2PO1-130 Observation of non-Hall Transverse Voltage in Superconducting Thin-Film

Vladimir I. Kuznetsov and Viacheslav A. Tulin, Institute of Microelectronics Technology and High-Purity Materials,Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia

Presenting Author: V.I. Kuznetsov

The transverse current-voltage characteristics (CVC) of a tin thin-film are studied experimentally. The sample is a thin-film cross of a few micron dimensions with cuts situated in the places of crossing potential and current strips. Occurrence of nonmonotonic transverse voltage induced by vortex motion is observed. This voltage appears due to the interaction between a vortex and an antivortex. We have found that the increasing of the value of current leads to the change of the sign of the transverse voltage. We emphasise that the current direction is unchanged. It is assumed that the change of the sign of the transverse voltage is due to the change of the vortex motion trajectory. This phenomenon occurs at temperature and current close to the superconducting transition.

The appearance, the transformation of the transverse voltage peaks was found to depend on the temperature, the current, weak magnetic and microwave fields. If the value of current is increased more and more, the voltage steps on CVC are observed instead of transverse voltage peaks. It is assumed that this voltage steps are a result of emergence of nonequilibrium regions which is similar to phase slip centres in the centre of the cross.

2PO1-131 Vortex Liquid Resistivity in Disordered YBa₂Cu₃O_7-d Single Crystals

Magnus Andersson, Andreas Rydh, and Östen Rapp. Department of Solid State Physics, KTH, SE-100 44 Stockholm, Sweden

Presenting Author: M. Andersson

The dissipation in the vortex liquid state of high temperature superconductors is still a controversial issue. In a recent paper, we described a model which gives a consistent description of the vortex liquid resistivity in disordered YBa₂Cu₃O_7-d single crystals for all accessible magnetic fields (B < 12 T) and resistivities less than 10 % of the normal state resistivity [1]. This approach was based on the vortex glass model and introduced the "mean pinning energy", U₀(B,T), of the vortex liquid into the vortex glass correlation length, x _g(T). In this paper, we investigate the possible relation between U₀, which determines the vortex glass transition, and the activation energy for flux motion usually encountered in a pinned vortex liquid state. An attempt to reconcile these two different physical pictures of the vortex liquid resistivity will be discussed.

[1] A. Rydh, Ö. Rapp and M. Andersson, Phys. Rev. Lett. 83 (1999), 1850.

2PO1-132 The Effects of field and temperature on V-I curve for Ag-Bi2223 tape

Y.H. Zhang 1,2, S.Y. Ding 1, H. Luo 1, L. Qiu 1, X.F. Wu 1, P. Zhang 1, F.Y. Lin 1, Z.H. Wang 1, Z. Xu 2, and X.X. Yao 1. 1 Department of Physics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P.R. China. 2 Department of Materials and Engineering, Tongji University, Shanghai 200092, P.R. China.

Presenting Author: Y.H. Zhang

Electric transport measurement of different Ag-Bi2223 tapes was performed in different fields and temperatures by four probe method to study the effect of applied field and temperature, sweeping rate of applied current (dI/dt) and pinning strength on the V-I curve and critical current density jc. It is found that while the increasing field or elevating temperature shift the V-I curve towards small currents, the differential resistivity decreases gradually. The V-I curve shifts towards smaller currents with increasing dI/dt as well. When the applied field increases gradually, the V-I curve loses gradually its sensitive to the speed of applied current. It is shown that the critical currents measured with higher dI/dt are smaller than those with lower dI/dt. Critical current (I_c) measured by pulsed method is significantly smaller than those measured by usual procedure. It is also shown that at least for the silver sheathed Bi2223 tapes the ohm heat in the current contacts is not important.

