Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A20: Firstprinciples Modeling of Excitedstate Phenomena in Materials I: Manybody Perturbation Theory (Techniques and Applications)Focus

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Sponsoring Units: DCOMP DMP Chair: Andre Schleife, University of Illinois at UrbanaChampaign Room: BCEC 157A 
Monday, March 4, 2019 8:00AM  8:36AM 
A20.00001: Accurate CoreLevel Spectra from GW Invited Speaker: Dorothea Golze We present an accurate method for computing Xray photoelectron spectra based on the GW approximation that overcomes the limitations of density functional theory (DFT) approaches. The GW method is routinely used to predict charged valence excitations in molecules and solids. However, GW corelevel spectroscopy has thus far been poorly explored. This in parts related to the fact that numerically very efficient techniques such as the analytic continuation, which treat the frequency dependence on the imaginary axis, break down for innershell excitations. We implemented a fullfrequency approach on the real axis in the allelectron code FHIaims [1] using a localized basis to enable the treatment of core levels in GW [2]. Our scheme is based on the contour deformation technique and facilitates precise and efficient calculations of the selfenergy, which has a complicated pole structure for core states. We present benchmark studies for 1s excitations of small and mediumsized molecules and discuss the optimization of the starting point as well as selfconsistent approaches. We find that the absolute corelevel binding energies deviate on average by less than 0.5 eV from experiment outperforming the DFTbased Delta SelfConsistent Field approach. Relative core excitations are also well reproduced with average deviations of less 0.2 eV from the experimental reference. Furthermore, our calculations reveal that the GW excitation spectrum exhibits satellite features in addition to the photoelectric peak, which might provide access to interesting manybody physics. 
Monday, March 4, 2019 8:36AM  8:48AM 
A20.00002: Timedependent GW: Progress in Solving KadanoffBaym Equations with Dynamic GW Self Energy Diana Qiu, YangHao Chan, Felipe Da Jornada, Steven G. Louie The ab initio GW plus BetheSalpeter equation (GWBSE) approach has achieved great success in describing the linear absorption spectra of materials in equilibrium. However, many important photoprocesses and experiments of interest that involve ultrafast and highintensity pulses of light fall well outside the regime of equilibrium and linear responses. One can generalize the Green’s function formalism in manybody perturbation theory to nonequilibrium situations by solving the KadanoffBaym equations (KBE), but the presence of two time variables that need to be simultaneously evolved presents a significant computational challenge. In recent years, progress has been made in solving the KBE from ab initio in the static limit of the GW self energy, which greatly simplifies the time evolution. Here, we present progress on solving the KBE including dynamical effects in the self energy. We discuss timeevolution algorithms, memory effects, approximations to the dynamics, and convergence behavior. 
Monday, March 4, 2019 8:48AM  9:00AM 
A20.00003: Excitonic effects in shift currents of low dimensional materials from timedependent GW approach Yanghao Chan, Diana Qiu, Felipe Da Jornada, Steven G. Louie We present firstprinciples studies on shift currents with excitonic effects in 2D materials using an adiabatic timedependent GW approach. Shift current in a noncentrosymmetric semiconductor is a DC current generated by optical excitations from a nonlinear response processes. Using a newly developed realtime simulation method, we are able to include excitonic effects in the nonlinear responses from first principles, treating the complicated electronhole interaction at the GW plus BetheSalpeter equation level for the first time. We applied this method to study shift currents in monolayer GeS and found strongly enhanced responses due to excitonic effects. Most interestingly, the dominant contributions to the shift currents here are generated at ingap frequencies, which peaked at energies below the quasiparticle band gap by the exciton binding energies. 
Monday, March 4, 2019 9:00AM  9:12AM 
A20.00004: Correlation satellites in optical and loss spectra Pier Luigi Cudazzo, Lucia Reining The coupling of excitations leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. We propose a cumulant formulation for neutral electronic excitations which opens the way to describe effects such as double plasmon satellites or excitonexciton coupling. Our approach starts from the widely used GW plus BetheSalpeter approximation to manybody perturbation theory which is based on a quasiparticle picture, and it adds coupling to other excitations through a consistent inclusion of dynamically screened interactions. We show that this requires to consider scattering contributions that are usually neglected. The result is formulated in a way that highlights essential physics, that can be implemented as a postprocessing tool in widely used first principles codes, and that suggests which kind of materials and measurements should exhibit strong effects. This is illustrated using a model. 
