2PO1-60 Evidence for a field driven transition in YBa₂Cu₃O_7-x single crystal

M. Pissas 1, E. Moraitakis 1, G. Kallias 1, and A. Bondarenko 2. 1 Institute of Materials Science, NCSR Demokritos, 15310, Aghia Paraskevi, Athens, Greece. 2 Kharkov State University, 310077, Svoboda sq.4, Kharkov, Ukraine.

Presenting Author: M. Pissas

Isothermal global magnetic measurements in a detwinned YBa₂Cu₃O_7-x single crystal revealed a jump in the slope of the increasing part of the virgin magnetization curves. The variation of the magnetic field at which this feature occurs decreases as the temperature increases.

The abrupt change in the slope is broadened or even disappears at low temperatures (T<20 K). At high temperatures, near the line defined by the points of the fishtail peak, the feature is also hard to discern. The observed feature may be attributed to a recently proposed disorder-induced transition (or crossover) from a relative ordered vortex lattice to a highly disordered vortex solid.

2PO1-61 Measurements of the exponent m in the vortex-glass phase for single crystal YBa₂Cu₃O_7-d thin films

A. Tirbiyine 1, A. Taoufik 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. Tirbiyine

The J-E characteristics were measured for YBa₂Cu₃O_7-d thin films at temperatures well below the superconducting transition temperature and in high magnetic field.

The nonlinear J-E curves show scaling behavior near the vortex-glass transition and the critical exponents are extracted. The functional form of the J-E curves is described by E/Jµ exp(-J₀/J)m (1) and the exponent µ values are comprised between 0.09 and 1.2. The functional changes to power-law behavior E(J,T=T_g)» J^(z+1/d-1) (2) at the vortex-glass transition temperature T_g, our results show that 4<z<6.

In this communication, we present the J-E characteristic results obtained for a high quality single crystal YBa₂Cu₃O_7-d thin film as a function of the applied magnetic fields up to 14 T, temperatures between 25 and 90 K and the current density from J » 0 to 10¹¹A/m² using a pulsed current of duration t = 10 ms. The results are analyzed in terms of the Fisher theory. We have determined the exponent m characterizing the vortex-glass phase by adjustment of the data with the equation (1).

2PO1-62 Field Independent Vortex Solid-to-Liquid Transition for B½ ½ ab in Oxygen Deficient Single Crystals of YBa₂Cu₃O_7-d

Björn Lundqvist, Osten Rapp, and Magnus Andersson, Department of Solid State Physics, Royal Institute of Technology, SE-100 44 Stockholm, Sweden

Presenting Author: B. Lundqvist

Single crystals of YBa₂Cu₃O_7-d (YBCO) provide a suitable test system for vortex dynamics in high temperature superconductors, since the electrical anisotropy g =(m_c/m_ab)^1/2 can easily be varied over a large range by changing the oxygen content. Here the vortex solid-to-liquid transition, determined from a vortex-glass analysis of the resistivity using the Vogel-Fulcher relation, has been studied for magnetic fields 0 < B² * 12 T parallel with the ab plane of oxygen deficient single crystals of YBCO. Strikingly, for high magnetic fields a field independent solid-to-liquid transition is observed. Possible explanations for this phenomenon will be discussed.

2PO1-63 Magnetic Field Dependence of the Muon Spin Rotation (m SR) Lineshape in the Bragg Glass Phase of Disordered Type-II Superconductors

Byron C. Den Hertog and Michel J.P. Gingras, Department of Physics, University of Waterloo, Waterloo ON, Canada N2L 3G1

Presenting Author: B.C. den Hertog

Recent experimental work has found that the muSR lineshapes of YBCO_6.95 and YBCO_6.60 vary systematically with the strength of the applied magnetic field. Data analysis reveals that the extracted London penetration length and coherence length are both field dependent. Such dependencies have been suggested to be due to non conventional pairing in these systems. However, it is now generally believed that one can tune via the magnetic field a phase transition between a Bragg glass to a vortex glass phase in short coherence type-II superconductors above a critical field H_c (H_c1<H_c<<H_c2). In order to make progress in our understanding of the intrinsic and macroscopic properties of non s-wave superconductors in an applied field, it is important to first understand how the extrinsic m SR lineshape evolves with disorder and large length scale London physics as the system is driven from the Bragg glass to the vortex glass. We present here analytical results from variational replica symmetry breaking analysis and numerical results from Monte Carlo simulations of a Lawrence-Doniach model with point impurities of relevance to YBCO materials. Our results will be compared and contrasted to experiment.

