arXiv subjects

J. J. Rehr

Publications and source records attributed to J. J. Rehr.

At least 19 recordsLinked to original sources

arXiv2025

Non-resonant two-photon x-ray absorption in Cu

We present a real-space Green's function theory and calculations of two-photon x-ray absorption (TPA). Our focus is on non-resonant K-shell TPA in metallic Cu, which has been observed experimentally at intense x-ray free electron laser (XFEL) sources. The theory is based on an independentparticle Green's function treatment of the Kramers-Heisenberg equation and an approximation for the sum over non-resonant intermediate states in terms of a static quadrupole transition operator. XFEL effects are modeled by a partially depleted d-band. This approach is shown to give results for K-shell TPA in quantitative agreement with XFEL experiment and with a Bethe-Salpeter Equation approach. We also briefly discuss many-body corrections and TPA sum-rules.

cond-mat.str-el↗

arXiv2022

Real-space Green's function approach for intrinsic losses in x-ray spectra

Intrinsic inelastic losses in x-ray spectra originate from excitations in an interacting electron system due to a suddenly created core-hole. These losses characterize the features observed in x-ray photoemission spectra (XPS), as well Here we present a complementary {\it ab initio} real-space Green's function (RSGF) generalization of the Langreth cumulant in terms of the dynamically screened core-hole interaction $W_c(ω)$ and the independent particle response function. We find that the cumulant kernel $β(ω)$ is analogous to XAS, but with the transition operator replaced by the core-hole potential with monopole selection rules. The behavior reflects the analytic structure of the loss function, with peaks near the zeros of the dielectric function, consistent with delocalized quasi-boson excitations. The approach simplifies when $W_c(ω)$ is localized and spherically symmetric. Illustrative results and comparisons are presented for the electron gas, sodium, and some early transition metal compounds.

cond-mat.other↗

arXiv2022

High-temperature self-energy corrections to x-ray absorption spectra

Effects of finite-temperature quasiparticle self-energy corrections to x-ray absorption spectra are investigated within the finite-temperature quasiparticle local density GW approximation up to temperatures $T$ of order the Fermi temperature. To facilitate the calculations, we parametrize the quasiparticle self-energy using low-order polynomial fits. We show that temperature-driven decrease in the electron lifetime substantially broadens the spectra in the near-edge region with increasing $T$. However, the quasiparticle shift is most strongly modified near the onset of plasmon excitations.

cond-mat.mtrl-sci↗

arXiv2022

Ab initio multiplet plus cumulant approach for correlation effects in x-ray photoelectron spectroscopy

The treatment of electronic correlations in open-shell systems is among the most challenging problems of condensed matter theory. Current approximations are only partly successful. Ligand field multiplet theory (LFMT) has been widely successful in describing intra-atomic correlation effects in x-ray spectra, but typically ignores itinerant states. The cumulant expansion for the one electron Greens function has been successful in describing shake-up effects but ignores atomic multiplets. More complete methods, such as dynamic mean-field theory can be computationally problematic. Here we show that separating the dynamic Coulomb interactions into local and longer-range parts with ab initio parameters yields a combined multiplet plus cumulant approach that accounts for both local atomic multiplets and satellite excitations. The approach is illustrated in transition metal oxides and explains the multiplet peaks, charge-transfer satellites and distributed background features observed in XPS experiment.

cond-mat.mtrl-sci↗

arXiv2021

Full spectrum optical constant interface to the Materials Project

Optical constants characterize the interaction of materials with light and are important properties in material design. Here we present a Python-based Corvus workflow for simulations of full spectrum optical constants from the UV-VIS to hard x-ray wavelengths based on the real-space Green's function code FEFF10 and structural data from the Materials Project (MP). The Corvus workflow manager and its associated tools provide an interface to FEFF10 and the MP database. The workflow parallelizes the FEFF computations of optical constants over all absorption edges for each material in the MP database specified by a unique MP-ID. The workflow tools determine the distribution of computational resources needed for that case. Similarly, the optical constants for selected sets of materials can be computed in a single-shot. To illustrate the approach, we present results for nearly all elemental solids in the periodic table, as well as a sample compound, and compared with experimental results. As in x-ray absorption spectra, these results are interpreted in terms of an atomic-like background and fine-structure contributions.

