Superconductivity fluctuations in electrical and thermoelectrical properties of granular ceramic superconductors: Homogeneous versus fractal behavior

Both the electrical-resistivity and the thermoelectric-power temperature-derivative diver- gences are examined in typical granular ceramic superconductors (YBa2Cu3O7-y and Bi1.75Pb0.25Ca2Sr2Cu3O10-z). Critical exponents are extracted in the region above the critical temperature. The dimensionality of anomalous superconductivity fluctuations is deduced. The same universal behavior is found for the excess resistivity and the excess thermoelectric power in a given system. It is shown that a homogeneous regime and a self-similar (fractal) behavior can be distinguished depending on the samples. The results are discussed in terms of superconductivity paths occurring through surface connections and of different values of the superconductivity S and percolation P coherence-length ratio