S. Kokkaliaris 1, A.A. Zhukov 1, P.A.J. de Groot 1, R. Gagnon 2, L. Taillefer 2, and T. Wolf 3. 1 Physics Dept., University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom. 2 Physics Dept., McGill University, Montreal, Quebec, H3A 2T8, Canada. 3 Forschungszentrum Karlsruhe, Institut für Technische Physik, Postfach 3640, D-76021 Karlsruhe, Germany.

Presenting Author: S. Kokkaliaris

We have performed partial magnetization loops in pure YBa₂Cu₃O_7-d single crystals with various types of defects. The obtained results have demonstrated that the critical current density in detwinned samples depends strongly on the magnetic history of the superconductor. As shown by S. Kokkaliaris et al. (Phys. Rev. Lett. 82, 5116 (1999)), this effect can be attributed to the presence of metastable topological disorder in the vortex lattice that is a precursor of the transition from a dislocation-free Bragg glass to a disordered vortex phase. The transition line is found to shift to lower fields with decreasing oxygen content and finally disappear for high densities of oxygen vacancies. The effect of columnar defects has also been investigated and found to be the suppression of the Bragg glass - disordered phase transition and the elimination of the memory effects even for low irradiation doses.

S.A. Aruna 1, H.M. Shao 1, Y.M. Cai 2, L.J. Shen 2, C.J. Wang 2, L. Chen 1, T. Yang 1, and X.X. Yao 1. 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, 210009 Nanjing, China.

Presenting Author: S.A. Aruna

Excess conductivities of superconducting Hg-1223 thin films irradiated with proton particles at various doses and of unirradiated ones have been studied from the data of resistivity versus temperature. The superconductivity fluctuation contributions to the transport properties of the superconducting Hg-1223 thin films are calculated and compared in this paper. The logarithmic plots of excess conductivity and reduced temperatures (T-T_c)/T_c reveal two cross-over temperatures with three exponents. Except for the unirradiated sample, which shows a distinct cross-over from two-dimensional to three-dimensional conductivity, all the irradiated samples fit only the two-dimensional Aslamzov-Larkin equation above the cross-over temperature. Interlayer coupling strength estimates show a decreasing trend with an increase in dose, corresponding to a decrease in carrier concentration.

A. Soibel 1, E. Zeldov 1,2, M. Rappaport 1, Y. Myasoedov 1, T. Tamegai 3, S. Ooi 3, M. Konczykowski 4, V. Geshkenbein 5,6. 1 Department of Condensed Matter Physics, The Weizmann Institute of Science, Rehovot 76100, Israel. 2 Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974. 3 Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. 4 CNRS, URA 1380, Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 5 Theoretische Physik, ETH-Honggerberg, CH-8093 Zurich, Switzerland. 6 L. D. Landau Institute for Theoretical Physics, 117940 Moscow, Russia.

Presenting Author: E. Zeldov

The vortex-lattice melting is a first-order transition in a pure high-temperature superconductor, and it turns into a continuous transition in presence of strong disorder. There is currently no microscopic understanding of the process in the interesting case of moderate disorder, which turns out to be the experimental situation in practical single crystals. We have developed a very sensitive differential magneto-optical method, which provides resolution of local magnetic fields as low as 30 mGauss and allows direct visualization of the nucleation and propagation process of the melting transition in Bi₂Sr₂CaCu₂O₈ crystals. We find that the disorder substantially modifies the detailed local behavior, yet the system still undergoes thermodynamic first-order transition. This situation raises a new scope of uninvestigated issues regarding the local nucleation mechanism, the coexistence of the solid and liquid phases, as well as the stability, surface tension, and dynamics of the solid-liquid interface. The obtained 'movies' of the melting transition show the influence of the disorder on a microscopic level and reveal a variety of complicated phenomena, including solid-liquid phase separation, position dependent hysteresis, interface coarsening, pinning, and instabilities. Possible formation of a solid-liquid laminar state due to an intrinsic instability at the first-order transition is analyzed theoretically.

Hideaki Sakata 1, Morimi Oosawa 1, Ken Matsuba 1, Nobuhiko Nishida 1, Hiroyuki Takeya 2, and Kazuto Hirata 2. 1 Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan. 2 National Research Institute for Metals, Tsukuba 305-0047, Japan.

