Determination of Hund's coupling in 5d Oxides using Resonant Inelastic X-ray Scattering

We report resonant inelastic X-ray scattering (RIXS) measurements on ordered double perovskite samples containing Re${}^{5+}$ and Ir${}^{5+}$ with $5d^2$ and $5d^4$ electronic configurations respectively. In particular, the observed RIXS spectra of Ba${}_2$YReO${}_6$ and Sr${}_2$MIrO${}_6$ (M=Y, Gd) show sharp intra-t${}_{2g}$ transitions, which can be quantitatively understood using a minimal `atomic’ Hamiltonian incorporating spin-orbit coupling ($\lambda$) and Hund’s coupling ($J_H$). Our analysis yields $\lambda =0.38(2)$eV with $J_H=0.26(2)$eV for Re${}^{5+}$, and $\lambda =0.42(2)$eV with $J_H=0.25(4)$eV for Ir${}^{5+}$. Our results provide the first sharp estimates for the Hund’s coupling in $5d$ oxides, and suggest that it should be treated on equal footing with spin-orbit interaction in multi-orbital $5d$ transition metal compounds.