cond-mat.mtrl-sci↗

arXiv2021

Web-Based Methods for X-ray and Photoelectron Spectroscopies

We present a simplified web-based application for simulating x-ray and photoelectron spectra of transition metals, built around the notion that web-based applications lower the bar for novice users. The application provides a simple interface to simulate x-ray absorption spectroscopy, resonant inelastic x-ray scattering, and angle-resolved photoemission spectroscopy, incorporating the effects of local electronic interactions, which give rise to multiplets, spin-orbit coupling, crystal field effects, and ligand hybridization/charge transfer. Results can be obtained that highlight the key role of photon polarization.

cond-mat.str-el↗

arXiv2021

Advanced calculations of x-ray spectroscopies with FEFF10 and Corvus

The real-space Green's function code FEFF has been extensively developed and used for calculations of x-ray and related spectra, including x-ray absorption (XAS), x-ray emission (XES), inelastic x-ray scattering, and electron energy loss spectra (EELS). The code is particularly useful for the analysis and interpretation of the XAS fine-structure (EXAFS) and the near-edge structure (XANES) in materials throughout the periodic table. Nevertheless, many applications, such as non-equilibrium systems, and the analysis of ultra-fast pump-probe experiments, require extensions of the code including finite-temperature and auxiliary calculations of structure and vibrational properties. To enable these extensions, we have developed in tandem, a new version FEFF10, and new FEFF based workflows for the Corvus workflow manager, which allow users to easily augment the capabilities of FEFF10 via auxiliary codes. This coupling facilitates simplified input and automated calculations of spectra based on advanced theoretical techniques. The approach is illustrated with examples of high temperature behavior, vibrational properties, many-body excitations in XAS, super-heavy materials, and fits of calculated spectra to experiment.

cond-mat.mtrl-sci↗

arXiv2021

Real-space Green's function approach for x-ray spectra at finite temperature

There has been considerable interest in properties of condensed matter at finite temperature, including non-equilibrium behavior and extreme conditions up to the warm dense matter regime. Such behavior is encountered, e.g., in experimental time resolved x-ray absorption spectroscopy (XAS) in the presence of intense laser fields. In an effort to simulate such behavior, we present an approach for calculations of finite-temperature x-ray absorption spectra in arbitrary materials, using a generalization of the real-space Green's function formalism. The method is incorporated as an option in the core-level x-ray spectroscopy code FEFF10. To illustrate the approach, we present calculations for several materials together with comparisons to experiment and with other methods.

cond-mat.mtrl-sci↗

arXiv2020

Real-time coupled-cluster approach for the cumulant Green's function

Green's function methods within many-body perturbation theory provide a general framework for treating electronic correlations in excited states. Here we investigate the cumulant form of the one-electron Green's function based on the coupled-cluster equation of motion approach in an extension of our previous study. The approach yields a non-perturbative expression for the cumulant in terms of the solution to a set of coupled first order, non-linear differential equations. The method thereby adds non-linear corrections to traditional cumulant methods linear in the self energy. The approach is applied to the core-hole Green's function and illustrated for a number of small molecular systems. For these systems we find that the non-linear contributions lead to significant improvements both for quasiparticle properties such as core-level binding energies, as well as the satellites corresponding to inelastic losses observed in photoemission spectra.

physics.chem-ph↗

arXiv2018

Dispersing and non-dispersing satellites in the photoemission spectra of aluminum

Satellites in electronic spectra are pure many-body effects, and their study has been of increasing interest in both experiment and theory. The presence of satellites due to plasmon excitations can be understood with simple models of electron-boson coupling. It is far from obvious how to match such a model to real spectra, where more than one kind of quasi-particle and of satellite excitation coexist. Our joint experimental and theoretical study shows that satellites in the angle-resolved photoemission spectra of the prototype simple metal aluminum consist of a superposition of dispersing and non-dispersing features. Both are due to electron-electron interaction, but the non-dispersing satellites also reflect the thermal motion of the atoms. Moreover, besides their energy dispersion, we also show and explain a strong shape dispersion of the satellites. By taking into account these effects, our first principles calculations using the GW+C approach of many-body perturbation theory reproduce and explain the experimental spectra to an unprecedented extent.