2PO1-64 Nonsingular Vortices in (s+d)-wave Superconductors

Alexander S. Mel’nikov, Igor M. Nefedov, Denis A. Ryzhov, Ilya A. Shereshevskii, and Peter P. Vysheslavtsev, Institute for Physics of Microstructures, Russian Academy of Sciences, 603600, Nizhny Novgorod, GSP-105, Russia

Presenting Author: D.A. Ryzhov

The structure of the vortex solution in (s+d)-wave superconductors is analyzed within the Ginzburg-Landau model generalized for two order parameter components. The appearance of non-zero s-wave component in the core of d-wave vortex is considered. This effect results in breaking of the fourfold symmetry of the vortex solution. The vortex structures with the breaking fourfold symmetry are theoretically investigated and observed in numerical experiments. The ranges of parameters, where different structures of nonsingular vortices are realized, are found. The features of motion of such vortices are discussed.

2PO1-65 Low-Energy Structures in Scanning Tunneling Spectra of Vortex Cores in Bi₂Sr₂CaCu₂O_8+d

B.W. Hoogenboom 1, M. Kugler 1, Ch. Renner 2, B. Revaz 1, I. Maggio-Aprile 1, and Ø. Fischer 1. 1 Department of Condensed Matter Physics, University of Geneva, 24 Quai E.-Ansermet, CH-1211 Genève 4, Switzerland. 2 NEC Research Institute, 4 Independence Way, Princeton, NJ 08540, USA.

Presenting Author: B.W. Hoogenboom

Scanning Tunneling Spectroscopy allows to study the changes in the quasiparticle local density of states due to the presence of vortex cores in Bi₂Sr₂CaCu₂O_8+d . The vortex core spectra are characterized by a gap-like structure similar to the pseudogap measured above T_c. We also observe small spectral structures inside the gap in the vortex cores. These low-energy structures are weaker than what one expects for localized quasiparticle excitations. In fact the increase of the zero-bias conductance in the vortex cores (with respect to the superconducting spectra) is larger than that of the mentioned low-energy structures. The spatial dependence of these features as well as the zero bias conductance is reported and analyzed.

2PO1-66 Vortex Core Shape in Bi₂Sr₂CaCu₂O_8+d Single Crystals

B.W. Hoogenboom 1, M. Kugler 1, Ch. Renner 2, B. Revaz 1, I. Maggio-Aprile 1, and Ø. Fischer 1. 1 Department of Condensed Matter Physics, University of Geneva, 24 Quai E.-Ansermet, CH-1211 Genève 4, Switzerland. 2 NEC Research Institute, 4 Independence Way, Princeton, NJ 08540, USA.

Presenting Author: B.W. Hoogenboom

Scanning Tunneling Spectroscopy measurements on Bi₂Sr₂CaCu₂O_8+d allow to obtain images of vortex cores with high spatial resolution. At an applied field of 6 T the vortices do not arrange in a regular lattice. This implies a strong pinning of the vortices by defects and inhomogenities. This pinning also seems to have a large impact on the shape of the vortex cores: they show highly irregular shapes, and often consist of two or more smaller elements. A striking observation is that of vortices divided between two positions as an intermediate state during the movements from one position to the other.

Both the irregular vortex shape and the apparent split vortices show a delocalization of vortices on different pinning sites. The origin of this behaviour may be thermal movement or quantum tunneling. Quantum effects can be expected to be of importance because of the smallness of the vortex cores (~20 Å) and the close spacing of the pinning sites. Such an interpretation is in agreement with the experiments which show an apparent temperature independence of this phenomenon.

2PO1-67 Giant Peak Effect in an Ultrapure YBa₂Cu₃O₇ Crystal: Melting of a Bragg Glass

X.S. Liang 1, J. Shi 1,2, R. Liang 3, D.A. Bonn 3, and W.N. Hardy 3. 1 Physics Dept., Brown University. 2 Wuhan University. 3 Physics Dept., UBC.

