“Persistent” Insulator: Avoidance of Metallization at Megabar Pressures in Strongly Spin-Orbit-Coupled Sr₂IrO₄

Here we report a rare insulating state that persists up to at least 185 GPa in the antiferromagnetic iridate Sr₂IrO₄, which is the archetype of a spin-orbit-driven J_eff = 1/2 insulators. This study shows the electrical resistance of single-crystal Sr₂IrO₄ decreases initially with pressure, then reaches a minimum in the range, 32 - 38 GPa, and is followed by a rapid rise to fully recover the insulating state (~10⁷ W) with further pressure increases up to 185 GPa. The onset of the rapid increase in resistance is accompanied by a structural phase transition from the native tetragonal I4₁/acd phase to an orthorhombic Pbca phase (with much reduced symmetry) at 40.6 GPa, according to our synchrotron x-ray diffraction and Raman scattering data. This close correlation explains the existence of the rare insulating state at megabar pressures: the pressure-induced, severe structural distortions prevent expected metallization, despite a 26% volume compression in Sr₂IrO₄ at the highest pressure accessed in this experiment. It is striking that the high electrical resistance remains essentially unchanged with a tripling of very high pressure range from 61 GPa to 185 GPa.