3D–2D-like vortex transition above Bc2 in niobium films

Abstract

Isotropic superconducting Nb-films thicker than the penetration depth exhibit a magnetically anisotropic structure within the magnetic field range B c2 ⩽ B a⩽ B c3 , where B a is the applied field, and B c2 and B c3 are the upper and surface superconductivity critical fields, respectively. The anisotropic structure of the films consists of three layers: two superconducting surface layers and normal layer in between. Upon tilting the field with respect to the film surface, we have found that surface vortices undergo a 3D–2D-like dimensional crossover. Vortices residing on each of both film surfaces are coupled at fields almost parallel to the surface (3D), and decoupled at larger angles (2D) within the surface superconductivity state.