2PO1-90 Magnetic Relaxation Measurements on Multifilamentary BSCCO/Ag Tapes in High Magnetic Fields

Massimiliano Polichetti, Danilo Zola, and Sandro Pace, Department of Physics, University of Salerno & INFM, Via S. Allende, Baronissi, Salerno, Italy

Presenting Author: M. Polichetti

To study the vortex dynamic in BSCCO/Ag multifilamentary tapes, the analysis of the magnetic decay has been performed at different temperatures and dc magnetic fields. In particular, the magnetization (M) have been measured as function of the time (t) at the temperatures T in the range [4.2K - 45 K], in order to investigate the difference in the flux dynamic response depending on the value of T respect to the the two-dimensional vortex lattice melting temperature (T[2d,m]) reported in literature. The M(t) curves have been recorded for about 5000 seconds, at different dc magnetic fields up to 12 T. The experimental data show that differences in the M(t) behaviors, depending on the field and on the elapsed time since the beginning of the relaxation process, appear between the curves at the lowest temperatures and the ones closer to T[2d,m].

2PO1-91 Upper critical field and irreversibility field of over-doped Tl₂Ba₂CuO_8+d superconductors

D.N. Zheng 1, J.Q. Li 1, C.T. Lin 2, P.J. Wu 1, S.L. Yan 3, L. Fang 3, and Z.X. Zhao 1. 1 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China. 2 Max-Planck-Institut fur Festkorperforschung, D-70569 Stuttgart, Germany. 3 Department of Electronics, Nankai University, Tianjin 300071, P.R. China.

Presenting Author: D.N. Zheng

The resistivity determined upper critical field of over-doped Tl₂Ba₂CuO_8+d superconductors shows peculiar upturn temperature dependence, in contrast to the prediction of conventional mean field theory. Much of attention has been drawn on this issue. A number of theoretical models based on different ideas, including bi-polaron pairing, temperature dependent effective mass and strong fluctuations, have been put forward. We have carried out systematic magnetic measurements on over-doped Tl₂Ba₂CuO_8+d single crystal samples. An irreversibility line was identified. The irreversibility line is considerably lower than the upper critical field B_c2 and shows the upturn curvature. We proposed that B_c2 determined from resistive transitions is not the true upper field but the irreversibility field instead. We presented a phenomenological model to explain why heavily over-doped Tl₂Ba₂CuO_8+d samples show sharp resistive transitions at the irreversibility field. Transport measurements have been carried out to verify the model. TEM work has also been performed on the samples and new structure features due to excess of oxygen were observed.

2PO1-92 Suppression of thermal fluctuation and improvement of the irreversibility line by Sr substitution in Hg_0.7Pb_0.3Ba₂Ca₂Cu₃O_y superconductor

Yi Zhuo 1,2, Chang Uk Jung 1, Jae-Hyuk Choi 1, Mun-Seog Kim 1, N.P. Kiryakov 3, M.S. Kuznetsov 3, Sergey Lee 3, and Sung-Ik Lee 1. 1 National Creative Research Initiative Center for Superconductivity, Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea. 2 Department of Physics, Shenyang Normal University, Shenyang 110031, China. 3 Department of Chemistry, Moscow State University, Moscow 119899, Russia.

Presenting Author: C.U. Jung

The nearly single phase compounds, Hg_0.7Pb_0.3Ba₂Ca₂Cu₃O_y and Hg_0.7Pb_0.3Sr₂Ca₂Cu₃O_y, were synthesized. The effects of Sr substitution on the Ba site were studied. The grain-aligned samples were obtained in a magnetic field of 7 T. The substitution of Sr for Ba shortens the c-axis lattice parameter by about 0.07 nm. The irreversibility line for the Sr-substituted compound is much steeper compared to the pristine sample. From the analysis of the reversible magnetization, we found that the fluctuation magnetization is much suppressed in the Sr-substituted compound. The suppression of thermal fluctuation and the improvement of the irreversibility line results from increase of interlayer coupling.

2PO1-93 Paramagnetic Meissner Effect in Melt-textured YBa₂Cu₃O₇

F.T. Dias 1, P. Pureur 1, P. Rodrigues, Jr. 2, and X. Obradors 3. 1 Instituto de Física, Universidade Federal do Rio Grande do Sul, P.O. Box 15051, 91501-970 Porto Alegre, Brazil. 2 Departamento de Física, Universidade Estadual de Ponta Grossa, 84031-510 Ponta Grossa, Brazil. 3 Institut de Ciencia de Materials, CSIC, Campus de la UAB, 08193 Bellaterra, Catalunya, Spain.

