A.R. Jurelo

Coherence transition in granular high temperature superconductors

Abstract We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa 2 Cu 3 O 7− δ , YBa 2 (Cu 2.98 Zn 0.02 )O 7− δ and GdBa 2 Cu 3 O 7− 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, T co , which is close to the point where resistivity vanishes. The specific heat results, when analyzed as d C /d T , show a weak but reproducible cusp-like anomaly at T co . This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.

Simulations and Tests of MCP-BSCCO-2212 Superconducting Fault Current Limiters

Superconducting fault current limiters (SCFCLs) represent a promising solution to the problem of increasing short- circuit currents in the grid. The SCFCL is based on the fast transition from the superconducting state to the normal state, causing a sudden increase in the impedance of the network. In this paper, we simulate the behavior of resistive-type SCFCL modules. The SCFCL modules are based on MCP-BSCCO 2212 coils. The superconductor acts as a nonlinear resistance that varies with the current and the temperature. The behavior of the simulated curves is consistent with the experimental results. Short-circuit currents as high as 37 kApeak were limited to about 10% of their peak values in the first half cycle.

Fluctuation conductivity and the dynamical universality class of the superconducting transition in the high-Tc cuprates

Systematic measurements of the fluctuation conductivity in several high-temperature superconducting cuprates show the occurrence of a genuine critical regime. The conductivity exponent is consistent with the predictions of the 3D-XY universality class and yields a dynamical critical exponent z = 1.5.

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.

Analysis of fluctuation conductivity of polycrystalline Er1 xPrxBa2Cu3O7 d superconductors

In this work we report on conductivity fluctuation measurements in polycrystalline samples of the Er1 xPrxBa2Cu3O7 d superconductor (x =0.00, 0.05 and 0.10). Samples were prepared by a standard solidstate 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 paracoherent to a coherent state of the granular array.

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.

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.

FLUCTUATION OF THE MAGNETOCONDUCTIVITY IN POLYCRYSTALLINE Y0.95Tb0.05Ba2Cu3O7-δ SUPERCONDUCTORS

The electrical magnetoconductivity near the superconducting transition in granular samples of Y0.95Tb0.05Ba2Cu3 O7-δ was experimentally studied. The measurements were performed using low magnetic fields up to 50 Oe, applied parallel to the current. Results showed that the transition proceeds in two stages as seen by the temperature derivative of the resistivity near TC. The analysis of the conductivity in the normal phase has evidenced contributions from Gaussian and critical fluctuations. In the regime of near zero resistance state, the occurrence of a coherence transition was evidenced, which was related to the connective nature of the granular samples. We have obtained the field independent exponents λC = 4.6 ± 1.1 and λC ≅ 6.7 ± 0.7 for fluctuation conductivity in the coherence transition.