cond-mat.str-el↗

arXiv2017

Cumulant Green's function calculations of plasmon satellites in bulk sodium: influence of screening and the crystal environment

We present ab initio calculations of the photoemission spectra of bulk sodium using different flavors of the cumulant expansion approximation for the Green's function. In particular, we study the dispersion and intensity of the plasmon satellites. We show that the satellite spectrum is much more sensitive to many details than the quasi-particle (QP) spectrum, which suggests that the experimental investigation of satellites could yield additional information beyond the usual studies of the band structure. In particular, a comparison to the homogeneous electron gas (HEG) shows that the satellites are influenced by the crystal environment, although the crystal potential in sodium is weak. Moreover, the temperature dependence of the lattice constant is reflected in the position of the satellites. Details of the screening also play an important role; in particular, the contribution of transitions from 2s and 2p semi-core levels influence the satellites, but not the QP. Moreover, inclusion of contributions to the screening beyond the RPA has an effect on the satellites. Finally, we elucidate the importance of the coupling of electrons and holes by comparing the results of the time-ordered (TOC) and the retarded (RC) cumulant expansion approximations. Again, we find small but noticeable differences. Since all the small effects add up, our most advanced calculation yields a satellite position which is improved with respect to previous calculations by almost one eV. This stresses the fact that the calculation of satellites is much more delicate than the calculation of a QP band structure.

cond-mat.str-el↗

arXiv2017

Finite temperature Green's function approach for excited state and thermodynamic properties of cool to warm dense matter

We present a finite-temperature extension of the retarded cumulant Green's function for calculations of exited-state and thermodynamic properties of electronic systems. The method incorporates a cumulant to leading order in the screened Coulomb interaction $W$ and improves excited state properties compared to the $GW$ approximation of many-body perturbation theory. Results for the homogeneous electron gas are presented for a wide range of densities and temperatures, from cool to warm dense matter regime, which reveal several hitherto unexpected properties. For example, correlation effects remain strong at high $T$ while the exchange-correlation energy becomes small. In addition, the spectral function broadens and damping increases with temperature, blurring the usual quasi-particle picture. Similarly Compton scattering exhibits substantial many-body corrections that persist at normal densities and intermediate $T$. Results for exchange-correlation energies and potentials are in good agreement with existing theories and finite-temperature DFT functionals.

cond-mat.str-el↗

arXiv2016

Particle-hole cumulant approach for inelastic losses in x-ray spectra

Inelastic losses in core level x-ray spectra arise from many-body excitations, leading to broadening and damping as well as satellite peaks in x-ray photoemission (XPS) and x-ray absorption (XAS) spectra. Here we present a practical approach for calculating these losses based on a cumulant representation of the particle-hole Green's function, a quasi-boson approximation, and a partition of the cumulant into extrinsic, intrinsic and interference terms. The intrinsic losses are calculated using real-time, time-dependent density functional theory while the extrinsic losses are obtained from the GW approximation of the photo-electron self-energy and the interference terms are approximated. These effects are included in the spectra using a convolution with an energy dependent particle-hole spectral function. The approach elucidates the nature of the spectral functions in XPS and XAS and explains the significant cancellation between extrinsic and intrinsic losses. Edge-singularity effects in metals are also accounted for. Illustrative results are presented for the XPS and XAS for both weakly and more correlated systems.