Presenting Author: X.S. Ling

We report an observation of a giant peak effect at the vortex solid-liquid transition in an untwinned ultrapure YBa₂Cu₃O₇ crystal, resolving a long standing issue of the origin of the peak effect in weak-pinning type-II superconductors.

2PO1-68 Effect of oxygen ordering on the hysteretic magnetization of YBa₂Cu₃O_7-d single crystals

S. Kokkaliaris 1, S.N. Gordeev 1, P.A.J. de Groot 1, R. Gagnon 2, L. Taillefer 2, P.F. Henry 3, and M.T. Weller 3. 1 Physics Dept., University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom. 2 Physics Dept., McGill University, Montreal, Quebec, H3A 2T8, Canada. 3 Chemistry Dept., University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom.

Presenting Author: S. Kokkaliaris

We have investigated the effect of oxygen ordering on the hysteretic magnetization of high quality YBa₂Cu₃O_7-d single crystals with various oxygen contents (0.03 < d < 0.09) in the region of optimal doping and in a broad temperature range for fields up to 12 T. Comparison of results obtained on samples annealed at low (1 bar) and high (100 bar) oxygen pressure demonstrates that for temperatures below 70 K the position of the second magnetization peak is independent of the oxygenation method. This suggests that the formation or not of oxygen vacancy clusters due to the different annealing treatments does not significantly affect the peak position at these temperatures, a result also supported by ageing experiments. We suggest that this might be due to similar pinning properties of a more homogeneous and cluster configurations of oxygen vacancies at these temperatures. For T > 70 K, however, oxygen ordering effects become important as demonstrated by the suppression of the magnetization peak for the high pressure annealed samples, which demonstrates that clusters are more robust against thermal depinning.

2PO1-69 Determination of U(x,t) and j in MTG YBCO bulk by magneto-optic imaging

Y.H. Liu 1, L. Lu 1, Z.Q. Luo 1, K. Luo 1, Z.X. Gao 1, L. Xiao 2, H.T. Ren 2, Y.L. Jiao 2, and M.H. Zheng 2. 1 Department of Physics, Peking University, Beijing 100871, P.R. China. 2 General Research Institute for Non-Ferrous Metals, Beijing 100088, P.R. China.

Presenting Author: Y.H. Liu

The MTG YBCO bulk is the important material for superconducting magnetic suspended train. In order to investigate the rule of the activated flux creep in the MTG YBCO bulk, we measure the spatial distribution and time dependence of the magnetic induction, determine the activation energy U(x,t) and current density j, then fit the model of vortex dynamics using magneto-optic imaging. In slab geometry the flux motion is governed by ¶ B/¶ t= -¶ D/¶ x, where D=Bn is the flux current density. In the magneto-optic imaging experiments, only the component of the magnetic induction normal to the sample surface, B_Z(x,t) and ¶ B_z(x,t)/¶ t, could be determined. We have D(x,t) = -(^x¶ B_z(x,t)/^a¶ t)dx and U(x,t)/k_BT = -ln((4p h D/Af ₀B_z(¶ B_z/¶ x)).

The critical temperature T_c of a MTG YBa₂Cu₃O₇ + 40 mol. % Y₂BaCuO₅ + 0.8 wt. % Pt bulk is 91 K measured by ac magnetic susceptibility. The sample was cut and polished into a rectangular slab with dimensions of 2b x 2a x 2d = 3.20 x 1.28 x 0.50 mm³. After the sample was zero-field cooled from room temperature to below T_c, an external magnetic field 40.0 mT perpendicular to the broad surface of the sample was applied, then switched off the external magnetic field. Once the critical state was established, the magneto-optic images on the sample surface were recorded. Using the B_z(x,t)data, ¶ B_z(x,t/¶ t, D(x,t) and U(x,t) were calculated. At different spatial points the figures show a linear dependence of U/k_BT on lnt with a constant slope U(x,t)=k_BTln(t/t₀). U(x,t) is almost constant within ± k_BT throughout the sample similar to single crystals, and this is consistent with the concept of self-organized criticality. The activation energy U is plotted as a function of the current density j at the middle between the sample edge and center at four different temperatures. The good fit for each temperature was obtained using expression U=U_cln(jc/j), the deviation may be explained by the rather complicated flux motion in the MTG bulk.