Near-Surface Effects in Superconducting Niobium-Zirconium Alloys

Abstract

At a given magnetic field, critical currents may be induced to flow in the surface layers of a superconductor by the application of a relatively small‐amplitude alternating‐current field. Many measurements of this induced current have been made as a function of the intensity of the direct‐current field, and the frequency and amplitude of the alternating‐current field. The results of these measurements may be interpreted by assuming that there is a near‐surface layer of a few hundred angstroms thickness which is able to carry extraordinary current densities from 3 to 20 times the bulk current densities at a given field. Current densities calculated for this surface layer exceed 10⁷ A/cm² at low fields. Details of the observations vary with sample history, but the general features of these measurements have been observed in all samples tested.