Structure of the Intermediate State in Superconductors

A niobium powder method has been used to display the arrangement of normal and superconducting domains in the intermediate state of a superconductor. Patterns have been observed on samples of tin, indium, lead, vanadium, and tantalum, and have been studied in some detail for tin. It has been found possible to produce patterns which are plane parallel lamina in a flat plate. From their spacing the surface energy at a superconducting-normal interface was evaluated. For tin, the surface energy parameter, $\Delta$, is of the order of 3×${10}^{-5\mathrm{}}$ cm. Both the magnitude of $\Delta$ and its variation with temperature are consistent with theories of its origin advanced by Ginsburg and Landau, Bardeen, and Lewis. Similar results were obtained for indium and lead, but hard superconductors like tantalum, and especially vanadium, give coarser patterns, indicating a very much larger surface energy.