Use of out-of-plane resistivity for determination of the 2D or 3D character of superconductive fluctuations for Bi-2212 crystals in the mixed state and origin of anomalous high out-of-plane resistivity for weakly oxidized crystals

In-plane and out-of-plane resistivity versus temperature measurements have been carried out on different kinds of as-grown Bi-2212 crystals: rho(ab)(T ) and rho(c)(T); rho(c)(T) has been specially used for determination of the 2D or 3D character of super conductive fluctuations. For weakly oxidized c rystals, 2D fluctuations exist in a large temperature range (30-35 K) and 3 D fluctuations exist only in a narrow temperature range (0-5 K). On the con trary, for highly oxidized crystals (with a metallic out-of-plane conductiv ity) 3D fluctuations exist in a large temperature range (30 K). For weakly oxidized crystals, rho(c)(T) increases, with decreasing temperature, quicke r than expected from the Arrhenius law (anomalous high resistivity along c) . Other authors have modelized this anomalous resistivity introducing elect ron scattering by Cooper pairs in the mixed state, but the fit is rather po or, while a Vogel-Fulcher model gives a good fit in a large temperature ran ge (200 K). So, another origin for anomalous high resistivity along c is su ggested: an increase, of chemical origin, of the Fermi level with decreasin g temperature. (C) 2000 Elsevier Science Ltd. All rights reserved.