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

## TL;DR

This study uses resonant inelastic X-ray scattering to measure Hund's coupling in 5d oxides, providing first sharp estimates and highlighting its importance alongside spin-orbit interaction.

## Contribution

It offers the first precise estimates of Hund's coupling in 5d oxides using RIXS, emphasizing its significance in multi-orbital transition metal physics.

## Key findings

- Hund's coupling in Re$^{5+}$ is 0.26 eV
- Hund's coupling in Ir$^{5+}$ is 0.25 eV
- Spin-orbit coupling is around 0.38-0.42 eV

## Abstract

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.