1PO2-30 Dynamic Response due to Depinning and the Amplitude Effects in AC Susceptibility of HTS

Yu Wang, Wei Wang, Kaixuan Chen, Guo Lu, Lixin Xue, Chuanyi Li, and Daole Yin. Department of Physics, Peking University, Beijing 100871, China

Presenting Author: D. Yin

We study the dynamic response of vortices under alternating driving forces. Some significant amplitude effects of susceptibility are observed even in small field hac. These phenomena can not be understood with the static Bean model. We discuss it with a recently found response function and show its connection with the used empirical power law E(J)=EC(J/J_c)ⁿ. This new dynamic response function predicts a power law shift of the peak position T_p of the out-of-phase susceptibility c ", which is observed in both p-type and n-type HTS. A discussion with magnetic relaxation is also given.

1PO2-31 Sample Size Dependence of the Second Magnetization Peak in Bi₂Sr₂CaCu₂O₈

Yan Mei Wang 1, Alex Zettl 1, Tsuyoshi Tamegai 2, and Shuuichi Ooi 2. 1 Department of Physics, University of California-Berkeley & Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA. 2 Dept of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.

Presenting Author: Y.M.A. Wang

The origin of the Second Magnetization Peak (SMP) in Bi₂Sr₂CaCu₂O₈ (BSCCO) is not clear. It has been suggested that it originates from a 2D-3D phase transition of the vortex solid, or alternatively that it arises from extrinsic processes such as a flux-jump instability. We have studied the SMP in BSCCO samples of various sizes ranging down to 20x20m m². The samples are fabricated using Ar-ion milling into disk and square shapes. All samples originate from the same flat, slightly over-doped single crystal. The local magnetization of BSCCO is measured using a GaAs/AlGaAs Hall sensor with an active area of 10x10m m². We observe the SMP for both the disk and square samples below sample-size-dependent temperatures. The SMP is observed below 22K and 30K for the 20x20m m² and the 65x65m m² samples, respectively. We interpret our results in terms of different mechanisms responsible for the SMP, including a flux-jump scenario.

1PO2-32 Square vortex lattice: elastic moduli and structural phase transitions

Anton Knigavko, Electrophysics Department, National Chiao Tung University, Hsinchu 30050, Taiwan, R.O.C., and Boris Ya. Shapiro, Physics Department, Bar-Ilan University, Ramat-Gan 529000, Israel

Presenting Author: A. Knigavko

Recent neutron diffraction experiments on borocarbide superconductors as well as on Ru₂SrO₄ have unambiguously shown that a square vortex lattice is the ground state of these systems in the appreciable range of external magnetic field values. To describe slight deviations out of equilibrium, and even some dynamical properties of the mixed state of superconductors the presentation of the vortex lattice as an elastic media is customary employed. While much is known about elastic moduli of the standard hexagonal vortex lattice and its uniaxially anisotropic version, the information of elastic properties of other vortex lattices is scarce. In this report we present results of the calculation of elastic moduli for the square lattice relevant to the case of borocarbide superconductors. Wave vector dependence of the two independent shear moduli is determined. Starting from the Ginzburg-Landau free energy functional we derive in the London approximation the effective functional in terms of vortex positions. Then we proceeded to the elastic energy by expanding around the square lattice which is known to be the ground state. Our results form the basis for considering the fluctuations in the mixed state of borocarbide superconductors. At the same time we have considered the structural phase transitions of ferroelastic type in the vortex lattice, which occur at such external magnetic fields when a shear elastic modulus vanishes.

