Suppression of surface barriers in superconductors by columnar defects - art. no. 134518

We investigate the influence of columnar defects in layered superconductors on the thermally activated penetration of pancake vortices through the sur face barrier. Columnar defects, located near the surface, facilitate penetr ation of vortices through the surface barrier, by creating "weak spots," th rough which pancakes can penetrate into the superconductor. Penetration of a pancake mediated by an isolated column, located near the surface, is a tw o-stage process involving hopping from the surface to the column and the de tachment from the column into the bulk; each stage is controlled by its own activation barrier. The resulting effective energy is equal to the maximum of those two barriers. For a given external field there exists an optimum location of the column for which the barriers for the both processes are eq ual and the reduction of the effective penetration barrier is maximal. At h igh fields the effective penetration field is approximately 2 times smaller than in unirradiated samples. We also estimate the suppression of the effe ctive penetration field by column clusters. This mechanism provides further reduction of the penetration field at low temperatures.