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

Abstract

When a type-1 superconductor with a surface nucleation field Hc2 ( T) > HC( T ) (thermodynamic critical field) is thermally cycled in an axially applied magnetic field H0 between the temperatures T(Hc3) and about T ( H2), experiments show that the magnetization changes reversibly. The latter is diamagnetic near T (Hc3) but can be paramagnetic just above T ( H2). 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 ξ /1.7 from the surface inside the cylinder (ξ is the coherence length).The temperature variation of the order parameter / at the surface of the cylinder, the magnetization m, and the temperature at which m = 0 for / ^ 0 are calculated for R » ξ 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.