2PO1-80 Critical currents and pinning mechanism of superconducting Nb-50% wt Ti Alloys

H.M. Shao 1, Y.M. Cai 2, S.A. Aruna 1, C.J. Wang 1, T. Yang 1, X.Z. Wu 3, L. Zhou 3, C.R. Li 3, T.J. Zhang 3, K.G. Wang 3. 1 Department of Physics and National Laboratory of Solid State Microstructures, Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, P.R. China. 2 Department of Basic Sciences, Nanjing University of Chemical Technology, Nanjing, 210009, China. 3 Superconducting Institute, Northwest Institute for Nonferrous Metal Research, Xian 710016, P.R. China.

Presenting Author: S.A. Aruna

The results of the transmission electron microscopy examination as well as the critical current measurements at 4.2K under fields of up to B_c2 for superconducting Nb-50% wt Ti multifilamentary wire optimized with a new method of multiple heat treatment and processing cycles are presented. The pinning mechanism of the optimized samples has been discussed to a great extent with respect to these results, demonstrating the theoretical basis for the optimization process. In order to further improve the T_c of the Nb-50% wt Ti alloys under high fields, it is necessary to control the flux shear motion at high fields besides those of both the fine-scale sub-band and a-Ti precipitates.

2PO1-81 Electrostatic analogies in the problems of vortex-defect interaction

A. Buzdin and M. Daumens, Centre de Physique Théorique et de Modélisation, Université Bordeaux 1, F-33405, Talence Cedex, France

Presenting Author: A. Buzdin

We consider the general case of vortex interaction with different types of mesoscopic defects. When the distances of interest are smaller than the London penetration depth, a useful analogy with electrostatic problems can be formulated. We demonstrate how this analogy permits to obtain the solutions to a number of problems of electromagnetic pinning of vortices (e.g. a vortex-elliptic hole interaction) and provide recommendations to optimize conditions to reach higher critical currents. According to our results, the segment-like defects are the most efficient for vortex pinning: the attraction region is of the order of the segment length whilst the excluded superconducting region is very small.

We also examine how the surface defects influence the field of the vortex entrance in superconductors and stress that such an effect is very important for layered superconductors as it could decrease many times theoretical entrance field.

2PO1-82 Nucleation of defects and depinning in vortex rows: the weakly disordered Frenkel-Kontorova model

R. Besseling 1, T. Dröse 2, and P.H. Kes 1. 1 Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands. 2 Institut fur Theoretische Physik, Hamburg University Jungiusstrasse 9, D-20355, Hamburg Germany.

Presenting Author: R. Besseling

Depinning of quasi 1-dimensional vortex filaments may occur in superconductors with an inhomeogeneous "pinscape" as well as in Josephson Junction Arrays. In both cases the pinning potential acting on the vortex row is periodic with random distortions. While depinning in periodic potentials is well understood, the effect of the random distortions on depinning/Commensurate-Incommensurate (C-I) transitions needs more investigation.

We study a chain of interacting particles in a randomly distorted periodic potential. We show that, below a critical disorder, (in)commensurability between the chain and the potential has a large effect on the nature of the depinning. In the C-phase depinning occurs via periodic interstitial/vacancy nucleation at a "weakest spot". This lowers the critical force f_c compared to the pure case and splits the moving chain in two regions with different density. In the I-phase, the disordered Peierls landscape pins the defects, thereby increasing f_c with respect to the pure case.

We map the system to the problem of a driven (kinked) line in a distorted washboard potential. The decrease/increase in f_c in the C/I phase with disorder is calculated and we show that the C-I transition is smeared when the forces for nucleation and depinning of defects become comparable.

2PO1-84 Effect of Field Nonuniform in SQUID Magnetometer on the Measurement of Superconductors having Strong Flux Pinning

S.Q. Guo, National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China

Presenting Author: S.Q. Guo

The effect of field nonuniform in SQUID Magnetometer on the measurement of superconductors, which have strong flux pinning, have been investigated. We found that:

1. When decreasing field, the nonuniform field in SQUID magnetometer greatly affects the measured of magnetic hysteresis loops. When the field non-uniform is 1% , the area of measured magnetic hysteresis loop may decrease more than 50%. Even when the field non-uniform is only 0.05% and the fields are higher than 1T, there still have more than a few percent errors in the measured magnetic hysteresis loop;

2. The higher of magnetic field, the larger errors of measured magnetic hysteresis loops. For example if 2T field appears 10% errors of measured magnetic hysteresis loop, then 4T field will appear more than 20% measured errors. It means that when the fields double, the measured errors will be more than double;

3. The demagnetic factor does not affect from the measuring magnetic hysteresis loops.

The physical explanation, which is totally different with the explanation in the Manual of SQUID Magnetometer, is pointed out.

The method to modification the experimental data is proposed. After the modification the errors of measured magnetic hysteresis loops will be less than 3%, even if the experimental data have more than 50% errors which are produced by the nonuniform field.

