Direct measurement of penetration length in ultrathin and/or mesoscopic superconducting structures

As the dimensions of thin superconducting structures become comparable with or less than the penetration length of magnetic flux into the structures, it becomes increasingly necessary to devise experimental tests of available theoretical models. One approach which we shall describe enables penetration lengths to be derived from the measurement of the effective area of planar, thin-film structures with linear dimensions in the range of 1–100μm. The effective area is defined by measurement of the inductive coupling of the structures to dc or low-frequency magnetic fields. The structures described consist of two parts: (1) an ultrathin annular superconducting film with transition temperature Tca (“washer”) and (2) surrounding the washer is a superconducting ring with transition temperature Tcs. Because the films are prepared in such a way that Tca