Q. R. Zhang

Mössbauer effect and structure of the YBa2Cu3−xSnxO7−y system

Abstract The series samples of YBa 2 Cu 3− x Sn x O 7− y was studied by transmission Mossbauer spectroscopy. Through combining the results of Mossbauer spectra with that of X-ray diffraction and XPS, we find Sn exists in this system in the form of Sn 4+ and that it occupies the sites of the Cu(1) and Cu(2).

Oxygen content is not the predominant factor for high Tc superconductivity in YBaCuO system

Abstract The relationship between the superconductivity and oxygen content of the nine substituted systems(Cu is substituted by Sn, AL, Zn, Ni; Ba by Sr, Ca, Y by Gd, Dy, Pr, respectively) is studied systematically. The experimental results reveal that the relationship is variable. In Zn-doped system, the oxygen content is hardly changed, but the T c is still supressed; in Ni-doped system, the oxygen content and T c are decreased simultaneously; in Dy- or Gd-doped system, the oxygen content and T c are hardly changed. The authors believe that the oxygen content does not predominate the superconductivity directly, the role of which is to adjust the coupling strength between positive and negative ions, and suggest that the coupling strength be one of the predominant factors for superconductivity.

Conductivity and dielectric constant in a wedge-shaped Pt-film percolation system☆

A wedge-shaped Pt-film percolation system was prepared by the magnetron sputtering method. A new nonlinear I-Vbehavior was found and can be well explained by the location-dependent hopping effect. Power-law behavior, σ(ω) α ωx and ϵ(ω) α ω−y, is observed near the percolation threshold. The exponents x and y are found to be 0.87±0.04 and 0.08±0.03, respectively, in agreement with the general scaling relation x + y = 1.

Thin silver films deposited in random fractal surfaces and their nonlinear dc I–V behavior

Abstract A nonlinear dc I–V behavior of thin Ag films, deposited in broken surfaces of α-Al 2 O 3 ceramics by using magnetron sputtering method, has been studied. To explain the phenomenon, a random tunneling junction network model is proposed and the numerical calculation is found to be in good agreement with the experiment.

Oxidation state of copper and superconductivity in the HgBaCaCuO system

Abstract The oxidation state of Cu in HgBa 2 Ca n −1 Cu n O 2 n + 2 + δ ( n = 1−5) phases has been analyzed in terms of the bond valence sums. The totally mobile hole concentration ( n H,t ) of type-I CuO 2 planes at the optimal level increases with the increasing number of CuO 2 layers ( n ) per unit cell from 1 to 3, and the largest n H,t is observed to be 0.350 when n = 3, which just corresponds to the highest T c in the Hg-based series. Thus the relationship between the maximum T c for each member and the number n is similar to the correlation between the n H,t and the n .

Observation of resistivity anisotropy on oriented YBa2Cu3O7-δ films

Abstract The difference between the longitudinal and transverse d.c. resistivity in oriented YBa2Cu3O7-δ films has been observed by special design of electrical contacts. It is attributed to the anisotropy of the in-plane resistivity, which at least indicates that some untwinned regions exist in the films. A sign change of the off-diagonal element of the resistivity tensor is also observed in the superconducting transition region for one of the films. Its possible origin is discussed.

CORRELATION BETWEEN ELECTRONIC STATES OF OXYGEN AND COPPER IN Y-BA-CU-O DOPED SYSTEMS

Abstract The electronic states of copper and oxygen in YBaCuO and five doped systems (Ba is doped by Sr, Ca;Cu by Zn, Al, Sn, respectively) are studied by XPS, the oxygen contents of samples are determined by thermogravimetric analysis. The experimental results show that there is strong correlation between the electronic states of copper and oxygen. The samples, in which the difference of two states of O 1s is indistinct and the oxygen content high, show partial Cu 3+ oxidation state. In contrast, the samples, in which the difference of two states of O 1s is clear and the oxygen content low, show entire Cu 2+ oxidation state. Comparing these results with the superconductivity of samples, it is found that Cu 3+ state has no direct relationship with superconductivity. The experimental results demonstrate that Cu 3+ state is only the result of the charge balance in the systems.

Electronic structure and superconductivity of the metallic glass

The x-ray photoelectron spectrum (XPS) and the superconducting critical temperature have been measured for the metallic glass with and 1.0. The results show that the density of electron states at the Fermi level increases while decreases as the amount of Fe increases. This implies the spin fluctuation role depresses effectively. The McMillan equation including the spin fluctuation effect has been used to estimate the behaviour. The doping effects of elemental Fe in this system are discussed.

Flux creep in YBa2Cu307-y epitaxial film

The dissipation behavior in the mixed state of YBa2Cu307-y thin film has been studied. It is found that a current-independent resistivity can be described by Arrhenius law and that the activation energy U0 as a function of magnetic field can be described by a power law dependence on H. All of the above results will be included in a vortex glass model.

Critical behavior of a metallic diffuse-fringe film percolation system

Preparation of a metallic diffuse-fringe film system and its electrical properties are reported. The anomalousR-I characteristics, third-harmonic coefficient and critical behavior are observed in the film system. Analysis shows that this behavior is caused by the location-dependent tunneling and hopping effects, which result from. the continuous variation of the diffuse fringe structure of the film due to the increase of the current.