2PO1-140 Effects of charge carriers on the critical vortex velocity in high-Tc superconductors

Ali E. Khalil, Physics Department, Xavier University, 3800 Victory Parkway, Cincinnati, OH 45207

Presenting Author: A.E. Khalil

A theoretical model is proposed to account for the magnetic field dependence of the critical vortex velocity predicted by Larkin-Ovchinnikov (LO) model in BSCCO compound. The temperature and field dependences of the critical vortex velocity were attributed to the physics of the vortex core in clean superconductors (HTSC) which is very different from the physics of the vortex core in dirty superconductors. In the current approach, the diffusion coefficient of quasiparticles and their scattering times were modelled. The quasiparticles diffusion coefficient Dqp was calculated based on the assumption that the quasiparticle motion is considered as a transport phenomenon on random walks. The quasiparticle scattering time was evaluated considering the hydrodynamic interaction of a moving gas of quasiparticles with the discrete states of the vortex velocity field. The predictions of the model are such that as the temperature of the sample increases and approaches the transition temperature, the quasiparticle excitations also increases due to the suppression of the order parameter. As a result the quasiparticle scattering rate decreases and their scattering time becomes longer as the applied magnetic field is increased. In this case, The critical vortex velocity decreases as the field increases. The predictions of the model and its explanation are in excellent agreement with the experimental observations recently reported in the literature. The present approach provides additional evidence to the validity of the flux-flow instability mechanism.

2PO1-141 Flux Flow and Quasiparticle Contributions in the Field Dependent Microwave Resistivity of YBCO and BSCCO Films

E. Silva 1, R. Marcon 1, D. Neri 1, R. Fastampa 2, M. Giura 2, and S. Sarti 2. 1 Dip. di Fisica E.Amaldi and Unità INFM, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy. 2 Dip. di Fisica and Unità INFM, Università La Sapienza, 00185 Roma, Italy.

Presenting Author: R. Fastampa

We present a wide set of measurement of the microwave resistivity at 48 GHz in YBCO and BSCCO films as a function of the temperature (60-100 K) and magnetic field (1-1000 mT). The markedly sublinear behavior of the real part of the resistivity vs. H, especially evident in BSCCO, cannot be explained in terms of flux motion alone. Including in a self-consistent treatment the contribution of the superfluid depletion in a superconductor with line nodes in the gap and the fluxon motion in its simplest formulation (free flux flow) allows to quantitatively describe all the measured curves, while the calculation for a superconductor without nodes in the gap fails to fit the data. The same model predicts the reactive part of the response to be of the same order of magnitude than the resistive part. The measurements allow to estimate the temperature dependence of the upper critical field (or of the fluxon viscosity). Good agreement is found with independent estimates.

2PO1-142 Investigation of the Geometrical Barrier in Bi-2212 Single Crystral

Zhi W. Lin 1, Gen D. Gu 2, and J. Graeme Russell 2. 1 Advanced Electronic Materials Group, School of Physics, University of New South Wales, Sydney, NSW 2052, Australia. 2 Advanced Electronic Materials Group, School of Physics, University of New South Wales, Sydney, NSW 2052, Australia.

Presenting Author: Z.W. Lin

It has been found that penetration of vortices into a weak bulk pinning high temperature superconductor crystal is governed by a geometrical barrier, and forms a dome-shaped flux profile across the crystal. We systematically investigated this geometrical barrier in a high purity Bi2Sr2CaCu2O8+x single crystal platelet using a powerful magneto-optical technique with a sensitive indicator film. The dome-shaped profile is clearly observed over the temperature range 20-70K, and possibly present below 20K, the lowest temperature reported. After discussing the influence of bulk pinning, surface and geometrical barriers on vortices motion, we confirm that the geometrical barrier dominates the motion of vortices and leads to the dome-shaped flux profile even at low temperatures. The experimentally derived flux densities agree well with the calculated flux profiles, although the roughness of the sample surface and irregularities influence the geometrical barrier, and consequently, produce an asymmetrical flux distribution.

