A. W. Mitchell

Superconductivity in YBa2−xSrxCu3O7−δ

We report structure, resistivity, and Meissner effect measurements on YBa2−xSrxCu3O7−δ for 0<x<2.0. We find a region of solid solubility extending at least to x=1.0 and a monotonic depression of Tc with x. Using arguments based on structural changes with Sr doping, we speculate that the depression of Tc is due to the local distortion of the lattice in the neighborhood of the Sr site and the introduction of additional oxygen vacancies.

Preparation of Bi‐Sr‐Ca‐Cu‐O superconductors from oxide‐glass precursors

A superconductor and precursor therefor from oxide mixtures of Ca, Sr, Bi and Cu. Glass precursors quenched to elevated temperatures result in glass free of crystalline precipitates having enhanced mechanical properties. Superconductors are formed from the glass precursors by heating in the presence of oxygen to a temperature below the melting point of the glass.

Tunneling spectroscopy in superconducting Ba1−xKxBiO3: Direct evidence for phonon-mediated coupling

Abstract Tunneling measurements have been performed on bulk samples of Ba1−xKxBiO3 (x=0.375) using the natural, surface oxide as the tunnel barrier and an In thin film counterelectrode. High bias conductance and second derivative measurements show reproducible features in the energy range 10 to 65 meV which are characteristic of phonon effect as seen in conventional superconductors. The structures appear at energies which correspond to peaks in the phonon density of states as measured by inelastic neutron scattering and Raman spectroscopy. Estimates of the electron-phonon coupling strength, λ, and the average phonon frequency, , from the tunneling data indicate that phonon mediated pairing is responsible for superconductivity in this compound. Strong lifetime broadening effects are observed in the gap-region conductance of these junctions indicating gapless superconductivity for samples with concentration, x=0.375.

New family of superconducting copper oxides: GaSr2Ln1−xCaxCu2O7

Abstract We have prepared a new copper oxide, GaSr 2 Ln 1− x Ca x Cu 2 O 7 (0 ≤ x ≤ 0.4), that becomes superconducting a high-pressure oxygen annealing. While the undoped parent compound, x = 0, does not show three-dimensional magnetic order above 4 K, probably due to the presennce of the thick ionic separating the CuO 2 layers, superconductivity with a high- T c (∼70 K) is possible for the doped compositions.

Synthesis of high‐density Ba1−xKxBiO3 superconducting samples

A synthesis procedure for producing high‐density Ba1−xKxBiO3 has been developed. A novel melt‐processing technique is described which yields strong, near 100% dense superconducting material. This material is much more amenable to experimental study than the powder samples produced by the previous synthesis methods.

Variable-range hopping conduction in Ba1−xKxBiO3−y system

Abstract Normal state resistivity of the Ba1−xKxBiO3−y system is reported for various K concentrations. In the concentration range where the material is superconducting, phonon-assisted variable-range hopping [exp(T0/T) 1 4 ] is observed between 160 and 300 K.

110 k superconductivity in crystallized Bi-Sr-Ca-Cu-O glasses

Abstract Superconducting transition temperatures near 110 K are observed in the Bi-Sr-Ca-Cu-O system with the nominal compositions Bi 2 Sr 2 Ca 3 Cu 4 O x and Bi 2 Sr 2 Ca 4 Cu 5 O x . Electrical resistivity, magnetization, and X-ray diffraction data are reported. Also discussed is the effect of heat treatment on the thermodynamic behavior of the superconducting phases in the rapidly quenched glasses.

Oxygen content and the synthesis of Ba1−xKxBiO3−y

Abstract In situ thermogravimetric analysis during the synthesis of Ba 1− x K x BiO 3− y shows the various processes occurring as a function of temperature and the gas composition of the ambient atmosphere. This allows the synthesis procedure to be optimized. Single-phase material can be produced with the metal-ion stoichiometry of the starting oxide mixture.

Structural modulations of Bi2Sr2CuO6 studied by in situ x‐ray diffraction

High‐temperature x‐ray diffraction has been carried out on a superconducting single‐phase sample of pseudo‐tetragonal Bi2Sr2CuO6+y, which was obtained from crystallization of thin glassy platelets. The sample displayed (00l) preferred orientation, which made it possible to determine the incommensurate modulation by conventional θ−2θ x‐ray scans. The existence of the modulation peaks (800u2009°C) indicates that the modulation cannot be caused solely by oxygen ordering. As temperature increased, it was observed that a decrease in the intensity of modulation lines was associated with an increase in intensity of fundamental reflection, which was similar to the order‐disorder phase transition. By removing 0.045(5) oxygen atom per formula unit, the components of modulation satellites along the b* and c* directions changed from bmod=0.183(9) and cmod=0.315(4) to 0.184(6) and 0.260(4), respectively. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation...

New family of planar cuprate superconductors: Effect of nonmagnetic chains and planes

Abstract The synthesis of layered cuprates with nonmagnetic and fixed oxidation state cations separating Cuue5f8O planes has, until now, led to nonsuperconducting materials. Recently we were able to introduce superconductivity in Yue5f8Srue5f8Cuue5f8Gaue5f8O system by appropriate doping and annealing.