Spin-Orbit Excitons in a Correlated Metal: Raman Scattering Study of Sr2RhO4

Abstract

Using Raman spectroscopy to study the correlated 4d-electron metal Sr2RhO4, we observe pronounced excitations at 220 meV and 240 meV with A1g and B1g 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 Sr2IrO4 and α-RuCl3. This observation provides direct evidence for the unquenched spin-orbit coupling in Sr2RhO4. A quantitative analysis of the data reveals that the tetragonal crystal field Δ in Sr2RhO4 has a sign opposite to that in insulating Sr2IrO4, 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 Sr2RhO4 may transform it into a quasi-two-dimensional antiferromagnetic insulator.