QUANTUM-RESOLVED INVESTIGATIONS OF FLUX DYNAMICS - COLLECTIVE AND SINGLE VORTEX EFFECTS

The dynamics of flux vortices in Pb films has been investigated with s ingle fluxon resolution. With increasing magnetic field, there is a cr ossover from a region where single vortex effects dominate to one wher e vortex motion becomes correlated. We have measured the flux bundle c orrelation radius directly and tracked single vortices or correlated b undles as they moved under the action of an applied dc current or in r esponse to a change in temperature. The signatures of flux flow and fl ux creep have been identified, enabling us to determine the pinning fo rce at individual pinning sites.