POSTER SESSION 2PO1: Vortex Physics

Tuesday, Feb. 22, 1:30 p.m. – 3:30 p.m., Hall D (GRB)

2PO1-1 Maximum Number of Flux Lines Inside Columnar Defects

M.M. Doria 1, S.C.B. de Andrade 2, and Edson Sardella 3. 1 Instituto de Fisica, Universidade Federal do Rio de Janeiro, C.P. 68528 Rio de Janeiro 21945-970, Brazil. 2 Departamento de Fisica, Pontificia Universidade Catolica do Rio de Janeiro, Rio de Janeiro 22452-970 RJ, Brazil. 3 Departamento de Fisica, Universidade Estadual Paulista C.P. 473, 17033-360, Bauru-SP, Brazil.

Presenting Author: M.M. Doria

The magnetic flux trapped inside a superconductor due to a Flux Line (FL) can be a multiple of hc/2e, but an energy requirement favors just hc/2e instead of any of its possible higher multiples. However this energetic balance changes in presence of Columnar Defects, which can trap multiple-quanta FL's, in case the radius of the Columnar Defect is larger than the coherence length. Columnar Defects are very long pinning centers that cross the superconductor from one side to the other, such as artificially made defects obtained by ion irradiation. Long ago Mkrtchyan and Shmidt calculated the maximum number of single FL's that enter a Columnar Defect and shown that this saturation limit only depends on the ratio between its radius and the coherence length. Beyond this limit the Columnar Defect turns into a repulsive center to all other exterior FL's. In this work we find new saturation limits different from those predicted by the Mkrtchyan-Shmidt theory, and show that their picture is just a first approximation that corresponds to not considering the pressure effect made by the remaining FL's exterior on the defect. For this we numerically solve the Ginzburg-Landau theory through Simulated Annealing in presence of a periodic lattice of Columnar Defects either with metallic or insulating cores and determine the number of trapped FL's as a function of the magnetic induction and radius.

2PO1-2 Vortex Coherence and Entangled Vortex States in High T_c Superconductors

Lia Krusin-Elbaum 1, G. Blatter 2, J.R. Thompson 3, D. Petrov 1, D. Lopez 1, R. Wheeler 4, J. Ullmann 5, and C.W. Chu 6. 1 IBM Research, Yorktown Heights, NY 10598, USA. 2 ETH-Honggerberg, CH-8093 Zurich, Switzerland. 3 Dept. of Physics, Univ. of Tennessee, Knoxville, TN 37996. 4 UES Inc., Dayton, OH 45432, USA. 5 Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 6 Texas Center for Superconductivity, Houston University, Houston, TX 77204, USA.

Presenting Author: L. Krusin-Elbaum

The loss of vortex coherence is a subject of heated debate, since melting of the vortex lattice is believed to be accompanied by a loss of the identity of individual vortex lines, which split into decoupled segments of finite length. Using columnar pins we can recouple vortex segments and maintain the line-like vortex structure far into the liquid regime. By manipulating (splaying) the defect configurations we (i) have a unique controllable tool to probe the c-axis vortex coherence, and (ii) can force topological vortex entanglement that involves the entire (uncut) line-like vortices, i.e., for which the coherence is preserved. We find that in a 3D YBCO a small angle splay entangles vortices above some (lower than matching) switching field. This gives rise to enhanced pinning, suppressed dynamics, and to a remarkable anisotropic dissipation in a viscous line liquid which disappears when the c-axis coherence vanishes. This fundamental result points to a field scale that switches dynamics to a regime governed by vortex recoupling. We show further that in a low anisotropy YBCO large-angle splay destroys vortex coherence (and reduces the pinning action to that of point defects) while in quasi-2D BSCCO and in highly anisotropic Hg-1223 it produces huge increases in pinning. We propose a unified explanation of these seemingly conflicting observations in terms of scaling description of splay and show that in high-anisotropy materials large splay angles are effectively reduced, thus preserving vortex coherence. The scaling proposal reconciles our observations with the concepts of topological entanglement.

