Orbital fluctuations in the S= 12 Mott insulator Sr2VO4

We report a high-energy inelastic neutron-scattering study on Sr2VO4 that shows a transition from an orbital liquid phase at high temperature to an orbital-order phase at low temperature with an unquenched orbital angular momentum. Susceptibility under pressure shows a collapse of the magnetic phase transition due to enhanced orbital fluctuations. One possible explanation for our data is the recently proposed magnetically hidden order of Kramers doublets in d1 electron systems.