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 Original Research Articl

In-plane and out-of-plane resistivity versus temperature measurements have been carried out on different kinds of as-grown Bi-2212 crystals: ρab(T) and ρc(T); ρc(T) has been specially used for determination of the 2D or 3D character of superconductive fluctuations. For weakly oxidized crystals, 2D fluctuations exist in a large temperature range (30–35 K) and 3D fluctuations exist only in a narrow temperature range (0–5 K). On the contrary, for highly oxidized crystals (with a metallic out-of-plane conductivity) 3D fluctuations exist in a large temperature range (30 K). For weakly oxidized crystals, ρc(T) increases, with decreasing temperature, quicker than expected from the Arrhenius law (anomalous high resistivity along c). Other authors have modelized this anomalous resistivity introducing electron scattering by Cooper pairs in the mixed state, but the fit is rather poor, while a Vogel–Fulcher model gives a good fit in a large temperature range (200 K). So, another origin for anomalous high resistivity along c is suggested: an increase, of chemical origin, of the Fermi level with decreasing temperature.