Electronic excitations in the edge-shared relativistic Mott insulator: Na$_2$IrO$_3$

TL;DR

This study investigates the excitation spectra of Na$_2$IrO$_3$, revealing distinct optical and RIXS features due to its relativistic electronic structure, highlighting differences from similar iridates and emphasizing the interplay of itinerant and local electronic properties.

Contribution

The paper provides detailed calculations of optical conductivity and RIXS spectra for Na$_2$IrO$_3$, demonstrating the importance of relativistic effects and electron-hole coupling in its excitation behavior.

Findings

Na$_2$IrO$_3$ shows a broad single peak in optical conductivity from interband transitions.

RIXS spectra reveal a two-peak spin-orbit exciton and a low-energy magnon peak.

A small peak near the absorption edge indicates coupling between electron-hole excitation and SO exciton.

Abstract

We have investigated the excitation spectra of $j_{eff}$=$\frac{1}{2}$ Mott insulator Na$_2$IrO$_3$. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity $\sigma(\omega)$ and resonant inelastic x-ray scattering (RIXS) spectra, which manifest different features from those of a canonical $j_{eff}$=$\frac{1}{2}$ system Sr$_2$IrO$_4$.Distinctly from the two-peak structure in Sr$_2$IrO$_4$, $\sigma(\omega)$ in Na$_2$IrO$_3$ has a broad single peak dominated by interband transitions from $j_{eff}$=$\frac{3}{2}$ to $\frac{1}{2}$. RIXS spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure arising from the crystal-field effect, and the magnon peak at energies much lower than in Sr$_2$IrO$_4$. In addition, a small peak near the optical absorption edge is found in RIXS spectra, originating from the coupling between the electron-hole ($e$-$h$) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in Na$_2$IrO$_3$.