Zhang Yi-tong

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.

Flux creep, flux pinning and critical current density of Y-, Gd-, and Tl-based superconductive thin films

Magnetic hysteresis loops have been measured for YBa2Cu3O6+x, GdBa2Cu3O6+x and Tl2Ba2Ca2Cu3O10-y films at different temperatures with a vibrating sample magnetometer. Based on Bean model, magnetic critical current density Jcm has been derived approximately. The field and temperature dependence of Jcm for all samples can be written as: Jcm = Jc0(T) × f(B,T), where f(B,T) = 1 - Aln(B/B0(T)), which is similar to the transport critical density strongly affected by flux creep: Jct = Jc0(B,T)(1 - (kBT/U0)ln(BΩd/Ec)). The extraordinary similarity suggests that Jcm is determined not only by flux pinning but also by flux creep. In the first formula, f(B,T) may be correlated with the effect of flux creep on Jcm, and Jc0(T) is determined by flux pinning. Jc0(T) is independent of magnetic field and is proportional to (Tc - T). Similar results have also been found for other samples. It may be the common characteristic of high-Tc superconductors. Magnetic relaxation of YBa2Cu3O6+x and GdBa2Cu3O6+x films has been measured at LN2 temperature. Using the equation of Hagen et al., effective activation energy U0 has been deduced, which is about 0.3 eV, one order bigger than the value obtained by A = kBT/U0. U0 is almost independent of the field in the field range selected.

Microstructure Study in Slow Neutron Irradiated Bi-2223

The effect of slow neutron irradiation at a low fluence of about 108n/cm2 on the microstructures of Bi-2223 polycrystalline samples was observed by transmission electron microscopy. By comparative study of the defect structures in a given area in one sample, we get evidence for the flux pinning enhancement by low fluence slow neutron irradiation.

CRITICAL CURRENT DENSITY OF Y-Ba-Cu-Sn-O SUPERCONDUCTORS

The magnetic curves of bulk Y-Ba-Cu-Sn-O have been studied in the temperature range 60-77 K and in fields up to 2.4 T. The results indicate that Jc varies with the magnetic field as a power law and decreases exponentially with temperature. The critical current density Jc can be improved by the substitution of Sn for Cu.

INVESTIGATION OF THE EFFECT OF SLOW NEUTRON IRRADIATION ON HALOGEN-DOPED HIGH-T c SUPERCONDUCTORS AND ITS MECHANISM

Irradiation effect of low-fluence (~108 n/cm2) slow neutrons on halogen-doped superconductors is presented in this paper. And the mechanism of the effect is also described from the viewpoint of nuclear physics for both fast and slow neutrons on high-temperature superconductors (HTSC). It is shown experimentally and theoretically that slow neutrons of low fluence has a similar irradiation effect to that of fast neutron beams with an energy En > 0.1 MeV and fluence 1016 - 1018 n/cm2. However, quite different mechanisms are involved in them: Fast neutrons transfer their energies through elastic scattering in HTSC, whereas slow neutrons give off their energies during the slow neutron capture (n, γ) reaction.

Measurement of anisotropic activation energies in high-Tc superconductors

For single crystalline and melt-textured samples there exist several different activation energies for fluxoid creep, which are related to various vortex orientations and hopping directions. We discuss how to obtain these anisotropic activation energies from magnetic relaxation data and present measured results of a melt-textured YBCO sample.

Thermally activated flux motion in high-Tc superconductors

Using the modified Anderson-Kim theory of flux creep and the Bean model, we derive an analytical expression for magnetic relaxation of high-Tc superconductors, which is applicable in the whole flux creep region. The high driving force limit of the expression gives the usual logarithmic decay law and its low driving force limit takes the exponential form. Theoretical analysis is in agreement with the existing experimental results.

Memory Effect in Ceramic Superconductors

We measured the time decay of the magnetic moment of high Tc superconductors YBa2Cu3O7-δ ceramic samples and found the memory effect which has hitherto been reported only in single crystals. The temperature and field ranges, especially the memory time are different from those of the latter. We believed that the memory phenomenon in ceramic samples at liquid nitrogen temperature can only be understood in a combined picture of both flux creep and glass behaviour.