Spin-orbit excitations and electronic structure of the putative Kitaev magnet $\alpha$-RuCl$_3$

TL;DR

This study uses optical spectroscopy and Raman scattering to investigate the electronic structure of $ ext{RuCl}_3$, revealing strong spin-orbit coupling and localized orbital excitations, supporting its candidacy as a Kitaev quantum spin liquid material.

Contribution

The paper provides detailed experimental analysis of the electronic structure of $ ext{RuCl}_3$, estimating spin-orbit coupling strength and clarifying energy scales, advancing understanding of its magnetic properties.

Findings

Identification of orbital excitations involving localized total angular momentum states.

Estimation of spin-orbit coupling strength and electronic energy hierarchy.

Comparison with density functional theory clarifies optical response features.

Abstract

Mott insulators with strong spin-orbit coupling have been proposed to host unconventional magnetic states, including the Kitaev quantum spin liquid. The 4$d$ system $\alpha$-RuCl$_3$ has recently come into view as a candidate Kitaev system, with evidence for unusual spin excitations in magnetic scattering experiments. We apply a combination of optical spectroscopy and Raman scattering to study the electronic structure of this material. Our measurements reveal a series of orbital excitations involving localized total angular momentum states of the Ru ion, implying that strong spin-orbit coupling and electron-electron interactions coexist in this material. Analysis of these features allows us to estimate the spin-orbit coupling strength, as well as other parameters describing the local electronic structure, revealing a well-defined hierarchy of energy scales within the Ru $d$ states. By comparing our experimental results with density functional theory calculations, we also clarify the overall features of the optical response. Our results demonstrate that $\alpha$-RuCl$_3$ is an ideal material system to study spin-orbit coupled magnetism on the honeycomb lattice.