Influence of magnetic field on electronic conduction of (Bi,Pb)–Sr–Ca–Cu–O granular superconductors

Granular superconductors are composed of superconducting grains between a few and hundreds nanometres in size, embedded in an insulating, semiconducting or metallic matrix. Their properties are very interesting thanks to the presence of Coulomb effects, electron tunnelling and Josephson coupling between granules. The presence of a magnetic field may cause either a decrease or increase in the material's resistivity.In this paper the electrical properties of (Bi,Pb)–Sr–Ca–Cu–O granular superconductors obtained by the solid state crystallization method were studied. The granules were in the range from 10 nm to 30 nm. All the studied materials below the transition temperature contained granules in the superconducting state. They showed positive magnetoresistance. The hopping conductivity model with the exponential temperature dependence of resistivity was applied. The obtained values of the exponents were lower than those typical for granular systems, but they increased with an increase in the magnetic field.