2PO1-110 Vortex dynamics of driven disordered vortex lattice

Zhengkuan Jiao and Yigang Cao, Department of Physics, Zhejiang University, Hangzhou 310027, P.R. China

Presenting Author: Z. Jiao

Using a simple repulsive vortex -vortex interaction model, we numerically investigate the vortex dynamics of the driven vortex lattice in a disordered two-dimensional superconductor. At high driving force we see evidence of an ordering of the moving vortices in the flux flow regime. At low driving force and temperature, evidence of plastic and filamentary flow is seen. Scaling fit between force and velocity above the critical driving force can be obtained in the elastic regime, but fail in the plastic one. Our results are in agreement with previous simulated results and recent experiments.

2PO1-112 The Role of Splayed Disorder and Channel Flow on the Dynamics of Driven 3D Vortices

Charles M. Palmer and Thomas P. Devereaux, Dept. of Physics, Univ. of Waterloo, Waterloo, ON, N2G 1Y2, Canada

Presenting Author: C.M. Palmer

Three-dimensional molecular dynamics simulations are used to investigate vortex motion in the presence of both columnar and splayed columnar disorder as a function of temperature, the Lorentz force, and defect and vortex densities. For BF > B, no noticeable differences for splayed and column disorder are observed. For BF < B, systems with columnar disorder appear prone to vortex channel flow, and display critical currents which are an order of magnitude smaller than those of columnar or point disorder. Commensuration effects are also examined.

2PO1-113 Zero field vortex dynamics in YBCO-thin films; a complete experimental survey.

Orjan Festin 1, Peter Svedlindh 1, Radoslav Chakalov 2, and Zdravko Ivanov 2. 1 Department of Materials Science, Uppsala University, Box 534, SE - 751 21 Uppsala, Sweden. 2 Department of Physics, Chalmers University of Technology, SE - 412 96 Gothenburg, Sweden.

Presenting Author: Ö. Festin

In this work, epitaxially grown YBCO-thin films (d<100nm) have been investigated using electrical transport, magnetic ac conductance and flux noise measurements. The measurements have been performed in zero field and in a narrow temperature range between the Kosterlitz-Thouless (T_KT) and Ginzburg-Landau (T_c0) transition temperatures. Within this temperature range the superconducting properties of a film are entirely described by spontaneously generated vortices. By combining all sets of data, it is possible to extract key information relevant to the superconducting transition.

By using predictions of the Ginzburg-Landau Coulomb gas model and experimental data for the temperature dependence of the normalized resistance (R/R_N) and for the I-V characteristics it is possible to obtain values for the transition temperatures T_KT and T_c0. This information is then used analyzing the data obtained from ac conductance and flux noise measurements. The results indicate a crossover from a high temperature regime where 2D vortex fluctuations dominate to a low temperature regime with 3D vortex fluctuations controlling the superconducting properties.

2PO1-114 Influence of pinning states on dynamic phase transition in a driven vortex lattice

Valeriy G. Prokhorov, Superconductivity Dept., Institute of Metal Physics, 36 Vernadsky Blvd., Kiev 252142, Ukraine, and Young P. Lee, Physics Dept., Sunmoon University, Asan, Choongnam, 336-840, Korea

Presenting Author: V.G. Prokhorov

We have investigated the effect of an applied current on the vortex dynamics in superconducting amorphous Nb₃Ge and crystalline Pb₇₈Bi₂₂ thin films. We have observed a principal difference between scenarios of the vortex dynamic phase transition at high currents for studied samples. In amorphous Nb₃Ge films the transition of flux line lattice from the pinning state to the moving coherent state is carried out escaping the appearance of the plastically deformed or "melting" phase. The magnetic field dependence of the flux flow resistance exhibits the weak nonlinear behavior and can be described in the framework of the Larkin-Ovchinnikov theory for superconductors with collective pinning. In Pb₇₈Bi₂₂ films we find the extended current's ranges with the plastically deformed motion of FLL, which is ended by the sharp transition in the ordered driven state. In contrast to Nb₃Ge films, the flux flow resistance exhibits the linear behavior versus magnetic field and is precisely governed by Bardeen-Stephen model. We compare our data to recent theories of ordering and phase transitions in the moving vortex lattice.

*2PO1-115 Local Studies of Instabilities and Memory Effects in NbSe₂

Yossi Paltiel 1, Eli Zeldov 1,2, Youri N. Myasoedov 1, Hadas Shtrikman 1, Shobo Bhattacharya 3,4, Mark J. Higgins 3, Zhili L. Xiao 5, Eva Y. Andrei 5, Peter L. Gammel 2, and David J. Bishop 2. 1 Department of Condensed Matter Physics, The Weizmann Institute of Science, Rehovot 76100, Israel. 2 Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA. 3 NEC Research Institute. 4 Independence Way, Princeton, NJ 08540, USA. 4 Tata Institute of Fundamental Research, Mumbai-400005, India. 5 Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08855, USA.

