NONLINEAR LOGARITHMIC TIME DECAY OF MAGNETIZATION IN SUPERCONDUCTING RB3C60

We have measured the time decay of the magnetization for a well characterized superconducting fullerene Rb3C60. The temperature dependence of the magnetic relaxation rate dM(t)/dln(t) exhibits a maximum at a characteristic temperature T(max), which decreases with increasing field. At low temperatures a nonlinear logarithmic time decay of the magnetization is observable only after several hours, but is clearly observable after much shorter times at high temperatures. This nonlinear behavior suggests the existence of a vortex glass state in Rb3C60 and is consistent with the collective creep model.