2PO1-133 Where Does the Applied Current flow in Ag-Bi₂Sr₂Ca₂Cu₃O_y tapes

H. Luo, Y.H. Zhang, S.Y. Ding, L. Qiu, F.Y. Lin, P. Zhang, X.F. Wu, Z.H. Wang, and X.Y. Yao, Department of Physics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P.R. China

Presenting Author: H. Luo

We have made numerical calculation and electric transport measurement of Ag Sheathed Bi₂Sr₂Ca₂Cu₃O_y (Ag-Bi2223) tapes to study the path where the applied transport current flows. The effect of sweeping rate of applied current (dI/dt) on the V-I characteristic curve was measured by four probe method. Experimental result shows that the V-I curve shifts towards smaller currents, indicating that the critical currents of these Ag-Bi2223 tapes decrease apparently with increasing dI/dt. To understand the result, the calculation was performed based on the flux creep with logarithmic barrier in bulk superconductor. At the boundary condition of fixed dI/dt, the time and space evolution of the transport current density, j(x,t), in the body and the V-I curve were calculated. Comparing the numerical V-I curve with the measured one, we find satisfactory agreement between the numerical and measured results. Hence, this work provides strong evidence that the applied current flows in body of our Ag-Bi2223 tapes, excluding in either in the surfaces or in percolation paths.

2PO1-134 Effects of the magnetic field orientation on the flux motion in the Bi-2212 whisker

Yuqin Zhou, Ping Zheng, Zhaojia Chen, and Wenhu Wang, Institute of Physics & Center for Condensed Matter, Beijing, China

Presenting Author: Y. Zhou

Bi-2212 superconducting whiskers were obtained by annealing a melt-quenched glass plate in a stream of oxygen for a long time. Different magnetic field orientation were applied to examine the effects on the flux motion. The flux flow resistance in H//c is much larger than that in H//a and H//b. Resistance peak instead of normal tail was found in the R-T curves regardless of the field orientation. This phenomenon was tentatively interpreted by K-T transition.

2PO1-135 Critical dynamics of superconducting HgBa₂CaCu₂O_6+d

Jin-Tae Kim 1, W.N. Kang 2, S.H. Chung 1, Sung-Ik Lee 2, Y.K. Park 1, and J.-C. Park 1. 1 Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon 305-606, Korea. 2 National Creative Research Initiative Center for Superconductivity, Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea.

Presenting Author: J.-T. Kim

The in-plane longitudinal resistivity r _xx(T, H) of HgBa₂CaCu₂O_6+d (Hg-1212) film has been measured as a function of magnetic field H (0 < H < 9 T) and temperature T near T_c. Zero resistance temperature is 123.5 K. The resistivity and X-ray data show no indication that secondary phases exist. The extracted fluctuation conductivity exhibits a dynamic scaling over a wide range of T and H, consistent with three-dimensional XY model; the static critical exponent n is 1.1 ± 0.1 and dynamic critical exponent z = 1.6 ± 0.1.

The c-axis distance of unit cell of Hg-1212 is 12.7 Å which is larger than the value 11.7 Å of YBa₂Cu₃O_7-d (YBCO), therefore the interlayer coupling of Hg-1212 is weaker. The static critical exponent n for Hg-1212 is 1.1 whereas YBCO has 0.669, indicating that weaker interlayer coupling increases the value of n . The value of z (= 1.6) is close to the value 1.5 of YBCO, apparently consistent with the dynamic universality class of superfluid ⁴He. The interlayer coupling strength influences most likely the static critical fluctuation.

2PO1-136 Magnetoresistance in epitaxial YBa₂Cu₃O_7-d thin films

Z.H. Wang, Department of Physics, Nanjing University, Nanjing 210093, China

Presenting Author: Z.H. Wang

It is well known that the relationship between dissipative flux motion and the flux pinning mechanism has been discussed in mixed state of high temperature superconductors (HTSCs). Although some models were proposed to describe the dissipation of HTSCs, they have broken qualitatively down for the complete shape of the resistive transition in the presence of magnetic fields. In this paper, we report the resistive transition and the magnetoresistive curve of an epitaxial YBa₂Cu₃O_7-d thin film in the case of the field applied parallel and perpendicular to the c-axis up to 8T, respectively. When the peak temperature T_p where dR/dT has a peak is defined as the critical temperature, we find that the dissipative resistivity blow T_p can be well fitted by thermally activated flux motion in the presence of a magnetic field. The temperature and field dependence of the pinning potential follows U(H,T)=U₀(1-T/T_p(H))ⁿH^-a , with n=1.5 for H//ab-plane and 1.4 for H//c-axis, showing a 3D behavior. The difference between formula of the pinning potential in this paper with that in literatures is that the temperature T_p is a function of both the magnetic field and current density.

