1PO2-51 Room temperature oxidation of sintered YBCO using µs pulsed electrolysis

Francesco Celani 1, Daniele Di Gioacchino 1, and Paolo Tripodi 2. 1 INFN-LNF, Via E. Fermi 40, 00044 Frascati, Italy. 2 SRI International, 333 Ravenswood Ave, 94025 Menlo Park, CA.

Presenting Author: D. Di Gioacchino

Sintered YBCO oxygen poor has been oxygenated at room temperature using µs pulsed electrolysis in acqueous solution. An increasing of superconducting properties by the measurements of the ac higher harmonic susceptibility has been found. A quantification of the pinning potential and critical current, near T_c, using a numerical solution of the ac magnetic field diffusion equation have been done.

1PO2-52 Improvement of the Flux Pinning properties of Hg based Single Crystals by a substitution of Ba by Sr

Manuel Angst 1, Janusz Karpinski 1, Serguei Kazakov 1, Hansjörg Schwer 1, Roman Puzniak 2, Andrzei Wisniewski 2, and Michel Willemin 3. 1 Solid State Physics Laboratory, ETH, 8093 Zurich, Switzerland. 2 Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland. 3 Physik Institut der Universität Zürich, 8057 Zürich, Switzerland.

Presenting Author: M. Angst

Mercury based cuprate superconductors have very high transition temperatures, but their flux pinning properties are inferior to the one of, e.g., YBa₂Cu₃O_7-x. An important parameter controlling the pinning properties of the material is the effective mass anisotropy of superconducting carriers. All the intrinsic parameters controlling the flux pinning, as expressed in the position of the irreversibility line, can be changed by chemical substitutions. Single crystals of mercury based cuprate superconductors were grown under high pressure. In some crystals, Hg was partially substituted by Re or Pb and Ba by Sr. With Re substitution, it was possible to obtain crystals with up to seven CuO₂-layers. The flux pinning properties of the grown crystals were measured by SQUID magnetometry, and their irreversibility lines were determined and compared. While substitutions at the mercury site did not change the irreversibility line significantly, a substitution of Ba by Sr enhanced the irreversibility field considerably. The intrinsic superconducting parameters, such as the penetration depth, the coherence length, and especially the effective mass anisotropy, were determined from torque magnetometry measurements performed in the region of reversible magnetic behavior. The relations between the irreversibility line, the effective mass anisotropy and structural parameters are shown.

1PO2-53 Critical currents and pinning mechanism of superconducting Nib₃Sn Multifilamentary tape

Y.M. Cai 1, H.M. Shao 2, S.A. Aruna 2, C.J. Wang 2, T. Yang 2, and J.R. Fang 3. 1 Department of Basic Sciences, Nanjing University of Chemical Technology, Nanjing 210009, China. 2 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. 3 Changsha Research Institute of Mining and Metallurgy, P.O. Box 67, Changsha 410012, China.

Presenting Author: S.A. Aruna

The results of the transmission electron microscopy examination as well as critical current measurements at 4.2K under the fields of up to nearly Bc2 for superconducting Nib₃Sn multifilamentary tape optimized with a new method of multiple heat treatment and processing cycles are presented in this paper. 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.

1PO2-54 Dip Anomaly in ac Screening Response and Peak Effect in LuNi₂B₂C and YNi₂B₂C Single Crystals

Vladimir N. Narozhnyi 1,2, Günter Fuchs 1, Konstantin Nenkov 1, Dieter Eckert 1, Angelika Teresiak 1, Karl-Hartmut Müller 1, and Paul C. Canfield 3. 1 Institute for Solid State and Materials Research, P.O. Box.270016, D-01171 Dresden, Germany. 2 Institute for High Pressure Physics Russian Academy of Sciences, Troitsk, Moscow Region, 142092, Russia. 3 Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.

Presenting Author: V.N. Narozhnyi

Simultaneous measurements of ac susceptibility and dc magnetization have been carried out for LuNi₂B₂C and YNi₂B₂C single crystals in magnetic fields H up to 50 kOe (H½ ½ [001], [100], [110]) and temperatures T>4.5 K. A pronounced dip anomaly in the ac screening response in the superconducting mixed state has been found for both compounds. For all three directions of H the positions of dip anomalies Hdip» (0.8¸ 0.9)H_c2(T) correspond well with the positions of the maxima in magnetic hysteresis loops observed slightly below H_c2(T) by dc magnetization measurements at 30£ H£ 50 kOe. Therefore the nature of the observed dip anomaly in the ac screening response is connected with the peak effect of the superconducting critical current. The value of the dip anomaly increases with increasing magnetic field (or decreasing temperature) and is observable at T£ 12.5 K and H310 kOe.

An unexpected considerable anisotropy of the dip anomaly has been found for the H directed in the basal tetragonal plane of LuNi₂B₂C, the anomaly being more pronounced for H½ ½ [110]. At the same time, the dip anomaly is practically isotropic for H in the basal plane of YNi₂B₂C. Most probably the observed for LuNi₂B₂C anisotropy of the dip anomaly is connected with earlier reported stronger H_c2(T) anisotropy in the H_c2(T) in basal plane for LuNi₂B₂C in comparison with YNi₂B₂C. No indication of dip anomalies has been observed for LuNi₂B₂C and YNi₂B₂C polycrystalline samples.

