Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B60: First-Principles Simulations of Excited-State Phenomena: X-Ray SpectraFocus
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Sponsoring Units: DCOMP Chair: Li Yang, Washington University, St. Louis Room: Room 419 |
Monday, March 6, 2023 11:30AM - 12:06PM |
B60.00001: Invited Talk: Eric ShirleyOn electron spectroscopy using theory, x-ray absorption and resonant Auger techniques Invited Speaker: Eric L Shirley A variety of theoretical methods treat electron excitation spectra (x-ray absorption, photoemission, Auger electron spectroscopy, or XAS, PES, AES). These include excited-state density-functional theory, the GW/Bethe-Salpeter (BSE) framework, real-space multiple-scattering methods, and the cumulant-based methodology meant to tackle satellites in spectra. Different methods treat different aspects and are also used in combination. This talk will feature a BSE-centered survey of work in many materials. We begin with the isoelectronic series: Ge, GaAs, ZnSe, CuBr; showcasing the successes and challenges of the BSE methodology, as well as a "chemical alchemy" effect borne out by standard extended x-ray absorption fine structure (EXAFS) analysis. The role of Debye-Waller effects is also of interest in these systems. We next turn to molybdenum disulfide, where the effects of satellites on PES, XAS and resonant AES are analyzed to varying degrees, and to Ag, where resonant AES can give insight into the unoccupied band states, allowing one to distill all dipole-allowed partial densities of states. We then apply theoretical methods to glean insight from core-excitation spectra in lead titanate, lead tungstate and lithium peroxide, illustrating the methods' role in helping identify crystal structures, including as a function of temperature and pressure. Along the way, throughout the talk, our current shortcomings can help suggest the path forward in the future. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B60.00002: CleaRIXS: A fast and accurate first-principles method for simulation and analysis of resonant inelastic x-ray scattering Subhayan Roychoudhury, David Prendergast Resonant inelastic x-ray scattering (RIXS), which probes the occupied and unoccupied electronic subspaces in an interrelated way, is one of the most information-rich experimental techniques employed in the investigation of electronic structure. By combining two diverse and established approaches to modeling electronic excited states (namely the response-based approach and the constrained-occupation approach), we introduce an ab initio, accurate, and efficient computational framework for simulation and analysis of RIXS spectra for electronic transitions. The core-hole linear-response RIXS (CleaRIXS) [1] method not only ensures accurate incorporation of the interaction of electrons with core and valence holes, but also automatically maps the salient RIXS features to the relevant electronic transitions. Through comparison with previous methanol C K-edge RIXS measurements, we show the efficacy of the formalism in modeling different regions of the RIXS spectrum and in gaining physical insight about their origins. The importance of including the electron-hole interactions outside the core region is explored, in addition to the connection between CleaRIXS and determinant-based methods for simulating x-ray absorption and nonresonant x-ray emission. CleaRIXS provides a robust and extendable framework for prediction and interpretation of RIXS processes and for the simulation of complex electronic excited states in general. |
Monday, March 6, 2023 12:18PM - 12:30PM |
B60.00003: Excitonic Effects in X-ray Absorption Spectra of Fluoride Salts and Their Surfaces Ana Sanz Matias Given their natural abundance and thermodynamic stability, fluoride salts may appear as evolving components of electrochemical interfaces in Li-ion batteries and emergent multivalent ion cells. This is due to the practice of employing electrolytes with fluorine-containing species (salt, solvent, or additives) that electrochemically decompose and deposit on the electrodes. Operando X-ray absorption spectroscopy (XAS) can probe the electrode–electrolyte interface with a single-digit nanometer depth resolution and offers a wealth of insights into the evolution and Coulombic efficiency or degradation of prototype cells, provided that the spectra can be reliably interpreted in terms of local oxidation state, atomic coordination, and electronic structure about the excited atoms. To this end, we explore fluorine K-edge XAS of mono- (Li, Na, and K) and di-valent (Mg, Ca, and Zn) fluoride salts from a theoretical standpoint and discover a surprising level of detailed electronic structure information about these materials despite the relatively predictable oxidation state and ionicity of the fluoride anion and the metal cation. Utilizing a recently developed many-body approach based on the ΔSCF method, we calculate the XAS using density functional theory and experimental spectral profiles are well reproduced despite some experimental discrepancies in energy alignment within the literature, which we can correct for in our simulations. We outline a general methodology to explain shifts in the main XAS peak energies in terms of a simple exciton model and explain line-shape differences resulting from the mixing of core-excited states with metal d character (for K and Ca specifically). Given ultimate applications to evolving interfaces, some understanding of the role of surfaces and their terminations in defining new spectral features is provided to indicate the sensitivity of such measurements to changes in interfacial chemistry. