Zero resistance above 90K in superconducting oxides ErBaCuO and YbBaCuO

Superconductivity transition has been observed by electrical resistance and a.c. magnetic susceptibility measurements in ErBaCuO and YbBaCuO systems. The highest T_c(ϱ=0) is 90.3K for ErBaCuO and 91K for YbBaCuO.     

High Tc superconductivity of a TlBaCaCuO compound

A TlBaCaCuO_4.5+x compound has been investigated from the point of view of superconductivity. Depending on the heat treatment, one part of the samples exhibits superconductivity with an onset of 121 K and a zero resistivity of 106 K, while the other part of them shows only a sharp drop in resistivity at 130 K which hints at the existence of superconducting domains. This picture was confirmed by magnetic and ESR measurements giving possibility for estimation of the critical magnetic fields.     

Pd addition in high Tc YBaCuO oxide

A small amount of Pd addition increases T_c for the YBa₂Cu₃O_7−y oxide. The maximum offset, midpoint and onset temperatures of the YBa₂(Cu_0.999Pd_0.001)₃O_7−y oxide are 92.1, 95.0 and 105 K, respectively. Excess addition of Pd decreases T_c. The T_c change is consistent with the results of the lattice constants.     

The superconductivity,structure and microstructure of BaYScCu oxides

We added small amount of Sc (0.05–0.35at.%) to BaYCu oxide and found that the superconductivity deteriorated drastically when the Sc content reached 0.325at.%. X-ray and electron diffraction showed that the orthorhombic perovskite structure changed to multi-phase structure with unidentified phases. The microstructures observed by TEM are of needle like morphology. Their phases are not yet identified.     

Zero resistance at 148.5K in fluorine implanted YBaCuO compound

Fluorine ions with an energy of 180keV at a dose of 1×10¹⁵ ions/cm² have been implanted into YBa₂Cu₃O_x. After annealed at 800°C, the resistance of the new Y-Ba-Cu-F-O compound drops to zero at 148.5K.     

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba₂YCu₃O_7−δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.     

Compositionally dependent superconducting transition temperature of YBaCu oxides

The superconducting transition temperatures of a family of YBaCu oxides have been explored by dc resistivity measurements. Substitutions of magnetic and non-magnetic ions on Y, Ba and Cu sites are reported. For some compositions, achieving high superconducting transition temperatures is strongly dependent on growth conditions for the material.     

Superconductivity of LaMCuO system (MBa,Sr and Ca

Superconductivity of the compounds of LaMCuO system with K₂NiF₄ type structure has been studied. For the sintered compounds with MBa, Sr and Ca, the highest onset transition temperatures, T_ci, estimated from resistivity measurement are about 36.9K, 43.0K and 23.5K, respectively and the highest temperatures where the resistivity vanishes, T_cf, are about 27.8K, 34.0K and 15.5K, respectively. Almost complete Meissner effect has been confirmed by low frequency ac susceptibility measurement. Crystals of the compounds with MBa and Sr have been prepared. As the result of the preliminary experiment for a crystal with Ba, it is found that T_cf of the crystal is higher than any one of the sintered specimens with MBa, while the T_ci of the crystal is nearly the same as the highest one of the sintered specimens.     

Atomic fraction around defects associated with nanoparticles in AlCuMg alloys

A positron annihilation spectroscopy analysis method to obtain a quantitative determination of the chemical composition around defects inside nanoparticles is presented here. This methodology is applied to AlCuMg alloys to study the rapid hardening phenomena associated with solute-vacancy aggregation. Coincidence Doppler Broadening (CDB) and lifetime spectroscopy measurements of reference samples of pure elements with and without defects were analyzed to give quantitative information of the average chemical environment around vacancies, i.e. the atomic fraction of the first neighbors of these defects, in the alloys studied. The accuracy and reproducibility of the methodology is confirmed not only by good fits to the experimental data but, in most cases, by the consistency between the mean lifetime values predicted, using the CDB estimation, and the mean lifetime values independently measured. Discrepancies in the methodology are expected when there is poor CDB contrast between elements, i.e. having similar electronic structure (for example, Al and Mg). The criterion for establishing the statistical accuracy of the separation of elements in these special cases is discussed. The methodology can be applied not only to study homogeneous materials as metallic alloys, but also to study the depth profile in thin films.     

Lattice dynamics and debye temperature of AlCu,AlSi and AlGe alloy systems

The lattice dynamics and specific heat at constant volume for AlCu alloy system is studied using our previous treatment based on the microscopic electronic theory. Considering the volume and electron density effect on the dynamical matrix of the pure constituent, we obtain the band and local mode frequencies at the temperature-dependent specific heat in the Al_1−x Cu_x solid solution. Then, using the data about the mean elastic wave and the specific heat extrapolated to the absolute zero temperature, the Debye temperature is presented for the Al_1−xCu_x, Al_1−xSi_x and Al_1−xGe_x alloy systems. The Debye temperature decreases as a function of the Cu atomic fraction x for Al_1−xCu_x alloy, and remarkably for Al_1−xSi_x and Al_1−xGe_x solid solutions.     