1PO2-55 Vortex bulk-nucleation and edge-tunneling in superconducting thin films

Giancarlo Jug, Dipartimento di Scienze, Universita' dell'Insubria, 22100 Como, Italy, and Roberto Iengo, SISSA, 34014 Trieste, Italy

Presenting Author: G. Jug

Vortex dissipative dynamics implies a non-zero resistance in superconductors. Even in the absence of an external magnetic field (B=0), we argue that vortex pair nucleation can induce a supercurrent decay; this becomes observable for the high-T_c superconducting thin films at high current densities. We present a general theory [1] for dissipative pair nucleation in films and work out the signature of an underlying 2D pinning array (oscillations of the resistance as function of the current) [2]. Without pinning, our functional form for the decay rate agrees with experiments. Our theory is extended to estimate vortex edge-tunneling effects [3], seen to be also important, and we discuss the effect of a longitudinal non-hermitean bias on the periodic pinning-potential induced localization/delocalization of vortices in the films.

[1] R.Iengo and G.Jug, Phil.Mag.B 78 (1998) 13; R.Iengo and G.Jug,Phys.Rev.B 52 (1995) 7536.

[2] ibid. 54 (1996) 9465.

[3] ibid. 54 (1996) 13207.

1PO2-56 The Langevin simulation of Vortex matter's response in glass state to external force and memory effect

G. Liu and R.S. Han, Department of Physics, Peking University, Beijing 100871, P.R. China

Presenting Author: G. Liu

Start from the vortex glass and Bose glass theory, we get the Langevin equation and Fokker-Planck equation for vortices with many-body effect been taken into account. We used the Langevin equation's Monte-Carlo simulation to investigate the response of vortices to an external force. We find that when J<J_c, the vortex line behavior as a elastic string at the very beginning of the motion for both vortex glass and Bose glass. And the motion is mainly within the pinning well with the typical time Tr. After t>Tr, the vortex glass will distort between two adjacent pining well, while there is no such motion in Bose glass. Finally, on a longer time scale, the vortex glass will distort further through thermal hopping of vortex segments and reach to the metastable state. There is no memory effect in Bose glass which is the direct result of plastic creep and is common in vortex glass. And the inertial effect is not dominated which support the opinion of the formation and growth of easy-flow channels.

1PO2-57 Asymmetric Pinning in Superconductors with Interpenetrating Arrays of Weak and Strong Pinning Centers

B.Y. Zhu 1, B.R. Zhao 1, L. Van Look 2, and V.V. Moshchalkov 2. 1 National Laboratory for Superconductivity, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China. 2 Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium.

Presenting Author: B.Y. Zhu

Molecular dynamics simulations of the current-voltage characteristics, V(I), have been performed for regular pinning arrays consisting of two interpenetrating triangular lattices of weak and strong pinning centers. In this case, an applied Lorentz force F_L provides the depinning force which is directed either from weak to strong pinning site or vice versa, depending on the polarity of F_L. This leads to a pronounced asymmetry of the critical currents I_c F_L and I_c (-F_L), revealed in our numerical simulations for the vortex lattice originally coinciding with the array of strong pinning centers. The dependence of the asymmetry on the shift of the two pinning sublattices is also investigated.

1PO2-58 Comparison of two methods of flux creep analysis: Flux diffusion equation versus current-voltage curve approach

D.V. Shantsev 1, A.V. Bobyl 1, Y.M. Galperin 2, and T.H. Johansen 2. 1 Ioffe Physico-Technical Institute, St. Petersburg, Russia. 2 Department of Physics, University of Oslo, Norway.

Presenting Author: D.V. Shantsev

We compare two commonly accepted approaches for flux creep simulations in superconductors. The first approach is based on a direct analysis of the equations for flux diffusion, while the second one employs phenomenological nonlinear current-voltage curves incorporated into the Maxwell equations. To compare the methods we conduct numerical simulations. The current-dependence of the pinning energy in the diffusion method was adjusted to the chosen current-voltage curve. Nevertheless, the two methods give clearly different results, the difference being most pronounced in the areas of low flux density. The results of both simulations of time-dependent flux and current distributions are compared with magneto-optical data obtained for a narrow YBaCuO current-carrying strip with different magnetic pre-history. We conclude that the simulation based on the flux diffusion equation provides an adequate description of the experimental results. Moreover, our simulations based on B-independent current-voltage curves do not reproduce some features of flux and current distributions, specifically in the regions of large flux density gradient.

1PO2-59 Brownian Motion of Coupled Vortices in Type-II Superconductor: Response to External Fforces as a Interacting Bosons System

Liu Gang, Han Rushan, and Daole Yin, Department of Physics, Peking University, Beijing 100871, P.R. China

Presenting Author: L. Gang

The many-body interaction's influence on overdamped Brownian motion of vortices in Type-II superconductor is investigated. When the vortices are under the influence of thermal fluctuation and dissipation, they behave as zero-temperature interacting bosons which is similar to the mapping of vortex line on boson that used by Nelson etc. Using the Green's function method for boson, and in the low density limit, the N-particles Fokker-Planck equation that describe the dynamical behavior of the system is substituted by an effective Fokker-Planck equation. And the I-V characteristic is investigated by this equation with random pinning potential been taken into account. Comparing with single vortex limit, we find the repulsive interaction among vortices increases the probability of a vortex escape from the pinning well.