Non-resonant inelastic x-ray scattering involving excitonic excitations

TL;DR

This paper presents a local many-body approach to accurately describe non-resonant inelastic x-ray scattering involving excitonic excitations, highlighting the tunability of momentum transfer and its use in probing local symmetries in transition metal compounds.

Contribution

It introduces a quantitative method for analyzing non-resonant inelastic x-ray scattering that utilizes momentum transfer and directional dependence to determine local symmetries.

Findings

Accurate description of $d$-$d$ excitations in NiO and CoO.

Momentum transfer magnitude can be tuned for different excitations.

Direction of $oldsymbol{q}$ can determine local symmetry.

Abstract

In a recent publication Larson \textit{et al.} reported remarkably clear $d$-$d$ excitations for NiO and CoO measured with x-ray energies well below the transition metal $K$ edge. In this letter we demonstrate that we can obtain an accurate quantitative description based on a local many body approach. We find that the magnitude of $\vec{q}$ can be tuned for maximum sensitivity for dipole, quadrupole, etc. excitations. We also find that the direction of $\vec{q}$ with respect to the crystal axes can be used as an equivalent to polarization similar to electron energy loss spectroscopy, allowing for a determination of the local symmetry of the initial and final state based on selection rules. This method is more generally applicable and combined with the high resolution available, could be a powerful tool for the study of local distortions and symmetries in transition metal compounds including also buried interfaces.