Abstract
Superconducting transitions in a series of Tl-In alloys have been studied by measurements of the incremental A.C. permeability of cylindrical specimens in an axial field H. For H intermediate between H[c2], the upper critical field and H[c3], the surface nucleation field, the results are interpreted in terms of surface superconductivity of the kind first suggested by St. James and de Gennes. Firstly, the measurements enable values to be determined; high nucleation fields have been observed for some freshly electropolished or etched surfaces, and a series of controlled experiments have shown that these are caused by a gradient in chemical concentration near the surface of the specimen, established by electrochemical or chemical processes during electropolishing or etching. Secondly, analysis of the permeability changes during transition in terms of a dissipative surface current mechanism suggested that, at least for normal and stable surfaces, the concept of a critical current density is approximately valid. The dependence of this critical current density on H has been determined; it is about two orders of magnitude less than that predicted by a recent theory of Abrikosov. Unstable surfaces possessing anomalously high nucleation fields could carry larger currents in larger A.C. measuring fields, and thus possess no critical current density. A frequency effect on the transition has also been observed and discussed. Transitions in the mixed state of type II superconductors and in the intermediate state of a type I superconductor have also been studied. Some of the features observed are; dependence of the transitions on frequency and on tbs amplitude of the measuring field, and irreversibilities in the transitions, according to whether H is increasing or decreasing. Such features are discussed in terms of the existence of an ohmic dissipative mechanism in the bulk of the superconductor with a surface current operating at the same time.