DIRECT OBSERVATION OF VORTEX DYNAMICS IN SUPERCONDUCTING FILMS WITH REGULAR ARRAYS OF DEFECTS

The microscopic mechanism of the matching effect in a superconductor, which manifested itself as the production of peaks or cusps in the cri tical current at specific values of the applied magnetic field, was in vestigated with Lorentz microscopy to allow direct observation of the behavior of vortices in a niobium thin film having a regular array of artificial defects. Vortices were observed to form regular and consequ ently rigid lattices at the matching magnetic field, at its multiples, and at its fractions. The dynamic observation furthermore revealed th at vortices were most difficult to move at the matching field, whereas excess vortices moved easily.