  The spin-orbit entangled (SOE) Jeff-state has been a fertile ground to study
novel quantum phenomena. Contrary to the conventional weakly correlated
Jeff=1/2 state of 4d and 5d transition metal compounds, the ground state of
CuAl2O4 hosts a Jeff=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 Jeff=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 higher
than Pc=8 GPa. This phase transition from the Mott insulating Jeff=1/2 to the
S=1/2 states is a unique phenomenon and has not been reported before. Our study
offers a rare example of the SOE Jeff-state under strong electron correlation
and its pressure-induced transition to the S=1/2 state.
