  Cs3C60 is a correlated superconductor under pressure, but an insulator under
ambient conditions. The mechanism causing this insulating behavior is the
combination of Mott localization and the dynamic Jahn-Teller effect. We show
evidence from infrared spectroscopy for the dynamic Jahn-Teller distortion. The
continuous change with temperature of the splitting of infrared lines is
typical Jahn-Teller behavior, reflecting the change in population of
solid-state conformers. We conclude that the electronic and magnetic
solid-state properties of the insulating state are controlled by molecular
phenomena. We estimate the time scale of the dynamic Jahn-Teller effect to be
above 10^(-11) s and the energy difference between the conformers less than 20
cm-1.
