Spin-orbit coupled systems in the "atomic" limit: rhenates, osmates, iridates

TL;DR

This paper develops a theoretical framework for interpreting RIXS spectra in heavy oxides with strong spin-orbit coupling, supported by new experimental data and ab initio calculations, revealing insights into electronic excitations and magnetic interactions.

Contribution

It provides an exact diagonalization approach for RIXS spectra in localized $t_{2g}$ systems, combines experimental and ab initio data, and explores the interplay of local and band electronic structures in heavy oxides.

Findings

RIXS spectra can be understood through a simple model of single- and multi-particle excitations.

New high-resolution RIXS data on Ba$_2$YReO$_6$ shows previously unresolved peak splitting.

Inter-site exchange interactions are significantly weaker in rhenates and iridates compared to osmates.

Abstract

Motivated by RIXS experiments on a wide range of complex heavy oxides, including rhenates, osmates, and iridates, we discuss the theory of RIXS for site-localized $t_{2g}$ orbital systems with strong spin-orbit coupling. For such systems, we present exact diagonalization results for the spectrum at different electron fillings, showing that it accesses "single-particle" and "multi-particle" excitations. This leads to a simple picture for the energies and intensities of the RIXS spectra in Mott insulators such as double perovskites which feature highly localized electrons, and yields estimates of the spin-orbit coupling and Hund's coupling in correlated $5d$ oxides. We present new higher resolution RIXS data at the Re-L$_3$ edge in Ba$_2$YReO$_6$ which finds a previously unresolved peak splitting, providing further confirmation of our theoretical predictions. Using ab initio electronic structure calculations on Ba$_2$${\cal M}$ReO$_6$ (with ${\cal M}$=Re, Os, Ir) we show that while the atomic limit yields a reasonable effective Hamiltonian description of the experimental observations, effects such as $t_{2g}$-$e_g$ interactions and hybridization with oxygen are important. Our ab initio estimate for the strength of the intersite exchange coupling shows that, compared to the osmates, the exchange is one or two orders of magnitude weaker in the rhenates and iridates, which may partly explain the suppression of long-range magnetic order in the latter compounds. As a way to interpolate between the site-localized picture and our electronic structure band calculations, we discuss the spin-orbital levels of the ${\cal M}$O$_6$ cluster. This suggests a possible role for non-dispersive intra-cluster excitons in Ba$_2$YIrO$_6$ which may lead to a weak breakdown of the atomic $J_{\rm eff}=0$ picture and to small magnetic moments.