[1] L. Krusin-Elbaum et al., Phys. Rev. Lett. 81, 3948 (1998);

[2] L. Krusin-Elbaum et al, Nature 389, 243 (1997);

[3] D. Lopez, L. Krusin-Elbaum et al., Phys. Rev. Lett. 79, 4259 (1997);

[4] L. Krusin-Elbaum et al., Phys. Rev. Lett. 76, 2563 (1996).

2PO1-3 Influence of the tilt on vortex pinning in irradiated BSCCO single crystals

V. Ta Phuoc, R. de Sousa, L. Ammor, A. Ruyter, E. Olive, and J.C. Soret, Laboratoire d’Electrodynamique des Matériaux Avancés, UFR Sciences, Université F. Rabelais, Parc de Grandmont, 37200 Tours, France

Presenting Author: V. Ta Phuoc

In-plane transport measurements have been performed on Bi₂Sr₂Y_xCa_1-xCu₂O₈ single crystals with columnar defects along the c-axis. For filling fractions f=B/Bf <1, the vortex phase diagram is investigated as a function of the angle between the column direction and the applied magnetic field. Below the Bose Glass (BG) transition temperature T_BG, a non ohmic behaviour associated with vortex variable range hopping [1] is observed below a critical angle q _c(T). The value of the corresponding glassy exponent m =1/3 does not depend on the magnitude [2] and direction of the field. This indicates that the vortex interactions are not relevant in this part of the phase diagram [1,3]. Moreover, the activation energy weakly decreases with increasing tilt, and then falls down to zero close to the critical tilt. Such a behavior agrees with a predicted transverse Meissner effect. Near T_BG, data verify the angular scaling rules, and the extracted critical exponents are n '» 1 and z'» 5.3. Furthermore, the scaling functions and the critical exponents are field independent over the range investigated in our experiment. Finally, above q _c, the signature of a smetic-A like vortex liquid phase is evidenced below T_BG.

[1] D.R. Nelson, V.M. Vinokur, Phys. Rev. B 48 (1993), 13060.

[2] V. Ta Phuoc, A. Ruyter, L. Ammor, A. Wahl, J.C. Soret, Phys. Rev. B 56 (1997), 122.

[3] U.C. Taüber, Hongjie Dai, D.R. Nelson, C.M. Lieber, Phys. Rev. Lett. 74 (1995), 5132.

2PO1-4 Pinning Effect of Columnar Defects as a Probe of Anisotropy of Bi₂Sr₂Ca_n-1Cu_nO_2n+1 Thin Films

Laurent Martel, Alberto Pomar, Zhi Zhong Li, and Helene Raffy, Laboratoire de Physique des Solides, Bat 510, Universite Paris-Sud, 91405 Orsay, France

Presenting Author: H. Raffy

We have studied the influence of columnar defects produced by heavy-ion irradiation on the superconducting properties of Bi₂Sr₂Ca_n-1Cu_nO_2n+1 thin films (2212 and 2201) and 2212/2201 multilayer. We have explored both the vortex solid and liquid phases by measuring, respectively, the critical current J_c(T,H) and the magnetoresistance, R(T,H) at different field-sample orientations. First, a sufficiently high dose (B_fi>1T, where B_fi is the matching field) is needed to observe an enhancement of pinning by the columnar defects and to overwhelm the effects of other pinning centers such as natural defects. Second, angle resolved measurements lead to different results in very anisotropic 2212 films and 2212/2201 multilayer on the one hand, and in weakly anisotropic 2201 films on the other. In the case of 2212 and multilayer this increase of pinning efficiency is isotropic, i.e. there is no additional peak in J_c(q ) or a minimum in R(q ) when the H is parallel to the defects in contrast with the results reported for single crystals. However, for the less anisotropic system 2201, a directional effect is observed in a given T and H range indicating an anisotropic pinning enhancement by the irradiation. It is demonstrated that columnar defects allow us to probe the anisotropy of the films and the vortex dimensionality.

