W. Widder

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.

Optical investigation of the metal-insulator transition in NdBa2Cu3Ox A comparison with YBa2Cu3Ox

Abstract We have performed reflectance measurements on the (001) surfaces of twinned single crystals of NdBa 2 Cu 3 O x . The oxygen concentration x was varied between 6 ≤ x ≤ 7. The results were compared to former investigations on YBa 2 Cu 3 O x . The different dependence of the electronic transport properties on the oxygen content of both compounds can be explained by the different distributions of oxygen amongst the CuO chains.

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.

Optical investigation of Y1−zPrzBa2Cu3O7−δ single-domain crystals. A comparison between impurity-free and Al doped samples

Abstract We have determined the polarized reflectance spectra of the (001) plane of different single-domain Y 1− z Pr z Ba 2 Cu 3 O 7−δ crystals, grown in a non-corrosive BaZrO 3 crucible. Separating the conductivity of the CuO chains and CuO layers we are able to determine the influence of Pr doping onto the two different kinds of CuO networks. For a comparison we also present data from crystals grown in Al 2 O 3 crucibles. For an explanation of those results we propose a quite different state of oxygen order in the chain-plane.

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”.

Low temperature magnetocaloric effects in Dy1Ba2Cu3O7−δ and Y1Ba2Cu3O7−δ high-Tc single crystals

Abstract The heat release Q has been measured in high temperature superconductors during and after a change of the external magnetic field μ 0 H a ≤ 0.5 T applied parallel to the c -axis at 0.1 K ≤ T ≤ 0.45 K. Two Y123 and one Dy123 twinned single crystals were investigated. An analysis of the heat release based on the Bean model and on a model for thin superconductors in perpendicular geometry recently proposed by Brandt et al. has been carried out. During field sweep we measured Q ∝ H a 2 d H a /d t . This dependence can be understood within the Bean model for longitudinal geometry. The critical current densities obtained with this method are in good agreement with those from magnetization measurements. We also present measurements, and a first quantitative analysis, of dissipation due to the time relaxation of the flux line lattice after a field sweep. For the heat release a t −1 dependence has been found which corresponds to a logarithmic time dependence of the magnetization. The normalized relaxation rate we obtained is in agreement with literature values.