Vortex penetration into micron-sized Bi2Sr2CaCu2O8+δ -: art. no. 184506

The onset of vortex penetration into micron-sized Bi2Sr2CaCu2O8+delta disks and squares is studied using local magnetization measurements. The vortex penetration field H-p is observed to exceed the lower critical field H-c1 at all temperatures below the superconducting transition temperature T-c. The magnitude of H-p is determined by sample size and geometry. Below 15 K, H-p is approximately proportional to the sample width D, consistent with the Bean bulk pinning model. Above 15 K, H-p is a function of the sample thickness-to-width ratio t/D, and agrees with the Bean-Livingston surface-barrier effect.