The effect of Sc on the superconductivity of the BaYCu oxides

The oxide compounds of Ba_0.6Y_0.4−xSc_xCu (x=0.05−0.35) have been prepared. It was found that the superconducting transition temperatures of the samples vary with the contents of Sc and when Sc>0.325, the samples become nonsuperconducting.     

Anisotropic low-field giant magnetoresistance in [NiFeCo/Cu] multilayers

[NiFeCo(3 nm)/Cu(2 nm)] multilayers having in-plane uniaxial magnetic anisotropy were prepared by using a carousel type sputtering apparatus. The MR (magnetoresistance) ratio was about 8% and the MR curve along the easy axis showed a sharp split hysteresis with a very small transient field, ΔH ∼ 3 Oe, though the switching is irreversible. The experimental results are discussed by using magnetic phase diagrams calculated by Dieny et al. (1990) and Folkerts (1991).     

Superconductivity in the LnBaCuO system (Ln = Y,Ho, Er)

Superconductivity around 90K is observed in mixed and single phase system of LnBaCuO (Ln = Y, Ho, Er). Resistivity, X-ray powder diffraction and AC susceptibility measurements were carried out on samples of the mixed phase systems Ln_2−xBa_xCuO_4−y (Ln = Ho, Er), Y_1.2Ba_0.8Cu_1+xO_4+x−y(0<x<5) and single phase system Ba₂LnCu₃O₇ (Ln = Y, Ho, Er). Ln_2−xBa_xCuO_4−y consists of Ba₂LnCu₃O₇, BaCuLn₂O₅, Cu₂Ln₂O₅ and starting materials and shows a small diamagnetic signal below T_c in the temperature dependent AC susceptibility. Y_1.2Ba_0.8Cu_1+xO_4+x−y is a mixture of Ba₂YCu₃O₇, BaCuY₂O₅ and CuO and also indicates a small diamagnetic signal at 92K. Interesting resistivity anomalies occur in the Y_1.2Ba_0.8Cu₄O_7−y and Ho_0.8Ba_1.2CuO_4−y composites including large drops in the resistivity at ∼220 and ∼120K, respectively. The pure compounds, Ba₂LnCu₃O₇ (Ln = Y, Ho, Er) show sharp resistive superconducting transitions and bulk Meissner effects of 90%.     

Study of the crystalline structure of the superconducting phase of the YBaCuO ceramic compound

We present the results of a study of the crystalline structure of the high-temperature superconducting (above 110K) phase in the single-phase ceramic compounds YBaCuO. We have concluded that the crystal symmetry of this HTS phase with chemical composition Y_1.2Ba_0.8CuO_4−δ is orthorhombic type with crystallographic lattice constants a=0.3815 nm, b=0.3457 nm, c=2.650 nm.     

A new superconducting oxide GdBaCuO with Tc (ρ=0) above 92K

A new superconducting oxide Gd_xBa_1−xCuO_3−y with T_c (ρ=0) above 92K has been successfully synthesized by the ceramic technology. The samples with composition of x=0.5, 0.625 and 0.9 are all observed with high T_c superconductivity. The highest T_c for samples measured is observed in Gd_0.5Ba_0.5CuO_3−y which has a resistive midpoint of 95K with a width of 1.5K, and a zero-resistance temperature of 92.5K. The results of a.c. magnetic susceptibility measurement are also reported.     

Anomalous behavior of phonons in NbCu metallic superlattices

Brillouin and Raman scattering investigations have been performed on NbCu metallic superlattices. Anomalies have been observed in the surface acoustic phonons and bulk acoustic phonons versus superlattice wavelength. These anomalies occur simultaneously at the same superlattice wavelength ≈ 19 Å.     

Anisotropy of the superconducting critical magnetic field HC2 of LaMCuO system(M = Sr and Ba)

Preliminary measurement of the superconducting critical magnetic field H_c2 of LaMCuO system (M = Sr and Ba) with layered perovskite structure has been carried out. The results show a quite large anisotropy of H_c2 indicating that the present system has a two dimensional character of the electron transport. The measured conductivities seem to be consistent with the values estimated from the observed H_c2 by use of an ideal two dimensional band with free electron mass.     

Growth of CuO chains on Ag(110) surfaces

The (AgO) chains grown along the 〈001〉 direction on Ag(110) disappeared by the reaction with Cu atoms at room temperature, and new one-dimensional chains grew in the 〈110〉 direction. The STM images at different bias potentials suggest the growth of (CuO) chains in the 〈110〉 direction on the Ag(110) surface. The (CuO) chains on the Ag(110) surface disappeared by heating to 470 K, but they reappeared by exposing to O₂ at room temperature.