Theory of resonant inelastic x-ray scattering in iridium oxide compounds: Probing spin-orbit-entangled ground states and excitations

We determine how the elementary excitations of iridium-oxide materials, which are dominated by a strong relativistic spin-orbit coupling, appear in resonant inelastic x-ray scattering (RIXS). Whereas the RIXS spectral weight at the L₂ x-ray edge vanishes in the limit of cubic symmetry, we find it to be strong at the L₃ edge. Applying this to Sr₂IrO₄, we observe that RIXS, besides being sensitive to local doublet-to-quartet transitions, meticulously maps out the strongly dispersive delocalized excitations of the low-lying spin-orbit doublets.