Unveiling charge, spin, and orbital excitations in correlated d-electron materials by resonant inelastic x-ray scattering

Resonant inelastic x-ray scattering (RIXS) is a powerful technique for momentum- and energy-resolved measurements of charge, spin, and orbital excitations in correlated materials. The dispersion relations of these excitations encode the strength and range of atomic-scale Coulomb and exchange interactions, which are of primary interest in research on the origin of collective instabilities, such as superconductivity and magnetism.



Recently, we have performed O K-edge RIXS experiments on the correlated 4d-electron metal Sr₂RhO₄, which revealed a spin-orbit entangled collective excitation [1]. The dispersion relation of this mode is opposite to that of the 5d antiferromagnetic Mott insulator Sr₂IrO₄, where the spin-orbit excitons are dressed by magnons. We find that a tight-binding model accounting for the different ground states of the materials accurately captures the distinct dispersions. The presence of propagating spin-orbit excitons in Sr₂RhO₄ implies that spin-orbit coupling in the material is unquenched, and that collective instabilities in correlated 4d metals must be described in terms of spin-orbit entangled electronic states.



In most 3d-electron materials, spin-orbit coupling plays only a minor role, whereas their electronic and magnetic structures are predominantly shaped by strong electron-electron correlations, as for instance in the case of cuprate superconductors. Remarkably, the newly discovered rare-earth nickelate superconductors exhibit several formal analogies to cuprate superconductors. Yet, using x-ray absorption spectroscopy (XAS) and RIXS at the O K- and Ni L-edge, we observe that the cuprate-typical 3d-2p hybridization between the transition metal ion and oxygen is diminished in nickelates, while the rare-earth 5d electrons contribute to their low-energy electronic structure [2]. Furthermore, a combination of muon-spin rotation (μSR) spectroscopy and thermodynamic properties measurements indicate a profound analogy between the magnetic correlations in parent (undoped) nickelates and doped cuprates [3].