2PO1-85 The Effects of Initial Condition on The Flux Creep in High-Temperature Supercondutors

C.D. Wei, Department of Physics, Peking University, Beijing 100871, China

Presenting Author: C.D. Wei

The numerical solutions of the flux creep equation for Anderson-Kim type activation energy and periodic pinning potential were studied. The relaxation of B(x,t) was calculated for five characteristic points A, B, C, D and E on the magnetization curve, for points B, C, D and E the flux has penetrated to the center of the sample, for point A the flux only partially penetrate the sample. The relaxation of B(x,t) depends on the position x and initial condition, the relaxation rate is maximum near the center and decrease toward the edge of the sample. For points B, C, D and E, in the plot of [B(x,t)-B(x,t_a)]/[B(x,0)-B(x,t_a)] versus ln(t), the data approximately fall on a single curve for different position x, where t_a is a constant. But point A does not show such scaling behavior. These results indicate that if we measure the relaxation of B(x,t) for different positions x and derive the thermal activation energy U from the measured relaxation data, the values of U should be approximately uniform if the relaxation start from the magnetic induction profile corresponding to points B, C, D or E. If the relaxation of B(x,t) starts from a profile similar to that corresponding to point A, the values of U derived from the relaxation data should have larger difference for different position x. The initial condition has strong effects on the relaxation behavior of the superconductors. The relationships between this scaling behavior and the relaxation of average magnetic moment have also discussed.

2PO1-86 Magnetic Hysteresis And Pinning Properties of HTSC

Yu Li Tang, Xiaonong Xu, and Xin Jin, Department of Physics and National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, 210093, P.R. China

Presenting Author: Y.L Tang

A simple method based on the experiential expression proposed by M.E. McHenry et al. is applied to calculate the activation energy in HTSC. Different magnetic hysteresis loops were measured with different temperature under the same outer field scan velocity, and these results are used to investigate the pinning properties of HTSC. Result shows that this method is simpler than ac susceptibility one, moreover it could be extended to larger temperature scale.

2PO1-87 Influences of Sample Geometry on the Derived Activation Energy of HTSC by dc and ac Magnetic Measurement

Xiaonong Xu, Yuli Tang, Xin Jin, and Xixian Yao, Physics Department and Center of Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, P.R. China

Presenting Author: X.N. Xu

The most effective experimental method of studying the electromagnetic properties of HTSC is by magnetic measurement. However, the theoretical formulas for the activation energy by ac or dc magnetic measurement were deduced for the sample with infinite length in one or two dimensions. In certain situations, false information will be obtained if the actual sample shape is different from the ideal model. The average field inside the sample is indirectly obtained from the measurement of the average magnetization and the applied field. The average field and its time-variation determine the activation energy by dc magnetic relaxation, dc dynamic relaxation and ac magnetic susceptibility. ac determined current density is based on the relation among the full penetration field, sample geometry and current density. In this paper, the correlation formula among the average magnetic field, applied field and average magnetization, and the correlation formula between the full penetration field and current density for the superconductors with continuously varied geometry parameters are deduced by numerical calculation. In addition, the influences of the sample geometry on dc and ac magnetic measurements are studied.

2PO1-88 Collective Flux Creep in YBaCuO (1-2-3): Influence of the Microstructure

Kamal Frikach 1, Sadok Senoussi 2, Mosbahb Faycal 3, and Pierre Manuel 4. 1 Physics Dept., Sherbrooke University, Quebec, Canada. 2 Laboratoire de Physique des Solides (associé au CNRS, URA 0002), Université Paris Sud, 91405 Orsay Cedex, France. 3 Université de Constantine, Département de Physique, Algerie. 4 EDF, 1. Avenue du Général de Gaulle, 92141 Clamart Cédex, France.

Presenting Author: K. Frikach

We studied the flux creep in various YBa₂Cu_3-xFe_xO_7-y superconductors (aligned powder, single crystals, thin films and melt textured samples) prepared by various techniques. The field dependence (0< H< 60 kOe) of the pinning energy exhibits two or three distinct pinning regimes depending on the material. The magnetic relaxation rate has a minimum value at a threshold field H_th of the order of 0.4 x H_m2 where H_m2 denotes the field of the second peak of the fishtail anomaly. All the present materials deviate from the linear logarithmic relaxation S and the magnitude of this deviation has a minimum at approximately the same field, H_th. This behavior is consistent with the existing collective creep model, in the field domain where J grows with H. However, the high H behavior (H> H_m2) is ascribed to the onset of thermally activated dislocations in the vortex lattice, and the breakdown of the elastic approximation usually assumed in the collective creep theory. Much attention is paid to the magnetic relaxation at short time scales, down to 0.2s.

2PO1-89 Flux creep in heavily lead doped Bi2212 single crystal

K.K. Uprety, J. Horvat, X.L. Wang, M. Ionescu, H.K. Liu, and S.X. Dou, Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia

Presenting Author: K.K. Uprety

A comparative study of normalised flux creep rate S(T,H) between heavily lead doped Bi2212 single crystal and pure Bi2212 single crystal has been performed. A sharp anomaly in S(T) was observed at a temperature T = T_CR, a cross over temperature below which collective pinning theory is valid for Bi2212 pure single crystal. Observed S(T) over a wide temperature range for the lead doped sample indicated a shift of T_CR to a higher temperature. X-ray measurements showed a decrease of c-axis parameter with increasing lead doping for this sample. The reduction of c-axis parameter improved the inter layer coupling of the pancake vortices, enhancing the vortex pinning and causing the shift of the peak in S(T) towards a higher temperature. The critical current density J_c as a function of field also indicated strong pinning in the lead doped sample, as compared to pure Bi2212 single crystals.