Pressure-induced collapse of spin-orbital Mott state in the hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$

TL;DR

This study investigates how high pressure induces structural and electronic phase transitions in the hyperhoneycomb iridate $eta$-Li$_2$IrO$_3$, leading to the collapse of its spin-orbital Mott state and formation of Ir$_2$ dimers.

Contribution

The paper provides the first detailed high-pressure structural analysis showing Ir$_2$ dimerization and the collapse of the $J_{ m eff}$=1/2 state in $eta$-Li$_2$IrO$_3$, combining neutron diffraction, RIXS, and electronic structure calculations.

Findings

Ir$_2$ dimers form on zig-zag chains above 4 GPa.

Collapse of the $J_{ m eff}$=1/2 state under pressure.

Structural transition involves Ir-Ir bond shortening.

Abstract

Hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$ is a three-dimensional analogue of two-dimensional honeycomb iridates, such as $\alpha$-Li$_2$IrO$_3$, which recently appeared as another playground for the physics of Kitaev-type spin liquid. $\beta$-Li$_2$IrO$_3$ shows a non-collinear spiral ordering of spin-orbital-entangled $J_{\rm eff}$ = 1/2 moments at low temperature, which is known to be suppressed under a pressure of $\sim$2 GPa. With further increase of pressure, a structural transition is observed at $P_{\rm S}$ $\sim$ 4 GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of $\beta$-Li$_2$IrO$_3$ above $P_{\rm S}$ was refined, which indicates the formation of Ir$_2$ dimers on the zig-zag chains, with the Ir-Ir distance even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular orbital state comprising the two local $d_{zx}$-orbitals on the Ir-O$_2$-Ir bond plane, which conflicts with the equal superposition of $d_{xy}$-, $d_{yz}$- and $d_{zx}$- orbitals in the $J_{\rm eff}$ = 1/2 wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering (RIXS) measurements and the electronic structure calculations are fully consistent with the collapse of the $J_{\rm eff}$ = 1/2 state. A subtle competition of various electronic phases is universal in honeycomb-based Kitaev materials.