No abstract

By using two randomly oriented polycrystalline YBa2Cu3O7 − δ samples with masses as big as 0.63 g and 0.90 g, but almost optimally doped (Tc0 ≃ 90.8 K and 92.0 K) and with excellent stoichiometric homogeneity, the in-plane fluctuation-induced diamagnetism was determined, for the first time in any superconductor, well inside the so-called short wavelength regime in the zero-magnetic-field limit, which corresponds to reduced temperatures, ϵ ≡ ln (T/Tc0), above typically ϵ = 0.1. It is then shown that these measurements may be explained in terms of the Schmidt limit of the Gaussian-Ginzburg-Landau approach for bilayered superconductors by introducing a total-energy cut-off in the fluctuation spectrum.

No abstract

We report detailed experimental results on the fluctuation-induced in-plane conductivity ⌬ ab , magnetoconductivity ⌬ ab , and diamagnetism ⌬ ab , of high-quality Bi 2 Sr 2 CaCu 2 O 8 crystals. The data were obtained with magnetic fields H applied perpendicularly to the superconducting ͑CuO 2 ͒ planes and up to 0 Hϭ5 T, which not too close to the transition ͓for reduced temperatures ϵ(TϪT C0 )/T C0 տ10 Ϫ2 ͔ may be considered in the weak magnetic field limit. In the mean field region ͑MFR͒ above the transition, these data are analyzed in terms of thermal fluctuations of the superconducting order parameter amplitude ͑OPF͒, on the grounds of the existing theoretical approaches for layered superconductors that take into account the presence of two superconducting layers in the layer periodicity length, s, which for these compounds is equal to one-half the crystallographic unit-cell length in the c direction. These results show that, due to its strong dependence, ⌬ ab is dramatically affected by the presence of small T C inhomogeneities, associated with small oxygen content inhomogeneities uniformly distributed in the crystals. These inhomogeneity effects are taken into account, consistently with our ⌬ ab and ⌬ ab results, by using an effective medium approach proposed by Maza and Vidal. In this way, the amplitude and the behavior of the three observables studied here are explained in terms of the direct OPF effects, at a quantitative level, confirming then the absence of appreciable indirect contributions ͓as, for instance, the Maki-Thompson and the density-of-states ͑DOS͒ terms͔. These last results may suggest unconventional ͑non 1 s 0 ͒, pair breaking, wave pairing in these compounds, as first proposed from OPF analyses by Veira and Vidal. The resulting values of the in-plane and out-of-plane coherence length amplitudes are, respectively, ab ͑0͒ϭ͑0.9Ϯ0.1͒ nm and c ͑0͒Շ0.05 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 fluctuations on the magnetization below the transition. These results also confirm at a quantitative level that in Bi-2212 compounds the effective number of fluctuating CuO 2 planes per periodicity length above the superconducting transition is N e Ϸ2 and that the OPF's are essentially two dimensional over the entire MFR. ͓S0163-1829͑96͒07833-2͔

We report high-resolution data of the electrical resistivity and of the magnetic susceptibility rounding effects above the superconducting transition in single-phase polycrystalline YBa,Cu,O,.* compounds. These data may be explained quantitatively and consistently, over two orden of magnitude in reduced temperature, by direct three-dimensional thermodynamic fluctuations of the amplitude of a two-component order parameter in unconventional (non-rwave pairing) superconductors. The resulting order parameter correlationlengthamplitudesareEab(0) = ( U + 3)A,andE,(0) = (1.4 +0.4).&, theLawrence-Doniach effective interlayer distance is d. = (8 3' 2) A, and the Ginzburg reduced temperature is = (0.7 * 0.3) X

No abstract

No abstract

We present measurements of pool boiling heat transfer from ceramic Y,Ba,Cu,O,_, to liquid nitrogen in the range of heat flux density up to q. = 35 W cm-'. Along the convective pool boiling regime, a linear relationship between the interface overtemperature, T w , andq, has been observed, with a value of the heat transfercoefficienth = 1.3 W cm-' K-'. In the nucleate regime, the q. dependence on T, follows quite well a power law, with an exponent close to 0.7. In the low heat flux regime (9, 5 5 W cm-'), the heat transfer curve shows a double hysteresis, which seems to be closely related to the porosity of the samples. Experimental set-up and procedureCeramic Y,Ba,Cu,O,_, samples, with 6 0.1, were prepared using the usual solid state synthesis procedure.