Xu Xiao-nong

Influence of Weak Self-Field Effect on Calculated Activation Energy for Superconducting Thin Film

When the applied held B-a = mu(0)H(a)(H-a > H-p) is perpendicular to the surface of a disk-shaped superconducting thin him with the full penetrated held H-p, the radius R, and the thickness d, a quantitative relation between the average field (B) and the magnetization M is described by [B] = mu(0)[H-a + (0.81d/R)M] for R much greater than d, where the term mu(0)(0.81d/R)M also is the average self-field of the film. Combining this relation with the magnetic flux conservation equation and the Arrhenius relation, the effective activation energy is extracted from the experimental magnetization relaxation and the dynamic magnetization relaxation, respectively.

Field and Current Distribution in Superconducting Film Under Partial Field Penetration

Using finite-element analysis in a self-consistent way with the self-field of the current considered for superconducting thin film, the field and current distributions under partial field penetration are calculated based on the field dependent critical current density function. The field penetrated depth is non-linear with the applied field. Due to the limited resolution of the vibrating sample magnetometer (VSM), the full penetrated field measured by VSM may be much smaller than its actual value.

Highly Oriented HgBaCaCuO Superconducting Thin Films Prepared by Laser Ablation

Pb-doped HgBa2CaCu2Oy films were prepared on the (100) SrTiO3 substrate by using laser ablation followed by post-Hg-vapor annealing. These films are dominated by c-axis-oriented Hg-1212 phase as indicated by x-ray diffraction and electromagnetic measurements. The transition temperature Tc0 is 110 K. The critical current density Jc achieved 106 A/cm2 at 77 K and zero applied field.

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.

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.

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.

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.

New Phenomenon of the Hysteresis of 4He in Vycor Glass

The low temperature part of hysteresis curves of the freezing and melting processes of 4He in vycor glass with the lowest temperature 0.4K were measured, as the pressure ranged from 36.45 to 55.18 (105 Pa). Some novel characteristics of these curves were observed in such high temperature and pressure range. They may result from the existence of superfluid 4He.