Presenting Author: K. Hirata

We have performed scanning tunneling measurements on YNi₂B₂C single crystals grown by TSFZ method in the magnetic fields up to 3T, applied along the a- and c-axis. The images of the vortex lattice, obtained on the cleaved surfaces of the crystals, show a vortex lattice transition with increasing a magnetic field. In the magnetic field parallel to the a-axis, a nearly square vortex lattice with a rhombic unit cell of a constant apex angle 85-degree above 1.0T has transformed continuously into a hexagonal lattice below 1.0T. This shows quite a contrast to the transition in the fields along c-axis, which occurs at much lower field around 0.1T. These results are qualitatively consistent with the predictions of the London model with non-local corrections. The unconventional steep increase in the quasi-particle density of states outside the core has also been found with increasing a magnetic field well below the upper critical field.

M. Pannetier, R. Surdeanu, R.J. Wijngaarden, J.M. Huijbregste, B. Dam, and R. Griessen, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, NL-1081 HV Amsterdam, The Netherlands

Presenting Author: M.T. Pannetier

In a recent work [1], it has been shown from transport measurements that artificial reversible pinning can be created by the application of a magnetic tape on the surface of a thin superconducting film.

We study the influence of a periodic array of magnetic stripes, generated from a signal pre-recorded on a magnetic tape, on the pinning of vortices. With our high resolution magneto-optical technique [2], we visualize in two dimensions the penetration of the flux in a thin YBa₂Cu₃O_7-x film partially covered by a magnetic tape. In particular, we have studied the evolution of the flux front entry in the superconducting stripe under and next to the magnetic tape. We present results obtained for various temperatures and various applied fields.

[1] Y. Yuzhelevski and G. Jung, Physica C 314 (1999) 163-171.

[2] R. Surdeanu et al, Phys. Rev. Lett. 83 (1999) 2054.

Juan A. Herbsommer, Javier Luzuriaga, and Gladys Nieva, Centro Atomico Bariloche and Instituto Balseiro, Nieva, Argentina

Presenting Author: J.A. Herbsommer

Bitter decorations of the vortices in twinned single crystal YBCO have been obtained to study the disordered structure formed on successive cooldowns of the sample under the same conditions of field and temperature. Between succesive decorations, the iron dots could be removed completely, giving a surface clean enough for further decoration. It was found that successive realizations of the disordered vortex state were very similar at long range, and differed mostly on short scales, compared to the average vortex separation. We have quantified the correlation between successive structures, and double sided decorations have been also performed to compare with the extent of spatial correlation in the applied field direction. The coincidence of the position of the vortices in successive realizations of the structure, is unexpected, and indicates an unusual behavior for a glassy system. Our technique makes it possible to image the position of the vortices in detail and repeatedly, providing us with a model amorphous system in which to study the reproducibility of the disordered structure.

B.L.T. Plourde and D.J. Van Harlingen, Department of Physics and Science and Technology Center for Superconductivity, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

Presenting Author: B.L.T. Plourde

The behavior of vortices in thin superconducting strips in a perpendicular magnetic field is a complex problem due to the large demagnetizing effects. These geometrical barriers are frequently encountered in transport measurements on high-T_c superconductors. We have studied the flux dynamics in strips patterned from both Nb and weak-pinning amorphous MoGe films using a Scanning SQUID Microscope (SSM). The unparalleled flux sensitivity of the SSM allows us to image the vortices in the strip under a variety of field and cooling conditions with single vortex resolution for low flux density. We are able to apply transport currents while imaging the strip and observe the shift of the vortex distributions due to the Lorentz force. Surface steps etched into the strips significantly alter the flux patterns and introduce asymmetry in the vortex motion under applied transport currents. The change in vortex line energy across the step impedes flux moving from the thin to the thick regions of the strip. We have correlated the vortex distributions from the SSM images with measurements of the transport characteristics of the strips.

