Collective creep of vortex bundles in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ 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 μ≃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 μ≃1.4. At even higher fields, μ decreases again to about 1 as the size of the bundles grows.