Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D39: Firstprinciples modeling of excitedstate phenomena in materials III: GW+BSE for Polarons and Optical ExcitationsFocus

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Sponsoring Units: DCOMP DMP DCP Chair: Sahar Sharifzadeh, Boston Univ Room: 703 
Monday, March 2, 2020 2:30PM  3:06PM 
D39.00001: Polarons from first principles Invited Speaker: Feliciano Giustino Polarons are among the most wellknown quasiparticles in solid state physics, and are key to understanding fundamental concepts such as the electron mass enhancement in semiconductors and the formation of Cooper pairs in superconductors. Interest in polaron physics has been reignited by recent angleresolved photoelectron spectroscopy studies, which revealed polaronic signatures in the band structures of several metal oxides and twodimensional semiconductors. In this talk I will describe our recent work aimed at describing polarons and their spectroscopic signatures from first principles. In the first part of the talk I will outline a general manybody framework to compute and analyze polaron satellites in photoelectron spectra using the cumulant expansion approach [1,2]. I will discuss applications to titanium dioxide and europium oxide, and show that the calculations are able to reproduce very closely measured angleresolved photoelectron spectra. In the second part of the talk I will address the question on how to compute the wavefunction of a polaron. I will describe a new approach to the polaron problem that overcomes some of the limitations of explicit supercell calculations [3]. This approach enables systematic calculations of wavefunctions and formation energies for both small and large polarons, and can be used to analyze the electronphonon coupling mechanisms responsible for electron or hole selftrapping. I will illustrate these concepts using lithium fluoride and lithium oxide as examples, and I will discuss the connection with previous work on the polaron problem based on model Hamiltonians. 
Monday, March 2, 2020 3:06PM  3:18PM 
D39.00002: Theory and FirstPrinciple Calculation of Photoemission Spectra from Optically Excited States Ting Cao, Keshav M Dani, Tony F Heinz The behaviors of optically excited states, such as excitons, not only give rise to a variety of fascinating phenomena in condensed matter, but also play vital roles in modern optoelectronics and energy harvesting. In this talk, I will present our recent development on the theory and firstprinciple methods in the study of the photoemission spectra of lowdimensional materials. By performing calculations based on manybody perturbation theories, we show that, in monolayer transition metal dichalcogenides, the excitons hold unique energy dispersions and spectra weights in photoemission, which unveil the fundamental physical properties of these excited states. We further connect our theoretical works to experimental results and explore their potential applications in other systems. 
Monday, March 2, 2020 3:18PM  3:30PM 
D39.00003: First Principles Studies of Photoluminescence of Functional Materials Yu Jin, Marco Govoni, Giulia Galli In order to predict the optoelectronic properties of several classes of functional materials, an accurate description of absorption and photoluminescence processes is necessary. Building on our previous work on calculations of absorption spectra from first principles [1], we present a method to compute photoluminescence spectra based on the solution of the generalized quantum Liouville equation, including electronphonon interaction [2]. We present results for the photoluminescence spectra of organic/inorganic perovskites and of optically controllable defects in semiconductors. 
Monday, March 2, 2020 3:30PM  3:42PM 
D39.00004: Deep ultraviolet luminescence and chargetransfer excitons in atomically thin GaN quantum wells Woncheol Lee, Dylan Bayerl, Nocona Sanders, Zihao Deng, Emmanouil Kioupakis We investigate the electronic, excitonic, and optical properties of atomically thin GaN quantum wells embedded in AlN or AlGaN barriers using firstprinciples calculations based on density functional theory (DFT) and manybody perturbation theory. The strong quantum confinement results in deep ultraviolet luminescence. Also, the quasi2D structural characteristic produces strongly bound excitons, which are even stable at room temperature. We also investigate the properties of pairs of atomically thin GaN wells, separated by polar AlN barriers. The perpendicular electrical polarization produces chargetransfer excitons, in which electrons and holes are spatially separated in the two different GaN wells. Compared to direct excitons, the reduced overlap of chargetransfer excitons enables exciton lifetime that are 34 orders of magnitude longer. By adjusting the separation distance between electrons and holes through variations of the well and barrier thickness we can control the exciton lifetime and the binding energy simultaneously. 
Monday, March 2, 2020 3:42PM  3:54PM 
D39.00005: Analysis of diagonal G and subspace W approximations within fully selfconsistent GW calculations for bulk semiconducting systems Yashpal Singh, LinWang Wang Fully selfconsistent GW (scGW) methods are now available to evaluate quasiparticle and spectral properties of various molecular and bulk systems. However, such techniques are computationally demanding and act as a bottleneck to include vertex function. In contrast, routinely used singleshot G_{0}W_{0} approximation has an undesirable dependency on the choice of xcfunctional. In this work, we consider AlAs, AlP, GaP, and ZnS as our prototype systems to perform scGW calculations by expressing the full G matrix using a planewave basis set. To reduce the computational cost, we present a framework within our scGW scheme to consider diagonal G and subspace W approximations. We analyse our results obtained from the above techniques by comparing against our fully scGW calculations and other similar approaches including experiments. The sub 2% difference in the values of the bandgap obtained from fully scGW and subspace W methods shows an encouraging direction to incorporate vertex function that could potentially improve overestimated scGW bandgaps. 
