Multiple Spin-Orbit Excitons and the Electronic Structure of $\hat{I}\pm$-RuCl${}_3$

Abstract

The honeycomb compound $\hat{I}\pm$-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 $\hat{I}»$ and a large cubic crystal field. The low-energy electronic structure of $\hat{I}\pm$-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 $\hat{I}»$ 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 $\hat{I}»$ = 0.16 eV and $\hat{I}”$ =42(4) meV for the observed non-cubic crystal-field splitting, supporting the validity of the $j=1/2$ picture for $\hat{I}\pm$-RuCl${}_3$. The unusual strength of the double excitation is related to the underlying hopping interactions which form the basis for dominant Kitaev exchange.

7 pages, 4 figures