Fluctuation conductivity in NdBa2Cu3O7-delta and REBa2Cu3O7-delta superconductors where RE is a natural mixture of rare earth elements

We have studied fluctuations on the electrical conductivity of granular sam ples of NdBa2Cu3O7-delta and RE-Ba2Cu3O7-delta superconductors, wherein RE stands for a natural mixture of rare earth elements. Special attention is t aken above T-c, where Gaussian and critical regimes are observed. Far from T-c, we can observe the regime dominated by Gaussian fluctuations, with exp onent approximately lambda ((3D))(G) congruent to 0.50, which is the predic ted one for homogeneous 3D fluctuations. Inside the critical region, we hav e observed two power-law regimes, one of them consistent with predictions f or the 3D-XY universality class with exponent lambda ((1))(cr) congruent to 0.34 +/- 0.02. Below T-c, on approaching the zero resistance state, our re sults show a power-law behavior with critical exponents s = 2.8 +/- 0.1 for NdBa2Cu3O7-delta and s = 4.0 +/- 0.4 for REBa2Cu3O7-delta. We have previou sly shown that this corresponds to a phase transition from a paracoherent t o a coherent state of the granular array. (C) 2001 Elsevier Science B.V. Al l rights reserved.