Quasimolecular electronic structure of the spin-liquid candidate Ba$_3$InIr$_2$O$_9$

TL;DR

This study investigates Ba3InIr2O9, revealing its quasi-molecular electronic structure and insulating magnetic properties, suggesting it as a cluster Mott insulator with potential for unconventional magnetism in 5d compounds.

Contribution

It establishes Ba3InIr2O9 as a cluster Mott insulator with a quasi-molecular electronic state, providing new insights into its magnetic interactions and electronic excitations.

Findings

Ba3InIr2O9 is a cluster Mott insulator with quasi-molecular dimer states.

Resonant inelastic x-ray scattering shows a rich excitation spectrum below 1.5 eV.

Low-energy excitations indicate proximity to a hopping-induced phase transition.

Abstract

The mixed-valent iridate Ba3InIr2O9 has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits Ir$^{4.5+}$ ions in face-sharing IrO6 octahedra forming Ir2O9 dimers with three t2g holes per dimer. Our results establish Ba3InIr2O9 as a cluster Mott insulator. Strong intra-dimer hopping delocalizes the three t2g holes in quasi-molecular dimer states while inter-dimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of Ba3InIr2O9 emerges from spin-orbit entangled quasi-molecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of 5d compounds. Using single-crystal x-ray diffraction we find the monoclinic space group C2/c already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering (RIXS) reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasi-molecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of Ba3InIr2O9 to a hopping-induced phase transition based on the condensation of a quasi-molecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding j=1/2 orbital and the third hole in a bonding j=3/2 orbital.