Hysteresis in Superconductors. III. Effect of Surface Energies on the Shielding and Field-Retaining Properties of Hollow Cylinders of Type-I Superconductors

Abstract

The diamagnetic and field-retaining properties of thin-walled type-I hollow cylinders can be changed by more than an order of magnitude through the introduction of surface strains. High-purity hollow cylinders of Pb and Sn in which the surface strains are minimal shield external fields of only a few percent of $H_c$ and do not retain appreciable fields in the void when an external field exceeding $H_c$ is removed isothermally. The shielding and field-retaining properties are influenced by localized phenomena. The introduction of local surface strains increases the local shielding properties during the application of an external field and leads to the retention of large axial field gradients in the void after the applied field is removed. Fields of both senses can be retained in the void simultaneously when cycles are performed involving both senses of the applied field. Some experiments are summarized showing spontaneous field enhancement when type-I superconductors are cooled in applied fields. A number of unexpected reversibility relations are associated with these phenomena. The effects can be accounted for by including surface and interface energy terms in the thermodynamic state function of the superconductor. An example of how hysteresis arises from equilibrium changes is described.