Temperature dependence of the superconducting giant-vortex state. Theory and experiment

Abstract

When a type-I superconductor with a surface nucleation field H/sub c/3(T) > H/sub c/(T) (thermodynamic critical field) is thermally cycled in an axially applied magnetic field H/sub 0/ between the temperatures T (H/sub c/3) and about T (H/sub c/), experiments show that the magnetization changes reversibly. The latter is diamagnetic near T (H/sub c/3) but can be paramagnetic just above T (H/sub c/). This behavior is explained by assuming that the fluxoid quantum number b is fixed at the transition from the normal to the superconducting state and retained at lower temperatures. The value of b is determined almost entirely by the flux at the transition which is enclosed by a contour located at a distance xi/1.7 from the surface inside the cylinder (xi is the coherence length). The temperature variation of the order parameter f at the surface of the cylinder, the magnetization m and the temperature at which m = 0 for f not = 0 are calculated for R >> xi. Conservation of the fluxoid quantum number while T is varied causes the two opposing surface currents to become imbalanced. This is the source of the observed para- and diamagnetism.