Nb nanoporous ultrathin films: matching effects and interconnected wires

When the applied magnetic field is higher than the lower critical field but below the upper critical field, a type-II superconductor allows magnetic flux to penetrate it in the form of vortices, a tiny normal area surrounded by supercurrents. Driven by the Lorentz force of a passing external current or by thermal activation, vortices can move. Their motion induces energy dissipation and eventually can destroy the super- conductivity. Recent advances in nanofabrication have led to tremendous possibilities for implementing superconducting pinning structures and controlling the motion of vortices. The dynamics of vortices in confined superconducting geometries has gener- ated much interest, including studies of fundamental properties about vortex matter and devices based on the motion of the vortices. During the past decades, a lot of efforts have been devoted to introducing artificial pinning centers into superconductors to stabilize and pin the vortex lattice against the external driving force, thus giving rise to higher critical currents. This is of practical importance since super- conductors are required to maintain high critical currents for potential technological applications. Generally there are two different kinds of artificial pinning centers. [edited by the author]