TIME RELAXATION OF PERSISTENT CURRENTS IN YBA2CU3O7-X CRYSTALS WITH COLUMNAR DEFECTS

We have measured the persistent currents and their time relaxation in YBa2Cu3O7-x, single crystals with correlated disorder introduced by ir radiation with high-energy heavy-ions. For vortex density lower than t he defect density, and for applied fields either parallel or nonparall el to the columnar defects, we find clear evidence of a pinning-crosso ver between a regime of strong individual pinning at low temperatures and a regime of weaker collective pinning at high temperatures. We exp lore the vortex dynamics above and below this crossover.