Collective creep of vortex bundles in YBa2Cu3O7 crystals

We present evidence for the crossover between creep of individual vortices and collective creep of vortex bundles from measurements of the relaxation of the persistent currents in YBa2Cu3O7 single crystals over long periods of time. In the single-vortex regime at low fields the relaxation is fast and is characterized by an exponent μ0.16, as predicted in the collective-creep scenario. With increasing field, collective bundling of vortices induces a slowdown of the relaxation, with a maximum μ1.4. At even higher fields, μ decreases again to about 1 as the size of the bundles grows. © 1994 The American Physical Society.