Silhanek, K., J. Guimpel, L. Civale, H. Lanza, and P. Levy. "Thickness effects on the pinning by columnar defects." PHYSICA C 341 (2000): 1217–1218.
Abstract: We compare the angular dependence of the pinning of equivalent columnar defects in GdBa2Cu2O7 thin films and a thicker YBa2Cu3O7 single crystal. The tracks' related enhancement of the critical currents is much smaller for the case of the thin films. We interpret this thickness dependence as resulting from a competition between the pinning of the correlated structures, related to total sample thickness, and surface boundary conditions, which force the vortices to depin from the columnar defects near the sample surface.