H. Bach

Residual resistivity and oxygen stoichiometry in Pr2−xCexCuO4+δ single crystals

Abstract Using an improved high-temperature annealing procedure we homogeneously vary the oxygen content of Pr 2− x Ce x CuO 4+δ single crystals and change the superconducting transition temperature in small steps. Minute changes of the oxygen concentration δ give rise to a strong increase of the residual resistivity but leave the Hall and the Seebeck coefficient nearly unchanged. A strong correlation of the change of the residual resistivity and the change of T c suggests that the point-defect scattering rate on oxygen interstitials or oxygen vacancies is an important parameter determining T c in the electron-doped high- T c systems.

Pressure dependence of the superconducting transition temperature of Bi- and Tl-based high-Tc superconductors

Abstract We report low field magnetization measurements of a single crystal Bi 2 Sr 2 Ca 1 Cu 2 O 8 and polycrystalline samples of the Bi 2 Sr 2 Ca 2 Cu 3 O 10 and the Tl 2 Ba 2 Ca 2 Cu 3 O 10 -phase under applied hydrostatic pressure up to 10 kbar. The pressure shift of T c for all compounds is positive with an initial slope d T c /d p = +0.19 K/kbar for the Bi(2212) and the d T c /d p = +0.16 K/kbar for the Bi(2223)-phase. For the Tl(2223)-phase we find a very strong shift of T c with pressure with a slope of d T c /d p = +0.50 K/kbar. A comparison of the pressure shift of T c for the different Y-, Bi- and Tl-based high- T c superconductors indicates that the shift is not determined by parameters of the CuO-layers alone but depends on details of the crystal structure. We discuss several mechanisms which are important for determining the absolute value of d T c /d p .

Electronic charge transport and magnetism of the system (La1−xSrx)2NiO4+δ

Abstract We report measurements of the electrical resistivity, the Seebeck-coefficient and the magnetic susceptibility of samples (La1−xSrx)2NiO4+δ in the concentration range 0 ≤ χ ≤ 0.5. All compounds exhibit semiconducting behaviour at low temperatures with thermally activated or variable range hopping type of conductivity. The comparison of the temperature dependence of the conductivity and the Seebeck-coefficient reveals the polaron character of the charge carriers. From the susceptibility measurements at high temperatures we derive an exchange interaction Jaf≈ 140 K for the NiO2-planes in La2NiO4. We develop a magnetic phase diagram for (La1−xSrx)2NiO4 with an antiferromagnetic range for x≤0.04 and a spin glass range for x > 0.04. The results are discussed in comparison with those of the high temperature superconducting system (La1−xSrx)2CuO4.

Magnetism and superconductivity in YBa2(Cu1-x(Fe,Ni,Zn)x)3O7−δ

We study the superconductivity in YBa 2 CU 3 O 7−δ with up to 15at% Fe, Ni and Zn-substitution for Cu. The Fe- and Ni-ions develop a magnetic moment consitent with a 2+ local moment state. In the Zn-system we find evidence that the reduction of the electronic density of states at the Fermi-level causes the strong suppression of the superconducting transition temperature.

Crystal growth of high-TC superconductors Pr2 − xCexCuO4 + δ with substitutions of Ni and Co for Cu

Abstract We report the crystal growth of the electron-doped high-TC superconducting Pr2 − xCexCuO4 + δ by a modified CuO self-flux flow method. We found that for the crystal growth process at different Ce concentrations the maximum growth temperature is the most important parameter and report about optimized crystal growth procedures for the Ce-concentration range x = 0−0.18. Using this flux growth method only Ni and Co atoms can be substituted for Cu in the Pr2 − xCex(Cu1 − yMy)O4 + δ crystals, the other substitutional elements that we have tested (M = Fe, Zn, Ga, and Sn) do not dissolve in the T′ phase. We introduce a new reduction treatment for single crystalline samples which ensures a homogeneous and variable oxygen stoichiometry over the whole crystal.

Electrical resistivity study of metallic Pr2−xCexCuO4+δ-single crystals over a broad concentration and temperature range

Abstract We report electrical resistivity measurements on single crystals of the electron doped high- T c superconductor Pr 2− x Ce x CuO 4+δ which have been made metallic in the concentration range 0.04 x x T towards a non-universal power law with increasing Ce-concentration. At high temperatures we always observe a cross-over to a linear temperature dependence. Measurements of the resistivity on single crystals with substitutions of Ni on the Cu-position revealed a change of the elastic as well as the inelastic scattering rate. The systematic development of the resistivity with changing charge carrier concentration in Pr 2− x Ce x CuO 4+δ is compared to that known for the hole doped high- T c , system La 2− x Sr x CuO 4+δ .

Growth of single crystals of rare earth chalcogenides

Abstract Rare earth chalcogenides usually melt congruently with melting points between 1900 and 2800 K. The melts are very corrosive and on melting in some cases a part of the chalcogen may be lost. Thus we used sealed crucibles of molybdenum or tungsten and were able to grow single crystals more than 1 cm in size and of high purity. Stoichiometry, trace impurities, structure and lattice imperfections were determined.

Anomalous temperature behaviour of the elastic constants of antiferromagnetic Mn-Invar alloys

The temperature dependences of the elastic constants of the antiferromagnetic Invar alloys Fe60Mn40 and Co46Mn54 have been investigated between 4.2 K and 800 K. Anomalies in the temperature behaviour of the sound velocities have been found in the vicinity of the magnetic phase transition indicating a strong magnetoelastic coupling in these antiferromagnetic Invar systems. The Landau theory has been adapted to the particularities of the systems under study for a phenomenological interpretation of the experimental data. Comparing our results with data obtained on ferromagnetic Invar alloys, we conclude that our phenomenological model is able to describe consistently the elastic anomalies in ferromagnetic and antiferromagnetic Invar systems.

Resistance anisotropy in strongly anisotropic high-Tc superconductors

Abstract Non-superconducting metallic single crystals from the n-type high-Tc system Pr2−xCexCuO4 exhibit a two-dimensional weak-localization behavior at low temperatures. We use the negative magnetoresistance from the coherent backscattering of electrons in the CuO2 planes as a probe for the microscopic current paths, when the macroscopic current direction is along the c-axis. The anisotropy of the magnetoresistance for the magnetic field parallel and perpendicular to the CuO2 planes provides clear evidence that the voltage drop develops mainly in the (ab) planes. Hence metallic bridging defects between the CuO2 layers are of dominant importance for the c-axis conductivity. We demonstrate that the peculiar anisotropy of ϱc(T, H) at the superconducting transition of Bi(2212) crystals can be explained following the same lines.

Crystal growth, characterization and physical properties of Bi2Sr2CaCu2O8+ϰ

In order to improve the crystal quality and size of the high T c . superconductor Bi 2 Sr 2 CaCu 2 O 8+x , the process of solidification near the melting point in air has been studied. The crystal structure formed from the melt depends on the oxygen stoichiometry. After quenching to room temperature, non-superconducting crystals with a higher oxygen deficiency solidify. During annealing they transform into the usual quasi-tetragonal superconducting structure. We have grown single crystals by a modified Bridgman-Stockbarger technique with a very slow lowering rate. After about 10 days, we obtained single crystals which then were annealed at a low oxygen pressure