Multiple Spin-Orbit Excitons and the Electronic Structure of $\alpha$-RuCl$_3$

TL;DR

This paper investigates the electronic structure of $ ext{α-RuCl}_3$, revealing multiple spin-orbit excitons and clarifying the spin-orbit coupling strength, supporting the $j=1/2$ model and its relevance to Kitaev physics.

Contribution

It provides a detailed analysis of the spin-orbit excitons in $ ext{α-RuCl}_3$ using infrared and Raman data, resolving previous ambiguities about the spin-orbit coupling and crystal field effects.

Findings

Infrared features are due to single, double, and triple spin-orbit excitons.

Spin-orbit coupling constant λ is 0.16 eV.

Non-cubic crystal-field splitting Δ is 42 meV.

Abstract

The honeycomb compound $\alpha$-RuCl$_3$ is widely discussed as a proximate Kitaev spin-liquid material. This scenario builds on spin-orbit entangled $j = 1/2$ moments arising for a $t_{2g}^5$ electron configuration with strong spin-orbit coupling $\lambda$ and a large cubic crystal field. The low-energy electronic structure of $\alpha$-RuCl$_3$, however, is still puzzling. In particular infrared absorption features at 0.30 eV, 0.53 eV, and 0.75 eV seem to be at odds with theory. Also the energy of the spin-orbit exciton, the excitation from $j = 1/2$ to 3/2, and thus the value of $\lambda$ are controversial. Combining infrared and Raman data, we show that the infrared features can be attributed to single, double, and triple spin-orbit excitons. We find $\lambda$ = 0.16 eV and $\Delta$ =42(4) meV for the observed non-cubic crystal-field splitting, supporting the validity of the $j= 1/2$ picture for $\alpha$-RuCl$_3$. The unusual strength of the double excitation is related to the underlying hopping interactions which form the basis for dominant Kitaev exchange.