Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B39: Firstprinciples modeling of excitedstate 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: Manybody 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 ManyBody Perturbation theory beyond the GW approximation Vojtech Vlcek I will present new stochastic approaches for the computation of electronic excitations within the manybody perturbation theory. The new methods go beyond the popular G_{0}W_{0} approximation and include nonlocal vertex corrections in the screened Coulomb interaction (G_{0}W_{0}^{tc}) as well as in the selfenergy (G_{0}W_{0}^{tc}Γ_{X}). I will discuss a stochastic implementation in realtime 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: groundstate and quasiparticle properties. Wenjie Dou, Tyler Takeshita, Ming Chen, Roi Baer, Daniel Neuhauser, Eran Rabani We develop a stochastic resolution of identity approach to the realtime secondorder Green’s function (realtime sRIGF2) theory, extending our recent work for imaginarytime Matsubara Green’s function (J. Chem. Phys.151, 044114 (2019)). The approach provides a framework to obtain the quasiparticle spectra across a wide range of frequencies as well as predict ionization potentials and electron affinities. To assess the accuracy of the realtime sRIGF2, we study a series of molecules and compare our results to experiments and to a manybody perturbation approach based on the GW approximation, where we find that the realtime sRIGF2 is as accurate as selfconsistent 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 electronhole interactions in singleparticle excitations within the GW approach Meng Wu, Zhenglu Li, Steven Louie There are a number of schemes in the literature to do “selfconsistent” GW calculations at different levels going beyond the G_{0}W_{0} approximation. For singleparticle excitations (e.g., the quasiparticle bandgap in semiconductors), a straightforward selfconsistent update of both the singleparticle Green’s function G and the screened Coulomb interaction W generally gives less satisfactory results than those from the G_{0}W_{0} approach as compared to experiment, which is due to an underscreening introduced and accumulated in the treatment of dielectric screening at the random phase approximation (RPA) level, where electronhole interactions are neglected. In this work, we investigate the importance of electronhole interactions in modifying W and hence the GW selfenergy, as well as in reshaping singleparticle excitations at the GW level. We present our theoretical formalism, along with firstprinciples results for several conventional semiconductors. 
Monday, March 2, 2020 12:27PM  12:39PM 
B39.00005: CoreLevel Spectra for Disordered Systems from GW Dorothea Golze, Patrick Rinke We apply our recently developed GW corelevel method to predict highly accurate Xray photoelectron spectra (XPS) of disordered carbonbased 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 selfenergy than for valence states [2]. In addition, partial selfconsistent schemes and relativistic corrections are crucial. For a benchmark set of small molecules, we find that GWcomputed absolute and relative corelevel binding energies deviate only 0.3 and 0.2 eV from experiment, respectively. Corelevel 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 SrTiO_{3} and MgO: a firstprinciples study including manybody effects Vijaya Begum, Markus Ernst Gruner, Rossitza Pentcheva Using density functional theory calculations combined with manybody perturbation theory we investigate the optical and XAS spectra of two paradigmatic oxides, SrTiO_{3} and MgO. For both systems taking into account quasiparticle (GW) and in particular excitonic effects (BetheSalpeter equation) is decisive to obtain good agreement with experiment. For the cubic phase of SrTiO_{3} [1], the theoretical optical spectrum shows a prominent excitonic peak at 6.58 eV which involves interband transition between O2p and Tie_{g} states. The main effect of the tetragonal distortion below 105 K is observed around this peak due to splitting of the e_{g} bands. The optical spectrum of MgO shows the best agreement with experiment using a hybrid exchangecorrelation functional. Furthermore, the xray absorption spectra of the O and Mg Kedge are in good agreement with experiment. The analysis of the origin of the peaks in kspace indicates a strong hybridization of the respective unoccupied p and dstates, 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 manybody intrinsic, extrinsic, and interference effects in XAS Joshua Kas, John Rehr, John Vinson Recently, a realtime cumulant approach has been successful in describing manybody effects in xray photoemission 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 realtime 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 realtime density response to the sudden appearance of an exciton, which is in turn described by solutions of the BetheSalpeter equation at a specific energy. We apply the method to the Kedge 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 Ramantofluorescence crossover in BaFe_{2}As_{2} Keith Gilmore, Jonathan Pelliciari, Thorsten Schmitt Resonant inelastic Xray 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 BaFe_{2}As_{2} at the Fe L_{3} edge, which exhibits a Ramantofluorescence crossover as the absorption threshold is traversed. By combining core and valence level BetheSalpeter solvers, we evaluate the RIXS cross section of BaFe_{2}As_{2} from first principles. Our calculations capture the Ramantofluorescence crossover as well as the main loss features observed in the experiment. By i) considering additionally the absorption and nonresonant 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 manybody effects in the RIXS spectra of BaFe_{2}As_{2}. This analysis is applicable to other strongly correlated metals. 
Monday, March 2, 2020 1:15PM  1:27PM 
B39.00009: Electronic structure of 3dtransition 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 openshell character. These challenges are particularly noteworthy when modeling their excited state properties. The GW approximation using atomcentered 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 (oneshot, eigenvalue selfconsistent) with different starting points when applied to quasiparticle spectra of 3d transition metal dioxide cluster anions ScO_{2}^{}, TiO_{2}^{}, VO_{2}^{}, CrO_{2}^{}, MnO_{2}^{}, FeO_{2}^{}, NiO_{2}^{}, and CuO_{2}^{}. 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 longrange correlations in transition metal compounds: Firstprinciple 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 meanfield theory (DFT+DMFT), which can treat the strong onsite correlations between the 3d electrons to all orders but omits the longrange intersite correlations. In this talk I discuss the recently developed multitier combination of the GWapproximation and dynamical meanfield theory [1], a parameterfree abinitio method which yields a fully selfconsistent description of both long and shortrange correlations. A systematic study of the cubic perovskites Sr(V,Mo,Mn)O3 reveal that the longrange 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 longrange 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 electronhole 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 electronhole 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 selfconsistency 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: Lowcost alternatives to the BetheSalpeter equation: a simple hybrid functional for excitonic effects in solids Jiuyu Sun, Carsten A. Ullrich The BetheSalpeter 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 momentumdependent model dielectric function or just by a single parameter. Combined with a semilocal exchangecorrelation 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 widegap insulators in close agreement with standard BSE. We also present applications to more complex systems. 
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