1PO2-40 Quantum limit of quasi-particle spectrum in the vortex states of d-wave superconductors

Masaru Kato 1, Yuzuriha Hiroyuki 1, and Kazumi Maki 2. 1 Dept. of Mathematical Sciences, Osaka Prefecture University, Sakai Osaka 599-8531, Japan. 2 Dept. of Physics and Astronomy, USC, Los Angeles, CA 90089-0484, USA.

Presenting Author: M. Kato

Making use of the Bogoliubov-de Gennes equation, we obtain quasi-particle spectrum and the vortex core structure in quasi-2D d-wave superconductors for small p_{Fx 0}(quantum limit). Here p_F is the Fermi momentum and x ₀ is the coherence length. During our numerical calculation, the number of quasi-particles is conserved. In particular we obtain the fourfold symmetric distortion of the vortex core which gives rise to the core interaction energy which favors a square lattice. We find this distortion is nearly temperature independent and becomes small for smaller p_{Fx 0}.

1PO2-41 Magnetic skyrmions and other nonconventional fluxons in p - wave superconductors

Baruch Rosenstein and Anton Knigavko, Electrophysics Department, National Chiao Tung University, Hsinchu 30050, Taiwan, R.O.C.

Presenting Author: B. Rosenstein

Topological analysis of the Ginzburg-Landau theory, proposed for UPt3 by Machida and coworkers, shows that there exists a new class of solutions carrying two units of magnetic flux: the magnetic skyrmions. These solutions do not have singular core like Abrikosov vortices and at low magnetic fields they become lighter for strongly type II superconductors. The existence of these topological defects at small fields explains the magnetization curve. Similar objects are expected to exist in U_1-xTh_xBe₁₃.

1PO2-42 Vortex Core States in the Evolution from BCS to BEC Superconductivity

R.D. Duncan and C.A.R. Sa de Melo, Georgia Institute of Technology, School of Physics, 837 State Street, Atlanta GA 30332

Presenting Author: R.D. Duncan

We discuss preliminary results concerning vortex core states in superconductors away from the BCS limit. Vortex core states are studied as a function of density and interaction strength during the evolution from BCS to BEC superconductivity. Connections with experimental results in short coherence length superconductors are also made.

1PO2-43 Low temperature vortex phase diagram of Bi₂Sr₂CaCu₂O₈: a magnetic penetration depth study

Ruslan Prozorov 1, Russell W. Giannetta 1, Tsuyoshi Tamegai 2, P. Guptasarma 3, and David G. Hinks 3. 1 Loomis Laboratory of Physics, University of Illinois at Urbana-Champaign, 1110 West Green St., Urbana, IL 61801. 2 Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan. 3 Chemistry and Materials Science Division, Argonne National Laboratory, Argonne, IL 60439

Presenting Author: R. Prozorov

We report measurements of the magnetic penetration depth 1m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero–field cooled sample, results in a non-monotonic 1m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.

*1PO2-44 Genesis of the Peak-Effect from the Melting Transition in YBa₂Cu₃O_y Single Crystals

Alexander A. Zhukov 1, Stelios Kokkaliaris 1, Peter A.J. de Groot 1, Louis Jansen 2, Eric Mossang 2, Robert Gagnon 3, Louis Taillefer 4, Thomas Wolf 5, and Hidehito Asaoka 6. 1 Department of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, United Kingdom. 2 Grenoble High Magnetic Field Laboratory, MPIF-CNRS, BP 166, 38042 Grenoble Cedex 09, France. 3 Department of Physics, McGill University, Montreal, Canada H3A 2T8. 4Department of Physics, University of Toronto, M5S 1A7, Canada. 5 Forschungszentrum Karlsruhe, Institute für Technische Physik, D-76021 Karlsruhe, Germany. 6Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-11, Japan.

Presenting Author: P.A.J. de Groot

We investigated magnetisation curves in magnetic fields up to 23T for three YBa₂Cu₃O_y single crystals with various oxygen contents using a highly sensitive magnetic torque rig. The latter together with the high quality of the studied crystals have allowed us to find several new previously unreported anomalies. We have observed a very narrow peak with a width even smaller than the one seen in conventional superconductors. At low temperatures the peak broadens and transforms into the conventional fishtail peak. The transformation is realised through the double peak structure. With increasing temperature, above the multicritical point, the sharp peak converts into a magnetisation jump due to melting of the vortex lattice. The B-T vortex phase diagram and its transformations with oxygen content are also found

*1PO2-45 Strong Quantum Fluctuations of Vortex Movements in C-axis-oriented YBCO Ultrathin Films

Chunguang Li 1, Duo Jin 1, Zhaojia Chen 1, Yusheng He 1, Guangcheng Xiong 2, Guijun Lian 2, Daole Yin 2, Sheng Luo 3, and C.E. Gough 4. 1 Institute of Physics & Center for Condensed Matter Physics, CAS, P.O. Box 2711, Beijing, 100080, P.R. China. 2 Department of Physics, Peking University, Beijing, 100871, P.R. China. 3 Department of Physics, University of Science and Technology Beijing, Beijing, 100083, P.R. China. 4 School of Physics and Astronomy, Birmingham University, Birmingham B15 2 TT, United Kingdom.

Presenting Author: Y. He

Measurements of the magnetic relaxation rate Q(T) and transport characteristics have been performed on YBCO films with thickness between 2.4 and 200nm. Strong quantum fluctuations of the vortex movements which occur at temperatures as high as 9~14 K is detected in all ultrathin films (2.4~40nm), while the thick sample (200nm) does not show any quantum fluctuations at the experimental temperature down to 4.2K. The experimental results are discussed with the theory based on dissipative tunneling of collectively pinned vortex [Blatter et al., Rev. Mod. Phys. 66, 1125(1994)]. It is suggested that the strong quantum fluctuations in the ultrathin films are due to the higher zero temperature normal state resistivity and the higher active energy of those samples.

