Magnetic order, disorder, and excitations under pressure in the Mott insulator Sr2IrO4

Abstract

Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, Sr2IrO4 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 Sr2IrO4 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 ∼10 GPa with reduced structural anisotropy in the IrO6 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 Jeff=1/2 pseudospins, the spin excitations are quasielastic and continuous to 10 meV, potentially hosting a gapless quantum spin liquid in Sr2IrO4.