R. Santiago 1, A. Krupski 1, C. Boekema 1, and D.W. Cooke 2. 1 Physics Dept., San Jose State University, San Jose, CA 95192-0106. 2 Los Alamos National Laboratory, Los Alamos, NM 87545.

Presenting Author: R. Santiago

The vortex states of Bi2212 and Tl2223 have been studied by muon-spin resonance (muSR). The transverse field muSR data are analyzed by the Maximum-Entropy (ME) technique. Below T_c, two major muSR signals overlap--one asymmetric signal reflecting the vortex field distribution, and one stemming from the grain boundaries. As predicted for d-wave symmetry, and as recently found for R1237 vortex states, twin-peak signatures in the main vortex signals below and at 10 K and at 5 kOe have been observed for Bi2212 and Tl2223. In contrast to the observed low field tails in the R1237 vortex field distributions, no such tail has been observed in the Bi2212 and Tl2223 vortex states near 5 K. This is consistent with vortex-pancake formation and the absence of CuO chains in these cuprates. ME-transforms reflecting the temperature-dependent field distributions will be discussed. The cuprate vortex states may not be typical type II, as has been assumed for the d-wave prediction calculations. However, d-wave symmetry probably occurs in cuprate superconductivity.

*Research is supported by NSF, LANL-DOE, WiSE@SJSU and SJSU Graduate Studies.

Qiang-Hua Wang, Department of Physics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China and Z.D. Wang, Department of Physics, University of Hong Kong, Pokfulam, Hong Kong, China

Presenting Author: Z.D. Wang

The Ginzburg-Landau theory with a Zeeman coupling between the magnetic field and the internal motion of Cooper pairs is studied in the context of a d_x2-y2+id_xy-wave superconductor. From a careful treatment of the energy gain due to the Zeeman coupling, the energy loss due to the excess superfluid, and the Doppler energy shift for quasi-particle excitations, we obtain a consistent picture to explain the reported abnormal and conflicting thermal conductivities at a few Kelvins (K. Krishana et al, Science 277, 83(1997)) and at sub-Kelvins (H. Aubin et al, Phys. Rev. Lett. 82, 624(1999)) in BSCCO cuprates.

Alexander A. Zhukov 1, Stelios Kokkaliaris 1, Peter A.J. de Groot 1, Mark J. Higgins 2, Shobo Bhattacharya 2, Robert Gagnon 3, and Louis Taillefer 4. 1 Department of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, United Kingdom. 2 NEC Research Institute, 4 Independence Way, Princeton, NJ 08540. 3 Department of Physics, McGill University, Montreal, Canada H3A 2T8. 4 Department of Physics, University of Toronto, M5S 1A7, Canada.

Presenting Author: A.A. Zhukov

We report the observation of a negative dynamic creep rate, i.e. increasing irreversibility in the magnetic hysteresis loop for decreasing sweep rate of the magnetic field, in both 2H-NbSe₂ and pure YBa₂Cu₃O_y single crystals. This phenomenon is found to appear on the increasing branch of the peak-effect that corresponds to a state, which is intermediate between the dislocation-free Bragg glass and a highly disordered vortex phase. The origin of this anomalous creep is shown to be connected to a negative differential resistance resulting from the N-like shape of the current voltage characteristics. This J(E) shape gives an explanation to the voltage oscillations, instabilities and hysteresis effects detected in transport measurements.

Alexander A. Kordyuk 1, Gernot Krabbes 2, Volodymyr V. Nemoshkalenko 2, and Roman V. Viznichenko 1. 1 Institute of Metal Physics, 36 Vernadsky str., Kyiv 252680, Ukraine. 2 Institute of Solid State and Materials Research Dresden, Helmholtzstrasse 20, 01171 Dresden, Germany.

Presenting Author: A.A. Kordyuk

We have studied the magnetic flux penetration into HTS bulks by means of resonance oscillations technique and levitation force measurements. Two types of a.c. losses during the vortex penetration into melt-processed HTS quasi-single crystals were distinguish with the resonance oscillations technique. The first one is well known hysteretic losses which can be described very well with the critical state model. The values of critical current density which were obtained within the developed approach are in a good agreement with ones obtained with other techniques. The second a.c. loss component has been found to be related with the flux-flow motion of the perpendicular to HTS surface vortices. The influence of surface treatment on AC loss in melt-processed quasi-single crystal HTS has been also investigated. We have found that the dependencies of a.c. loss on magnetic field amplitude become rather complicated after surface polishing. The experimental data show well distinguished dynamic crossover from absence of barrier at low rate of field variation to its appearence at higher rate. An explanation of such a dynamic surface barrier appearance, based on consideration of along surface vortex propagation, was suggested.

Abdel Fattah Khoder, Université Libanaise, Faculté des Sciences,B.P. 826, Tripoli, Lebanon, and Michel Couach, CEA Grenoble, Département de Recherche Fondamentale sur la Matière Condensée, 38054 Grenoble Cedex 9, France

Presenting Author: A.F. Khoder

The differential susceptibility X'_H in the reversible mixed state of Bi₂Sr₂CaCu₂O₈ single crystals is measured for magnetic field as high as 4 Tesla perpendicular to the layers. Fluctuations regime, around T_c, is observed but anomalous behavior of X'_H=dM/dH is evidenced far from T_c following a classical behavior where M(H) seems to obey a T linear lnH dependence. The low temperature (T<0.7 T_c) behavior in the present compound has been already observed in oxygen off stoechiometric YBa₂Cu₃O_7-d and has been associated to extra condensation of charges in the normal layers by continuous proximity effect enhanced condensation in the non superconducting layers in the highly anisotropic superconductors.

