Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y22: FirstPrinciples Modeling of ExcitedState Phenomena in Materials VII: XRay SpectroscopyFocus Live

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Sponsoring Units: DCOMP DCP DMP Chair: Yuan Ping, University of California, Santa Cruz 
Friday, March 19, 2021 11:30AM  12:06PM Live 
Y22.00001: Cumulant Green’s function methods for excited state properties of functional materials Invited Speaker: John Rehr Many interesting properties of functional materials depend on their excited state properties, such as dynamic response and thermodynamic behavior. Often this behavior depends on details of excitations in the system such as phonons and plasmons, which lead to inelastic losses and damping effects. These excitations can be probed by photoemission spectra, where they show up as satellites beyond the quasiparticle peak [1]. These manybody effects are neither amenable to density functional theory nor extensions based on quasiparticle approximations. Here we discuss an approach based on the cumulant Green’s function, which provides a unified treatment of such dynamic correlation effects [2]. The approach is illustrated with several applications. Remarkably, a cumulant calculated in linear response within a quasiboson approximation, is adequate to explain the loss spectra and chargetransfer excitations in many systems. Finitetemperature exchangecorrelation potentials, thermodynamic properties, and the finitetemperatureTDDFT kernel, and can also be obtained [3,4]. Finally, some extensions are briefly discussed, including nonlinear contributions to the cumulant [5]. 
Friday, March 19, 2021 12:06PM  12:18PM Live 
Y22.00002: Valence and corelevel nonlinear optics in solids from realtime TDDFT C Das Pemmaraju Velocitygauge realtime TDDFT within the generalized KohnSham (GKS) formulation provides a convenient framework for accessing both linear and nonlinear optical response in solids across nearinfrared to soft Xray energy ranges [1, 2]. In this work the GKS TDDFT description of loworder nonlinear response properties such as second harmonic generation and twophoton absorption is investigated in materials like hexagonal BN where excitonic effects are important. Additionally the relevance of this approach to the emerging field of nonlinear Xray spectroscopy is discussed. Results from TDDFT are compared to benchmark manybody perturbation theory results or experimental data where available. 
Friday, March 19, 2021 12:18PM  12:30PM Live 
Y22.00003: Excitation Pathways in Resonant Inelastic Xray Scattering of Solids Christian Vorwerk, Francesco Sottile, Claudia Draxl Resonant inelastic xray scattering (RIXS) is a powerful spectroscopic technique that offers an elemental and orbitalselective probe of the electronic excitations over a huge energy range. In this talk, we present a novel manybody approach to determine RIXS spectra in solids, yielding an intuitive expression for the RIXS cross section in terms of pathways between intermediate manybody states containing a core hole, and final manybody states containing a valence hole. Explicit excited manybody states are obtained from the diagonalization of the BetheSalpeter equation in an allelectron framework. For the paradigmatic examples of the fluorine K edge of LiF and the carbon K edge in diamond, we show how the excitation pathways determine the spectral shape of the emission, and demonstrate the nontrivial role of electronhole correlation in the RIXS spectra. Finally, we discuss for the example of Ga_{2}O_{3} how RIXS can be employed to determine the nature of bound valence excitons and probes the valence hole distribution of the bound excitons in this material. 
Friday, March 19, 2021 12:30PM  12:42PM Live 
Y22.00004: Theoretical Xray Absorption Spectroscopy of Liquid Water by the GW plus BetheSalpeter equation (GWBSE) method Fujie Tang, Chunyi Zhang, Zhenglu Li, Steven G Louie, Roberto Car, Diana Qiu, Xifan Wu Oxygen Kedge xray absorption spectroscopy (XAS) provides an important local probe of the Hbond network in liquid water. Theoretical calculations of XAS spectra demand accurate modeling of both the molecular structure and the electronhole excitation process. We generate the water structure from pathintegral DeePMD calculations, whose neural network potential is trained on stateoftheart DFT data at the SCAN0 hybrid functional level. Based on the above equilibrated trajectory, the XAS spectra of liquid water are computed by solving the BetheSalpeter equation (BSE) as implemented in the BerkeleyGW package. Our calculated XAS spectra of water agree well with experiments. Our results indicate that selfconsistent GW quasiparticle wavefunctions and localfields effects in electronic screening are crucial in the GWBSE approach in order to yield XAS spectra that are in quantitative agreement with experiments. 
