RIXS interferometry and the role of disorder in the quantum magnet Ba$_3$Ti$_{3-x}$Ir$_{x}$O$_9$

TL;DR

This study uses RIXS to investigate how disorder affects magnetism in Ba3Ti(3-x)IrxO9, revealing complex local moments and magnetic interactions influenced by cation disorder and site mixing.

Contribution

It demonstrates the application of RIXS interferometry to distinguish excitations at different Ir sites and uncovers the impact of disorder on magnetic properties in this compound.

Findings

Ir-Ti site mixing is pronounced.

Coexistence of singlet states and spin-orbit moments.

Disorder suppresses long-range magnetic order.

Abstract

Motivated by several claims of spin-orbit driven spin-liquid physics in hexagonal Ba$_3$Ti$_{3-x}$Ir$_x$O$_9$ hosting Ir2O9 dimers, we report on resonant inelastic x-ray scattering (RIXS) at the Ir L3 edge for different x. We demonstrate that magnetism in Ba$_3$Ti$_{3-x}$Ir$_x$O$_9$ is governed by an unconventional realization of strong disorder, where cation disorder affects the character of the local moments. RIXS interferometry, studying the RIXS intensity over a broad range of transferred momentum q, is ideally suited to assign different excitations to different Ir sites. We find pronounced Ir-Ti site mixing. Both ions are distributed over two crystallographically inequivalent sites, giving rise to a coexistence of quasimolecular singlet states on Ir2O9 dimers and spin-orbit entangled j=1/2 moments of 5d$^5$ Ir$^{4+}$ ions. RIXS reveals different kinds of strong magnetic couplings for different bonding geometries, highlighting the role of cation disorder for the suppression of long-range magnetic order in this family of compounds.