Modeling multiplet effects in ‭X-ray spectroscopies

Electron correlations can strongly effect material properties. For the ground-state the monopole part (spherical average) of the local Coulomb interaction can turn band-metals into Mott insulators. The multipole part of the Coulomb interaction defines local multiplets and effectively changes hopping parameters. Multiplet interactions between local electrons can lead to a state described as a Hund's metal. Optically excited states in solids can also be strongly modified by electron correlations. Without electron electron interactions optical excitations are described by a convolution of the occupied and unoccupied density of states. Correlations lead to additional features within the spectrum. Strong interactions produce bound-states or excitons. These bound-states can show a multitude of multiplets related to the multipole part of the Coulomb interaction. Interesting effects happen when electron correlations and band-formation compete in interaction strength. This can lead to rich phase diagrams for the ground-state. For optical excitations this leads to the formation of resonances. These resonances can be understood as a local bound exciton interacting with a continuum at the same energy. 

In this talk I will discuss how to describe band-transitions, excitons and resonances in correlated quantum materials such as transition metal and rare earth compounds. We will focus on the description of the multiplet line-shapes of resonances in core level x-ray spectroscopies including XAS, XPS and RIXS. An important question adressed is how the high energy x-ray excitations relate to the low energy states of interest.

The theoretical methods discussed during the talk are implemented in the software package Quanty (www.Quanty.org), an opensource script-language that implements functions to calculate response functions of a variety of quantum materials on a post DFT or Hartree-Fock level.