Intermediate State of Thin Superconductors

The purpose of this study was to examine the state of a thin type-I superconductor in a magnetic field to determine its dependence on the specimen thickness and on the value of the Ginzburg-Landau parameter kappa. Three materials (aluminum, indium, and In0.99Pb0.01) with values of the Ginzburg-Landau parameter ranging from 0.19 to 0.34 were studied. Specimen thicknesses ranged from 200 to 200,000 angstrom. All of the materials studied were type-I or nonlocal superconductors in the bulk. Yet, it has been predicted that they would behave in ways characteristic of type-II or local superconductors if the specimen were sufficiently thin. For intermediate thicknesses the specimens were expected to be in one of many possible states. We have inferred from critical field studies that the structure of the intermediate state in thin type-I superconductors is equivalent to the type-II vortex state for very thin films (d << xi(0)), and to the type-I macroscopic domain state for very thick films d greater than or similar to 2 xi(t). For thicknesses between these limits the intermediate-state structure takes on many forms as the area of each normal domain and the amount of flux threading it increases with increasing thickness.