Pairing and coherence transitions in granular GdBa2Cu3O7-delta

We report on fluctuation magnetoconductivity measurements in a granular sam ple of GdBa2Cu3O7-delta. 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. T he critical thermodynamics for this transition is that of the 3D-XY univers ality class, with dynamic exponent z = 1.5. At a lower temperature occurs t he coherence transition, where the fluctuating phases of individual grains become long-range ordered and a zero resistance state sets in. (C) 2001 Els evier Science BN. All rights reserved.