Thermal fluctuation effects on the magnetization above and below the superconducting transition in Bi2Sr2CaCu2O8 crystals in the weak magnetic field limit

Abstract

We present detailed experimental data of the magnetization, Mab(T,H), of Bi2Sr2CaCu2O8 crystals on both sides of the superconducting transition, for magnetic fields, H, applied perpendicularly to the ab (CuO2) planes and for amplitudes up to μ0H=5 T, which not too close to the superconducting transition correspond to the weak magnetic field amplitude limit. These data are analyzed in terms of thermal fluctuations in this weak H limit: In the reversible mixed state below the transition, by taking into account the fluctuations of the vortex lines positions, as first proposed by Bulaevskii, Ledvij, and Kogan. Above the transition, by taking into account the Cooper pairs created by thermal fluctuations, through a generalization of multilayered superconductors of the Schmidt-like approach. These simultaneous, quantitative and consistent analyses of Mab(T,H) above and below the transition allow us to estimate the effective number of independent fluctuating superconducting CuO2 planes in the periodicity length s=c/2, c being the unit-cell length, and to separate for the first time the in-plane correlation length amplitude, ξab(0), and the parameter related to the vortex structure, η. We found ξab(0)=(0.8±0.1) nm and η=0.15±0.05, this last value being well within the one calculated by Fetter by applying the London model to a triangular vortex lattice. For the in-plane magnetic penetration depth, we found a temperature behavior compatible with the clean BCS weak coupling limit, and an amplitude (at T=0 K) of λab(0)=(180±20) nm. © 1996 The American Physical Society.

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