Resistive transition in disordered superconductors with varying intergrain coupling

Abstract

The effect of disorder is investigated in granular superconductive materials with strong and weak links. The transition is controlled by the interplay of the \emph{tunneling} $g$ and \emph{intragrain} $g_{intr}$ conductances, which depend on the strength of the intergrain coupling. For $g \ll g_{intr}$, the transition involves first the grain boundary, while for $g \sim g_{intr}$ the transition occurs into the whole grain. The different intergrain coupling is considered by modelling the superconducting material as a disordered network of Josephson junctions. Numerical simulations show that on increasing the disorder, the resistive transition occurs for lower temperatures and the curve broadens. These features are enhanced in disordered superconductors with strong links. The different behaviour is further checked by estimating the average network resistance for weak and strong links in the framework of the effective medium approximation theory. These results may be relevant to shed light on long standing puzzles as: (i) enhancement of the superconducting transition temperature of many metals in the granular states; (ii) suppression of superconductivity in homogeneously disordered films compared to standard granular systems close to the metal-insulator transition; (iii) enhanced degradation of superconductivity by doping and impurities in strongly linked materials, such as magnesium diboride, compared to weakly-linked superconductors, such as cuprates.