Spin-orbital liquid state assisted by singlet-triplet excitation in $J~=~0$ ground state of Ba$_3$ZnIr$_2$O$_9$

TL;DR

This study reveals that Ba$_3$ZnIr$_2$O$_9$ exhibits a spin-orbital liquid state driven by singlet-triplet excitations in a nominally non-magnetic $J=0$ ground state, highlighting the role of strong spin-orbit coupling and interdimer interactions.

Contribution

It demonstrates the realization of a spin-orbital liquid in a $J=0$ iridate compound due to competing SOC and superexchange interactions, supported by experimental and theoretical analysis.

Findings

Ir local moments are spontaneously generated despite a $J=0$ ground state.

The compound exhibits quantum fluctuations and a spin-orbital liquid phase.

Singlet-triplet excitations are stabilized by strong SOC and interdimer interactions.

Abstract

Strong spin-orbit coupling (SOC) effects of heavy $d$-orbital elements have long been neglected in describing the ground states of their compounds thereby overlooking a variety of fascinating and yet unexplored magnetic and electronic states, until recently. The spin-orbit entangled electrons in such compounds can get stabilized into unusual spin-orbit multiplet $J$-states which warrants severe investigations. Here we show using detailed magnetic and thermodynamic studies and theoretical calculations the ground state of Ba$_3$ZnIr$_2$O$_9$, a 6$H$ hexagonal perovskite is a close realisation of the elusive $J$~=~0 state. However, we find that local Ir moments are spontaneously generated due to the comparable energy scales of the singlet-triplet splitting driven by SOC and the superexchange interaction mediated by strong intra-dimer hopping. While the Ir ions within the structural Ir$_2$O$_9$ dimer prefers to form a spin-orbit singlet state (SOS) with no resultant moment, substantial interdimer exchange interactions from a frustrated lattice ensure quantum fluctuations till the lowest measured temperatures and stabilize a spin-orbital liquid phase.