Numerical simulation of core-level spectroscopies

Abstract

Core-level X-ray spectroscopies are widely used for investigating electronic structures of transition metal (TM) compounds. In core-level spectroscopies an incident photon excites a core electron where the created core hole acts as a positive test charge. The electronic properties can be determined by core-level spectra, influenced by the reaction of the system to the core hole. For the investigation of TM compounds different kinds of core-level X-ray spectroscopies are available. X-ray photoemission spectroscopy (XPS) provides photoelectron spectra, whereas X-ray absorption spectroscopy (XAS) provides absorption spectra due to the excitation of core electrons to valence states. Resonant inelastic X-ray scattering (RIXS) is a second-order optical process where the incident photon gets absorbed, such as in XAS, but the spectra are generated by relaxation processes. We present within this thesis a computational study of Cu L-edge RIXS for LaCuO3 and NaCuO2 which are two typical high-valence transition-metal oxides. Basis of the approach is a theoretical framework based on the local-density approximation and dynamical mean-field theory (LDA+DMFT) which is applied on the Anderson impurity model (AIM) to calculate spectral functions of various core-level X-ray spectroscopies. Recent experiments revealed unusual coexistence of bound and continuum excitations in the L-edge RIXS spectra where the underlying physics is still a challenging part of research. This study is motivated by these experiments and we analyze in detail the behavior of the fluorescence-like (FL) feature and show how it is connected to the details of the electronic and crystal structure. On the studied compounds we demonstrate how material details determine whether the electron-hole continuum can be excited in the L-edge RIXS process.