Superfast Vortex Creep in YBa₂Cu₃O_7-δ Crystals with Columnar Defects: Evidence for Variable-Range Vortex Hopping

We observe a large peak in the vortex creep rate of YBa2Cu3O7-δ Crystals with Columnar Defects. The peak appears 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 μ∼1/3 and large creep rate are characteristic of variable range vortex hopping. © 1997 The American Physical Society.