L. Bauernfeind

Ruthenium-based layered cuprates RuSr2LnCu2O8 and RuSr2(Ln1+xCe1−x)Cu2O10 (LnSm, Eu and Gd)

Abstract We synthesized layered cuprates where the intermediate layer between the CuO 2 blocks consists of vertex sharing RuO 6 octahedra. The (idealized) stoichiometries of these compounds are RuSr 2 LnCu 2 O 8 (Ru-1212) and RuSr 2 (Ln 1+ x Ce 1− x ) Cu 2 O 10 (Ru-1222), where Ln is one of the three neighboring lanthanides Sm, Eu and Gd. The analogy to the isostructural Nb based compounds, as well as simple valence counting, suggest that Ru is pentavalent in these compounds. Both 1212 and 1222 compounds are semiconducting when prepared in air and rapidly cooled to room temperature. Metallic behavior and superconductivity can be achieved for various compositions upon appropriate annealing.

Superconductors consisting of CuO2 and RuO2 layers

We have synthesized compounds which may be viewed as stacking of pairs of CuO2 layers alternating with RuO2 layers. There are two structural variants, RuSr2GdCu2O8-δ and RuSr2Gd1+xCe1−xCu2O10−δ, which are denoted shortly 1212 and 1222 structures. In their stacking sequence, they differ solely by the sheets separating the CuO2 bilayers. The synthesis of these phases is accompanied by the formation of small amounts of SrRuO3 impurities, which appear to have a much more devastating effect on the superconducting properties of the 1212 phase as compared to the 1222 phase. We present an investigation of the influence of various synthesis procedures on the appearance of impurity phases and on superconductivity in the 1212 system.

Superconductivity at 40 K in the 1212 system (Pb1-xVx)Sr2(Ca1-zYz)Cu2O7-δ

Abstract Samples of nominal composition (Pb 1− x V x )Sr 2 (Ca 1− z Y z )Cu 2 O 7−δ (0.1≤ x ≤0.7, 0≤ z ≤1) synthesized at 950°C in air were investigated. None of the as-prepared samples and of the samples post-treated in oxygen was superconducting. Some of the samples post-treated in argon at 500°C exhibited superconductivity; the main phase of the sample with the nominal composition (Pb 0.7 V 0.3 )Sr 2 (Ca 0.4 Y 0.6 )Cu 2 O 7−δ had a tetragonal unit cell with a =3.822 A, c =11.818 A. This sample showed a T c,onset of 40 K as measured by the temperature dependence of the resistance.

Growth and characterization of (Y, Pr)-123 single crystals: influence of Al contamination and post-annealing under oxygen pressure

Abstract (Y 1− x Pr x )Ba 2 Cu 3− z Al z O 7−δ (0 ≤ x ≤ 1) single crystals of large surface area (up to 9 × 8 mm 2 ) and thickness between 0.3 and 1.5 mm were grown in Al 2 O 3 crucibles using a self-flux method. The crystals were characterized by means of AC susceptibility, X-ray diffraction, energy dispersive X-ray spectrometry, optical reflectance measurements and Raman spectroscopy. To ensure that the crystals are fully oxygenated, post-annealing of the crystals was performed under high oxygen pressure (100–200 bar) in a home-made autoclave. With increasing Pr content a strong reduction of T c , a softening of the Raman B 1g mode, a hardening of the Raman A 1g mode, a red shift of the plasma edge and a strong decrease of the in-plane optical conductivity in the infra-red region were observed. The Raman data indicate that the valency of Pr is close to +3. EDX measurements as well as Raman measurements show, that the Al contamination of the crystals (which results from crucible corrosion) increases with increasing Pr content. The complete disappearance of the free carrier concentration which was determined by optical reflectivity measurements is affected by the Al contamination.

