Pressure-induced transition from Jeff=1/2 to S=1/2 states in CuAl2O4

The spin-orbit entangled (SOE) J(eff) state has been a fertile ground to study quantum phenomena. Contrary to the conventional weakly correlated J(eff) = 1/2 state of 4d and 5d transition metal compounds, the ground state of CuAl2O4 hosts a J(eff) = 1/2 state with a strong correlation of Coulomb U. Here, we report that, surprisingly, Cu2+ ions of CuAl2O4 overcome the otherwise usually strong Jahn-Teller distortion and instead stabilize the SOE state, although the cuprate has relatively small spin-orbit coupling. From the x-ray absorption spectroscopy and high-pressure x-ray diffraction studies, we obtained definite evidence of the J(eff )= 1/2 state with a cubic lattice at ambient pressure. We also found the pressure-induced structural transition to a compressed tetragonal lattice consisting of the spin-only S = 1/2 state for pressure P-c > 8 GPa. This phase transition from the Mott insulating J(eff) = 1/2 to the S = 1/2 states is a unique phenomenon. Our study offers an example of the SOE J(eff) state under strong electron correlation and its pressure-induced transition to the S = 1/2 state.