C.D. Wei

Fishtail effects in YBa2Cu3Oy crystal prepared by melt-textured growth method

Abstract The magnetization curves of a YBa 2 Cu 3 O y crystal prepared by a melt-textured growth method were measured; the fishtail effects were observed over the temperature range from 30 K to 87 K. The magnetization curves and the volume pinning force exhibit a scaling behavior, indicating that a single mechanism is involved for all the temperatures studied. The fishtail effects can be described by collective-pinning theory very well at high temperature.

Scaling properties of the magnetization curves of the DyBa2Cu3Oy crystal prepared by the melt-textured growth method

Abstract The magnetization curves of the DyBa 2 Cu 3 O y crystal prepared by the melt-textured growth method were measured from 10 to 85 K with the magnetic field parallel to the c -axis. A scaling behavior of the magnetization curves was observed and explained in terms of an extended Bean model.

Order-disorder and superconductivity in Ca-doped LaBa2Cu3O7−δ

Abstract An analytical electron microscopy study has been undertaken on six superconductors with nominal compositions LaBa 2− x Ca x Cu 3 O 7−δ ( x =0, 0.25, 0.5, 0.75, 1.0 and 1.5). Energy dispersive X-ray spectroscopy (EDXS) analysis shows that the Ca-dopant saturates at doping level x =0.5 for superconducting phases (1-2-3 phases) in these materials, and that further Ca-doping changes only the actual ratio of La/Ba cations and increases the amount of secondary phase Ca 2 CuO 3 . An 80 K T c plateau is found corresponding to this saturation. Convergent-beam electron diffraction (CBED) and transmission electron microscopy (TEM) results show not only that these high- T c 1-2-3 phases are tetragonal with P4/mmm symmetry, but also, based on the observation of several superstructures, that there exists an order-disorder transition in these Ca-doped 1-2-3 phases induced by the variations of La/Ba cation ratios over certain sites in the 1-2-3 unit cell. The high- T c superconductivity in these tetragonal 1-2-3 phases suggests that the Ca cations introduce holes into the bridging site between two conductive Cu-O 2 sheets. This alternative “bridging” hole picture is more general in the high- T c copper oxide family than the “transferring” picture associated with the orthorhombic 1-2-3 phases.

Scaling law and flux pinning in neutron-irradiated YBa2Cu3Oy crystal prepared by melt-textured growth method

Abstract The magnetization curves of neutron-irradiated YBa 2 Cu 3 O y crystal prepared by melt-textured method were measured from 1.5 K to 85 K for H ∥ C . The volume pinning force can be described by a Fietz-Webb separable variable form, which provides strong evidence that flux pinning is the mechanism which determines J c in this material.

Scaling properties of the magnetization curves of the neutron-irradiated YBa2Cu3Oy crystal prepared by melt-textured growth method

Abstract The magnetization curves of neutron-irradiated YBa 2 Cu 3 O y crystal prepared by melt-textured method were measured from 5 K to 85 K for H ‖ C . The virgin magnetization curves, the complete hysteresis loops and the critical current density all exhibit scaling behavior. A new scaling law for the volume pinning force was observed. The scaling behavior observed was explained in terms of the extended Bean model.

Antiferromagnetic ordering observed in the high Tc superconductor GdBa2Cu3O7-x

Abstract Magnetic measurements are performed on the high Tc superconductor GdBa 2 Cu 3 O 7-x . Susceptibility goes through a maximum at 2.6 K, indicating an antiferromagnetic ordering. Magnetic hysteresis loop tends to saturation in high fields at 1.5 K. Magnetic properties of the sample before and after vacuum annealing are compared.

Scaling behavior of virgin magnetization curve of DyBa2Cu3Oy crystal prepared by melt‐textured growth method

The magnetization curves of a DyBa2Cu3Oy crystal prepared by the melt‐textured growth method were measured from 10 to 85 K with the magnetic field parallel to the c axis. The scaling behavior of the virgin magnetization curves is observed and explained in terms of an extended Bean model.

COMPLEX SUSCEPTIBILITY AND CRITICAL CURRENT OF HIGH-TEMPERATURE SUPERCONDUCTORS

The ac complex susceptibility χ′−iχ″ of several bulk samples with very high critical current density Jc have been measured. The results show that these samples are all polycrystalline bulk material, the peak of χ″ still related to losses within “the weak link structure” of the intergrain material. But the intergrain coupling strength of the high Jc samples is much stronger than ordinary high-Tc superconductors. A parameter β=(Tc2(0) −Tc2(h0))/h02/3 has been proposed to characterize the coupling strength.

Raman studies on LaBa2−xCaxCu3Oy system

Abstract Raman studies on LaBa 2−x Ca x Cu 3 O y system with X =0, 0.25, 0.75, 1.0 and 1.5 have been performed. The Raman peaks for LaBa 2 Cu 3 O y with the tetragonal phase structure have been identified with 104, 152, 342, 432 and 488 cm -1 . The results indicated that the phonon peaks associated with OII (Z)-OIII (Z) vibration in the OII-CuII-OIII plane became into two broad, weakened bands as Tc raising due to the increasing of the Ca content. It may imply that the deformation of OII-CuII-OIII plane do not play a key rule in the high - Tc superconducting mechanism.

Antiferromagnetic ordering in the high-Tc superconductors (Y1−xGdx) Ba2Cu3O7−y

Abstract A systematic study of structure, superconductivity, and magnetic properties of (Y 1−x Gd x ) Ba 2 Cu 3 O 7−y was carried out. The susceptibility of YBa 2 Cu 3 O 7−y above Tc is nearly independent of temperature. For all Gd-substituted samples, Gd exhibite the same effective magnetic moment of 7.75 μ B . Antiferromagnetic ordering temperature is 2.3K and 1.7K for x=1.0 and 0.8, respectively. Antiferromagnetic ordering was not observed for x ≤ 0.6 down to 1.5K.