2PO1-5 Influence of annealing on the flux pinning of heavy-ions irradiation YBa₂Cu₃O₇ crystal

M. Abdel-Salam 1, G.K. Perkins 1, S. Hebert 1, F. Damay 1, J. Driscoll 2, C. Chen 3, and A.D. Caplin1. 1 Centre for HTS, Blackett Laboratory, Imperial College, London SW7 2BZ, United Kiingdom. 2 Dept. of Materials, Imperial College, Prince Consort Rd, London SW7 2BP, United Kingdom. 3 Clarendon Laboratory, Physics Dept., Oxford University, Park Rd, Oxford OX1 3PU, United Kingdom.

Presenting Author: M. Abdel-Salam

Changes in superconducting properties due to Pb+ heavy ion irradiation followed by thermal annealing were investigated in high quality YBa₂Cu₃O₇ single crystals grown by the self flux method. The crystal were irradiated with 6 GeV Pb+ ions parallel to the c-axis. The irradiateds crystal were annealed at 350, 400, 450C in an oxygen pressure of 0.2 atm for 48 hrs. After each annealing step, the transition temperature(T_c) and the critical current density (J_c) were determined again; throughout the sequence, the T_c varied by no more than about 1K. The J_c's calculated from magnetization curves at 30K, 50K, 60K, 77K, and 80K, taking into account the changes in current-carrying length-scale, show substantial enhancement induced by annealing. We discuss these effects in terms of the structure of the irradiation columns and the cation and oxygen disorder associated with them.

2PO1-6 Universal behaviour of Bose glass exponents in 250 MeV Ag⁺¹⁷ ion irradiated YBCO melt textured samples

Ujjual Divakar 1, G.L. Bhalla 1, G.C. Trigunayat 1, and Ravi Kumar 2. 1 Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India. 2 Nuclear Science Centre, Aruna Asaf Ali Marg, New Delhi 110067, India.

Presenting Author: K.M.U. Divakar

Columnar defects in crossed geometry and along the C-axis of YBa₂Cu₃O₇ melt textured samples have been created by 250 MeV Ag⁺¹⁷ ion irradiation. Angle dependent magnetoresistance measurements were carried out to study the pinning behaviour of the correlated columnar defects. The Bose glass theory was invoked to account for the experimental results . The Bose glass temperature TBG was found to be a maximum along the direction of columnar defects. Angular critical scaling laws were applied to determine the critical exponets and these values were found to be universal below a certain critical angle. The critical angles were found to be ~ 30 degree in case of parallel linear defects and ~ 20 degree in case of crossed linear defects.

2PO1-7 Bose-glass transition in irradiated Bi₂Sr₂Ca_1-xY_xCu₂O₈ single crystals

R. De Sousa 1, L. Ammor 1, J.C. Soret 1, V. Ta Phuoc 1, A. Ruyter 1, E. Olive 1, and A. Wahl 2. 1 LEMA Université F. Rabelais - UFR Sciences, Parc de Grandmont, 37200 Tours, France. 2 CRISMAT CNRS URA 1318, 6 Bld du Maréchal juin, 14050 Caen Cedex, France.

Presenting Author: R. De Sousa

In this study, transport measurements are used over a wide range of filling fractions 0.026<f=B/BF <1 to investigate vortex dynamics in irradiated Bi₂Sr₂Ca_1-xY_xCu₂O₈ single crystals. Three samples were irradiated with 5.8 GeV Pb ions parallel to the c-axis, with matching fields Bf =0.75T for sample 1 (x=0), 1.5T for the sample 2 (x=0) and 0.75T for the sample 3 (x=0.36). We find that isotherm I-V curves investigated near the superconducting transition are consistent with the Bose-glass scaling theory1. The critical exponent values z'=5.3±0.05 and n '=1.3±0.07 extracted are field and matching field-independent, and are similar for all the samples. The experimental phase diagram, derived from scaling analysis, reveals various vortex pinning regimes strongly depending on filling fractions f. The results show that defect-vortex interactions only dominate the flux line dynamics between two characteristic temperatures T_0» 0.72T_c (f» 0.8) and T_1» 0.83T_c (f=1/3). In this range the Lindemann criterion, which accounts for the contribution of correlated disorder, seems well adapted to describe the Bose-glass line of such superconducting compounds [1,2]. Moreover, above f=0.8 and T₁=0.83T_c the vortex system is increasingly dominated by vortex-vortex interactions and thermal fluctuations. In these domains, the Bose-glass melting line (high temperature regime) and the depinning line (high field regime), respectively, merges the pristine line determined from low resistivity criterion R» 1m W . We obtain good quantitative agreement between our experimental data and the predictions of the Bose-glass theory.

