Covalency-driven collapse of strong spin-orbit coupling in face-sharing iridium octahedra

Abstract

We report $\text{ab-initio}$ density functional theory calculation and Raman scattering results to explore the electronic structure of Ba${}_5$CuIr${}_3$O${}_{12}$ single crystals. This insulating iridate, consisting of face-sharing IrO${}_6$ octahedra forming quasi-one-dimensional chains, cannot be described by the local $j_{\mathrm{eff}}$=1/2 moment picture commonly adopted for discussing electronic and magnetic properties of iridate compounds with IrO${}_6$ octahedra. The shorter Ir-Ir distance in the face-sharing geometry, compared to corner- or edge-sharing structures, leads to strong covalency between neighboring Ir. Then this strong covalency results in the formation of molecular orbitals (MO) at each Ir trimers as the low-energy electronic degree of freedom. The theoretically predicted three-peak structure in the joint density of states, a distinct indication of deviation from the $j_{\mathrm{eff}}$=1/2 picture, is verified by observing the three-peak structure in the electronic excitation spectrum by Raman scattering.

5 pages, 5 figures, with a Supplementary Information

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