Critical Currents in the Superconducting Surface Sheath

Results are presented of magnetization measurements on Pb-Tl cylinders in axial magnetic fields of such magnitude that superconductivity is confined to the superconducting surface sheath. Specimens whose Ginzburg-Landau $\kappa$ values range from 0.6 to 1.4 have been investigated. For these cylindrical specimens a nonzero magnetization exists beyond the upper critical field $H_{c2\mathrm{}}$ up to a field $H_{c3\mathrm{}}$ (1.7 to 1.75 $H_{c2\mathrm{}}$) identifiable with the surface nucleation field (1.695 $H_{c2\mathrm{}}$). Evidence is presented that indicates the observed magnetization results from partial shielding of the interior of the cylinders by critical currents induced in the multiply-connected surface sheath. Our experimental results are in quantitative accord with recent theoretical calculations of Fink and Barnes concerning the influence of critical sheath currents on the effective bulk-magnetization characteristics of a cylinder. It is observed that diamagnetic ($M<\mathrm{}0\mathrm{}$) and paramagnetic ($M>\mathrm{}0\mathrm{}$) shielding occur in uniformly increasing and decreasing fields, respectively, which is consistent with the existence of a multiconnected superconducting region which can carry a total current. The surface localization of the superconductivity involved is demonstrated by the proximity effects of electroplated normal metals on the specimens. Cu plating reduces both the magnetization and $H_{c3\mathrm{}}$ (∼1.4 $H_{c2\mathrm{}}$). Owing, perhaps, to their magnetic properties, electroplated Ni and Cr have a much more drastic effect than electroplated Cu. To the limit of experimental sensitivity, all traces of superconductivity above $H_{c2\mathrm{}}$ are removed by Ni and Cr platings.