Friday, March 19, 2021 12:42PM  12:54PM Live 
Y22.00005: Theoretical Prediction of Core Electron Binding Energies in Solids, Surfaces and Molecules Juhan Kahk, Johannes Lischner Core level Xray Photoelectron Spectroscopy (XPS) is one of the most widely used experimental techniques in the study of materials and molecules. The key quantity in the analysis of experimental spectra is the core electron binding energy. Measured binding energies allow the identification of the elemental composition of the probed region, and small shifts in binding energies can also be used to make inferences about the local chemical environments of the atoms. However, this information can be difficult to interpret, especially for complex systems. 
Friday, March 19, 2021 12:54PM  1:06PM Live 
Y22.00006: The pumpprobe xray absorption spectroscopy as an ultrafast thermometer of out of equilibrium systems Oleh Matvyeyev, Andrij Shvaika, James Freericks We propose to use pumpprobe xray absorption spectroscopy (XAS) as an ultrafast thermometer of the electronic subsystem, when it is driven out of equilibrium. The idea is inspired by the fact that the shape of the socalled satellite peak in the xray photoemission spectra (XPS) depends strongly on temperature in equilibrium. This satellite peak also demonstrates a strong dependence on time in timeresolved spectroscopy, and it is connected to the total energy of the system. Further, by comparing the energy gain during the nonequilibrium process to the equilibrium energy, allows one to measure in situ the effective temperature of the thermalized electron state. XAS is closely connected to XPS and might be an effective thermometer of the nonequilibrium system and it can be performed in many experiments where ultrafast xrays are available. 
Friday, March 19, 2021 1:06PM  1:18PM 
Y22.00007: Timedependent scattering approach to compute RIXS spectra Krissia de Zawadzki, Alberto Nocera, Adrian Feiguin Reliable methods to calculate Resonant Inelastic XRay Scattering (RIXS) spectra in strongly correlated manybody systems have been constrained to relatively small problems, as they rely on evaluating a response function (KramersHeisenberg formula) that is obtained from a timedependent perturbative analysis of the scattering problem. This requires the knowledge of all eigenstates of the manybody Hamiltonian to account for intermediate processes involving excitations of core electrons. We introduce a novel approach recasting such calculation as an effective timedependent problem in which incoming photons scatter with the system and outgoing photons are captured by a detector. The spectral density is obtained by solving the timedependent Schroedinger equation explicitly involving the photon degree of freedom. We demonstrate the formalism with an application to Mott insulating Hubbard chains using the timedependent density matrix renormalization group method. Our approach can readily be applied to systems out of equilibrium without modification and generalized to other spectroscopies. 
Friday, March 19, 2021 1:18PM  1:30PM Live 
Y22.00008: A novel equationofmotion coupledcluster framework for robust modeling Xray nonlinear spectroscopies Kaushik Nanda, Anna Krylov Nonlinear Xray spectroscopies such as resonant inelastic Xray scattering (RIXS) are powerful probes to investigate the structure of matter and dynamics with unprecendented resolution, especially when augmented with reliable theoretical tools. However, modeling nonlinear Xray spectra is challenging due to the difficulties in describing the physics of underlying coreelectron processes. In particular, the presence of the valence ionization continuum in which corelevel states are embedded renders standard theoretical methods, designed for modeling valenceelectron processes, inadequate. The ionization continuum is also responsible for the nonconvergent behavior of the response states in standard ab initio calculations for modeling nonlinear spectra. We will present a novel equationofmotion coupledcluster framework based on the corevalence separation scheme that provides convergent calculations of Xray response calculations and facilitates robust modeling of RIXS spectra for both closed and openshell species. We will illustrate the capabilities of our novel approach by measuring its performance in modeling the RIXS spectrum of the transient aqueous OH radical formed in the ionization of water against experiments. 
Friday, March 19, 2021 1:30PM  1:42PM Live 
Y22.00009: Timeresolved XAS of Fe/MgO heterostructure Tun Sheng Tan, Joshua Kas, Fernando Vila, John Rehr, Markus Gruner, Andrea Eschenlohr, Uwe Bovensiepen, Carolin SchmitzAntoniak, Katharina Ollefs, Klaus SokolowskiTinten, Nico Rothenbach, Rossitza Pentcheva, Heiko Wende A recent study of energy transfer due to laser excitation in a Fe/MgO heterostructure showed that the dynamic behavior is dominated by phononmediated processes [1]. We present an analysis of the spectral changes in the transient xray absorption spectra (XAS) using a finitetemperature realspace Green’s function approach. We approximate the phonon contribution via a harmonic dynamical matrix sampling method. Our theory accounts for the spectral differences shortly after laser excitation and at long times. 
