Abstract
We have experimentally studied the electrical conductivity near the normal-superconducting transition of two $\text{Y}_{0.25}\text{Gd}_{0.25}\text{Er}_{0.25}\text{Nd}_{0.25}\text{Ba}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ polycrystalline samples with different oxygen contents. The oxygen content for one of the samples is close to the optimum. The oxygen content of the other sample is slightly less than the optimum. The results show that resistive transition proceeds in two stages, as shown by the temperature derivative of the resistivity near $T_{C}$ . Fluctuations in electrical conductivity were investigated by the determination of the logarithmic temperature derivative of the conductivity. In the normal phase, i.e., above $T_{C}$ , Gaussian fluctuation regimes were identified. On approaching the zero-resistance state we observed, for both samples, a power-law behavior that corresponds to a phase transition from a paracoherent to a coherent state of the granular array. In general, our results could suggest that partial substitution of Y by Gd, Er, and Nd, in polycrystalline $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ , produces or magnifies granularity effects at microscopic length scale in a manner relevant to the criticality. In addition, the results indicate that the degree of oxygen is relevant below $T_{C}$ , where predominate the effects related to the grain coupling phenomenology.
Published Version
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