Superconductive supercooling and superheating of small cadmium spheres: Size effects.

Abstract

Supercooling and superheating have been observed for samples composed of small cadmium spheres of uniform well-defined sizes. The data obtained for large sphere diameters are independent of size and represent, by all indications, ideal bulk properties. The supercooling data for such spheres have been used to obtain the Ginsburg-Landau parameter k and, in conjunction with BCS expressions, the penetration depth X and the coherence length ε0. The temperature dependence of the supercooling field H3c shows an anomaly for t ~0.8, very similar to that previously reported for A1 and Zn. The superheating field of these “bulk” spheres shows strong effects of the nonlocal electrodynamics below Tc. Spheres of diameters equal to or smaller than 10u show strong size effects in the supercooling and superheating fields, especially near Tc. The results are examined in the light of existing theories of critical fields for small spheres. A systematic decrease of the critical temperature with sphere size has also been observed. It seems related to a decrease in the gap anisotropy, produced by boundary scattering. These results are analyzed in terms of the theory of Markowitz and Kadanoff..