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.

The characterization and pyrolysis behavior of a set of pitches prepared from anthracene oil have been described. The pitches were obtained from four successive cycles of a sequential process that begins with blowing air through the heated anthracene oil, to bring about recombination reactions. Reaction products are distilled to give a pitch residue and a lighter fraction. Thermal treatment/distillation cycles of this reaction product yield a pitch and a distillate fraction (unreacted anthracene oil) during each subsequent stage. Products obtained during the process have been characterized by elemental analysis, Fourier transform infrared (FTIR) and ultraviolet (UV)-fluorescence spectroscopy, and size-exclusion chromatography (SEC). The pyrolytic behavior of the anthracene oil derivatives was examined using a thermogravimetric balance. Thermal treatment of the anthracene oil and its (distilled) reaction products at 440−460 °C under 5 bar pressure leads to a partially anisotropic pitch with the formation of a liquid crystal phase (mesophase). The formation and evolution of these mesophases were analyzed by optical microscopy.

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