Some Surface Current Phenomena in Type II Superconductors

Abstract

For a cylindrical sample of an imperfect type II superconductor in an external field H_ext parallel to its axis, it is found that in the mixed state no flux enters or leaves the specimen within an interval H_ext±ΔH; the positive or negative sign applies when H_ext is reached in a decreasing or increasing field, respectively. Similarly, when the sample is cooled in a field H_ext, an increase of the external field less than ΔH does not change the trapped flux. ΔH depends only on H_ext and not on B, the bulk flux density, which itself depends on the magnetic history of the specimen and its defect structure (flux pinning, bulk critical current). ΔH is affected by various surface treatments, such as machining and chemical polishing, but not significantly by metal plating. The results can be explained in terms of an intrinsic surface critical current, and comparison with existing theoretical calculations yields reasonable agreement.