Super-Fast Vortex Creep in YBa2Cu3O7-delta Crystals with Columnar Defects: Evidence for Variable Range Vortex Hopping

We observe an enormous peak in the vortex creep rate of YBa2Cu3O7-delta crystals with columnar defects. The peak articulates at the crossover from the regime where vortices are well localized on columnar pins to a regime where vortex-vortex interactions dominate, and it is most prominent (nearly 12%) at low fields, near T ~ 40K. Near this crossover, the current dependent creep activation energy U(J) shows distinct and unusual behavior (pinch). On the high-J side of the pinch, the glassy exponent u ~ 1/3 and large creep rate are characteristic of variable range vortex hopping.