P. Rodrigues

State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil

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Publications (16)12.46 Total impact

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    ABSTRACT: A detailed study of the effect caused by the partial substitution of Er by Ce on fluctuation conductivity of the Er1-xCexBa2Cu3O7-delta 0
    Modern Physics Letters B 01/2011; 25:1671-1681. · 0.48 Impact Factor
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    ABSTRACT: We have experimentally studied the pressure dependence of the fluctuation conductivity in melt-processed YBa2Cu3O7 (YBCO) for currents applied parallel or perpendicular to the Cu-O2 atomic layers. Results show that the asymptotic critical regime is described by the 3D-XY universality class for both in-plane and off-plane conductivity components. The dynamical exponent for the in-plane conductivity component is z = 1.5, independently of the applied pressure. However, a pressure induced crossover in the dynamical universality class is observed in the c-axis fluctuation conductivity, where the exponent changes gradually from z = 1.5 to z = 2. The Ginzburg number increases with pressure for both crystalline orientations. This behavior suggests that the in-plane and c-axis components of the G-L coherence length decrease upon pressure application. The fluctuation spectrum for the Gaussian regime in the planar conductivity keeps the three dimensional character. However, filamentary fluctuations are observed along the c-axis above the Ginzburg temperature. Our results for the out-of-plane fluctuation conductivity in YBCO suggest that the c-axis transport is predominantly coherent in this system.
    Physics of Condensed Matter 01/2011; 83(4):423-428. · 1.28 Impact Factor
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    ABSTRACT: We have studied fluctuations on the electrical conductivity in granular Tm1-xPrxBa2Cu3O7-delta (0
    Modern Physics Letters B 01/2011; 25:1203-1210. · 0.48 Impact Factor
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    ABSTRACT: Single phase polycrystalline Gd1-xPrxBa2Cu3O7-delta (0
    Modern Physics Letters B 01/2010; 24:2465-2475. · 0.48 Impact Factor
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    ABSTRACT: We have studied fluctuations on the electrical conductivity of granular compounds of Er1-xTbxBa2Cu3O7-δ (0 ≤ x ≤ 0.10) superconductors. From the temperature derivative of the resistivity, we have found that the transition proceeds in two intragranular-intergranular stages, with intergranular character accentuated for higher concentrations of terbium (x ≅ 0.10). The data also revealed the occurrence of critical regimes, as well as fluctuation regimes beyond 3D-XY. However, for higher concentrations of terbium, it was not possible to observe fluctuation regimes beyond 3D-XY, probably because TbBa2Cu3O7-δ is a metastable phase. Below TC, on approaching the zero resistance state, our results showed a power-law behavior with critical exponents consistent with a phase transition from a paracoherent to a coherent state of the granular array.
    Modern Physics Letters B 01/2009; 23:1367-1375. · 0.48 Impact Factor
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    ABSTRACT: Systematic measurements of the in-plane and out-of-plane fluctuations magnetoconductivity in a melt-textured sample of YBa2Cu3O7-δ are presented. The experiments were performed in fields ranging from 0.02 T to 5 T applied perpendicular to CuO2 atomic (ab) planes. The measurements of the in-plane fluctuations were performed in zero applied field. The method of analysis is based on the determination of the quantity χ-1σ = -(d)/(dT)ln Δ σ , where Δσ = σ - σR is the fluctuation conductivity. In the normal phase, contributions from Gaussian regimes are clearly evidenced. These regimes are unstable regarding the application of magnetic fields perpendicular to the ab plane. Below TC, in approaching the zero resistance state, our results show a power-law behavior, that is indicative of a phase transition from paracoherent to a coherent state of the granular array.
    Modern Physics Letters B 01/2008; 22:1717-1725. · 0.48 Impact Factor
  • A. R Jurelo, P Rodrigues, J Flemming
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    ABSTRACT: We have studied fluctuations on the electrical conductivity of granular samples of NdBa2Cu3O7−δ and REBa2Cu3O7−δ superconductors, wherein RE stands for a natural mixture of rare earth elements. Special attention is taken above Tc, where Gaussian and critical regimes are observed. Far from Tc, we can observe the regime dominated by Gaussian fluctuations, with exponent approximately λG(3D)≅0.50, which is the predicted one for homogeneous 3D fluctuations. Inside the critical region, we have observed two power-law regimes, one of them consistent with predictions for the 3D-XY universality class with exponent λcr(1)≅0.34±0.02. Below Tc, on approaching the zero resistance state, our results show a power-law behavior with critical exponents s=2.8±0.1 for NdBa2Cu3O7−δ and s=4.0±0.4 for REBa2Cu3O7−δ. We have previously shown that this corresponds to a phase transition from a paracoherent to a coherent state of the granular array.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2001; 349(1):75-82.
  • F. T. Dias, P. Pureur, P. Rodrigues, X. Obradors
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    ABSTRACT: The paramagnetic Meissner effect was observed in field-cooled magnetization measurements in melt-textured YBa2Cu3O7−δ samples. Fields from 2 Oe up to 50 kOe were applied and paramagnetic moments were detected below Tc in the low, intermediate and high field ranges. Results are discussed.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2000; 341:1377-1378.
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    ABSTRACT: We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa2Cu3O7−δ, YBa2(Cu2.98Zn0.02)O7−δ and GdBa2Cu3O7−δ. The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, Tco, which is close to the point where resistivity vanishes. The specific heat results, when analyzed as dC/dT, show a weak but reproducible cusp-like anomaly at Tco. This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.
    Physica C Superconductivity 01/1999; · 0.72 Impact Factor
