[en] We work out a detailed magnetoresistance, superconducting fluctuation and magnetic irreversibility study of an Y_0.95Pr_0.05Ba₂Cu₃O_7-δ single crystal and polycrystalline samples. Our data reveal, besides the usual magnetic irreversibility regimes related to the intergrain and intragrain flux dynamics, a whole set of fluctuation regimes and two close genuine superconducting transitions. We suggest that this split superconducting transition is an intrinsic effect related to the condensation of two independent superconducting fluids, presumably having different orbital symmetries

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[en] In this work we report on conductivity fluctuation measurements in polycrystalline samples of the Er_1-xPr_xBa₂Cu₃O_7-δ superconductor (x =0.00, 0.05 and 0.10). Samples were prepared by a standard solid state reaction technique. The results were analyzed in terms of the temperature derivative of the resistivity and of the logarithmic temperature derivative of the conductivity that allowed the identification of power-law divergences of the conductivity. The results show that the transition proceeds in two stages: pairing and coherence transition. Also, our results, from the critical exponent analysis, show a two-peak splitting at pairing transition, indicating possibly a phase separation. On approaching the zero resistance state, our results show a power-law behavior that corresponds to a phase transition from para coherent to a coherent state of the granular array. (author)

[en] The discovery of the spatial uniform coexistence of superconductivity and ferromagnetism in ruthenocuprates, RuSr₂GdCu₂O₈ (Ru-1212), has spurred an extraordinary development in the study of the competition between magnetism and superconductivity. However, several points of their preparation process and characterization that determine their superconductive behaviour are still obscure. The improvement of sample preparation conditions involves some thermal treatments in inert atmosphere. Anelastic spectroscopy measurements were made using an inverted torsion pendulum, operating with an oscillation frequency of 38 Hz, temperature in the 90 and 310 K range, heating rate of 1 K/min, and vacuum better than 10^-3 Pa. The results show anelastic relaxation peaks at 210 K related to the presence of interstitial oxygen atoms. The peaks decrease significantly with the oxygen loss caused by the heat treatments in vacuum, appearing again after the annealing of the sample in an oxygen atmosphere. These observed peaks are clearly related to the additional oxygen atoms, with activation energy 0.13 and 0.36 eV, and can be explained in terms by diffusional jumps of interstitial oxygen in the RuO₂ planes.

[en] Systematic conductivity measurements near T_c in granular (Bi,Pb)-Sr-Ca-Cu-O superconductors are presented, and focus is given on the interplay between thermal fluctuations and disorder at micro- and mesoscopic levels. Experiments show that the resistive transition is a two-step process. In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. Both are affected by local disorder. Particularly, the critical regime is characterized by a power law with exponent λ_cr∼3, which we interpret as resulting from microscopic granularity. Below T_c and in the regime describing the approach to the zero-resistance state, fluctuation conductivity diverges as expected in a paracoherent-coherent transition of a mesoscopic granular superconductor. The results show that, instead of trivially rounding the transition, disorder at micro- and mesoscopic levels preserves a true critical phenomenology in granular superconductors. copyright 1997 The American Physical Society

[en] In this work we report the dimensional dependence of local properties of polycrystalline and top seeded melting textured YBa₂Cu₃O_7-x/Ag superconducting samples. The cross sectional area of superconducting bars was successively decreased and the resistance-temperature (R x T) curves for different cross section area was obtained. The results show that for a given current, the R x T curves, specially the onset of zero resistance (T_C0), are dependent on the bridges dimensions. The results obtained are in accordance with a percolation model considering a random mixture of normal and superconducting elements.

[en] The transverse magnetoresistance of RuSr₂GdCu₂O₈ is studied as a function of the applied field in several fixed temperatures spanning the superconducting and purely magnetic phases. In low applied fields, the magnetoresistance is positive and results from suppression of superconducting fluctuations. In the high field-high temperature regimes, a spin-disorder contribution to the magnetoresistance is identified. This fact indicates that the magnetic Ru-O₂ atomic layers have a conducting character