2PO1-143 Superconductor Strip Surface-Barrier and Response to a Normally Incident Magnetic Field

D. Agassi and J.R. Cullen, Naval Surface Warfare Center, Carderock Division, Bethesda, MD 20817, USA

Presenting Author: D. Agassi

The surface barrier and the associated current distribution of a superconductor strip in the presence of a normally incident static magnetic field are of considerable importance in instances such as the response to a weak incident ac electromagnetic field. The commonly used expression for thin strips (e.g., Zeldov et. al., Phys. Rev. Lett. 73, 1428 (1994)) predicts a parabolic-like shape for the surface barrier with the minimum at the strip's center line. The associated screening current distribution is predicted to peak at the strip's edge; however, the diverging expression there is quantitatively unreliable. By employing the exact magnetic response of a strip of arbitrary dimensions to a normally incident field (Agassi and Cullen, Physica C, in press) it is shown that in the high aspect-ratio strip limit the surface barrier entails indeed a center-strip minimum. Otherwise there are two minima, symmetrically located with respect to the strip's center line. Characteristics of this barrier will be discussed. The associated, calculated, non-divergent screening current is found to peak at the strip's edges. However, the predicted total screening current exceeds the value predicted by the 'canonical' result (Zeldov et.al., above) by about a factor 5.

2PO1-144 Voltage response of the AC Susceptibility and the Pinning Effects in Hg0.69Pb0.31Ba2Ca 2Cu3O8+d

Y.L. Tang 1, D.W. Lu 1, X.X. Xu 1, D.K. Sun 1, X. Jin 1, C.C. Lam 2, Y.S. Chen 2, and K.C. Hung 2. 1 Department of Physics and National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, 210093, P.R. China. 2 Department of Physics and Material Science, City University of Hong Kong, Hong Kong.

Presenting Author: Y.L. Tang

In this paper, we studied the voltage response of AC susceptibility, and on this base, we advanced a new method to obtain an analytical form of the pinning barrier and the true critical current density Jc. Then, we discussed the pinning properties and the dynamical response in our Hg0.69Pb0.31Ba2Ca 2Cu3O8+d sample.

2PO1-145 Observation of Periodic Magnetization Jumps Well Below TC of a Superconducting Nb Film with a Square Lattice of Ni Dots

A. Terentiev 1, D.B. Watkins 1, L.E. De Long 1, D.J. Morgan 2 and J.B. Ketterson 2. 1 Department of Physics and Astronomy, University of Kentucky Lexington, KY 40506-0055. 2 Department of Physics and Astronomy, Northwestern University Evanstron, IL 60208.

Presenting Author: A. Terentiev

The magnetization of a square lattice (1.2 µm spacing) of 120 nm diameter, ferrmomagnetic Ni dots embedded in a superconducting Nb film exhibited periodic (13.5 Oe interval) step-jumps between 8.5 K and the zero-field transition temperature Tc ~ 8.8 K. This behavior is typical of "matching anomalies" previously observed only very close to Tc in patterned superconducting films when the Abrikosov vortex density is uniform and an integral multiple of the density of periodic pins (dots or antidots). A novel, "saw-tooth" pattern of quasiperiodic magnetization instabilities was observed below ~ 3 K, persisting to an abrupt, temperature dependent cutoff field of order 102 Oe. The field dependent periods of the anomalies corresponded to the first, second and third multiples of the expected matching period ~ 13 Oe, suggesting that a terrace structure made up of domains of three different matched vortex densities is established near the film edge.

1 Supported by U.S. Department of Energy Office of Basic Energy Sciences, Division of Materials Research Grant #DE-FG02-97ER45653.

2 Supported by U.S. NSF Materials Research Center Grant #DMR-9309061.

2PO1-146 SO(5) Theory of Insulating Vortex Cores in the High Tc Materials

Brian M. Andersen, Henrik Bruus and Per Hedegard, Department of Physics, University of Copenhagen, Niels Bohr Institute for APG, Universitetsparken 5, 2100 Copenhagen, Denmark

Presenting Author: Brian M. Andersen

By using the formalism of the SO(5) theory for high Tc superconductors we investigate the existence of localized resonances in the cuprate vortex cores. Contrary to traditional s- or d-wave superconductors, several STM experiments have shown that for instance BSCCO seems to have featureless vortex cores.

A natural consequence of the SO(5) model is that high Tc superconductors can have antiferromagnetic vortex cores. Intuitively one can think of the usual potential well of the vortex cores in the conventional superconductors as being transformed into a potential barrier expelling all charge carriers from the cores in some of the high Tc materials. Explicitly we incorporate the antiferromagnetism into a generalized set of Bogoliubov-de Gennes equations and solve these numerically on a lattice.

Besides STM experiments the results seems to agree well with recent neutron scattering experiments on optimally doped YBCO.