Haiying Chen

Structural and magnetic properties of (Gd, La)1.85Ce0.15CuO4

Abstract Structural and magnetic properties of Gd 1.85− x La x Ce 0.15 CuO 4 are studied. The samples re single phase with the T′ structure when x ≤1.2. Lattice parameters increase linearly with increasing La content. Weak ferromagnetism associated with the canting of the copper moments is observed in the compounds with x ≤0.2. The weak ferromagnetism disappears when the lattice parameter is a≥3 A . The structure of Gd 1.85− x La x Ce 0.15 CuO 4 is unstable at high temperature. No superconductivity can be induced down to 1.5 K by reduction annealing in N 2 .

Distribution of magnetic flux in high-Jc YBa2Cu3O7 superconductors

Abstract We present high-resolution Bitter patterns which reveal the flux distribution in high-J c melt-textured growth superconductor YBa 2 Cu 3 O 7 at 64K. Glass type of spatial configuration of vortex was observed and it supports the vortex glass model. In addition to the conventional pinning sites, such as twin boundaries, grain boundaries, microcracks etc., the dispersed small particles, ≤0.5μm in size, of the second phase also cause flux pinning. The strongest pinning appears in the vicinity of large grains of second phase, where the crystal lattice is seriously distorted.

Weak ferromagnetism and superconductivity in (Gd,R)1.85Ce0.15CuO4 with R = Pr, Nd, and Sm

Abstract Gd 1.85− x R x Ce 0.15 CuO 4 compounds with R = Pr, Nd, and Sm are prepared by regular solid state reaction. Structure deformation, weak ferromagnetism, and superconductivity are studied. Experimental results showed that weak ferromagnetism appears in the deformed compounds. Superconductivity appears in the composition where weak ferromagnetism disappears, but there is no direct correlation between structure deformation and superconductivity. Structure instability prevents a proper reduction to introduce superconductivity.

Effect of Ni and Zn substitution on magnetic ordering in Gd2CuO4

Abstract Ceramic samples of Gd 2 Cu 1−x Ni x O 4 with x =0, 0.03, 0.05, 0.07, 0.09 and 0.11, and Gd 2 Cu 1−x Zn x O 4 with x = 0.01, 0.03 and 0.05, were prepared. Magnetic measurements showed that the antiferromagnetic ordering temperature of Cu decreased due to the substitution of Cu by Ni or Zn. The effect of Ni on suppressing antiferromagnetic ordering is larger than that of Zn. The results are compatible with the substitution effect on superconductivity in Nd 1.85 Ce 0.15 CuO 4 .

Magnetic interaction between Cu ions in LaxSr14-xCu24O41 (x = 0-6.0)

Abstract The compound La x Sr 14− x Cu 24 O 41 contains a simple CuO 2 chain and a two-leg Cu 2 O 3 ladder. All samples follow the Curie–Weiss law at high temperature. Six holes are inherently doped in Sr 14 Cu 24 O 41 . With increasing x , holes were reduced and the Weiss temperature changed from negative to positive indicating that the magnetic interaction between Cu ions in a chain changes from antiferromagnetic (AF) to ferromagnetic (FM). Magnetic susceptibility χ is described by a weakly interacting FM chain for x ≥4. A long-range AF ordering with FM correlation within the chain is observed for x ≥5. Our results suggest that doped holes are mainly in the CuO 2 chain.

A nonlinear response function for the scaling behavior of vortex matter near the transition temperature

Abstract We show that the electromagnetic response of high-temperature superconductors with inhomogeneties or defects as pinning centers can be well described by a universal nonlinear response function. This function is compatible with the different model barriers U ( J ), suggested so far. It is able to give a consistent description of the vortex system near the transition temperature. A comparison with the scaling behavior of the experimental measured isothermal current–voltage curves with YBCO samples shows reasonable agreement.

Superconducting fluctuation of the YBCO films

Abstract We measured the resistivity of several c-axis-oriented YBCO films under the magnetic fields, the temperature ranged up to ln(T c /T) = 0.68 and magnetic field up to 7 T. By fitting the data to the Aslamazov-Larkin (A-L) and Maki-Thompson (M-T) terms of the fluctuation contribution, we got reasonable parameters value in the theory. By measuring the magnetoresistivity up to 164 K we didnt find any proof of any rising trend with temperatures increasing of the fluctuating resistivity.

Anisotropic vortex structure in YBa2Cu3O7 thin films at liquid nitrogen temperature

Abstract Bitter-pattern observations of the vortices for flux trapped at the liquid nitrogen temperature in YBa 2 Cu 3 O 7 thin films with the c -axis and 〈110〉 direction parallel to the surface are reported. The penetration-depth anisotropy is estimated to be 5 from the vortex shape. The shape and distribution of the spots give a visual description of the homogeneity in the film.