Rosângela Menegotto Costa

Scaling beyond 3D XY in the fluctuation conductivity of YBa2Cu3O7−δ

Abstract Systematic measurements of the in-plane fluctuation magnetoconductivity in a YBa 2 Cu 3 O 7−δ single crystal are presented. Fields up to 500 mT were applied either parallel or perpendicular to the Cu–O planes. The data reveal the occurrence of Gaussian-fluctuation regimes far from the transition. Closer to T c , the results clearly show the occurrence of a genuine critical regime, where the exponent is consistent with predictions for the 3D XY universality class with model- E dynamics. Still closer to T c , evidence is found for a fluctuation regime beyond 3D XY . This new scaling can be interpreted as revealing an ultimate first-order character of the superconducting transition in YBa 2 Cu 3 O 7−δ .

Effects of Ag addition on some physical properties of granular YBa2Cu3O7-δ superconductor

The effects of Ag addition on some physical properties of YBa2Cu3O7-δ superconductor has been studied. The samples were produced by different routes and characterized by scanning electron microscopy, differential thermal analysis, thermogravimetric analysis, X-ray diffraction and nanoindentation. The superconducting properties were studied by dc magnetization and electrical resistivity measurements. The X-ray patterns revealed that all samples were polycrystalline and corresponded to the orthorhombic YBa2Cu3O7-δ phase. However, for some samples, silver appears to be present within the grains. The superconducting properties were not significantly influenced by Ag addition. Hardness and elastic modulus were also obtained by instrumented indentation. Ag addition was found to be effective in improving the mechanical properties, probably by filling the pores in the grain boundaries. Hardness profiles indicated values between 3.1 and 3.4 GPa at deep tip penetration depths. The highest elastic modulus of 146 GPa was attained for samples prepared using Y2O3, BaCO3, CuO and Ag2O as precursors, while the lowest modulus (125 GPa) was obtained for the pure YBa2Cu3O7-δ sample.

FLUCTUATION CONDUCTIVITY IN MELT-TEXTURED YBa2Cu3O7-δ

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 , 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.

ANALYSIS OF FLUCTUATION CONDUCTIVITY OF GRANULAR Gd1-xPrxBa2Cu3O7-δ SUPERCONDUCTOR

Single phase polycrystalline Gd1-xPrxBa2Cu3O7-δ (0 ≤ x ≤ 0.10) samples have been prepared by standard solid state reaction technique and characterized by SEM, X-ray diffraction and electrical transport experiments. The samples were granular and homogeneous. It was observed that the critical temperature decreases and that the transition width increases with increasing Pr doping. Systematic measurements of fluctuation conductivity are reported. Special attention is taken above the critical temperature, where Gaussian and critical regimes are observed. Below the critical temperature, on approaching the zero resistance state, our results show a power-law behavior consistent with a phase transition from a paracoherent to a coherent state of the granular array.

Conductivity fluctuations of polycrystalline Ag-doped YBa2Cu3O7-δ superconductor

Systematic conductivity measurements near TC in granular Ag-doped YBa2Cu3O7-δ superconductors are presented. Silver doped YBa2Cu3O7-δ samples were produced by different routes and characterized by X-ray diffraction, dc magnetization and electrical resistivity. The results were analyzed in terms of the temperature derivative of the resistivity and of the logarithmic temperature derivative of the conductivity, that allowed identifying power-law divergences in the conductivity. The results show that the resistive transition proceeds in two stages. In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. Close to the zero-resistance temperature, a coherence transition related to the connective nature of the granular samples was evidenced. It was not observed significant differences in critical and Gaussian fluctuations between pure and Ag-doped YBa2Cu3O7-δ polycrystalline samples.