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 $L_2$ x-ray edge vanishes in the limit of cubic symmetry, we find it to be strong at the $L_3$ edge. Applying this to Sr${}_2$IrO${}_4$, 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.

Figure 1

RIXS spectra of Sr${}_2$IrO${}_4$ at the Ir $L_3$$t_{2g}$ edge. The left-hand panel shows the spectrum for incoming $\pi$ polarization, and the right-hand panel for incoming $\sigma$ polarization. The outgoing polarization is not measured. The intradoublet excitations are broadened by $J/2$.

Figure 2

Vertical cuts through Fig. 1. The left-hand panel shows spectra at several $\mathbf{q}$ for incoming $\pi$ polarization, and the right-hand panel for incoming $\sigma$ polarization.

Figure 3

Spectral weight of different excitations. The units on the vertical axis are chosen such that the $J_{\mathrm{eff}}=1/2$ to $3/2$ excitation has a spectral weight of unity.