Monday, March 2, 2020 3:54PM  4:06PM 
D39.00006: Spinwave dispersion of Cu_{2}MnAl, Ni_{2}MnSn, and Pd_{2}MnSn based on quasiparticle selfconsistent GW method Haruki Okumura, Kazunori Sato, Takao Kotani We calculated the spinwave dispersion and the stiffness constants of three metallic ferromagnetic Heusler alloys: Cu_{2}MnAl, Ni_{2}MnSn, and Pd_{2}MnSn. We determined the ground state by the quasiparticle selfconsistent GW (QSGW) method. In conjunction with the Wannier function, we obtain transverse dynamical spin susceptibility based on linear response method. It is found that the ground states within the QSGW are reasonably calculated to reproduce spinwave dispersion around Gamma point. In Cu_{2}MnAl, the magnetic moment in QSGW agrees with the experiment, but stiffness constant is underestimated. In Ni_{2}MnSn, the QSGW overestimates the moment, as seen in the itinerant ferromagnetic case as FCC Ni; however, the stiffness in QSGW agrees with the experiment. In Pd_{2}MnSn, the QSGW reproduces the spinwave throughout the Brillouin zone, and the stiffness is close to the experiments. This agreement is due to the reasonable exchange splitting of Mn 3d in accordance with the large screened Coulomb interaction in QSGW. 
Monday, March 2, 2020 4:06PM  4:18PM 
D39.00007: Selfconsistent GW method for solids: efficient implementation Andrey Kutepov An efficient implementation of the selfconsistent GW method in the FlapwMBPT code (https://www.bnl.gov/cmpmsd/flapwmbpt/) is presented. It features the evaluation of the polarizability and the selfenergy which scales only linearly with respect to the system size. The total computational time scaling measurements show it to be between linear and quadratic up to 72 atoms in silicon supercells. Application to such materials as CoSbS (24 atoms), supercells of La2CuO4 (up to 56 atoms), and SmB6 (7 atoms) illustrate the potential of the approach in computational material science. 
Monday, March 2, 2020 4:18PM  4:30PM 
D39.00008: Systematic QSGW calculations on the electronic structure of rareearth nitrides Kazunori Sato, Takao Kotani, Katsuhiro Suzuki Rareearth (RE) based light emitting materials are distinguished for their narrow line width, small temperature dependence and insensitivity to the environment. These characteristic properties mostly come from localized nature of 4fstates. Therefore, for accurate prediction and design of RE related functional material, reasonable description of 4fstates is indispensable. 
Monday, March 2, 2020 4:30PM  4:42PM 
D39.00009: Jigsaw Puzzle Orbitals for Electronic Structure Dimitar Pashov, Mark Schilfgaarde We present the jigsawpuzzle orbitals (JPOs), a recently developed basis set for solving the oneparticle Schrödinger equation with an optimally constructed minimal basis set. We illustrate some of its advantages with QSGW bandstructure calculations. A significant improvement in selfenergy interpolation is observed as well as physically intuitive dependency of the band gap on the projection range. 
Monday, March 2, 2020 4:42PM  4:54PM 
D39.00010: Firstprinciples Studies of Tl activated Scintillator Phosphor Materials: Towards an understanding of the Scintillation mechanism Andrew Canning, Mauro Del Ben, Jaroslaw Glodo Tl doped halide scintillator phosphors are amongst the most commonly used gamma ray detector materials for medical imaging, high energy physics and nuclear materials detection applications (e.g. CsI:Tl, NaI:Tl). Even so the complete scintillation process in these materials is still poorly understood. In particular in recent years there has been great interest in codoping these materials to try and improve their detection performance. We have performed firstprinciples studies based on GGA, hybrid functionals and the GW/BSE method in tandem with experiments to understand the scintillation mechanism in these materials and how it could be improved by codoping. In particular we have looked at the Tl exciton optical emission states and energy transfer mechanisms from the gamma ray to the Tl. Recently there has also been interest in new Tl bulk scintillators such as TLYC (Tl2LiYCl6) which we have also studied. 
Monday, March 2, 2020 4:54PM  5:06PM 
D39.00011: Excitation Pathways in Resonant Inelastic Xray Scattering
from ManyBody Perturbation Theory Christian Vorwerk, Francesco Sottile, Claudia Draxl Resonant inelastic xray scattering (RIXS) spectroscopy is a powerful tool to unravel the nature of elementary excitations in a wide range of crystalline materials. In the RIXS process, a core electron is excited through the absorption of an xray photon. Subsequently, a valence electron fills the core hole via the emission of a xray photon. The final manybody state contains an excited electron and a valence hole. Through resonant xray absorption and emission, RIXS offers an elemental and orbital selective probe of the electronic valence excitations. In this talk, we present a novel manybody approach to RIXS. We use explicit manybody excited states in the optical and xray region, as obtained from full diagonalization of the BetheSalpeter equation in an allelectron framework, to obtain an expression for the RIXS cross section. The RIXS cross section is expressed in terms of pathways between intermediate manybody states containing a core hole, and final manybody states containing a valence hole. We apply our indepth analysis to the RIXS spectra of the flour K edge of LiF and carbon K edge of diamond. Our results show that the excitation pathways determine the spectral shape of the emission, and the importance of electronhole correlation in the spectra. 
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