J. Schaf

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.

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.

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.

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.

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.

The effects of Sr and Ca on the magnetic irreversibility and fluctuation conductivity of YBCO-123

Abstract We have measured the magnetic irreversibility of the YBa 2−x D x Cu 3 O 7−δ ( D = Sr or Ca and x=0.5 or 1.0) polycrystalline superconductors in fields up to 7 kOe and of an oxygen depleted YBa1.75Sr0.25Cu3O6.85 polycrystalline sample in fields up to 40 kOe as well as the fluctuation magnetoconductivity of YBa 1.5 D 0.5 Cu 3 O 7−δ ( D = Sr or Ca ) up to 300 Oe. These partial substitutions of Ba as well as oxygen depletion cause reduction of Tc proportional to the impurity concentration and an increase of the granular superconducting character, maintaining almost unchanged the overall aspect of the magnetic irreversibility and fluctuation conductivity. Our results show that in a low field region the flux dynamics is dominated by the intergranular Josephson flux, that is strongly marked by the conflicting and frustrated grain coupling physics, the magnetic irreversibility totally resembling that of the spin glasses. But in fields above 10 kOe the flux dynamics is more conventional.

Mössbauer effect and magnetization studies of the Fe2+xCr1−xAl system in the L 21 (X2YZ) structure

We investigate the effect of deviations from the stoichiometric composition on the structural and magnetic properties of the Fe2+xCr1?xAl system. The ordered L 21 structure was identified in all prepared alloys, and was found to coexist with a tetragonally distorted face-centred tetragonal (fct) phase. The Curie temperature is strongly composition dependent, and achieves its highest value in the alloy which has the smallest cell volume. The saturation magnetization and saturation moments at 4.2?K increase with the increase of the Cr concentration. Conversely, the highest average Bhf value at 300?K was obtained for the more Fe-rich alloy. It may be inferred from our results that the Fe atoms are carrying a non-null magnetic moment, which depends strongly on the composition in this system.

Magnetic hyperfine fields on 111Cd in the Pd1+xMnSb heusler alloys☆

Abstract The time-differential perturbed angular correlation technique has been used to measure the magnetic hyperfine fields (mhf) acting on 111Cd impurity on the series of alloys Pd1+xMnSb with 0 ⩽ x ⩽ 1. We observe three diffrent mhfs which for small values of x are of the order of +250, −250 and −150 kOe, decreasing slowly for larger values of x. These three fields are associated with 111Cd occupying the sites of Mn, Sb and Pd, respectively. In addition, magnetization measurements on three of the allows (x = 0, 0.4 and 0.8) gave magnetic moments on the Mn atoms of 3.88 ± 0.08, 4.18 ± 0.08, (4.14 ± 0.08)μB and Curie temperatures of 470 ± 2,422 ± 5 and (288 ± 2) K, respectively. Our results together with the existing data for mhfs on various 5sp elements in the Pd2MnSb Heusler alloy are compared with the Blandin-Campbell model and show an agreement which is only qualitative.

Static critical exponents of the ferromagnetic transition in spin glass re-entrant systems.

The static critical phenomenology near the Curie temperature of the re-entrant metallic alloys Au(0.81)Fe(0.19), Ni(0.78)Mn(0.22), Ni(0.79)Mn(0.21) and amorphous a-Fe(0.98)Zr(0.08) is studied using a variety of experimental techniques and methods of analysis. We have generally found that the values for the exponents α, β, γ and δ depart significantly from the predictions for the 3D Heisenberg model and are intermediate between these expectations and the values characterizing a typical spin glass transition. Comparing the exponents obtained in our work with indices for other re-entrant systems reported in the literature, a weak universality class may be defined where the exponents are distributed within a certain range around average values.

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.