Spin-orbit excitons in a correlated metal: Raman scattering study of Sr2RhO4

TL;DR

This study uses Raman spectroscopy to reveal spin-orbit excitons in the correlated metal Sr2RhO4, providing evidence for unquenched spin-orbit coupling and insights into its electronic ground state and potential phase transitions.

Contribution

It presents the first direct observation of spin-orbit excitons in Sr2RhO4 and analyzes the crystal field effects influencing its metallic state.

Findings

Detected excitations at 220 meV and 240 meV with specific symmetries.

Identified these as transitions between spin-orbit multiplets.

Revealed the sign of the crystal field enhances planar orbital character.

Abstract

Using Raman spectroscopy to study the correlated 4$d$-electron metal Sr$_2$RhO$_4$, we observe pronounced excitations at 220 meV and 240 meV with $A_\mathrm{1g}$ and $B_\mathrm{1g}$ symmetries, respectively. We identify them as transitions between the spin-orbit multiplets of the Rh ions, in close analogy to the spin-orbit excitons in the Mott insulators Sr$_2$IrO$_4$ and $\alpha$-RuCl$_3$. This observation provides direct evidence for the unquenched spin-orbit coupling in Sr$_2$RhO$_4$. A quantitative analysis of the data reveals that the tetragonal crystal field $\Delta$ in Sr$_2$RhO$_4$ has a sign opposite to that in insulating Sr$_2$IrO$_4$, which enhances the planar $xy$ orbital character of the effective $J=1/2$ wave function. This supports a metallic ground state, and suggests that $c$-axis compression of Sr$_2$RhO$_4$ may transform it into a quasi-two-dimensional antiferromagnetic insulator.