Abstract
Systematic resistivity measurements in polycrystalline samples of the high- T c oxide superconductors RBa 2Cu 3O 7 (R = Y, Eu, Gd) are performed with several (low) current densities. Special attention is devoted to the temperature range close to the superconducting transition. The data allows the determination of temperature derivatives of the resistivity, which reveal the occurence of a two-step transition. This feature is typical of a granular system. Two scaling regimes for the fluctuation conductivity are also identified. Near the temperature where the grains become superconducting, the enhancement of the conductivity follows a power law with sample dependent exponents. Analysis of this result on the basis of the Aslamazov-Larkin theory indicates that a fractal topology might be adequate to describe the inhomogeneous superconducting state. In the second scaling regime, near the zero-resistance temperature, an apparently universal exponent is observed.
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