[1] D.R. Nelson, V.M. Vinokur, Phys. Rev. B 48 (1993), 13060.

[2] G. Blatter, M.V.Feigel'man, V.B. Geshkenbein, V.B Larkin, V.M. Vinokur, Rev. Mod. Phys 66 (1995) 1125.

2PO1-8 Field-induced Pinning in the Vortex Liquid Phase of Heavy-ion Irradiated Bi₂Sr₂CaCu₂O₈ Thin Films and Multilayers Bi₂Sr₂CaCu₂O₈/Bi₂Sr₂CuO₆

Alberto Pomar, Laurent Martel, Zhi Zhong Li, and Helene Raffy, Laboratoire de Physique des Solides, Bat 510, Universite Paris-Sud, 91405 Orsay, France

Presenting Author: A. Pomar

In-plane dissipation, R(H) in the vortex liquid phase is investigated in heavy-ion irradiated Bi₂Sr₂CaCu₂O₈ epitaxial thin films and a highly anisotropic Bi₂Sr₂CaCu₂O₈/Bi₂Sr₂CuO₆ multilayer. Samples have been irradiated with different doses Bfi, and orientations. For all the samples we observe a dip in the R(H) curves in a well-defined region of field (0.25B_fi<B<B_fi) and temperature (0.6T_c<T<0.95T_c). This decrease of dissipation is attributed to pinning of vortices by the columnar defects produced by irradiation. From the position of the dip and using T_c and Bfi as normalizing parameters we obtain an universal (H,T) phase diagram with a new pinned state well within the vortex liquid phase. From the study of R(T,H) curves we find that the onset of magnetoresistance is described by an Arrhenius law, R=R₀exp(-U/T), indicating that the dissipation is due to thermal activation processes with an activation energy U. Importantly, we conclude from R(q ) curves, that the 2D angular scaling behavior is valid in the entire (H,T) liquid region. These results indicate that the dissipation is always due to mobile 2D vortices, even in the presence of columnar defects, and they are analyzed in terms of field-driven confinement of vortices at the columnar defects.

2PO1-9 Effect of Heavy Ion Irradiation on Flux Pinning Properties of Ag-Cu Alloy Sheathed Bi-2223 Tapes

Hiroshi Ikeda 1, Naoshi Kuroda 2, Tadashi Kambara 3, Kozo Yoshikawa 4, and Ryozo Yoshizaki 1. 1 Institute of Materials Science and Cryogenics Center, University of Tsukuba, Ibaraki 305-8577, Japan. 2 Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan. 3 Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-0198, Japan. 4 Takasago Research Center, Mitsubishi Heavy Industries, Ltd., Takasago, Hyogo 676-0008, Japan.

Presenting Author: H. Ikeda

We have investigated the superconducting properties of Ag-Cu alloy sheathed Bi-2223 tapes where Hf impurities are included in the pristine alloy. We introduced columnar defects in the tapes parallel to the aligned c-axis (normal to the tape surface) by Xe-ion irradiation.

We found that the irreversibility line in the heavy-ion irradiated tapes showed the shift toward higher fields. Moreover, the irreversibility curve of the irradiated sample is located in the higher temperature side of the irradiated Ag-sheathed tape (without H_f impurities in the tape) even in the higher magnetic field than the matching field.

In addition, the improved irreversibility line is observed in the configuration of the field parallel to the tape (i.e., perpendicular to the columnar defects). We observed the prominent difference at the high field part of the scaling plots of the reduced pinning force F_p for the magnetic field parallel and perpendicular to the tape surface. These behaviors suggest that the columnar defects become more effective pinning centers in cooperation with the point defects induced by Hf doping in the Ag-Cu alloy sheathed tape samples.