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    ABSTRACT: The field and temperature dependence of the magnetic irreversibility Tg(H) of Bi2Sr2Ca1Cu2Oy and YBa2Cu3O7-delta single crystals as well as oriented grain systems was investigated for different orientations of the applied field. The data reveal a clear anisotropy in the irreversibility lines.
    Physica C Superconductivity 01/1994; 235:2803-2804. · 0.72 Impact Factor
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    ABSTRACT: Systematic conductivity fluctuation measurements are reported for polycrystalline YBa2Cu3O7-° and single-crystal Bi2Sr2CaCu2O8. Gaussian and critical regimes are observed. For both systems, the critical exponents are consistent with s-wave pairing.
    Physica C Superconductivity 01/1994; 235:1939-1940. · 0.72 Impact Factor
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    ABSTRACT: We report on systematic conductivity fluctuation measurements on three different samples of YBa2Cu3O7-delta. We show, using the temperature derivative of the resistivity and the logarithmic derivative of the conductivity with respect to temperature, that the transition is a two-step process. In the normal phase, contributions from Gaussian and critical fluctuations are clearly evidenced. Far from Tc, the Gaussian exponents indicate that a fractal topology might be adequate to describe the space dimensionality of the fluctuation spectrum. Closer to Tc we observe a crossover to a three-dimensional (3D) homogeneous Gaussian regime. Still closer to Tc we unambiguously identify the exponent lambdacr~0.33, predicted by the simplest full dynamic scaling theory of critical superconducting fluctuations. The obtained exponent is consistent with a 3D, two-component, order parameter. Near the zero-resistance state, the temperature dependence of our data is rather consistent with power-law behavior, suggesting the occurrence of a phase-transition phenomenon related to the percolation granular network.
    Physical review. B, Condensed matter 06/1993; 47(17):11420-11425. · 3.77 Impact Factor
  • P. Rodrigues, L. Ghivelder, P. Pureur, S. Reich
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    ABSTRACT: Detailed resistivity measurements were made on YBa2Cu3O7Ag nonrandom composites, prepared by coating the superconducting grains with silver. We investigated samples with 6, 11 and 23 wt.% Ag, and YBa2Cu3O7 reference material. The results show that the resistive transition proceeds in two steps. As the temperature is lowered, the superconducting transition first stabilizes inside the grains, followed by the establishment of superconductivity through the grain boundaries, or weak-links. Silver addition to YBa2Cu3O7 clearly enhances the process dominated by the weak-links. Results with various current densities show that with low enough currents all samples exhibit the same transition temperature. The measured critical current density around Tc increases for greater Ag content. Above Tc, the analysis of the paraconductivity data indicates that the fluctuation network is defined in a space with fractal topology. Close to the zero-resistance temperature a power law regime is found, with the same exponent in all samples. This suggests that long-range superconducting order results from a thermally assisted percolation process through the weak-links between grains.
    Physica C Superconductivity 01/1993; · 0.72 Impact Factor
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    ABSTRACT: We investigated the superconducting properties of YBa2Cu3O7-delta-Ag (23 wt.%), prepared by coating the superconducting grains with silver. The sample was characterized by X-ray diffraction and scanning electron microscopy. Measurements of electrical resistivity rho(T) confirmed the metallic nature of the composite material and showed the appearance of a double peak structure in drho/dT, with broadening of the resistive transition. In the specific heat C(T), not previously measured in YBa2Cu3O7-delta-Ag composites, the anomaly at the superconducting transition was not readily detected in the data. The feature becomes visible only in the derivative dC/dt, which is a very useful way of analyzing the data. The low-temperature specific heat linear coefficient is strongly affected by the silver metal. The results can be interpreted as evidence of a higher nonuniformity of the superconducting properties of the composite material.
    Physica C Superconductivity 01/1992; 194:97-104. · 0.72 Impact Factor
  • J. Schaf, P. Rodrigues, P. Pureur
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    ABSTRACT: The ZFC-FC irreversibility line Tg(H) of YBa2Cu3O7-δ is investigated as a function of δ(0<δ<0.26). For low fields (H<1K0e) Tg(H) fits to a H2/3 - de Almeida Thouless-like line but for H>1k0e it fits to a H2 - Gabay-Toulouse-like line. This crossover is discussed in the light of similar observations in Spin Glass systems. The functional form of Tg(H) independends from δ bt the amplitude factors decrease strongly with δ. Work in part financed by the Brazilian agencies CNPq and FAPERGS.
    Physica C Superconductivity 12/1991; · 0.72 Impact Factor
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    ABSTRACT: Fluctuations in the electrical conductivity of polycrystalline Ho1−x Pr x Ba2Cu3O7−δ superconductors were investigated with Ho contents from x=0.01 up to 0.10. Samples were prepared by the standard solid-state reaction technique. The method of analysis is based on the determination of the quantity cs-1=-\fracddTlnDs\chi_{\sigma}^{-1}=-\frac{d}{dT}\ln\Delta\sigma, where Δσ=σ−σ R is the fluctuation conductivity. The results show that the resistive transition proceeds in two stages as seen by the temperature derivative of the resistivity near T C . In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. The pairing transition splitting, associated with Pr doping and related to the occurrence of a phase separation, as observed in studies with other rare earth elements, was not clearly observed. On approaching the zero-resistance state, our results show a power-law behavior that corresponds to a phase transition from a paracoherent to a coherent state of the granular array. This behavior was not affected by Pr doping. KeywordsHigh-T C superconductor-Polycrystalline-Holmium-Praseodymium-Critical phenomena-Fluctuation effects
    Journal of Superconductivity and Novel Magnetism 23(7):1307-1312. · 0.70 Impact Factor