of the BSE approach to include both electronic and lattice contributions to the dielectric screening. We show that in heteropolar semiconductors, lattice effects can significantly reduce the electron-hole interaction, even when the exciton binding energy is much larger than the characteristic phonon frequency. We demonstrate the importance of dynamical lattice screening for excitons in halide perovskites, and generalize our findings by extending the Wannier model to include lattice polarization effects, clarifying the relevance of this effect for general classes of heteropolar semiconductors. |
Monday, March 2, 2020 1:51PM - 2:03PM |
B39.00012: GW calculations and ultraviolet photoelectron spectroscopy of gas phase ion pairs - a window into the electronic structures of ionic liquids Juhan Matthias Kahk, Ivar Kuusik, Vambola Kisand, Johannes Lischner Room temperature ionic liquids have extremely low equilibrium vapor pressures, but in ultrahigh vacuum vapors consisting of neutral ion pairs can be detected. Spectroscopic measurements of these ion pairs, the fundamental building blocks of ionic liquids, can yield insights into the electronic structures of these unusual and technologically important materials. From a theoretical perspective, the description of these ion pairs is challenging due to the presence of long range charge transfer. For example, in density functional theory different exchange correlation functionals can produce qualitatively different ground state electronic structures. In this study, it is shown that the GW method yields a consistent description of the gas phase ion pairs that is only weakly dependent on the mean field starting point. The effect of different levels of self-consistency in the GW calculations is analyzed. Theoretical valence level photoelectron spectra are calculated, and it is found that G0W0 at a hybrid DFT starting point yields excellent quantitative agreement with experiment. In one instance, GW calculations highlighted the presence of a contaminant in the experimental spectrum that had not been previously recognized, and corroborated its assignment to a decomposition product. |
Monday, March 2, 2020 2:03PM - 2:15PM |
B39.00013: Low-cost alternatives to the Bethe-Salpeter equation: a simple hybrid functional for excitonic effects in solids Jiuyu Sun, Carsten A. Ullrich The Bethe-Salpeter equation (BSE) is the standard computational method for optical excitations in solids, including excitonic effects. We reduce the computational cost of the BSE by simplifying the dielectrically screened Coulomb interaction: instead of calculating the dielectric function from first principles, we replace it by a momentum-dependent model dielectric function or just by a single parameter. Combined with a semilocal exchange-correlation kernel, this defines a new hybrid functional for solids within generalized TDDFT. We perform a systematic assessment of these simplified approaches, and find that they yield optical absorption spectra and exciton binding energies of semiconductors and wide-gap insulators in close agreement with standard BSE. We also present applications to more complex systems. |
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