Vortex chain size effect and negative resistance in pin-free superconducting films

We investigate the static and dynamic behavior of vortices in a pin-free superconductor. A diffraction pattern is observed in the critical currentJ_cvs. applied fieldHparallel to the surface of a granular Al film. When one vortex chain is formed in the film,J_cbecomes nearly equal to zero, which verifies that the sample is actually pin-free. An interaction between a radiofrequency current and moving vortices occurs, and theI–V curve shows peculiar nonlinear effects such as real negative resistance. The results are analogous to dc and ac Josephson effects.