New light on magnetic excitations: indirect resonant inelastic X-ray scattering on magnons

TL;DR

This paper derives the theoretical framework for magnetic RIXS, showing it probes spin dynamics via four-spin correlations, with calculations matching experimental spectra and revealing selection rules for magnon scattering.

Contribution

It provides the first detailed derivation of the RIXS cross-section for magnetic excitations and identifies the specific spin correlations measured by this technique.

Findings

RIXS involves at least two magnons in scattering processes.

Scattering intensity vanishes at specific momenta, notably at q=0.

Calculated spectra agree well with experimental data.

Abstract

Recent experiments show that indirect resonant inelastic X-ray scattering (RIXS) is a new probe of spin dynamics. Here I derive the cross-section for magnetic RIXS and determine the momentum dependent four-spin correlation function that it measures. These results show that this technique offers information on spin dynamics that is complementary to e.g. neutron scattering. The RIXS spectrum of Heisenberg antiferromagnets is calculated. It turns out that only scattering processes that involve at least two magnons are allowed. Other selection rules imply that the scattering intensity vanishes for specific transferred momenta ${\bf q}$, in particular for ${\bf q}=0$. The calculated spectra agree very well with the experimental data.