Chemical Control of Underdoped and Overdoped States in Y(Ba2 ? ySry)Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba_2–y Sr _y )Cu₃O_{6 +} (0.9) compounds gives rise to high hole concentrations within both the CuO₂ planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.     

Superconductivity in the tetragonal phase of La1.9Bi0.1CuO4+δ annealed under high oxygen pressure

We have investigated the relation between the crystal structure and superconductivity in La_1.9Bi_0.1CuO₄₊ , in which the phase separation observed in La₂CuO₄₊ is suppressed. A phase diagram in theT– plane is given for La_1.9Bi_0.1CuO₄₊ with excess oxygen. For very small values, the crystal structure is orthorhombic, and an orthorhombic-tetragonal phase transition occurs markedly at 0.03 in the measured temperature range between 13 and 293 K. Superconductivity is observed in the range of 0.04<<0.11. This is clear evidence thathigh-T _c superconductivity also appears in the tetragonal phase.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

Thermoelectric power measurements of YBa2Cu3O7?δ up to 950°C and their application to test the band structure near EF

Four-probe resistivity () and thermoelectric power (TEP) measurements were carried out on samples of YBa₂Cu₃O_7– up to 950°C, in air and in flowing oxygen at 1 bar. Below 700 K the TEP is small and increases rapidly above it, reaching, at 1200 K, +140V/K in air and +120V/K in oxygen. At the changeover temperature (700 K) the slope of log vs.T changes abruptly. These results are interpreted in terms of a model of transport of carriers in a narrow band, which is full for = 1 and half-filled for = 0. Possible origins for such a narrow band are discussed in detail.     

Percolation Behavior of the Normal-State Resistivity and Superconductivity of the YBa2Cu3O7?δ-Ba2GdNbO6 Composite System

The percolation behavior of the normal-state resistivity and superconductivity of the YBa₂Cu₃O_7–Ba₂GdNbO₆ composite system were studied by X-ray diffraction and temperature-resistivity measurements. No detectable chemical reaction was observed between the YBa₂Cu₃O_7– superconductor and the ceramic insulator Ba₂GdNbO₆, even after severe heat treatment above 950°C. The normal-state and superconducting percolation threshold values were found to be 17 vol.% and 30 vol.% of YBa₂Cu₃O_7– respectively in the YBa₂Cu₃O_7–-Ba₂GdNbO₆ composite system. The values obtained for the critical exponents describing the normal-state pecolation behavior of the system matched fairly well with the theoretically expected values for an ideal metal-insulator composite system.     

Current Noise in HgBa2CuO4+δ High Tc Superconductor

Experimental results of current noise in magnetic fields up to 1 kG after zero field cooling and field cooling are reported in bulk HgBa₂CuO₄₊ prepared with vapor–solid reaction in a two-temperature zone reactor. Noise measurements performed in Hg-based superconductor materials are compared with previously studied superconductors, in particular, YBa₂Cu₃O_7–. A maximum in the noise is observed at the same resistance, approximately one order of magnitude lower than the normal state value and independently of the applied field. This result is consistent with experiments in bulk YBa₂Cu₃O_7– samples and with percolative model simulations of lattice junctions in which the maximum is obtained at the same value of the concentration of superconducting junctions and, thus, at the same macroscopic resistance. Maximum intensity is shown to increase with magnetic field up to 1 kG, with noise values slightly higher in the field cooling procedure. Our results attest that weak-link effects, as in YBa₂Cu₃O_7–, dominate superconducting properties of bulk HgBa₂CuO₄₊. With respect to YBa₂Cu₃O_7–, no saturation at high magnetic field in both noise and magnetoresistance measurements is observed.     

Correlation Between the Morphology of Ag and the Contact Resistivity of the Ag/YBa2Cu3O7?δ Thin Film Contact

The morphology of the silver films deposited and annealed on laser ablated YBa₂Cu₃O_7– thin films and the corresponding contact resistivity have been systematically investigated. A minimum contact resistivity of 6 × 10^–8 cm² was reached at 77 K by annealing Ag/YBa₂Cu₃O_7– contact at the optimum temperature. The effect of the annealing temperature on the contact resistivity was explained by considering the morphology of the silver films and the diffusion of silver into YBa₂Cu₃O_7– film, etc. The difference of the contact resistivity for Ag contact to polycrystalline, single crystal and thin film of YBa₂Cu₃O_7– were also explained.