Fluctuation-induced conductivity of polycrystalline Gd$_{1 - x}$Ce$_{x}$Ba$_{2}$Cu$_{3}$O$_{7 - \delta }$ superconductor

Abstract

This paper reports on measurements of the resistive transition in polycrystalline Gd$_{1 - x}$Ce$_{x}$Ba$_{2}$Cu$_{3}$O$_{7 - \delta }$ samples ($x = 0.000$, 0.025 and 0.050). The samples were produced by a standard solid state reaction method and in two different thermal routes. The microstructure was analyzed by X-ray diffraction. The samples were considered homogeneous since no extra peaks due to impurity phase were observed. To identify power-law divergences in conductivity, the results were analyzed in terms of the temperature derivative of the resistivity and the logarithmic temperature derivative of the conductivity ($d\ln \left( {\Delta \sigma } \right) / dT$). From the results, the occurrence of a two-stage transition besides the pairing transition splitting for sample with $x = 0.050$ was observed. Such splitting was associated with Ce doping and related with the occurrence of a phase separation. In the normal phase, Gaussian and critical fluctuations conductivity regimes were identified. On approaching the zero resistance state, our results showed a power-law behavior that corresponds to a phase transition from a paracoherent to a coherent state of the granular array.