Magnetic order, disorder, and excitations under pressure in the Mott insulator Sr$_2$IrO$_4$

TL;DR

This study investigates how pressure affects magnetic order, structural phases, and spin excitations in Sr$_2$IrO$_4$, revealing multiple magnetic phases and the potential emergence of a quantum spin liquid state.

Contribution

It provides the first detailed Raman scattering analysis of pressure-induced magnetic and structural phase transitions in Sr$_2$IrO$_4$, identifying a spin-disordered phase with possible quantum spin liquid characteristics.

Findings

Identification of three magnetically-ordered phases under pressure

Observation of a structural phase transition around 10 GPa

Detection of a spin-disordered phase with gapless excitations

Abstract

Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, Sr$_2$IrO$_4$ at high pressure is a rare example of a Mott insulator with a paramagnetic ground state. Here, using optical Raman scattering, we measure both the phonon and magnon evolution in Sr$_2$IrO$_4$ under pressure, and identify three different magnetically-ordered phases, culminating in a spin-disordered state beyond 18 GPa. A strong first-order structural phase transition drives the magnetic evolution at $\sim$10 GPa with reduced structural anisotropy in the IrO$_6$ cages, leading to increasingly isotropic exchange interactions between the Heisenberg spins and a spin-flip transition to $c$-axis-aligned antiferromagnetic order. In the disordered phase of Heisenberg $J_\mathrm{eff}=1/2$ pseudospins, the spin excitations are quasi-elastic and continuous to 10 meV, potentially hosting a gapless quantum spin liquid in Sr$_2$IrO$_4$.