Theoretical analysis of resonant inelastic x-ray scattering spectra of Ca2RuO4

We calculate the resonant inelastic x-ray scattering (RIXS) spectra of a 4d-electron Mott insulator Ca2RuO4 that exhibits spin-orbit entangled magnetic ordering. Considering the Ru L3-edge RIXS, we identify four differ-ent types of excitations: magnetic dipolar transitions, quadrupolar transitions changing total angular momentum by AJ = 2, spin-state transitions with AS = 1, and t2g -> eg orbital transitions across the crystal-field and Hund's coupling derived levels. We provide analytical expressions for the magnetic transitions and the AJ = 2 transitions in the tetragonal compression limit, which are generally applicable to other compounds with t42g configurations in a square lattice. Intriguingly, we find that upon tetragonal compression the dipole-forbidden AJ = 2 transitions acquire large spectral weights with pronounced polarization dependencies, which are con-sistent with the experimental observations. Our numerical simulations show that this is due to a constructive interference between dipolar and quadrupolar scattering channels.