Superfast Vortex Creep inYBa2Cu3O7−δCrystals with Columnar Defects: Evidence for Variable-Range Vortex Hopping

We observe a large peak in the vortex creep rate of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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 $T\sim 40\mathrm{K}$. Near this crossover, the current dependent creep activation energy $U\left(J\right)$ shows distinct and unusual behavior (“pinch”). On the high- $J$ side of the pinch, the glassy exponent $\mu \sim 1/3$ and large creep rate are characteristic of variable range vortex hopping.