1PO2-33 Angular-dependent Studies of Vortex Behavior in YNi₂B₂C Single Crystals

R. Chu, Q.Y. Chen, W.K. Chu, and J.H. Miller, Jr., Department of Physics and Texas Center for Superconductivity, University of Houston, 3201 Cullen Blvd, Houston, TX 77204-5932

Presenting Author: R.K. Chu

We have investigated the angular-dependent I-V curves on a-axis oriented YNi₂B₂C single crystals. A magnetic field of 3 T was applied with respect to the a axis varied from 0 to 90 degrees. The field-independent scaling behavior is observed with critical exponents (m = 1.27 ~ 0.05 and z = 5.86 ~ 0.09) for all orientations. At small angles, the results indicate a vortex-glass phase, which is caused by random point-like disorder. We find no evidence supports a transition to the Bose-glass phase, which is usually associated with line-like disorder.

1PO2-34 Two-Stage Melting of the Vortex Glass with Strong Point Disorder

Ryoko Sugano 1, Toshiyuki Onogi 1, Kazuto Hirata 2, and Masashi Tachiki 2. 1 Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama, 350-0395, Japan. 2 National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.

Presenting Author: R. Sugano

We study vortex phase diagram of a Bi₂Sr₂CaCu₂O_8+d model in the presence of strong point like disorder. Based on a vortex variable version of the Lawrence-Doniach model, we performed the Monte Carlo simulation. We found two successive phase transformations from strongly-pinned vortex glass (or Bragg glass at low field) to pancake vortex gas via an intermediate weakly-pinned (soft) glass, when temperature is increased. The first one is identified as an intra-layer depinning transition as pre-melting, which subdivides the glass phase into two phases. This phase boundary makes an almost vertical line near 30K. Subsequently, at higher temperature above 30K, the soft glass melts with inter-layer decoupling of vortex-lines, implying the loss of interlayer long-range coherence. These two-phase boundaries agree with recent experimental data.

At low temperature, we also found a field-induced dimensional transformation from three-dimensional Bragg glass to quasi-two-dimensional vortex glass. At the boundary between Bragg glass and vortex glass, an abrupt change of the interlayer coherence takes place with increasing field, suggesting a first-order phase transition. This work was supported by Center for Promotion of Computational Science and Engineering (Japan Atomic Energy Research Institute).

1PO2-35 Experimental evidence for vortex decoupling in a superconducting Bi₂Sr_2-xLa_xCuO_6+d thin film

Y.Z. Zhang 1,2, R. Deltour 2, and Z.X. Zhao 1. 1 National Laboratory for Superconductivity, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China. 2 Universite Libre de Bruxelles, Physique des Solides, CP 233, B-1050, Brussels, Belgium.

Presenting Author: Y.Z. Zhang

Resistive transitions and I-V isotherms of a Bi₂Sr_2-xLa_xCuO_6+d thin film have been measured in magnetic fields up to 5 T. A resistive transition showing a sharp jump to a very small value at a well defined temperature, T_c0(H), is observed. The T_c0(H) coincides with the temperature at which a (T,H) » 2 in the relation V(T, H) ~ I(T, H)a ^{(T, H)+1}. Using a Coulomb-gas (CG) model, we find that T_c0(H) can be considered as characterising a Kosterlitz-Thouless transition temperature, allowing all the resistive tails in different magnetic fields to fall onto a universal transition curve; besides, the scaling process also allows all the a (T, H) curves to fall onto a single a (T, H) scaling curve. These results suggest that T_c0(H) is related to the CG transition temperature above which the CG particles begin to be produced by the thermal unbinding of the vortex couplings. Testing the relation H ~ [1-T_c0(H)/T_c]ⁿ, we find that n = 4.5 corresponding to a quasi-2D characteristic as mentioned recently in the literature. We suggest that this CG scaling form may be universal for all the HTSCs provided that the vortex matter is in a quasi-2D state.

1PO2-36 Coupling Between the Layers in Vortex Liquid Regime of YBa₂Cu₃O_7-d Single Crystals

Z.H. Wang 1, H. Zhang 2, J. Fang 3, S. Wang 3, J.L. Chen 3, S.Y. Ding 1, and X.W. Cao 3. 1 Department of Physics, Nanjing University, Nanjing 210093, P.R. China. 2 Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, P.R. China. 3 Institute of Plasma Physics, Chinese Academy of Sciences, Hafei 230031, P.R. China.

