Spin-orbit-entangled electronic structure of Ba${}_2$CaOsO${}_6$ studied by O $K$-edge resonant inelastic X-ray scattering

Transition-metal ions with $5d^2$ electronic configuration in a cubic crystal field are prone to have a vanishing dipolar magnetic moment but finite higher-order multipolar moments, and they are expected to exhibit exotic physical properties. Through an investigation using resonant inelastic X-ray spectroscopy (RIXS), Raman spectroscopy, and theoretical ligand-field (LF) multiplet and $abinitio$ calculations, we fully characterized the local electronic structure of Ba${}_2$CaOsO${}_6$, particularly, the crystal-field symmetry of the 5$d^2$ electrons in this anomalous material. The low-energy multiplet excitations from RIXS at the oxygen $K$ edge and Raman-active phonons both show no splitting. These findings are consistent with the ground state of Os ions dominated by magnetic octupoles. Obtained parameters pave the way for further realistic microscopic studies of this highly unusual class of materials, advancing our understanding of spin-orbit physics in systems with higher-order multipoles.