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

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 alpha-RuCl3. This observation provides direct evidence for the unquenched spinorbit coupling in Sr2RhO4. A quantitative analysis of the data reveals that the tetragonal crystal field Delta in Sr2RhO4 has a sign opposite to that in insulating Sr2IrO4, which enhances the planar xy orbital character of the effective J 1/4 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.