Fluctuation superconductivity in uniform and nonuniform rings

Abstract

Due to fluxoid quantization, the average current around a superconducting ring close to its critical temperature is an oscillatory function of the magnetic field. For the case of uniform one-dimensional rings, there is an exact theory that enables us to evaluate this current. Using a scanning SQUID device, it has been possible to isolate magnetic signals that are seven orders of magnitude smaller than the applied field. This technique was applied to several samples, intended to be uniform. A numerical simulation method which we have recently developed enables us to evaluate the average fluctuating current also in the case that the ring is not uniform. We find that the behavior of most rings is indeed in agreement with the assumption that they are uniform, but for some of the rings nonuniformity must be invoked. The difference between the behavior of a uniform and a nonuniform ring is most pronounced near half-integer number of fuxoids, and close to the temperature at which a critical point has been found.