HOMOGENEOUS AND FRACTAL BEHAVIOR OF SUPERCONDUCTING FLUCTUATIONS IN THE ELECTRICAL-RESISTIVITY OF GRANULAR CERAMIC SUPERCONDUCTORS

Experimental data show that the electrical-resistivity temperature derivative diverges in a different manner in the granular ceramic superconductors YBa2Cu3O7-y and Bi1.75Pb0.25Ca2Sr2Cu3O10-y. Critical exponents are extracted and explained. The dimensionality of anomalous superconductivity fluctuations is deduced. A homogeneous case and a self-similar (fractal) behavior can occur and are found. They correspond to different values of the superconductivity s and some percolation p coherence-length ratio. The results indicate that superconductivity percolation is achieved through surface connection rather than through path connection. © 1990 The American Physical Society.