We first argue that the collective behaviour of the Cooper pairs created by thermal fluctuations well above the superconducting transition temperature, T C , is dominated by the uncertainty principle which, in particular, leads to a welldefined temperature, T C , above which the superconducting coherence vanishes. On the grounds of the BCS approach, the corresponding reduced-temperature, ε C ≡ ln(T C /T C ), is estimated to be around 0.55, i.e., above T C ≃ 1.7 T C coherent Cooper pairs cannot exist. The implications of these proposals on the superfluid density are then examined using the Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the thermal fluctuation effects on the electrical conductivity and on the magnetization in different lowand high-T C superconductors with different dopings which are in excellent agreement with these proposals and that demonstrate the universality of ε C . 74.20.-z Theories and models of superconducting state 74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.) 74.40.+k Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.)

By using high quality single crystals and epitaxial thin films, the in-plane paraconductivity in almost optimally doped YBa 2 Cu 3 O 7Ϫ␦ , with T c0 տ92 K, was determined well inside the so-called short-wavelength fluctuation regime, which corresponds to reduced temperatures, ⑀ϵln(T/T c0 ), above typically ⑀ϭ0.1. It is then shown that these data may be explained in terms of the Gaussian-Ginzburg-Landau approach for bilayered superconductors by introducing a total energy cutoff, instead of the momentum cutoff approximation always used until now. These results seem to confirm the absence of appreciable pseudogap effects on the in-plane resistivity in optimally doped YBa 2 Cu 3 O 7Ϫ␦ superconductors.

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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.

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