P. Rodrigues

Paramagnetic Meissner effect at high fields in melt-textured YBa2Cu3O7-δ

Abstract We observe the paramagnetic Meissner effect in melt-textured YBa2Cu3O7–δ samples after cooling in fields from 1 up to 50 kOe. The results suggest that the paramagnetic moments are primarily generated by compression of the magnetic flux into the sample in the course of the field cooling procedure. We also observe that the field-cooled moments relax towards positive values in experiments performed at constant field and temperature.

Splitting of the bulk resistive transition in high-Tc superconductors : evidence for unconventional pairing

Abstract We report on measurements of the resistive transition in several samples of polycrystalline YBa 2 Cu 3 O 7− y and Bi 2 Sr 2 Ca 2 Cu 3 O 10+ x , and single crystal Bi 2 Sr 2 CaCu 2 O 8+ x . The results are analyzed in terms of the temperature derivative of the resistivity, d ϱ /d T . In the ceramic samples, we systematically find that the transition proceeds as a two-stage process. Initially, superconductivity establishes in some disconnected regions of the homogeneous phase. Long range ordering is obtained at lower temperatures as a consequence of a paracoherent-coherent transition involving the granular array. A new, reproducible and general feature is also revealed in our results. The principal maximum of d ϱ /d T , which denotes the intragranular transition, shows a two-peak splitting. This characteristic, which is observed in all investigated samples strongly suggests the occurrence of a double bulk superconducting transition, which is in itself a signature of unconventional order parameter symmetry.

Fluctuation conductivity in NdBa2Cu3O7−δ and REBa2Cu3O7−δ superconductors where RE is a natural mixture of rare earth elements

Abstract We have studied fluctuations on the electrical conductivity of granular samples of NdBa 2 Cu 3 O 7− δ and REBa 2 Cu 3 O 7− δ superconductors, wherein RE stands for a natural mixture of rare earth elements. Special attention is taken above T c , where Gaussian and critical regimes are observed. Far from T c , we can observe the regime dominated by Gaussian fluctuations, with exponent approximately λ G (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 λ cr (1) ≅0.34±0.02. Below T c , on approaching the zero resistance state, our results show a power-law behavior with critical exponents s =2.8±0.1 for NdBa 2 Cu 3 O 7− δ and s =4.0±0.4 for REBa 2 Cu 3 O 7− δ . We have previously shown that this corresponds to a phase transition from a paracoherent to a coherent state of the granular array.

Critical and Gaussian conductivity fluctuations in YBa2Cu3O7−δ and Bi2Sr2CaCu2O8☆

Abstract Systematic conductivity fluctuation measurements are reported for polycrystalline YBa 2 Cu 3 O 7−° and single-crystal Bi 2 Sr 2 CaCu 2 O 8 . Gaussian and critical regimes are observed. For both systems, the critical exponents are consistent with s-wave pairing.

A 92.5 K REBa2Cu3O7-δ SUPERCONDUCTOR STARTING FROM NATURAL MIXTURE OF RARE EARTH ELEMENTS

The processing and characterization of a REBa2Cu3O7-δ, sample superconductor is described, where RE corresponds to a natural mixture of rare earth elements extracted from xenotime mineral. This mixture contains no Pr and, contrary to previous studies, can substitute the yttrium atom in YBa2Cu3O7-δ, (YBCO) without deleterious effects in structural and superconducting properties, as measured by X-ray diffraction, resistivity and magnetoresistivity. Our sample presented Tc=92.5 K and average lattice parameters a=3.821 A, b=3.896 A and c=11.686 A, which are very similar to YBCO properties. Considering that the process of separation of Y (and other pure rare earth elements) from any natural mixture involves high costs, our system represents a practical alternative to YBCO compound.

Preparation and characterization of PbTi0(3) ceramics modified by a natural mixture of rare earth oxides of xenotime

Lead titanate ceramics modified by xenotime (Xm) with nominal composition (Pb, Xm)TiO3, Xm 10 or 15 mol %, were prepared by the conventional oxide mixture technique. Xenotime is a natural mineral consisting of a mixture of rare earth oxides. Thermal, structural and electric properties were investigated through differential and gravimetric thermal analysis, X-ray diffraction and dielectric measurements as a function of temperature. The results of both compositions revealed a higher density and free of cracks ceramic body, compared to pure PbTiO3 prepared by the same procedure. On the other hand, the structural characteristics and Curie temperature are nearly the same as those of pure PbTiO3. The hysteresis loop measured at room temperature revealed a hard ferroelectric material with coercive field of 10.7 kV/cm and a remanent polarization of 0.2 ∝C/cm2. These finding reveal a material with properties that highlight potential to be used as electronic devices that operate at high temperature and high frequencies.

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.

Paramagnetic meissner effect in melt-textured YBa2Cu3O7−δ

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.

FLUCTUATION CONDUCTIVITY OF POLYCRYSTALLINEYbBa2Cu3O7 -δSUPERCONDUCTORS

We studied the effect of superconducting fluctuations on the electrical conductivity of granular samples of Y1-xTbxBa2Cu3O7-δ superconductors. For all samples we observed that the transition proceeds in two stages. Above the critical temperature, the Gaussian and critical regimes were observed, independently of the terbium concentration. In the regime of approach to the zero resistance state, and for low concentrations of terbium, an occurrence of a coherence transition was observed. For high concentrations of Tb, the results show a strong disorder at mesoscopic levels.

Fluctuation-induced conductivity in melt-textured Pr-doped YBa2Cu3O7-δ composite superconductor

ABSTRACT In this study, the effects of thermal fluctuations on the electrical conductivity in melt-textured YBa2Cu3O7-δ, Y0.95Pr0.05Ba2Cu3O7-δ and (YBa2Cu3O7-δ)0.95–(PrBa2Cu3O7-δ)0.05 composite superconductor were considered. The composite superconductor samples were prepared through the top seeding method using melt-textured NdBa2Cu3O7-d seeds. The resistivity measurements were performed with a low-frequency, low-current AC technique in order to extract the temperature derivative and analyze the influence of the praseodymium ion on the normal superconductor transition and consequently on the fluctuation regimes. The results show that the resistive transition is a two-step process. In the normal phase, above the critical temperature, Gaussian and critical fluctuation regimes were identified, while below the critical temperature, in the regime near the approach to the zero-resistance state, the fluctuation conductivity diverges as expected in a paracoherent-coherent transition.