Conductivity and Superconductivity in Granular Materials

Granular metals and superconductors are very interesting materials thanks to their untypical electrical properties caused by the presence of the Coulomb effects, electron and Cooper pair tunnelling and various aspects of disorder. Most typical and widely studied representatives of this group consist of low $T_c$ superconductor granules distributed in insulating matrix (e.g. lead in germanium matrix). In comparison with them a system of granules of high-$T_c$ superconductors embedded in an insulating matrix is more difficult to be experimentally realised. (Bi, Pb)-Sr-Ca-Cu-O materials obtained with solid state crystallisation method may be considered as an example of granular and disordered metals and superconductors. Solid state crystallisation of initially amorphous material leads to formation of 2201 and 2212 granules embedded in the insulating or semiconducting matrix. Further annealing causes increase in size and/or the number of conducting grains and decrease in the width of the insulating barriers between them. In both cases of low- and high-$T_c$ granular materials the main parameters, which determine their properties, are the amount of metallic phases and the tunnelling conductivity between the neighbouring grains. In this work some properties of low- and high-$T_c$ granular materials are discussed and compared.