FLUCTUATION-INDUCED INPLANE CONDUCTIVITY, MAGNETOCONDUCTIVITY, AND DIAMAGNETISM OF BI2SR2CACU2O8 SINGLE-CRYSTALS IN WEAK MAGNETIC-FIELDS

We report detailed experimental results on the fluctuation-induced in- plane conductivity Delta sigma(ab), magnetoconductivity Delta<(sigma)o ver tilde>(ab), and diamagnetism Delta chi(ab), of high-quality Bi2Sr2 CaCu2O8 crystals. The data were obtained with magnetic fields H applie d perpendicularly to the superconducting (CuO2) planes and up to mu(0) H=5 T, which not too close to the transition [for reduced temperatures epsilon=(T-T-C0)/T(C0)greater than or similar to 10(-2)] may be consi dered in the weak magnetic field limit. In the mean field region (MER) above the transition, these data are analyzed in terms of thermal flu ctuations of the superconducting order parameter amplitude (OFF), on t he grounds of the existing theoretical approaches for layered supercon ductors that take into account the presence of two superconducting lay ers in the layer periodicity length, s, which for these compounds is e qual to one-half the crystallographic unit-cell length in the c direct ion. These results show that, due to its strong epsilon dependence, De lta<(sigma)over tilde>(ab) is dramatically affected by the presence of small T-C inhomogeneities, associated with small oxygen content inhom ogeneities uniformly distributed in the crystals. These inhomogeneity effects are taken into account, consistently with our Delta sigma(ab) and Delta chi(ab) results, by using an effective medium approach propo sed by Maza and Vidal. In this way, the amplitude and the epsilon beha vior of the three observables studied here are explained in terms of t he direct OFF effects, at a quantitative level, confirming then the ab sence of appreciable indirect contributions [as, for instance, the Mak i-Thompson and the density-of-states (DOS) terms]. These last results may suggest unconventional (non (1)s(0)), pair breaking, wave pairing in these compounds, as first proposed from OFF analyses by Veira and V idal. The resulting values of the in-plane and out-of-plane coherence length amplitudes are, respectively, xi(ab)(0)=(0.9+/-0.1) nm and xi(c )(0)less than or similar to 0.05 nm. These coherence length amplitudes are consistent with the values that we have obtained before for other Bi-based crystals by analyzing the effects of the vortex position flu ctuations on the magnetization below the transition. These results als o confirm at a quantitative level that in Bi-2212 compounds the effect ive number of fluctuating CuO2 planes per periodicity length above the superconducting transition is N-e approximate to 2 and that the OPF's are essentially two dimensional over the entire MFR.