Substitutions for copper in YBa 2 (Cu 1− x M x ) 3 O 7− δ (1:2:3) were investigated. It was found that indirect methods to determine solubility into 1:2:3 typically used in the study of intermetallic compounds (observed variation with composition of superconducting transition temperature and of lattice parameters) are not entirely reliable. Optical micrography was used to asses the morphology of the sintered polycrystalline specimens. Differential thermal analysis provides melting point data signaling the presence of impurities which melt at lower temperatures. Electron microprobe studies were undertaken to determine actual M content of the superconducting phase and to check for the presence of impurity phases. Estimates of solubility limits are made. Transmission electron microscope studies were used to determine the twin structure and investigate the occurrence of a tweed pattern. Superconducting transition temperature as a function of x was determined below the solubility limits. In several cases T c 's observed in this work differ from some previous reports. Possible sources of these discrepancies are discussed. Variation of lattice parameters as a function of x is reported. Also described are electrical resistivity and DC susceptibility measurements for selected samples.

Substitutions for Cu in YBa2(Cu1−xMx)3O7−δ (M = Fe, Co, Al, Cr, Ni, and Zn)

The transport properties of YBa 2 Cu 3 O y with various y and YBa 2 Cu 3− x M x O y (M = Fe, Co, Mn) with various χ have been studied. The results obtained have been analyzed on the basis of the phenomenological theory of electron transport in the case of a narrow conductive band. The narrow-band model permits one to account for the specific features of the transport coefficients of HTSC and to determine the band-structure parameters from comparison of the experimental and calculated temperature dependences of the thermoelectric power. The dependences of band-structure parameters and superconductive properties on y and χ have been analyzed together. The differences in the influence of different 3d metals on the band structure were revealed. Taking into account the crystallochemical-studies data we drew the conclusion that the oxygen sublattice disordering has a determinant effect on band-structure parameters and superconductive properties of YBa 2 Cu 3 O y .

The electron transport phenomena in Y based HTSC's and their analysis on the basis of phenomenological narrow-band theory the band structure transformation with oxygen content and substitution for Cu

An experiment is described which attempts to replicate the results of Podkletnov et al. concerning an alleged detection of a gravity-like force above a spinning superconductor. The experiment is based on Podkletnov’s published descriptions plus personal communications but found no evidence of a gravity-like force to the limits of the apparatus sensitivity. A full description of the apparatus and operation is given.

Gravity modification experiment using a rotating superconducting disk and radio frequency fields

Zn, having a definite valence state and filled 3d levels, is used for substituting Cu sites in RBa 2 Cu 3− x Zn x O 7⇔ y ( R = Yb , Er , Y , Dy , Gd , Eu , Sm and Nd ). Substitution of Zn for Cu in these systems causes a rapid nearly linear suppression of the superconducting transition temperature T c . All the RBa 2 Cu 3− x Zn x O 7− y samples are essentially single phase and retain the orthorhombic structure in the range of substitution (0≤ x ≤0.1). The reduction of T c by Zn doping in RBa 2 Cu 3− x Zn x O 7− y was found to be strongly dependent on the ionic radius of the rare-earth element R. The superconducting-nonsuperconducting transition accompanied by a transition into the semiconducting phase in these systems is interpreted in terms of a Mott transition.

Effect of Zn doping on the superconductivity of RBa2Cu3−xZnxO7−y (R = Yb, Er, Y, Dy, Gd, Eu, SmandNd)

A series of YBa 2 Cu 3− x Co x O y ( x =0–0.50) samples have been studied by means of SEM, TEM, XRD and positron annihilation technology. Oxygen contents of the samples have been measured using a new volumetric method. The microstructure evolution of the Co-doped YBCO is very similar to that in the Fe-doped YBCO. YBa 2 Cu 3− x Co x O y undergoes a structure phase transition from orthorhombic to tetragonal between x =0.12 and 0.15. Twin structure predominates when x is low ( x x , tweed structure appears and its proportion increases till it becomes dominant. When x >0.12, the tweed structure disappears completely. The grain boundary density increases with x and shows a sudden drop when x >0.12. Similarly, the positron short lifetime component τ 1 shows a sudden drop between x =0.12 and 0.15. Therefore, positron can be used as a sensitive probe for the O–T phase transition in this system. A possible model is proposed to describe the relationship between positron short lifetime component τ 1 and the concentrations of oxygen vacancy and twin (tweed) boundary. The experimental results can be satisfactorily explained by this model.

The microstructure study of Co-doped YBCO system

We report magnetic and electric measurements on $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ with up to 15 at.% substitution of Ni, Fe, and Zn for Cu in the insulating state ($\ensuremath{\delta}\ensuremath{\simeq}1$) and the metallic state ($\ensuremath{\delta}\ensuremath{\simeq}0.1$). The concentration dependence of the Pauli susceptibility and the electrical conductivity indicates that the shift of the Fermi energy towards the mobility edge with increasing degree of substitution is one important mechanism for the suppression of the superconducting transition temperature. In the insulating antiferromagnetic state the Fe and Ni spins are very weakly coupled to the Cu spins. The Ni spins remain paramagnetic down to the lowest measuring temperature of 3 K, the Fe spins order at 5 K, independent of the Fe concentration. The N\'eel temperature depends weakly on the Fe and Zn concentration but decreases strongly with the Ni concentration. This different behavior reflects the different site preference for the substitution in the insulating state, namely Cu(2) for Ni and Cu(1) for Fe and Zn.

Influence of Ni, Fe, and Zn substitution on the superconducting and antiferromagnetic state of YBa2Cu3O7- delta.

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.

H.J. Wüller

Papers

Influence of Ni, Fe, and Zn substitution on the superconducting and antiferromagnetic state of YBa2Cu3O7- delta.

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