Presenting Author: Z.H. Wang

We attached two current contacts and two voltage contacts on both sides of MTG-YBCO single crystals and performed transport measurement applying the current parallel and perpendicular to the CuO₂ layers in case of the applied magnetic field up to 7.5T parallel and perpendicular to the c-axis, respectively. The results show that in the dissipative regime the voltage signal on the side of the applied current contacts is more than that on the opposite side, and the ratio of voltage signals increases at first and then decreases with decreasing the temperature. The field of layers-coupling temperature follows the formula H=H₀(1-T/T_c0)^1.32 consistent with the vortex-glass transition line. The critical temperature T_c^c along the c-axis is higher than that in ab-plane, indicating that the coupling between the layers is stronger as the temperature decreases and 3D character is in the lower temperature.

1PO2-37 Vortex Charge in High-T_c Superconductors Probed by Nuclear Quadrupole Resonance

Ken-ichi Kumagai 1, Koji Nozaki 1, and Yuji Matsuda 2. 1 Dept. of Physics, Hokkaido Univ. Sapporo 060, Japan. 2 Institute for Solid State Physics, Univ. of Tokyo, Roppongi, Minato-ku, Tokyo 106-8666, Japan.

Presenting Author: K. Kumagai

Recent theories predict a possibility that a vortex traps an electric charge. To examine the vortex charge in high-T_c superconductors, we performed the NQR experiments which can probe the local carrier density accurately. A finite shift of the NQR frequency which can be attributed to the vortex charging effect is observed in the vortex state of optimally doped YBCO. We show that the number of hole within the core decreases relative to the outside; the vortex core charges up. This tendency is inconsistent with the prediction of BCS theory and seems to suggest an importance of the electron correlation inside the core. We also discuss the relation between the vortex charge and the vortex Hall effect.

*1PO2-38 Theory of Vortex Lattice Effects on STM Spectra in D-wave Superconductors

Alexander S. Mel'nikov, Institute for Physics of the Microstructures, Russian Academy of Sciences, GSP-105, Nizhny Novgorod, 603600, Russia.

Presenting Author: A.S. Mel'nikov

Theory of scanning tunneling spectroscopy of low energy quasiparticle (QP) states in vortex lattices of d-wave superconductors is developed taking account of the effects caused by the band structure in the QP spectrum near the gap nodes. Different vortex lattice geometries are considered. The oscillatory structures in STM spectra, which correspond to van Hove singularities are analysed. Theoretical calculations carried out for finite temperatures and scattering rates are compared with recent experimental data for high temperature cuprates.

1PO2-39 Dynamics of bound states and charges in the vortex core

M. Eschrig 1, D. Rainer 2, and J.A. Sauls 1. 1 Department of Physics & Astronomy, Northwestern University, Evanston, IL 60208, USA. 2 Physikalisches Institut, University of Bayreuth, D-95440 Bayreuth, Germany.

Presenting Author: M. Eschrig

We present calculations for the ac response to electromagnetic fields of isolated vortex cores in a layered superconductor. The results are appropriate for high-T_c cuprates with a mean free path comparable to the coherence length. Our calculations cover ac fields with freqencies comparable to the superconducting gap.

The excitation spectrum and current response are obtained self-consistently with the order parameter response. Our results for the dynamics of the spectrum of Caroli-deGennes-Matricon bound states show the importance of the coupling between collective modes of the condensate and the excitation spectrum.

We discuss the charge dynamics in the vortex core. The local charges induced by the time dependent external fields extend over several coherence lengths and are easily larger than the static charge of the vortex core.

We discuss the influence of pinning on the current response and compare the dissipation of pinned and unpinned vortices.

The work was supported by the STCS through NSF grant no. 91-20000 and the Deutsche Forschungsgemeinschaft.