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B60.00004: Efficient core-excited state orbital representation for many-body X-ray absorption transitions David Prendergast, Subhayan Roychoudhury X-ray absorption spectroscopy (XAS) is an explicit probe of the unoccupied electronic structure of materials and an invaluable tool for fingerprinting various electronic properties and phenomena. Computational methods capable of simulating and analyzing such spectra are therefore in high demand for complementing experimental results and for extracting valuable insights therefrom. In particular, a recently proposed first-principles approach titled Many-Body XAS (MBXAS), which approximates initial and final states as Slater determinants constructed from Kohn-Sham (KS) orbitals optimized in the absence or presence of the relevant core-hole, respectively, has shown promising prospects in evaluating X-ray transition amplitudes. Here, we re-derive the MBXAS approach using a transition operator expressed entirely in the basis of core-excited state KS orbitals. This reformulation offers substantial practical and conceptual advantages: circumventing previous issues of convergence with respect to the number of unoccupied ground-state orbitals; reducing computational cost by rendering the calculation of such orbitals unnecessary altogether; and providing a direct pathway for comparing the many-body approximation with the so-called single-particle treatment. One can now directly quantify the importance in observed XAS intensity of the relaxation of the valence occupied subspace induced by the core excitation and observe the associated electronic structure changes using auxiliary orbitals to explain the observed spectral intensity. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B60.00005: Debye-Waller effects on x-ray near-edge spectra Eric L Shirley, Joseph C Woicik, Eric J Cockayne, Igor Levin We present an ad hoc method to include Debye-Waller effects in theoretical near-edge spectra that are obtained without use of real-space multiple scattering. Instead of considering the mean square fluctuations of interatomic distances in an atom-by-atom fashion, we model the fluctuations as a continuous function of distance that approaches a long-range limit. A near-edge spectrum is converted from the energy domain to the wave-vector domain and Fourier-transformed to a pseudo-real-space (PRS) domain. In the PRS domain, the real-space coordinate is the difference between any two scattering paths that a photoelectron can take to achieve a given final state. We window the real-space transform of the spectrum using overlapping triangle functions and convolve each portion with a distance-dependent Gaussian before returning to the energy domain. We will show results for simple metals and semiconductors. The fluctuations can be obtained from traditional models, molecular dynamics, extended x-ray absorption fine structure (EXAFS) data and crystallographic data. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B60.00006: A new formalism for calculating core electron binding energies in periodic solids Juhan Matthias Kahk, Johannes C Lischner In reference [1], a formalism for calculating core electron binding energies in periodic solids, as measured in experimental X-ray Photoelectron Spectroscopy (XPS), was presented. In this formalism, the core electron binding energy is calculated as the total energy difference between two N-1 electron systems: one with a core hole, and one with a hole in the highest occupied state. |
Monday, March 6, 2023 1:06PM - 1:18PM Author not Attending |
B60.00007: Full-potential multiple scattering calculations of solids and molecules within the FEFF10 code Joshua J Kas, John J Rehr The real-space multiple scattering FEFF codes have been in wide use for calculations of a multitude of spectroscopies, [1,2] with the most common being EXAFS and XANES. However, the limitation to spherical muffin-tin potentials has been a roadblock to accurate calculations of various interesting materials, such as layered materials, small molecules, and surface effects. Here we present an extension to the FEFF10 code that takes into account full, non-spherical potentials based on the treatment of Ankudinov et al. [3]. These potentials can be calculated within FEFF or taken from external DFT or quantum chemistry codes and used as input. We show that this approach yields accurate densities of states and spectra both for small molecules and solids, that compare well with those of other codes and with experimental results. |
Monday, March 6, 2023 1:18PM - 1:30PM |
B60.00008: Evaluation of the excitation spectra with diffusion Monte Carlo on an auxiliary bosonic ground state Fernando A Reboredo, Jaron T Krogel, Paul Kent Theoretical methods to evaluate the excitation spectra of an interacting fermionic system from the ground state of an auxiliary bosonic system are derived and tested in exactly soluble models. The ground state's instantaneous response to multiplication by an envelope function and the removal of an arbitrary effective potential is derived. The instantaneous imaginary time evolution of an arbitrary excitation is then obtained by sampling an observable on the ground-state distribution of walkers of an auxiliary bosonic system. We show that this approach can take advantage of and correct for approximate eigenstates obtained with mean-field calculations or truncated interactions. Relevant parts of the theory have been tested in soluble model systems and exhibit excellent agreement with exact analytical data, and the CI and the F12 approaches. In particular, for limited basis set expansions, this approach outperforms CI and F12 using the same input. Therefore, this auxiliary boson approach is expected to behave better than CI or variational Monte Carlo (VMC) for excitation spectra when the effects of Coulomb singularities are not fully captured with standard Jastrow factors. The potential use of this method to study realistic fermionic systems is discussed. