Collective creep of vortex bundles in YBa2Cu3O7 crystals.

Abstract

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 ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ 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 \ensuremath{\mu}\ensuremath{\simeq}0.16, as predicted in the collective-creep scenario. With increasing field, collective bundling of vortices induces a slowdown of the relaxation, with a maximum \ensuremath{\mu}\ensuremath{\simeq}1.4. At even higher fields, \ensuremath{\mu} decreases again to about 1 as the size of the bundles grows.