LOW-TEMPERATURE MAGNETIC-RELAXATION IN YBA2CU3O7-DELTA - EVIDENCE FOR QUANTUM TUNNELING OF VORTICES

Hall-probe measurements of an YBa2Cu3O7-delta crystal in the range 0.1-1 K show a temperature-independent time-logarithmic magnetic relaxation. These results rule out conventional thermally activated flux creep, which predicts a T2/3 dependence at the lowest temperatures if one takes into account the dependence of the barrier on driving force in a simple one-dimensional model. They are consistent with a phenomenological theory assuming a quantum tunneling process for vortex kink motion.