2PO1-137 Critical current and field – dependent resistivity of thin superconducting film

G.M. Maksimova, N.V. Zhelezina and I.L. Maksimov, Department of Physics, Nizhny Novgorod University, Nizhny Novgorod 603600, Russian Federation

Presenting Author: G.M. Maksimova

The resistive state of thin – superconductors is studied taking into account both edge barrier for the flux entry/exit and the bulk pinning. The critical current I_c(H) and current – voltage characteristics are found for a thin narrow superconducting film whose width is less or equal of the effective penetration depth of the magnetic field. The expression for I_c(H) derived analytically is compared with the experimental result. The viscous motion of the vortices is investigated in the hydrodynamic approximation. The effect of both the Ginsburg – Landau type nonlinearity and the bulk pinning on the resistive state characteristics are discussed. It is shown that the differential resistivity of the initial part of the current – voltage curve is decreasing function of the magnetic field if a transition to resistive state occurs from Meissner state. In this case the magnetoresistance of the film is negative. If the film passes into resistive directly from the mixed state field – dependent differential resistivity is described by the Bardeen – Stefen expression.

2PO1-138 Scaling of the Hall resistivity in twinned single crystal YBCO

G. D’Anna 1, V. Berset 1, L. Forro 1, A. Erb 2, and E. Walker 2. 1 IGA, DP, EPFL, CH-1015 Lausanne, Switzerland. 2 DPMC, Université de Genève, 1211 Genève 4, Switzerland.

Presenting Author: G. D'Anna

The vortex Hall effect is studied in a high quality superconducting YBCO twinned single crystal. We observe the Hall anomaly, that is, a sign reversal of the Hall effect in the mixed state, and show that the Hall resistivity scaling law remains unchanged by crossing either the vortex melting or the vortex Bose-glass transitions. When the magnetic field is parallel to the twin boundaries, we obtain the critical exponent 2, that is a constant Hall conductivity below and slightly above the Bose-glass transition. For a tilted magnetic field the exponent is 1.4. In this case, we observe a sharp change of behavior in the Hall conductivity right at the vortex lattice melting point, its slope becoming much larger in the vortex solid phase. This effect being strongly dependent on the current density, we interpret it as a result of vortex pinning. This demonstrates that the Hall conductivity is pinning dependent. A phenomenological model for the Hall resistivity scaling law is also given, directly inspired from the theory of percolation in metallic conductors.

2PO1-139 Nonlinear 2D current flow and electric field domains in inhomogeneous superconductors

A. Gurevich 1, and M. Friesen 1, and V.M. Vinokur 2. 1 Applied Superconductivity Center, University of Wisconsin, Madison, WI 53706. 2 Argonne National Laboratory, Argonne, IL 60439.

Presenting Author: A. Gurevich

New analytical methods of calculation of 2D current flow in inhomogeneous high- T_c superconductors (HTS) with highly nonlinear E-J characteristics were developed. The obtained current distributions around planar defects exhibit long-range current flow patterns and strong enhancement of electric field and dissipation [1,2]. A nonlinear effective medium theory was developed to calculate averaged global J(E,T,B) characteristics of HTS with random macroscopic inhomogeneities. We show that typical inhomogeneities of the local critical current density, J_c, revealed by magneto-optical imaging of HTS, can radically change the global J(E,T,B) curves, requiring re-interpretation of many past experimental results on vortex dynamics. Random inhomogeneities can cause new effects, such as, "fish-tail" effect in J_c(B) and negative differential conductivity, resulting in vortex Gunn effect, bistability and electric field domains [3]. This theory can also describe percolative current flow in BSCCO tapes and YBCO coated conductors.

[1] A. Gurevich and J. McDonald, Phys. Rev. Lett. 81, 2546 (1998).

[2] M. Friesen and A. Gurevich (1999, unpublished).

[3] A. Gurevich and V.M. Vinokur, Phys. Rev. Lett. 83, Oct. 18 (1999).