THERMAL FLUCTUATION EFFECTS ON THE MAGNETIZATION ABOVE AND BELOW THE SUPERCONDUCTING TRANSITION IN BI2SR2CACU2O8 CRYSTALS IN THE WEAK MAGNETIC-FIELD LIMIT

We present detailed experimental data of the magnetization, M(ab)(T,H) , of Bi2Sr2CaCu2O8 crystals on both sides of the superconducting trans ition, for magnetic fields, H, applied perpendicularly to the ab (CuO2 ) planes and or amplitudes up to mu(0)H=5 T, which not too close to th e superconducting transition correspond to the weak magnetic field amp litude limit. These data are analyzed in terms of thermal fluctuations in this weak H limit: In the reversible mixed state below the transit ion, by taking into account the fluctuations of the vortex lines posit ions, as first proposed by Bulaevskii, Ledvij, and Kogan. Above the tr ansition, by taking into account the Cooper pairs created by thermal f luctuations, through a generalization of multilayered superconductors of the Schmidt-like approach. These simultaneous, quantitative and con sistent analyses of M(ab)(T,H) above and below the transition allow us to estimate thr effective number of independent fluctuating supercond ucting CuO2 planes in the periodicity length s=c/2, c being the unit-c ell length, and to separate for the first time the in-plane correlatio n length amplitude, xi(ab)(0), and the parameter related to the vortex structure, eta. We found xi(ab)(0)=(0.8+/-0.1) nm and eta=0.15+/-0.05 , this last value being well within the one calculated by Fetter by ap plying thr London model to a triangular vortex lattice. For the in-pla ne magnetic penetration depth, we found a temperature behavior compati ble with the clean BCS weak coupling limit, and an amplitude (at T = 0 K) of lambda(ab)(0) = (180+/-20) nm.