Novel pinning phenomena in a superconducting film with a square lattice of artificial pinning centers

摘要

We study the transport properties of a superconducting Nb film with a squarelattice of artificial pinning centers (APCs) as a function of dc current, at atemperature close to the superconducting transition temperature of the film. Wefind that, at low dc currents, the differential resistance of the film showsthe standard matching field anomaly, that is, the differential resistance has alocal minimum at magnetic fields corresponding to an integer number of fluxlines per APC. However, at higher dc currents, the differential resistance ateach matching field turns to a local maximum, which is exactly opposite to thelow current behavior. This novel effect might indicate that the flux lines inthe APC system change their flow mode as the dc current is increased.