Pressure-driven collapse of the relativistic electronic ground state in a honeycomb iridate

TL;DR

This study reveals that the honeycomb iridate Li2IrO3 transitions from a relativistic $J_{eff}=1/2$ state to a quasi-molecular orbital state under low pressure, highlighting the complex interplay of electronic factors in iridates.

Contribution

It demonstrates the pressure-induced crossover from a $J_{eff}=1/2$ state to a QMO state in Li2IrO3, challenging previous assumptions about its electronic ground state.

Findings

Li2IrO3 is $J_{eff}=1/2$ at ambient pressure.

Under ~0.1 GPa pressure, Li2IrO3 transitions to a QMO state.

The electronic state is sensitive to small pressure changes, indicating a delicate balance of interactions.

Abstract

The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC). Such a localized electronic state carries an effective total angular momentum of $J_{eff}=1/2$. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these $J_{eff}=1/2$ moments are expected to be coupled through a special bond-dependent magnetic interaction, which is a necessary condition for the realization of a Kitaev quantum spin liquid. However, this relativistic electron picture is challenged by an alternate description, in which itinerant electrons are confined to a benzene-like hexagon, keeping the system insulating despite the delocalized nature of the electrons. In this quasi-molecular orbital (QMO) picture, the honeycomb iridates are an unlikely choice for a Kitaev spin liquid. Here we show that the honeycomb iridate Li2IrO3 is best described by a $J_{eff}=1/2$ state at ambient pressure, but crosses over into a QMO state under the application of small (~ 0.1 GPa) hydrostatic pressure. This result illustrates that the physics of iridates is extremely rich due to a delicate balance between electronic bandwidth, spin-orbit coupling, crystal field, and electron correlation.