Pairing transition, coherence transition, and the irreversibility line in granular GdBa2Cu3O7-delta

We report on electrical magnetoconductivity experiments near the supercondu cting transition of a granular sample of GdBa2Cu3O7-delta. The measurements were performed in magnetic fields ranging from 0 to 500 Oe applied paralle l to the current orientation. The results show that the transition proceeds in two steps. When the temperature is decreased we first observe the pairi ng transition, which stabilizes superconductivity within the grains at a te mperature practically coincident with the hulk critical temperature T-c. An alysis of the fluctuation contributions to the conductivity shows that the universality class for this transition is that of the three dimensional (SD )-XY model in the ordered case, with dynamic critical exponent z=3/2. Close to the zero-resistance state, the measurements reveal the occurrence of a coherence transition, where the phases of the order parameter in individual grains become long-range ordered. The critical temperature T-co for this t ransition is close to the point where the resistivity vanishes. A strong en largement of the fluctuation interval preceding the coherence transition is caused by the applied magnetic held. In this region, a 3D-Gaussian regime and an asymptotic critical regime were clearly identified. The critical con ductivity behavior for the coherence transition is interpreted within a 3D- XY model where disorder and frustration are relevant. The irreversibility l ine is determined from magnetoconductivity measurements performed according to the zero-field-cooled (ZFC) and held-cooled data collected on cooling ( FCC) recipes. The locus of this line coincides with the upper temperature l imit for the fluctuation region above the coherence transition. The irrever sibility line is interpreted as an effect of the formation of small cluster s with closed loops of Josephson-coupled grains.