Electronic conduction in (Bi,Pb)–Sr–Ca–Cu–O granular superconductors

Granular superconductors are very interesting materials owing to their untypical electrical properties caused by the presence of Coulomb effects, electron tunnelling, Josephson coupling between granules and various aspects of disorder. The electrical properties of (Bi,Pb)–Sr–Ca–Cu–O granular superconductors obtained by the solid state crystallization method have been studied in this paper. The materials may be considered as a system of granules of a high-temperature suprconductor embedded in the insulating matrix. The main parameter which determines the properties of granular materials is the tunnelling conductivity between the neighboring grains. Composed of small weakly coupled granules they are characterized by hopping conductivity exponential dependence. Samples containing relatively large granules of the 2212 phase (above 30 nm), with larger inter-grain conductivity, above the transition temperature, exhibit a logarithmic temperature dependence of resistivity.