Monday, March 4, 2019 9:12AM  9:24AM 
A20.00005: Faster and more accurate stochastic GW Vojtech Vlcek, Eran Rabani, Roi Baer, Daniel Neuhauser I will present recent developments in stochastic approach to the GW approximation, which further accelerate the calculation of quasiparticle energies and increase their accuracy. A new concept of sparse stochastic compression is used to speed up stochastic approaches and leads to an overall decrease of statistical errors in large finite and periodic systems. Computation of quasiparticle energies and gaps for systems with up to N_{e} >10,000 electrons is thus feasible with only small statistical fluctuation (± 0.05 eV) and consuming < 2000 core CPU hours. Further, I will present an efficient scissorslike GW selfconsistency approach that can be implemented at zero additional cost. This result is a simple modification of the timedependent G0W0 and enables an a posteriori selfconsistency cycle applicable to large systems. 
Monday, March 4, 2019 9:24AM  9:36AM 
A20.00006: Calculations of Multiplet Splittings in Open Shell Systems within the GW approximation Meisam Rezaei, Serdar Ogut In open shell systems, the coupling of the orbital and spin angular momenta of electrons can result in multiple energy eigenstates of the electronically excited system leading to the characteristic multiplet structures observed in photoemission experiments. While DFT is not capable of determining the resulting multiplet splittings accurately, recent studies have shown that the GW approximation can describe the multiplet structure with reasonable accuracy^{1}. In this work, we investigate the multiplet splittings for open shell molecules such as NO_{2}, NF_{2}, O_{2}, ClO_{2}, and single atoms by applying the oneshot GW approximation. We compare predictions obtained with G and W computed with the same hybrid functional starting points containing varying amounts of Fock exchange with those where G is still computed with a hybrid functional starting point but W is computed within PBE. We show that it is possible to achieve excellent agreement with experimental results for molecular systems using both types of approaches, but the amount of exact exchange needed for quantitative accuracy depends on the molecule as well as the choice of the GW method employed. 
Monday, March 4, 2019 9:36AM  9:48AM 
A20.00007: Electronic structure of 3dtransitionmetal oxide clusters with partially filled d shells from GW calculations YoungMoo Byun, Serdar Ogut The GW approximation using atomcentered localized basis sets is an emerging method for studying the excited properties of confined systems. However, G_{0}W_{0}@PBE, which is typically used for simple extended systems, fails to accurately describe the electronic excitations in 3dtransitionmetal oxide clusters due to dimensionality and correlation effects, and suffers from exhibiting multiple solutions of the quasiparticle equation due to complex selfenergy poles. G_{0}W_{0} on top of exact exchange (EXX) is one way to overcome these problems, but the predictions depend on the amount of EXX and the method suffers from convergence errors in openshell systems (e.g. due to spin contamination). In our previous work on early and late 3d transition metal oxide clusters, we showed that G_{n}W_{0}@PBE addresses all the above issues, and thus is an accurate and efficient GW method for these systems. In this work, we investigate various flavors of the GW method (oneshot, partially selfconsistent, quasiparticle selfconsistent) with different starting points for 3d transition metal oxide clusters, VO^{}, CrO^{}, MnO^{}, FeO^{}, CoO^{}, and NiO^{}, which display a number of high multiplicity states and exhibit moderate electronic correlation. 
Monday, March 4, 2019 9:48AM  10:00AM 
A20.00008: Spin wave spectrum of 3d ferromagnet based on QSGW calculations Okumura Haruki, Kazunori Sato, Takao Kotani We have developed spin wave (SW) spectrum calculation code, combining quasiparticle selfconsistent GW(QSGW) and maximum localized Wannier function (MLWF). With MLWF, we can overcome timeconsuming calculations for large qpoint SW spectrum. Since we adopt the linear response method for dynamical susceptibility^{[1]}, our SW spectrum includes Stoner excitation, which causes the damping of spin SW. Stoner excitation is important in weak ferromagnet such as Fe, while not important in strong ferromagnet such as Ni. In the local density approximation (LDA), our SW energy in Fe is smaller than experimental result or other frozen SW calculation. In the QSGW calculations, due to the corrections of exchange splitting, the calculated SW stiffness constant of Ni (481 meV･Å in average) is in good agreement with inelastic neutron scattering experiment (483 meV･Å), while the LDA result (834 meV･Å) is almost twice as large as experimental one. In the fcc Co, our LDA result corresponds to frozen SW calculation because of weak Stoner excitation coupling in this strong ferromagnet. In the case of cubic FeCo in ordered state, we found acoustic and optical mode in the SW spectrum. [1] C. Friedrich, et al., First Principles Approaches to Spectroscopic Properties of Complex Materials pp 259301 (2014). 
