Resistive transition in disordered superconductors with varying intergrain coupling

Abstract

The effect of disorder is investigated in granular superconductive materials withstrong- and weak-links. The transition is controlled by the interplay of the tunnelingg andintragrain gintr conductances, which depend on the strength of the intergrain coupling. For , the transition first involves the grain boundary, while forg ∼ gintr the transition occurs into the whole grain. The different intergrain couplings are consideredby modeling the superconducting material as a disordered network of Josephson junctions.Numerical simulations show that on increasing the disorder, the resistive transition occursfor lower temperatures and the curve broadens. These features are enhanced indisordered superconductors with strong-links. The different behavior is further checkedby estimating the average network resistance for weak- and strong-links in theframework of the effective medium approximation theory. These results may shedlight on long standing puzzles such as: (i) enhancement of the superconductingtransition temperature of many metals in the granular states; (ii) suppressionof superconductivity in homogeneously disordered films compared to standardgranular systems close to the metal–insulator transition; (iii) enhanced degradation ofsuperconductivity by doping and impurities in strongly linked materials, such asmagnesium diboride, compared to weakly linked superconductors, such as cuprates.