Pressure-driven orbital selective insulator-to-metal transition and spin-state crossover in cubic CoO

The metal-insulator and spin state transitions of CoO under high pressure are studied by using density functional theory combined with dynamical mean-field theory. Our calculations predict that the metal-insulator transition in CoO is a typical orbital selective insulator-to-metal transition, where the t2g orbitals of a Co $3d$ shell become metallic first around 60 GPa while the eg orbitals still remain insulating until 170 GPa. Further studies of the spin states of a Co $3d$ shell reveal that the orbital selective Mott phase in the intermediate pressure regime is mainly stabilized by the high-spin state of the Co $3d$ shell, and the transition from this phase to the fully metallic state is driven mainly by the high-spin to low-spin transition of the Co²⁺ ions. Our results are in good agreement with the most recent transport and x-ray emission experiments under high pressure.