cond-mat.mtrl-sci↗

arXiv2016

Efficient implementation of core-excitation Bethe Salpeter equation calculations

We present an efficient implementation of the Bethe-Salpeter equation (BSE) method for obtaining core-level spectra including x-ray absorption (XAS), x-ray emission (XES), and both resonant and non-resonant inelastic x-ray scattering spectra (N/RIXS). Calculations are based on density functional theory (DFT) electronic structures generated either by abinit or Quantumespresso, both plane-wave basis, pseudopotential codes. This electronic structure is improved through the inclusion of a GW self energy. The projector augmented wave technique is used to evaluate transition matrix elements between core-level and band states. Final two-particle scattering states are obtained with the NIST core-level BSE solver (NBSE). We have previously reported this implementation, which we refer to as ocean (Obtaining Core Excitations from Ab initio electronic structure and NBSE) [Phys. Rev. B 83, 115106 (2011)]. Here, we present additional efficiencies that enable us to evaluate spectra for systems ten times larger than previously possible; containing up to a few thousand electrons. These improvements include the implementation of optimal basis functions that reduce the cost of the initial DFT calculations, more complete parallelization of the screening calculation and of the action of the BSE Hamiltonian, and various memory reductions. Scaling is demonstrated on supercells of SrTiO_3 and example spectra for the organic light emitting molecule Tris-(8-hydroxyquinoline)aluminum (Alq_3 ) are presented. The ability to perform large-scale spectral calculations is particularly advantageous for investigating dilute or non-periodic systems such as doped materials, amorphous systems, or complex nano-structures.

physics.comp-ph↗

arXiv2014

Real time cumulant approach for charge transfer satellites in x-ray photoemission spectra

X-ray photoemission spectra generally exhibit satellite features in addition to the quasi-particle peaks due to many-body excitations, which have been of considerable theoretical and experimental interest. However, the satellites attributed to charge-transfer (CT) excitations in correlated materials have proved difficult to calculate from first principles. Here we report a real-time, real-space approach for such calculations based on a cumulant representation of the core-hole Green's function and time-dependent density functional theory. This approach also yields an interpretation of CT satellites in terms of a complex oscillatory, transient response to a suddenly created core hole. Illustrative results for TiO$_2$ and NiO are in good agreement with experiment.

cond-mat.str-el↗

arXiv2014

Cumulant expansion for phonon contributions to the electron spectral function

We describe an approach for calculations of phonon contributions to the electron spectral function, including both quasiparticle properties and satellites. The method is based on a cumulant expansion for the retarded one-electron Green's function and a many-pole model for the electron self-energy. The electron-phonon couplings are calculated from the Eliashberg functions, and the phonon density of states is obtained from a Lanczos representation of the phonon Green's function. Our calculations incorporate ab initio dynamical matrices and electron-phonon couplings from the density functional theory code ABINIT. Illustrative results are presented for several elemental metals and for Einstein and Debye models with a range of coupling constants. These are compared with experiment and other theoretical models. Estimates of corrections to Migdal's theorem are obtained by comparing with leading order contributions to the self-energy, and are found to be significant only for large electron-phonon couplings at low temperatures.

cond-mat.mtrl-sci↗

arXiv2014

Generalized particle/hole cumulant approximation for the electron Green's function

The cumulant expansion is a powerful approach for including correlation effects in electronic structure calculations beyond the GW approximation. However, current implementations are incomplete since they ignore terms that lead to partial occupation numbers and satellites both above and below the Fermi energy. These limitations are corrected here with a generalized cumulant approximation that includes both particle and hole contributions within a retarded Green's function formalism. The computational effort is still comparable to GW, and the method can be extended easily to finite temperature. The approach is illustrated with calculations for the homogeneous electron gas and comparisons to experiment and other methods.

cond-mat.str-el↗

arXiv2013

Charge transfer satellites in x-ray spectra of transition metal oxides

Strongly correlated materials such as transition metal oxides (TMOs) often exhibit large satellites in their x-ray photoemission (XPS) and x-ray absorption spectra (XAS). These satellites arise from localized charge-transfer (CT) excitations that accompany the sudden creation of a core hole. Here we use a two-step approach to treat such excitations in a localized system embedded in a condensed system and coupled to a photoelectron. The total XAS is then given by a convolution of a spectral function representing the localized excitations and the XAS of the extended system. The local system is modeled roughly in terms of a simple three-level model, leading to a double-pole approximation for the spectral function that represents dynamically weighted contributions from the dominant neutral and charge-transfer excitations. This method is implemented using a resolvent approach, with potentials, radial wave-functions and matrix elements from the real-space Green's function code FEFF, and parameters fitted to XPS experiments. Representative calculations for several TMOs are found to be in reasonable agreement with experiment.

cond-mat.str-el↗