Monday, March 4, 2019 10:00AM  10:12AM 
A20.00009: Tunable Low Temperature Phononinduced Electronic Bistability in Vanadium Dioxide Mark Schilfgaarde, Cedric Weber, Swagata Acharya, Mostafa Shalaby We use a joint theory and experimental approach to clarify the nature of the Metalinsulator transition in VO_{2}, by QSGW calculations in the frozen phonon approach, and pumpprobe ultrafast spectroscopy with THz radiation. We first show that the insulating state is induced by a Peierls instability, that is responsible for the orbital selection that drives the material to its insulating state, and is associated with a degenerate twoorbital electronic valence band. At the same time, Peierls excitations driven by the lattice dynamics. A 5.7 THz phonon modes split the degeneracy, in a particular, which in turn induces a rapid metallization. This mode is observed in THz pump measurements, far below the critical temperature in the M_{1} phase. Our combined approach sheds light in the nature of the transition, which is in our view mediated by a combination of the lattice dynamics and electronic excitations. Finally, we report a possibility in the theory for a novel mechanism to induce an electronic metalinsulator hysteresis via the electronphonon coupling to the 6.5 THz phonon mode. 
Monday, March 4, 2019 10:12AM  10:24AM 
A20.00010: Alternative approaches for calculations of exchange and correlation contributions to thermodynamic properties Joshua Kas, John Rehr The cumulant expansion of the oneelectron Green’s function has been found to be an improved method for calculations of excited states and spectra for a wide variety of systems [1]. Additionally, an extension of the cumulant Green’s function approach to finite temperatures has yielded total energies and exchange correlation potentials in good agreement with quantum MonteCarlo results [2]. Here we present an investigation of alternative thermodynamic pathways for properties such as the free energy, entropy, and heat capacity of the homogeneous electron gas. In addition we discuss approximate methods based on quasiparticle properties alone. These approximate methods improve the efficiency of the approach, and could be applied to real systems for which calculations of GW quasiparticle effects are computationally reasonable. 
Monday, March 4, 2019 10:24AM  10:36AM 
A20.00011: Electron Hydrodynamics of Graphene from a FirstPrinciples GW Approach: Electronic Compressibility, Backflows and Transport Coefficients Andrea Cepellotti, Steven G. Louie Doped graphene at low temperatures has experimental signatures of hydrodynamic behavior originating from the electronelectron interaction. Here, we use the GW approximation to study the electronic and thermal properties of graphene from firstprinciples within a Fermi liquid framework. We show how the electronic compressibility is modified by manybody contributions at finite temperature. Next, we solve the kinetic equation using an exact expression of the scattering matrix. We show how the commonlyused relaxation time approximation causes a severe underestimation of transport coefficients, and how a correct treatment of momentum dissipation is critical to the description of transport properties. We will present results for transport coefficients, including electrical conductivity, the electronic contribution to thermal conductivity and the viscosity coefficients, discussing corrections due to the interaction of carriers with the environment and the behavior of the electron liquid as a nonNewtonian fluid. 
Monday, March 4, 2019 10:36AM  10:48AM 
A20.00012: Cumulant Green’s function approach for phonon satellites in resonant inelastic Xray scattering Keith Gilmore, Andrey Geondzhian Resonant inelastic Xray scattering (RIXS) probes various excitations in materials, including plasmons, magnons and phonons. Calculating RIXS spectra is challenging since it is a secondorder process involving the coupling of coreexcited states to bosonic collective modes, and requires summation over virtual intermediate states. Using a cumulant expansion [1], we develop a tractable excitonic Green’s function formulation of the RIXS cross section. We demonstrate our methodology by reproducing the phonon contribution to the RIXS spectrum of acetone [2]. 
Monday, March 4, 2019 10:48AM  11:00AM 
A20.00013: Reexamining a local, realspace approach to screening John Vinson, Eric Shirley Various manybody perturbation theory techniques for calculating electron behavior rely on W, i.e., the screened Coulomb interaction. The exact screening requires complete knowledge of the dielectric response of the electronic system. As a simplification, calculations often begin with the RPA or random phase approximation. However, even at the RPA level the calculations are costly and scale poorly with system size. A local approach has been shown to be efficient while maintaining accuracy [1]. We present improvements to this scheme, including reconstruction of the allelectron character of the pseudopotentialbased wave functions, improved scaling with system size, and a parallelized implementation. We discuss applications to BetheSalpeter calculations of core and valence spectroscopies. 
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