Lev A. Dorosinskii 1, Huseyin Bocuk 1, Ugur Topal 1, Cengiz Birlikseven 1, Peter Kes 2, and Roel Drost 2. 1 National Metrology Institute, P.K. 21, 41470 Gebze-Kocaeli, Turkey. 2 Leiden University, Leiden Institute of Physics, Kamerlingh Onnes Laboratory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands, P.O. Box 9504, 2300 RA Leiden, The Netherlands

Presenting Author: L. Dorosinskii

Using the magnetooptic technique, the ac susceptibility of YBCO and BSCCO-2212 single crystals was measured locally in the frequency range from 50 Hz to 4 MHz. Crystals with various doping level, which was varied by annealing them in different conditions, were investigated. A shift of the apparent melting line with the ac-field frequency was observed in BSCCO crystals and characteristic time scales for vortex fluctuations in the viscous vortex liquid regime were estimated from the frequency dependence of the melting field. In underdoped crystals the apparent superconducting transition was observed to shift to higher temperatures at higher frequencies. We interpret this shift in terms of the phase fluctuations of the superconducting order parameter, which manifest themselves in ac-susceptibility measurements above T_c.

Tiziana Di Matteo, Massimiliano Polichetti, and Sandro Pace, Department of Physics, University of Salerno & INFM, Via S. Allende, Baronissi, Salerno, Italy

Presenting Author: T. Di Matteo

We investigate the temperature dependence of the higher harmonics of the AC susceptibility in high-T_c superconductors by solving numerically the non-linear magnetic diffusion equation, in the case of a superconducting sample having a slab geometry, at different frequencies of an applied AC magnetic field, and in presence of a DC bias magnetic field. In the diffusive term, both the thermally activated and flux flow processes have been included. In particular, we choose the temperature dependence of the collective pinning model and a realistic magnetic field dependence for the pinning potential and the critical current density. Finally, a comparison with the experimental curves of the higher harmonics of the complex susceptibility is presented.

Daniele Di Gioacchino 1, Paolo Tripodi 2, Francesco Celani 2, and Donglu Shi 3. 1 INFN-LNF, Via E. Fermi 40, 00044 Frascati, Italy. 2 SRI International, 333 Ravenswood Ave, 94025 Menlo Park, CA. 3 Cincinnati University, 493 Rhodes Hall, Cincinnati, OH.

Presenting Author: D. Di Gioacchino

Measuring the ac higher harmonic susceptibilities of the YBCO samples versus frequency and magnitude of the applied ac magnetic field and temperature near T_c, pinning potential and critical current have been carry out using the comparison between experimental data and numerical analysis of the magnetic diffusion equation inside the sample.

Sergio O. Valenzuela and Victoria Bekeris, Laboratorio de Bajas Temperaturas, Depto. de Física, F.C.E.y N., Universidad Nacional de Buenos Aires, Pabellón I, Ciudad Universitaria, 1428, Buenos Aires, Argentina

Presenting Author: S.O. Valenzuela

We report on marked thermomagnetic history and memory effects in the ac susceptibility of twinned YBa₂Cu₃O₇ single crystals. We varied the effective pinning character of the twin boundaries by tilting the applied dc-field from the c-axis direction. Our results present three angular regions below the peak effect zone characterized both by its thermomagnetic history dependence and by a distinct screening capability of the sample. For q < q _ch, vortices channel along twin boundaries reducing the screening; for q _ch < q < q _p the screening is enhanced due to twin planes acting as pinning centers; for larger angles, q > q _p, the characteristic peak effect observed in untwinned samples is recovered while the screening (at lower temperatures) becomes less effective due to the ordering of the vortex system. History effects are pronounced in the last angular region becoming less important at smaller angles. We relate this behavior to the loss of order in the vortex structure. We also found that, starting from a disordered state (ZFC), the ac-field induces an annealing of the vortex lattice in the penetrated outer zone of the sample reducing the pinning. The new state is stable leading to memory effects both in temperature and ac-field.

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

Presenting Author: F.Y. Lin

The measurement of ac susceptibility (ACS) in high T_c superconductor will meet several phases of the vortex matter. The numerical simulation of ACS especially need consider this problem. The former calculations, which use logarithmic U-J relationship or glass state one U=(U₀/m )((J_c/J)m -1), resolve the problem by introducing a factor of J/J_c in the flux creep velocity v(U,T)=v₀e^-U/kT , which will become v(U,T)=v₀(J/J_c)e^-U/kT. Now we use a model considering the backward jump of flux lines to solve this problem without introducing a J/J_c factor additionally. It is shown that consideration of the backward jump of flux lines will meet the experiments well. In this paper we also use the numerical simulation of ACS to determine the true critical current density J_c in Tl2212 thin film, which exclude the affect of the vortex creep.