Edge superconducting states

Abstract

Surface superconductivity usually refers to the nucleation of superconctivity near the outer surface of a bulk specimen. The same intrinsic phenomena takes place in a patterned thin film giving rise to localized superconducting edge states near any free boundary, thickness jump or local variation of the vector potential. We have investigated a thin superconducting film with an array of patterned microholes. A splitting of the critical field line is found when the field is perpendicular to the film. The new transition line (Hc3) exhibits two types of fine structure due to the quantization of the multiply connected edge states. We discuss these states in terms of multiquanta vortices nucleated inside the holes. We report Bitter decoration of the the corresponding vortex configuration. Because of the enhancement of the order parameter near the hole edges, the interaction between the vortices and the microholes is repulsive slightly below the bulk critical field (Hc2) and is much stronger than the intervortex interaction. We find a "localization transition": all vortices drop inside holes when the coherence length becomes larger than the interhole spacing. This transition explains the two types of oscillation found in the transition line and leads to a qualitative change of the creep below the second critical field.