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

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 similar to 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