Analysis of resonant inelastic x-ray scattering from Sr$_2$IrO$_4$ in an itinerant-electron approach

TL;DR

This paper models RIXS spectra of Sr$_2$IrO$_4$ using an itinerant electron approach, successfully explaining magnon and exciton peaks through a weak coupling theory aligned with experimental observations.

Contribution

It introduces a multi-orbital tight-binding model combined with Hartree-Fock and RPA calculations to interpret RIXS spectra in Sr$_2$IrO$_4$, highlighting the role of bound states and excitations.

Findings

Reproduces two-peak structure in RIXS spectra.

Identifies magnon and exciton modes as bound states.

Qualitative agreement with experimental data.

Abstract

We analyze the resonant x-ray scattering (RIXS) spectra from Sr$_2$IrO$_4$ in an itinerant electron approach. Employing a multi-orbital tight-binding model on the square lattice, we calculate the one-electron energy band within the Hartree-Fock approximation, which leads to an antiferromagnetic ground state. We then evaluate the two-particle Green's functions for the particle-hole pair excitations within the random phase approximation, which are connected to the RIXS spectra within the fast collision approximation. The calculated RIXS spectra exhibit two-peak structure with slightly different energies in the low-energy region, which are originated from the bound states in the two-particle Green's function. They may be interpreted as the split modes of magnon. We also obtain several $\delta$-function peaks, which arise from the bound states around the bottom of energy continuum. They may be called as the exciton modes. These characteristics are in qualitative agreement with the RIXS experiment, demonstrating that the weak coupling theory could explain both the magnon and the exciton peaks in the RIXS spectra on an equal footing.