Magnetization of superconducting RuSr2GdCu2O8 and of concurrent phases

Abstract The appearance of impurity phases belonging to the quaternary oxide system SrO–GdO 1.5 –CuO–RuO 2 in Ru-1212 samples is difficult to avoid. While the formation of the ferromagnetic compound SrRuO 3 may be suppressed by a precursor route involving Sr 2 GdRuO 6 , this latter compound shows magnetization anomalies below 30 K. We present evidence that magnetization anomalies observed for some Ru-1212 samples in this temperature range are not due to trace amounts of Sr 2 GdRuO 6 . Rather, the effect appears to be related to the establishment of inter-granular superconductivity and to the vortex dynamics in Ru-1212.

Critical state models for a granular ferromagnetic superconductor

Abstract Measurements of the magnetic moment M as a function of the magnetic field B can be used to extract information about the penetration behaviour of the magnetic field in a superconducting sample. In granular superconductors the response to an applied AC field is governed by both intra- and intergranular shielding currents. We have employed low-field AC susceptibility measurements to investigate the magnetic properties of a polycrystalline RuSr2GdCu2O8 weak ferromagnet in its superconducting state. For fields below ≃1 mT the results are consistent with the Bean critical state model. For B>1 mT the Kim model is more suitable for the description of the samples properties.

Nonlinear AC susceptibility and critical current of (Y1−xPrx)Ba2Cu3O7−δ ceramics

Abstract The intergranular critical current density JcJ(T) of (Y1−xPrx)Ba2Cu3O7−σ ceramic samples was determined at zero DC magnetic field by temperature-dependent AC susceptibility measurements χ(T) with different field amplitudes hAC as well as by isothermal low-field AC susceptibility measurements χ(hAC) (140 mG ≤ hAC ≤ 18.4 G). In the “low” temperature regime, the temperature dependence of jcJ was deduced from the initial slope of isothermal χ′(hAC) measurements. The shift of the intergrain peak in χ″(T) to lower temperatures with increasing field amplitude hAC was used to determine the temperature dependence of jcJ in the “high” temperature regime which could be fitted to a power-law behaviour j cJ (T) ∝ (1 − T T cJ ) β with β ≈ 1 . This temperature dependence of the intergranular critical current density jcJ indicates that the superconducting grains are connected by insulating interfaces (network of SIS junctions). jcJ was found to decrease monotonically with increasing Pr content x.

An attempt of carrier doping into (Ca1−xSrx)CuO2 by chemical substitutions under ambient pressure

Abstract In an attempt to introduce superconductivity into the infinite layer parent compound Ca 0.86 Sr 0.14 CuO 2 by chemical substitutions of the divalent Sr by monovalent metals, samples of nominal composition (Ca 1−x M x )CuO 2 (0⩽×⩽0.5) with M = Sr, Li, Na, Ag were investigated. Under our preparation conditions (ambient pressure) the infinite layer phase was not formed for the samples (Ca 1−x M x )CuO 2 with M = Li, Na, Ag.

The occurrence of 1212-type phase in the Pb-Sb-Sr-Ca-Y-Cu-O system

Abstract Samples of nominal composition Pb1−xSbxSr2Ca1−yYyCu2O7−δ with 0 ≤ x, y ≤ 1, synthesized at 900 °C in air, argon and oxygen, have been investigated. None of the samples sintered in air or in oxygen was superconducting. Some of the samples which were prepared in argon exhibited superconductivity. The superconducting signal was not due to the “1212-phase”, but probably due to a “2213-phase”.

Possible pitfalls in SQUID magnetometry of superconducting samples: The case of RuSr2GdCu2O8

Inhomogeneities of the field in the superconducting magnet of a superconducting quantum interference device (SQUID) magnetometer can create serious artefacts in the d.c. magnetization measurements of superconducting samples. We discuss the problem focusing on the procedure of calculating the magnetic moment of a sample from the measured SQUID output signal. The (weakly) ferromagnetic superconductor RuSr2GdCu2O8 has been reported sensitive to inhomogeneities of the SQUIDs superconducting magnet, which create artefacts in its measured magnetization. Indeed, we show that, because of the small values of the magnetic hysteresis width, field changes less than 100 μT over the scanning length inside the magnet are enough to create spurious signals in the measured magnetic moment of this compound.