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B60.00009: Spectral Density Calculations for Solids with Selected Configuration Interaction Luis Rangel DaCosta, kevin gasperich, Michel Caffarel, Anouar Benali In this presentation, we will discuss spectral density analysis of molecules and solids through many-body theory, using multireference wavefunctions in the framework of selected configuration interaction (sCI). This method allows controlled convergence to the full CI limit within a given basis set at a fraction of the cost by iteratively updating a reference wavefunction with a small set of determinants with large contributions to the wavefunction energy. sCI can calculate properties of ground-state and excited-state wavefunctions with similar levels of accuracy, making it a unique ab initio method. We implemented a Green's function approach to calculating spectral densities, for which we will discuss relevant computational pragmatic strategies and convergence behaviors. We successfully predict core and valence excitations for several molecules and will discuss spectral density results for the strongly correlated hydrogen chain and periodic silicon. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B60.00010: Ab initio Predictions of Circular Dichroism in Chiral Crystals Christian Multunas, Andrew Grieder, Junqing Xu, Yuan Ping, Ravishankar Sundararaman Chirality in materials is of increasing recent interest especially because of its strong coupling to spin dynamics, such as in the Chirality-Induced Spin Selectivity effect. Designing new chiral materials and benchmarking their optoelectronic properties, including circular dichroism (CD) and optical rotation, is therefore of increasing importance. Here, we present a framework for first-principles prediction of CD of chiral crystals starting from the independent particle approximation and identify key conceptual and computational extensions necessary beyond the well-established theory of CD in molecules. Specifically, we show that CD in general anisotropic crystals is a rank-2 tensor in terms of the wave propagation direction, and requires treatment of electric quadrupole matrix elements in addition to magnetic dipole ones. We validate our framework with excellent agreement with experimental measurements on molecular benchmarks, as well as crystals ranging in complexity from elemental chiral semiconductors, such as tellurium and selenium, to hybrid halide perovskites. Finally, we show that the impact of spin-orbit coupling on the CD of crystals with heavy elements is primarily due to changes in the electronic energies, rather than through spin matrix element contributions. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B60.00011: Photoexcitation and Ionization of the Oxygen Vacancy in MgO Christian W Vorwerk, Giulia Galli Oxygen vacancies are ubiquitous in oxides such as MgO and strongly influence the electronic structure and catalytic and transport properties of these materials. Due to their scientific and technological importance, the electronic excited states of oxygen vacancies in MgO have been studied with a variety of methods both by quantum chemists and solid-state physicists. However, despite considerable theoretical effort over the last decades, a microscopic understanding of absorption and emission processes of single oxygen vacancies in MgO are still under debate. Here, we present a balanced ab initio investigation of the photoexcitation and photoionization process of the oxygen vacancy in MgO. Using a newly developed embedding Bethe-Salpeter equation (eBSE) approach, implemented in the WEST code, we calculate energies and transition matrix elements of all relevant defect excitations. We find absorption and emission energies in good agreement with available experimental results and we provide a detailed understanding of the absorption and emission processes of the neutral and positively charged oxygen vacancy, reconciling different views present in the chemistry and condensed-matter physics community. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B60.00012: Applications of variational autoencoders for polymer nanocomposite structure generation Shizhao Lu, Arthi Jayaraman Polymer nanocomposites (PNCs) are used in automobile parts, aircraft design, materials for energy storage, soft electronics, and photonics applications. In these applications the desired PNC properties are a function of the PNC morphology (i.e., dispersed/aggregated nanorods with/without orientational alignment or percolation). Molecular simulations can be useful tools that can predict PNC morphology for variety of PNC design. However, generating numerous uncorrelated structures from simulations of PNCs at high (melt-like) packing fraction can be tedious and computationally intensive for some design parameters (e.g., long polymer chains, complex nanofiller design). In this talk, we present a novel deep learning workflow using variational autoencoders that takes as input a handful of uncorrelated structures to generate a larger ensemble of uncorrelated PNC structures with similar morphological features as the input. Our workflow will aid in the fast and automatic generation of configurations of 3D structures of amorphous soft material with prescribed structural characteristics / properties. |
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