Zhang Qirui

Effects of Ag doping on the critical current density of YBa2Cu3O7−y

Abstract A series of (YBa 2 Cu 3 O 7−y ) 1−x Ag x samples were prepared using solid state reaction method. On the basis of X-ray diffraction analysis, scanning electron microscopic observation, measurements of the resistivity; a.c. magnetic susceptibility as well as the critical current density, we find that enhancing the intergranular coupling by improving the properties of grainboundaries, while the properties of grains are kept unchanged, helps to increase the critical current density. The dependences of the critical current density on the Ag content, the temperature and the magnetic field are discussed. Our results suggest that eliminating grainboundaries together with improving the properties of grains may be an effective way in raising the critical current density in bulk materials.

Structural and Critical Behaviors of Ag Rough Films Deposited on Liquid Substrates

A new Ag rough film system, deposited on silicone oil surfaces by rf-magnetron sputtering method, has been fabricated. The chrysanthemum-like surface morphology at micron length scale is observed. It is proposed that the anomalous critical behavior mainly results from the relative shift between the Ag atom clusters and the substrate. The discussion of the deposition mechanism is also presented.

DIELECTRIC CHARACTERISTICS IN NANOSTRUCTURED γ-Fe2O3

Abstract The spectra of the dielectric constant and dielectric loss in nanostructured γ-Fe 2 O 3 have been measured by using the ac LRC method. The relaxation time of polarization and the conductance have also been calculated by means of the Debye mechanism, and two kinds of polarization processes with two different relaxation times have been proposed. ©1997 Acta Metallurgica Inc.

A new variational calculation for N-dimensional polarons in the strong-coupling limit.

A novel variational approach is presented for the calculation of the ground-state energy of the polaron in arbitrary N dimensions in the strong-coupling limit. By using the phonon coherent state to represent the wavefunction of phonons, a self-consistent integro-differential equation for the electron wavefunction is derived. The calculated results of the ground-state energy for N = 1, 2 and 3 agree well with the best results in the literature. It is also found that, for arbitrary N, the present results are less than the Feynman path integral ones by small percentages. It is proposed that this approach should be universal for systems involving polarons in the strong-coupling regime.

An influence of Fermi level on superconductivity in orthorhombic single phase Y1−xPrxBa2Cu3Oy

Abstract The Orthorhombic 123 phase compounds Y 1−x Pr x Ba 2 Cu 3 O y (x=0.1, 0.3, 0.5, 0.7 and 1.0) were synthesized. By Hall coefficient measurements of the samples, it is found that, with the increase of Pr content, the concentration of hole-type carriers decreases. Simultaneously, a transition from metal to semiconductor indicated by the R(T) curves occurs. The carrier concentration exhibits an activated behavior. A band structure model involved Pr 4+ as a donor (found by XPS) is used in discussing the effect of Pr substitution, suggesting that the position of Fermi level is very important to the high-T c superconductivity of the 123 phase compounds.

Flux Pinning in Y1-xMxBa2Cu3O7-δ (M=Gd, Eu and Sm) System

In this paper, the flux pinning effects in Y1-xMxBa2Cu3O7-δ (M=Gd, Eu and Sm) samples are reported. It is found that there is another pinning mechanism under higher magnetic fields in addition to that under lower fields. This pinning effect may stem from the lattice stress field resulting from the partial substitution of the rare earth elements with different atomic radii for Y.

Effects of substitution in different atomic sites in superconducting YBa2Cu3O7

Abstract By doping superconducting YBa 2 Cu 3 O 7−y samples in different atomic sites and measuring their crystal structure, thermal stability, XPS, Mossbauer spectrum and superconductivity, we have investigated the characteristics of electronic structure and superconductivity of the 123 compounds. We found the Cu O planes as well as the coupling between themselves is very important to high-T c superconductivity in this system. Meanwhile, there is a strong correlation between superconductivity and thermal stability of the samples, indicating the high-T c superconductivity in 123 compounds is phonon mediated partially.

Evidence of weakening of the Pr4f-O2p hybridization in Y0.4Pr0.6Ba2−xSrxCu3O7-δ: a photoemission spectrum study

Abstract We have performed a photoemission study on Y 0.4 Pr 0.6 Ba 2−x Sr x Cu 3 O 7-δ system. It was shown that the binding energies of Y3d, Ba3d, Sr3d and O1s decreased while the Pr3d core level increased with increasing Sr content. The result strongly suggests that Sr-doping leads to the weakening of Pr4f-O2p hybridization. We present a band picture to interpret the revival of superconductivity induced by the Sr substitution.

OXIDATION STATE OF CU AND SUPERCONDUCTIVITY IN BILAYER TL-BASED CUPRATES

The oxidation state of Cu in the homologous Tl2Ba2Can-1CunO2n+4+δ (n = 1 - 4) series has been analyzed in terms of the bond valence sums. It is found that the totally mobile hole concentration nH,t of the type-I CuO2 planes at the optimal level increases with increasing number of CuO2 layers per unit cell from 1 to 3, and the largest nH,t is observed to be 0.474 when n = 3, which just corresponds to the highest Tc in the bilayer Tl-based materials, and then it decreases with the increase of n. It is indicated that the correlation between the maximum value of Tc and n is similar to that between nH,t and n.

Flux pinning effect of small size defects in YBa2Cu3O7-δ

Abstract In this paper, it is reported that the small size defects, which result from the introductions of Y, Ba, Cu and O ionic vacancies and the partial substitutions of Ln (Gd, Sm and Eu) for Y in YBa 2 Cu 3 O 7−δ system, may pin the flux lines. From the experimental results, authors suggest tha the flux pinning effect at lower field relates to the magnetic interaction between the vortex and the surfaces of grains in the samples; and that the flux pinning at higher field belongs to the core interaction mechanism in the samples with ionic vacancies and stems from the elastic interaction between the vortex and the lattice stress field in the partially substituted samples.