Propagating Spin-Orbit Excitons in a Correlated Metal

The presence of strong spin-orbit coupling (SOC) can lead to highly exotic phases of matter in quantum materials. While SOC is considered as a dominant energy scale in 5d compounds such as Sr₂IrO₄, its strength decreases for lighter elements and its significance in shaping the electronic structure of 4d materials is a topic of active debate. Here, we present a comprehensive study of the solid-solution Sr₂Rh_1-xIr_xO₄, which is situated between the 4d-electron metal Sr₂RhO₄ and the 5d-electron Mott insulator Sr₂IrO₄. We investigate the evolution of the low-energy electronic structure in the substitution series using resonant inelastic x-ray scattering and Raman spectroscopy. A focus of our study is Sr₂RhO₄, where we detect similar spin-orbit entangled excitations as previously observed in Sr₂IrO₄, albeit on a smaller energy scale and with an opposite dispersion, which we rationalize in terms of the distinct ground states of the two materials. Our findings shed new light on the role of SOC in 4d compounds and we comment on the prospects of using cation substitution as a platform to tune the effective spin-orbit interaction strength.