1PO2-46 Measurement of the Inductance of Pinned Vortices in YBCO Films

Aaron A. Pesetski and Thomas R. Lemberger, Department of Physics, Ohio State University, Columbus, OH 43210-1106

Presenting Author: T.R. Lemberger

We have measured the magnetic penetration depth, l (T), of YBCO films in a perpendicular magnetic field from 0 to 6 Tesla from optimal doping through the superconductor-to-insulator transition at strong underdoping. Films do not degrade under repeated de- and re-oxygenation. Our deoxygenation procedure permits very small incremental reductions in oxygen content. The two-coil measurement apparatus accurately detects small changes in l (0). At moderate underdoping, the low-T slope of 1/l ²

However, there are many puzzles in the data. Extrapolating the low-T fit to the data predicts that vortices should be unpinned above about 45 K, while they remain pinned to much higher temperatures. The density of pinning sites implied by the fit is much larger than can be accounted for by microscopic studies of lattice defects. As a function of field, the sheet inductance of the film increases by about 10% between 0 and 300 Gauss, but it takes another Tesla to increase it by another 10%.

1PO2-47 Increasing of the superconducting pinning potential and critical current due to hydrogen doping in sintered YBCO

Paolo Tripodi 1, Daniele Di Gioacchino 2, and Francesco Celani 2. 1 SRI International, 333 Ravenswood Ave, 94025 Menlo Park, CA. 2 INFN-LNF, Via E. Fermi 40, 00044 Frascati, Italy.

Presenting Author: P. Tripodi

YBCO sintered samples have been hydrogenated by chemical absorption. Using experimental measurements of the higher harmonic ac susceptibilities versus frequency of the applied ac magnetic field and numerical analysis, an increasing of the pinning potential and critical current due to hydrogen have been found.

1PO2-48 Stopping of Levitation Force Relaxation in Superconductors: The Flux Stopping Effect

Boris M. Smolyak 1, Gregory N. Perelshtein 1, German V. Ermakov 1, and Yevgeniy V. Postrekhin 2. 1 Institute of Thermal Physics, Russian Academy of Sciences, Ekaterinburg 620219, Russia. 2 Texas Center for Superconductivity, University of Houston, Houston, TX 77204.

Presenting Author: B.M. Smolyak

A unique setup for measuring the vertical levitation force was used to examine the force relaxation caused by the flux creep in a melt-textured YBCO sample. We observed the effect of relaxation stopping, which took place when a small reverse manipulation was realized during magnetization of the sample: while magnetizing the sample moving it to and from the magnet, the sample was shifted a little backward. In this case current opposite to the main magnetization current of the sample passed in the near-surface layer (reverse-layer) of the superconductor. The time, during which the levitation force was constant, depended on thickness of the reverse-layer, i.e. the depth of the flux profile modulation. This depth can be characterized by the modulation coefficient: the ratio of the force acting on the reverse-layer to the levitation force without reverse magnetization. We found that the stable levitation time was an exponential function of the modulation coefficient. In our experiment the modulation coefficient ranged from 0.05 to 0.2 and the time, during which the force did not change, was 4 to 380 minutes respectively. Without reverse magnetization (in the case of unipolar magnetization) the levitation force decreased with time following the usual logarithmic law.

1PO2-49 Absence of pinning mechanism change in the second peak of YBa₂Cu₃O_6.5 at low temperature.

A.D. Alvarenga 1, S. Salem-Sugui Jr. 2, W.A. Ortiz 3, P. Paulikas 4, and B. Veal 4. 1 Centro Brasileiro de Pesquisas Fisicas, Rua Xavier Sigaud, 150, 22290-180 Rio de Janeiro, Brasil. 2 Instituto de Fisica, UFRJ, CP68528, 21945-970 Rio de Janeiro, Brasil. 3 Departamento de Fisica, UFSCar, 13565-905 São Carlos, Brasil. 4 Materials Sciences Division, Argonne National Laboratory, Argonne, IL 60439.

Presenting Author: A.D. Alvarenga

We have measured hysteresis curves and magnetic relaxation M(t) for a YBa₂Cu₃O_6.5 single crystal (T_c = 62.5 K) with the magnetic field H // c axis. Previous hysteresis measurements in this sample [1] showed the existence of the feature knew as second magnetization peak, Hp, from 60 K down to 1.8 K (Hp is the maximum diamagnetic value). Magnetic relaxation is measured for magnetic fields values running over the second magnetization peak at 1.8 K, 8.5 K and 20 K. The activation pinning energy U (H) is obtained from relaxation data following an approach presented in [ref. 2]. For each hysteresis curve, U (H) is plotted against M (t). At 8.5 and 20 K, U (H) ~ M^-a with 1.0<a <1.5 for H < Hp and 0.3<a <0.7 for H > Hp, as previously observed (3). The pinning crossover at Hp explains the second peak in this case [3]. At 1.8 K U (H) ~ M^-a with 1.1<a <1.5 for H < Hp and H > Hp. The later implies the existence of the second magnetization peak with only one pinning mechanism at 1.8 K.

[1] S. Salem-Sugui Jr, et al., Phys. Rev. B 60, 102 (1999).

[2] M. P. Maley, et al., Phys. Rev. B 42, 2639 (1990).

[3] Y. Abulafia, et al., Phys. Rev. Lett. 77, 1596 (1996).