Resonant inelastic scattering spectra at the Ir $L$-edge in Na$_2$IrO$_3$

TL;DR

This study models the RIXS spectra of Na$_2$IrO$_3$ using an itinerant electron approach, revealing magnetic and excitonic excitations that align qualitatively with experimental data.

Contribution

It introduces a multi-orbital tight-binding model combined with RPA calculations to interpret RIXS spectra in Na$_2$IrO$_3$, emphasizing the itinerant electron perspective.

Findings

Identification of magnetic excitations below 20 meV as bound states.

Detection of excitonic excitations between 0.4 and 0.8 eV.

Qualitative agreement with recent RIXS experimental results.

Abstract

We analyze resonant x-ray scattering (RIXS) spectra in Na$_2$IrO$_3$ on the basis of the itinerant electron picture. Employing a multi-orbital tight-binding model on a honeycomb lattice, we find that the zigzag magnetic order is the most stable with sizable energy gap in the one-electron band within the Hartree-Fock approximation. We derive the RIXS spectra, which are connected to the generalized density-density correlation function. We calculate the spectra as a function of excitation energy $\omega$, within the random phase approximation. The spectra consist of the peaks with $\omega$ $<20$ meV, and of the peaks with $0.4<\omega<0.8$ eV. The former peaks are composed of four bound states in the density-density correlation function, and may be identified as the magnetic excitations, while the latter peaks are composed of sixteen bound states below the energy continuum of individual electron-hole pair excitations, and may be identified as the excitonic excitations. The calculated spectra agree qualitatively with the recent RIXS experiment.