Numerical studies on the effect of normal-metal coatings on the magnetization characteristics of type-II superconductors.

Abstract

Magnetic properties of superconductors coated with metals of arbitrary resistivity rho(N) are calculated using the time-dependent Ginzburg-Landau equations in which both T-c and rho(N) vary. As rho(N) in the coating is reduced, the initial vortex penetration field H-p(rho(N)) does not decrease monotonically from the insulating (Matricon) limit to the extreme metallic (Bean-Livingston) limit, but has a minimum value H-p(min) below the extreme metallic value. The minimum occurs because the barrier is weakened by proximity-effect penetration of superelectrons into the coating which only occurs at finite resistivity. In an applied magnetic field, local depressions in psi nucleate in the coating which do not have the well-known quantum of magnetic flux (h/2e) until they have crossed the coating and entered the interior of the superconductor. When T=0 and T-c of the normal metal coating is zero, the minimum vortex penetration field H(p(min))approximate to 0.76 kappa(-1.17)H(c2) which occurs for a coating resistivity rho(N)approximate to 1.1 kappa(-0.6)rho(S). For T>0 the minimum is attenuated. Adding a thick weakly superconducting S' layer between the superconductor and normal metal coating reduces the irreversibility markedly.