Description of resonant inelastic x-ray scattering in correlated metals

TL;DR

This paper develops a first-principles methodology to interpret and calculate the full RIXS response in correlated metals, validated through comparison with experimental data on BaFe$_2$As$_2$, enhancing understanding of RIXS contributions.

Contribution

The paper introduces a new first-principles approach for calculating the full RIXS response in correlated metals, including indirect contributions and momentum dependence.

Findings

RIXS response in BaFe$_2$As$_2$ is dominated by the direct channel.

Finite core-hole lifetime broadening explains the Raman-like response below threshold.

The methodology accurately reproduces experimental RIXS spectra.

Abstract

To fully capitalize on the potential and versatility of resonant inelastic x-ray scattering (RIXS), it is essential to develop the capability to interpret different RIXS contributions through calculations, including the dependence on momentum transfer, from first-principles for correlated materials. Toward that objective, we present new methodology for calculating the full RIXS response of a correlated metal in an unbiased fashion. Through comparison of measurements and calculations that tune the incident photon energy over a wide portion of the Fe L$_3$ absorption resonance of the example material BaFe$_2$As$_2$, we show that the RIXS response in BaFe$_2$As$_2$ is dominated by the direct channel contribution, including the Raman-like response below threshold, which we explain as a consequence of the finite core-hole lifetime broadening. Calculations are initially performed within the first-principles Bethe-Salpeter framework, which we then significantly improve by convolution with an effective spectral function for the intermediate-state excitation. We construct this spectral function, also from first-principles, by employing the cumulant expansion of the Green's function and performing a real-time time dependent density functional theory calculation of the response of the electronic system to the perturbation of the intermediate-state excitation. Importantly, this allows us to evaluate the indirect RIXS response from first-principles, accounting for the full periodicity of the crystal structure and with dependence on the momentum transfer.