Ji Helin

Flux Creep Determination of Field Dependence of Magnetic Jc at Low Temperatures

The effective activation energy of one HgBaCaCuO 1223 superconductor as a function of current density was extracted from the magnetic relaxation measurements below 30 K, and found to obey a power law. We also obtained effective activation energy as a function of current density from the magnetic hysteresis loops (M-H). The data sets for the M-H loops at different temperatures overlap each other, and all fall on the smooth Ueff(J,0) curve based solely on the magnetization relaxation measurements. This indicates that the field dependence of magnetic critical current density is determined by the flux creep via nonlinear effective activation energy at low temperatures.

EXTENDED RATE-EQUATION FOR FLUX MOTION IN HIGH-TC SUPERCONDUCTORS

The rate equation for flux motion in high-Tc superconductors is extended to take the contribution of field sweep into account in this work. It is shown that the time and field sweep rate () dependence of magnetization are equivalent approaches to the study of flux creep properties in high-Tc superconductors. The agreement of the effective activation energy Ueff (J) extracted from the time relaxation and that from magnetic hysteresis loops measurements on a melt-textured YBa2Cu3O6+x sample supports this extended rate equation for flux motion in high temperature superconductors.

Study of halogen doping in polycrystalline YBa2Cu3O7-δ

This paper presents systematically the doping effects of halogens in YBa2Cu3O7-δ polycrystalline samples, with a series of concentrations of fluorine, chlorine, bromine or idodine. All samples were prepared by chemical method of citrate pyrolysis, which makes the halogen concentration in the samples more uniform. Samples doped with F have Tc as high as 94.5 K, Cl, Br and I dopings also raise Tc to a certain extent. In addition, the influence of halogen doping on critical current density, morphology, constitution and so forth is discussed.

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.

Formation and Growth of Stacking Fault in YBCO by Electron Irradiation

This paper studies a new electron irradiation effect of the formation, movement and growth of stacking fault in YBa2Cu3O6+x. The energy of electron irradiation is about 160 keV. It would be a promising way to enhance the density of pinning centers.

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.

Study of Flux Pinning in Zone-melted YBCO by SQUID

Magnetic hysteresis loops and magnetization relaxation of a zone-melted Y Ba2Cu3 O6+x sample were measured by superconducting quantum interference device (SQUID) at temperature from 4.5 to 70 K. The logarithmic behavior of magnetic decay at 4.5 K and the gradual deviation as the temperature is increased demonstrated the important role of nonlinear current density dependence of effective flux pinning potential U(J) in high temperature superconductors.

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.

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.