Friday, March 19, 2021 1:42PM  1:54PM Not Participating 
Y22.00010: Investigating antisite disorder in quantum spin liquid candidates using firstprinciples calculations Idris Boukahil, C Das Pemmaraju, Rebecca Smaha, Charles J Titus, Mingde Jiang, John P Sheckelton, Wei He, Jiajia Wen, Suyin Wang, Yusheng Chen, Thomas Devereaux, Young Sang Lee Materials based on the Znparatacamite family have generated intense research efforts because of their spin½ frustrated magnetism that may exhibit a quantum spin liquid (QSL) ground state. Intrinsic disorder in a potential QSL candidate must be characterized as this can introduce unwanted interactions that destroy QSL physics. Using firstprinciples calculations combined with experimental Xray absorption measurements, we focus on characterizing the local and longrange structures of two leading QSL candidates: herbertsmithite (Cu_{3}Zn_{0.85}Cu_{0.15}(OH)_{6}Cl_{2}) and Znsubstituted barlowite (Cu_{3}Zn_{x}Cu_{1x}(OH)_{6}FBr). Our results indicate that Znbarlowite is more resistant to antisite disorder compared to herbertsmithite while also providing additional structural advantages important for QSL. 
Friday, March 19, 2021 1:54PM  2:06PM Live 
Y22.00011: Realtime cumulant Green’s function including nonlinear corrections Joshua Kas, John Rehr, Lucia Reining The cumulant approximation to the one electron Green's function has recently gained attention as an improved method for including the effects of manybody excitations in calculations of xray spectra [1]. Multiple plasmon satellites in the XPS spectra of solidstate systems are well reproduced by the cumulant, but not the GW approximation. Approximations of the cumulant are usually based on an expansion to linear order in the screened coulomb interaction, but this may become inadequate at higher correlation. Recently, a new approach for including nonlinear contributions was presented, and a simple approximation was proposed based on realtime TDDFT [2]. Here we apply this new approximation and compare to the linear response form as well as to available experimental data. Extensions of the approximation are also briefly discussed. 
Friday, March 19, 2021 2:06PM  2:18PM Live 
Y22.00012: Corelevel spectra from GW for molecular and amorphous systems Dorothea Golze, Levi Keller, Patrick Rinke GW has become the method of choice for the calculation of valence photoemission spectra [1]. To apply GW also to deep core excitations as measured by Xray photoelectron spectroscopy, we recently advanced the GW methodology and our implementation by combining exact numeric algorithms in the real frequency domain [2] with partial selfconsistency [3] and relativistic corrections [3,4]. We benchmarked our corelevel GW implementation for 65 1s core excitations, for which we find that GW reproduces absolute molecular 1s excitations within 0.3 eV of experiment and relative binding energies with average deviations smaller than 0.2 eV [3]. We then combined GW with machine learning (ML). We computed 16,000 C1s and O1s excitations of organic molecules with GW and used them to train a Kernel Ridge Regression (KRR) ML model. The KRRML model predicts molecular corelevel energies within 0.1 eV of the GW reference data. We used these models as starting point to develop GWML schemes for amorphous carbon materials, which show promise as electrode material for biomedical devices. 
Friday, March 19, 2021 2:18PM  2:30PM Live 
Y22.00013: The application of multireference pumpprobe simulation method to XUV signatures of ultrafast alkyl halides photodissociation Han Wang, David Prendergast UV pumpXUV probe measurements have been successfully applied in the study of photoinduced chemical reactions. Although rich elementspecific electronic structure information is accessible within XUV (innershell) absorption spectra, it can be difficult to interpret the chemistry directly from the spectrum. Due to the multireference character of the excited states of heavier alkyl halide molecules, we developed and applied a multireference method to completely simulate UV pumpXUV probe measurements to study photodissociation in alkyl halides molecules. Fewest switches surface hopping (FSSH) trajectories were used to explore the coupled electronic and ionic dynamics upon photoexcitation. Interpretation of previous measurements is provided by associated multireference, restricted active space, innershell spectral simulations. This combination of FSSH trajectories and XUV spectra provides an interpretation of experimental transient features and validates the branching ratio between different spinorbit split states in the photodissociated products. This methodology should prove useful for interpretation of the increasing number of innershell probe studies of molecular excited states or for designing new experiments to control the direction of chemical reactions. 
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