Vortex Matter in Nanostructured Superconductors
Victor V. Moshchalkov
Nanoscale Superconductivity and Magnetism Group,
Laboratory for Solid State Physics and Magnetism, K. U. Leuven,
Celestijnenlaan 200 D, B-3001 Leuven, Belgium
Flux confinement phenomena have been studied in individual superconducting nanoplaquettes, their clusters and huge arrays (films with nanoengineered periodic pinning arrays (PPA)). In individual nanoplaquettes of the different form (loops, discs, triangles and squares) the superconducting critical temperature Tc(H) was measured resistively and also calculated from the linearized Ginzburg-Landau equations. Novel symmetry consistent vortex patterns have been identified for triangles and squares. To keep the imposed symmetry, vortex-antivortex pairs can be spontaneously formed, for example, in an equilateral triangle with two flux quanta applied, antivortex is formed in the centre and the three vortices sit in the three corners, thus complying with the symmetry (see also Nature 408, 833 (2000); PRL 86, 1323 (2001)). Stability of the vortex-antivortex molecules in type-I equilateral mesoscopic tiangles has been demonstrated (PRL 90, 147003 (2003)). In films with the PPA (lattices of antidots or magnetic dots) pronounced peaks at integer and rational fields have been revealed in dc- and ac-magnetisation and transport measurements. The peaks are attributed to certain stable vortex configurations. These configurations have been directly visualised by using scanning Hall probe microscopy (PRL 90, 237001 (2003)). The combination of the local probe techniques with the bulk probes has made it possible to identify correctly all relevant vortex patterns (multiquanta and composite vortex lattices, interpenetrating sublattices of strongly and weakly pinned vortices, etc) responsible for the strong enhancement of the critical current. In Pb films with a periodic array of magnetic dots with perpendicular magnetic anisotropy field-induced superconductivity has been observed. This effect can be induced in any superconducting film combined with a nanoengineered dipole field compensator (PRL 90, 197006 (2003)).