The dynamics of flux vortices in Pb films has been investigated with single fluxon resolution. With increasing magnetic field, there is a crossover from a region where single vortex effects dominate to one where vortex motion becomes correlated. We have measured the flux bundle correlation radius directly and tracked single vortices or correlated bundles as they moved under the action of an applied dc current or in response to a change in temperature. The signatures of flux flow and flux creep have been identified, enabling us to determine the pinning force at individiual pinning sites.

• Received 9 August 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.3854