Presenting Author: Y. Paltiel

Recent studies have shown a number of surprising vortex dynamics phenomena both in low and high temperature superconductors, which include: low frequency noise, slow voltage oscillations, history dependent dynamic response, memory of the direction, amplitude, duration, and frequency of the previously applied current, suppression of a large ac vortex response by a very small dc bias, and a strong frequency dependence. Taken together, these phenomena are incompatible with the current understanding of bulk vortex dynamics. We propose a generic mechanism to account for these observations in terms of the competition between the injection of a disordered vortex phase through the surface barriers at the sample edges, and the annealing of this metastable disorder by the transport current. The model is confirmed by investigating the current distribution across NbSe₂ single crystals using arrays of Hall sensors. For an ac current only narrow regions near the edges are in the pinned disordered phase resulting in a large response. In the presence of a dc bias a wide region of the sample is filled by the disordered phase preventing vortex motion. The resulting spatial variation of the disorder across the sample acts as an active memory of the previously applied current sequence.

2PO1-116 Nonlinear Magnetohydrodynamics of Type-II Superconductor

Vladimir N. Kopylov and Sergey A. Tenishev, Institute of Solid State Physics RAS, Chernogolovka, Moscow District, 142432, Russia

Presenting Author: V.N. Kopylov

We investigated breaking of superconductivity in Nb by current at temperatures in vicinity of critical temperature. To generate quasidirect current we used inductive method. Typical value of current in our sample reached 100A in the superconducting state. Signal from receiving coil wounded on the sample after amplifier fed to a sound card of a computer.

Simple consideration of the process for Type-I superconductor leads to consideration the wide class of static and dynamic periodic nonhomogenious current-field structures. On the other hand it is commonly accepted that resistive state of Type-II superconductor is connected with radial moving of vortex rings burning on a surface of a cylinder sample and disappearing on the axis. According to this model, current density and magnetic induction are smooth and monotonic functions only of radius and flow is laminar.

Our experiments demonstrated that this simple picture is not correct for Nb in vicinity of the T_c and destroying of superconductivity by current accompanied by changing of a series of stable current structures. The second result is observation of turbulent flow of current passing through a sample. The reason of such behavior is mutual influence of current and field distribution due to magnetoresistivity in the mixed state.

2PO1-117 Voltage noise behavior in highly textured YBa₂Cu₃O_7-d thin films under the applied magnetic field

A. Taoufik 1, A. Tirbiyine 1, S. Senoussi 2, and A. Ramzi 1. 1 Equipe des Matériaux Supraconducteurs à Haute Température Critique, Département de Physique, Faculté des Sciences, Université Ibn Zohr, B. P:28/S, Agadir, Maroc. 2 Laboratoire de Physique des Solides (associé au CNRS. URA. 0002), Université Paris Sud, Bâtiment 510, 91405 Orsay Cedex, France.

Presenting Author: A. Taoufik

The characteristic voltage noise of the normal conductor-superconductor transition region is investigated on a high quality single crystal YBa₂Cu₃O_7-d . The sample is a highly textured YBa₂Cu₃O_7-d . thin film deposited on the (100) surface of SrTiO₃ substrate by laser ablation method. Its dimensions are 400 nm of thickness, 7.53 µm of width. The resistance vanishes in zero magnetic field at T_{c 0} = 90 K. The voltage noise measurements are realized using the dc-four probe method for a large range of temperature and magnetic field. The effect of physical parameters on the voltage noise like temperature, current, angle between the applied magnetic field direction and the crystal c-axis and the applied magnetic field were studied.

The aim of this communication is to present the behavior of the voltage noise with the applied magnetic field H (H // c-axis) and the effect of temperature.

2PO1-118 Current effect on the voltage noise near the transition region in YBa₂Cu₃O_7-d single crystal thin films

A. Taoufik 1, S. Senoussi 2, A. Tirbiyine 1, and A. Ramzi 1. 1 Equipe des Matériaux Supraconducteurs à Haute Température Critique, Département de Physique, Faculté des Sciences, Université Ibn Zohr, B. P:28/S, Agadir, Maroc. 2 Laboratoire de Physique des Solides (associé au CNRS. URA. 0002), Université Paris Sud, 91405 Orsay Cedex, France.

Presenting Author: S. Senoussi

The voltage noise measurements have been realized on high quality single crystal YBa₂Cu₃O_7-d thin film deposited by the laser ablation on the surface (100) of a SrTiO₃ substrate. The resistance vanishes in zero magnetic field at T_{c 0} = 90 K and at 5 T, parallel to ab planes, the resistance disappears at T_{c 5} = 85 K. The voltage noise was measured using a dc four-probe method at a large magnetic field (0 – 14 Tesla).

We have observed a voltage noise peak near the transition temperature for all the magnetic field values studied.

We have investigated the physical parameters influence on the voltage noise behavior namely the applied magnetic field, the angle between its direction and the c-axis, temperature and the current effect.

Here, we present the current effect on this voltage noise V_n in an applied magnetic field of 5 T parallel to ab planes. The figure shows the voltage noise behavior for two current values obtained in magnetic field of 5 T parallel to the ab planes.

2PO1-119 Spectral Flow and Vortex Dynamics in a Temperature Gradient

A. Freimuth and M. Zittartz, II. Physikalisches Insitut, Universität Köln, 50937 Köln, Germany

Presenting Author: A. Freimuth

In the mixed state of superconductors the spectral flow of fermion zero modes in the vortex core couples the motion of vortices to that of the normal fluid. We show that this gives rise to a heat current perpendicular to the direction of vortex motion and therefore to longitudinal thermomagnetic effects like the thermopower and the Peltier-effect. An analysis of vortex motion in a temperature gradient on this basis is presented. It yields excellent agreement with experimental results on high temperature superconductors.