Emergent excitation at the magnetic metal-insulator transition in the pyrochlore osmate Cd2Os2O7

TL;DR

This study uses resonant inelastic X-ray scattering to explore the electronic and magnetic excitations at the metal-insulator transition in the 5d3 pyrochlore osmate Cd2Os2O7, revealing emergent magnetic excitations linked to spin-flip processes.

Contribution

First RIXS measurements on an osmium compound elucidate the electronic and magnetic energy scales controlling the MIT in Cd2Os2O7, highlighting the role of SOC in 5d systems with magnetic order.

Findings

Emergence of magnetic excitation at MIT involving spin-flip processes.

Magnetic groundstate characterized as all-in/all-out configuration.

Electronic and magnetic energy scales benchmarked against quantum chemistry calculations.

Abstract

The rich physics manifested by 5d oxides falls outside the Mott-Hubbard paradigm used to successfully explain the electronic and magnetic properties of 3d oxides. Much consideration has been given to the extent to which strong spin-orbit coupling (SOC), in the limit of increased bandwidth and reduced electron correlation, drives the formation of novel electronic states, as manifested through the existence of metal-insulator transitions (MITs). SOC is believed to play a dominant role in 5d5 systems such as iridates (Ir4+), undergoing MITs which may or may not be intimately connected to magnetic order, with pyrochlore and perovksite systems being examples of the former and latter, respectively. However, the role of SOC for other 5d configurations is less clear. For example, 5d3 (e.g Os5+) systems are expected to have an orbital singlet and consequently a reduced effect of SOC in the groundstate. The pyrochlore osmate Cd2Os2O7 nonetheless exhibits a MIT intimately entwined with magnetic order with phenomena similar to pyrochlore iridates. Here we report the first resonant inelastic X-ray scattering (RIXS) measurements on an osmium compound, allowing us to determine the salient electronic and magnetic energy scales controlling the MIT in Cd2Os2O7, which we benchmark against detailed quantum chemistry calculations. In particular, we reveal the emergence at the MIT of a magnetic excitation corresponding to a superposition of multiple spin-flip processes from an Ising-like all-in/all-out magnetic groundstate. We discuss our results with respect to the role of SOC in magnetically mediated MITs in 5d systems