P. Pureur

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.

Resistive transition and fluctuations of the conductivity in YBa2Cu3O7, EuBa2Cu3O7 and GdBa2Cu3O7 superconductors: Effect of inhomogeneities

Abstract Systematic resistivity measurements in polycrystalline samples of the high- T c oxide superconductors RBa 2 Cu 3 O 7 (R = Y, Eu, Gd) are performed with several (low) current densities. Special attention is devoted to the temperature range close to the superconducting transition. The data allows the determination of temperature derivatives of the resistivity, which reveal the occurence of a two-step transition. This feature is typical of a granular system. Two scaling regimes for the fluctuation conductivity are also identified. Near the temperature where the grains become superconducting, the enhancement of the conductivity follows a power law with sample dependent exponents. Analysis of this result on the basis of the Aslamazov-Larkin theory indicates that a fractal topology might be adequate to describe the inhomogeneous superconducting state. In the second scaling regime, near the zero-resistance temperature, an apparently universal exponent is observed.

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 .

Static critical behavior of the ferromagnetic transition in LaMnO3.14 manganite

Abstract The ferromagnetic phase transition in LaMnO 3.14 is investigated by measuring the DC magnetization as a function of magnetic field and temperature. Modified Arrott plot and Kouvel–Fisher analysis yield estimates for the critical exponents β , and γ , with values between the predicted for the Heisenberg model and mean-field theory. At low fields we found an anomalous small value of β , indicating that the critical behavior is influenced by the range of magnetic fields used.

Chiral susceptibility in canonical spin glass and re-entrant alloys from Hall effect measurements

The extraordinary Hall effect coefficients Rs(T) of the canonical spin glass alloys AuFe 8at% and AuMn 8at% and of an archetype re-entrant system AuFe 18at% were measured as functions of temperature. The data show a critical cusp-like term superimposed on a smooth background for the spin glasses, and change of sign with temperature for the re-entrant. The results can be interpreted consistently by invoking a chiral Hall effect contribution as proposed by Kawamura.

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.

Resistive transition and fluctuation conductivity in YBaCuOAg nonrandom composites

Abstract Detailed resistivity measurements were made on YBa 2 Cu 3 O 7 Ag nonrandom composites, prepared by coating the superconducting grains with silver. We investigated samples with 6, 11 and 23 wt.% Ag, and YBa 2 Cu 3 O 7 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 YBa 2 Cu 3 O 7 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 T c increases for greater Ag content. Above T c , 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.

Chiral anomalous Hall effect in reentrant AuFe alloys

The Hall effect has been studied in a series of AuFe samples in the re-entrant concentration range, as well as in part of the spin glass range. An anomalous Hall contribution linked to the tilting of the local spins can be identified, confirming theoretical predictions of a novel topological Hall term induced when chirality is present. This effect can be understood in terms of Aharonov-Bohm-like intrinsic current loops arising from successive scatterings by canted local spins. The experimental measurements indicate that the chiral signal persists, meaning scattering within the nanoscopic loops remains coherent, up to temperatures of the order of 150 K.

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.

Electrical and magnetic properties of the high-Tc oxide superconductor YBa2Cu3O7☆

Abstract We report on electrical and magnetic measurements on the high- T c oxide superconductor YBa 2 Cu 3 O 7 . Accurate resistance data allow the determination of the temperature derivative of the resistivity near T c . DC magnetic susceptibility and low field magnetic hysteresis cycles experiments are performed in the superconducting and normal states of the sample. The interpretation of the results is coherent with the description of the system as an inhomogeneous superconducting network where frustration is likely to occur.