Pairing and coherence transitions in granular GdBa2Cu3O7-δ

We report on fluctuation magnetoconductivity measurements in a granular sample of GdBa2Cu3O7-δ. The experiments show that the resistive transition is a two-stage process in this sample. The pairing transition occurs at a higher temperature, where superconductivity is established in the grains. The critical thermodynamics for this transition is that of the 3D-XY universality class, with dynamic exponent z=1.5. At a lower temperature occurs the coherence transition, where the fluctuating phases of individual grains become long-range ordered and a zero resistance state sets in