Upper critical field of granular SnGe samples near the superconductor-to-insulator transition

Abstract

We have measured the upper critical field, H-c2(T), in granular SnGe samples with normal state resistivities, rho(N), that range from 0.3 m Omega cm to 0.5 Omega cm. The samples demonstrate an evolution from superconducting to insulating in transport with increasing disorder. The temperature dependence of H-c2 shows a clear change in curvature between high rho(N) and intermediate rho(N) samples differing from the experimental results by Deutscher et al.. Our data is analyzed in terms of Ginzburg-Landau theory and grain structure induced disorder effect. The result is similar to the observation in high T-c superconductors that anisotropy induced fluctuation effect play an important role.