D. Giller 1, A. Shaulov 1, L. Dorosinskii 2, T. Tamegai 3, and Y. Yeshurun 1. 1 Institute of Superconductivity, Department of Physics, Bar-Ilan University, Ramat-Gan, Israel. 2 National Metrology Institute, P.K.21, 41470 Gebze-Kocaeli, Turkey. 3 Department of Applied Physics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku Tokyo 113-8656, Japan.

Presenting Author: Y. Yeshurun

We describe novel experiments in which the formation of vortex phases is monitored by means of high-temporal resolution magneto-optics system. This technique has been used to study the transient vortex states formed in Bi₂Sr₂CaCu₂O_8+d after a sudden application of a steady magnetic field H. We focus here on fields H and temperatures T corresponding to the disordered phase in the equilibrium H-T vortex phase diagram. We find that the injected flux lines are initially arranged in a mixed phase: Disordered phase near the sample edges and quasi-ordered phase near the sample center. The boundary between these two phases is clearly indicated by a break in the induction profiles across the sample. The location of this boundary exhibits non-monotonic time dependence. Initially, the quasi-ordered phase nucleates at the center and expands outwards. As the induction increases, the quasi-ordered phase retreats and the high current phase pushes its way towards the center. We propose that the time dependence of the boundary between the two phases is determined by the competing effects of the increase in the induction and the decrease of the current.

G. Yang 1, P. Corrigan 1, J.S. Abell 1, and C.E. Gough 2. 1 School of Metallurgy and Materials, The University of Birmingham, Birmingham, B15 2TT, United Kingdom. 2 School of Physics and Astronomy, The University of Birmingham, Birmingham, B15 2TT, United Kingdom.

Presenting Author: G. Yang

Magnetic flux penetration and flux density distributions in Bi₂Sr₂CaCu₂O_y single crystals have been investigated by the magneto-optical technique using an iron garnet film at various temperatures and magnetic fields. In as-grown crystals, preferential flux penetration along narrow bands in the a-direction are observed, independent of the geometry of the crystals. On annealing in flowing oxygen, flux penetration becomes more uniform but significantly anisotropic, reflecting an anisotropy of critical current in the ab-plane, with J_ca/ J_cb ~ 2. Comparison with ac susceptibility measurements near T_c, suggests that inhomogeneous flux penetration in the as-grown crystals is associated with inhomogeneous oxygen stoichiometry.

S. Kolesnik 1,2, V. Vlasko-Vlasov 3, U. Welp 3, G.W. Crabtree 3, T. Piotrowski 4, J. Wrobel 1, A. Klimov 1, P. Przyslupski 1, T. Skoskiewicz 1, and B. Dabrowski 2. 1 Institute of Physics, Polish Academy of Sciences, Warszawa, Poland. 2 Department of Physics, Northern Illinois University, DeKalb, IL, USA. 3 Materials Science Division, Argonne National Laboratory, Argonne, IL, USA. 4 Institute of Electron Technology, Warszawa, Poland.

Presenting Author: S. Kolesnik

Anisotropic arrays of antidots have been prepared on Y-Ba-Cu-O and Nb thin films by electron-beam lithography. Magnetization curves have been measured on the structured thin films at temperatures close to the critical temperature at different angles between the magnetic field direction and the normal to the sample surface. Magneto-optical investigations have been performed by use of an iron garnet film indicator.

For Y-Ba-Cu-O thin film, strong pinning in the material inhibits direct observation of matching peaks. Fast-Fourier-Transform analysis reveals periodic character of the magnetization curves with periods corresponding to anisotropic vortex lattices. For niobium thin film, matching peaks can be observed on the decreasing branches of the magnetization curves. Positions of the matching peaks and their angular scaling strongly suggest formation of anisotropic vortex lattices in the studied systems.

Magneto-optical investigations at lower temperatures show unidirectional flux entry and leaving along the shorter antidot lattice parameter. In higher magnetic fields, an anisotropic critical state is observed. The anisotropy parameter of critical currents is magnetic-field dependent. By changing the magnetic field value, the anisotropic critical state can be converted to an isotropic one, due to the interplay between the pinning by antidots and by other pinning centers in niobium.

Work supported in part by the Polish Committee for Scientific Research (KBN) under grant No. 2 P03B 10714.