Presenting Author: F.T. Dias

We report on ZFC (zero-field- cooled), FCC (field-cooled-cooling) and FCW (field-cooled-warming) magnetization experiments in a melt-textured YBa₂Cu₃O_7-d sample. Several fields up to 5 T were applied parallel and perpendicular to the c-axis. In the very low field limit (2 Oe < H < 10 Oe), the FC moment is positive in the whole temperature range below T_c and its magnitude diminishes when the field is increased. In fields between 10 and 20 Oe, the FC moment becomes diamagnetic and so remains up to about 300 Oe. In the interval 1000 Oe < H < 5000 Oe, the FC moment is no longer reversible and may be either positive or negative at low temperatures, depending on the sample’s cooling rate. In fields above 1 T and up to 5 T, the moment is positive in low temperatures. In the moderate and high field regions, we have observed strong time-dependent effects where the FC moment relaxes towards paramagnetic values.

We ascribe our low-field Paramagnetic Meissner Effect (PME) data to spontaneous orbital currents related to the existence of junctions in a Josephson medium. At intermediate and high fields, PME is likely to occur because of flux compression into the monolithic sample due to inhomogeneous cooling.

2PO1-94 Dynamic scaling behavior of a paramagnetic Meissner effect sample - evidence of a chiral - glass transition

Evie L. Papadopoulou, Per Nordblad and Peter Svedlindh, Departments of Materials Science, Uppsala University, Box 534, SE - 751 21 Uppsala, Sweden

Presenting Author: E.L. Papadopoulou

2PO1-95 Microscopic Theory for the Josephson Plasma in High-T_c Superconductors: Interplay Between the Josephson Current and the c-axis Optical Phonons

Tomio Koyama, Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendau 980-8577, Japan

Presenting Author: T. Koyama

A full quantum theory for the Josephson plasma is formulated in intrinsic Josephson-junction arrays. We investigate the 2nd-order tunneling process of electrons interacting with the electromagnetic field in serial tunneling-junctions. The frequency-dependent dielectric function is derived quantum-mecanically for arbitrary wave-numbers. Numerical results for the dielectric function are presented for both coherent and incoherent tunneling processes, from which the temperature dependent plasma frequency is calculated.

The c-axis optical phonons interact with the tunneling current and the electric field along the c-axis in the present systems. It has been observed that the phonon spectrum in YBCO and Bi-2212 is strongly affected by the superconductivity. We extend our theory to the cases in which the c-axis optical phonons are present. We assume that the tunneling electrons interact with the c-axis optical phonons via variations of the tunneling matrix elements and also the Josephson plasma couples electromagnetically with them. The dielectric function and the optical conductivity including the phonon effects are derived. It is shown that the strong supression of the peak intensity and the broadening of the phonon peaks are caused in the superconducting state. This result is consistent with the experimental observation in Bi-2212.

2PO1-96 Interlayer Carrier Dynamics in Underdoped High T_c Cuprates

Xianggang Qiu 1,3, T. Itoh 2, A.K. Kuma 2, M. Iwasaki 2, M. Kawasaki 2, H. Koinuma 1,2 Y. Segawa 3. 1 CREST, Japan Science and Technology Cooperation, 3-4-1 Okubo, Shinjuku, Tokyo 169, Japan. 2 Ceramics Materials and Structures laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Japan. 3 Photodynamics Research Center, the Institute of Physics and Chemistry (RIKEN), 19-1399 Koeji, Nagamachi, Aoba-ku, Sendai, Japan.

Presenting Author: X.G. Qiu

C-axis far-infrared reflectivity measurements have been conducted on (110) oriented YBa₂Cu₃O_6.5 thin films. It is revealed that in the normal state the carriers are confined in the CuO planes and that their transport across the c-axis is incoherent. A pseudogap develops in the normal state at temperatures well above T_c and it persists in the superconducting state. Upon decreasing the temperature below the critical transition temperature T_c, coherence is built up for the carrier transportation along the c-axis and a reflectivity edge associated with the Josephson plasma is observed around 100 cm^-1. The plasma frequency W_p increases with a decreasing temperature, representing the increasing superconducting carrier density and diminishing quasiparticle scattering. Similar measurements under high magnetic fields at different temperatures confirm the above picture and it is found that the magnetic field has rather small influence on the Josephson plasma resonance. Its effect is only significant near T_c when the residue pinning effect is relatively small.

2PO1-97 Collective Josephson Vortex Dynamics and Josephson Plasma Excitations in Layered Cuprate Superconductors

M. Machida 1, T. Koyama 2, and M. Tachiki 3. 1 Japan Atomic Energy Research Institute, 2-2-54, Nakameguro, Meguro-ku, Tokyo 153-0061, Japan. 2 Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan. 3 National Research Institute for Metals, Sengen 1-2-1,Tsukuba, Ibaraki 305-0047, Japan.

Presenting Author: M. Machida

In order to investigate collective dynamical states of Josephson vortices under the c-axis parallel applied current, we perform direct numerical simulations for the coupled Sine-Gordon equation.

Simulation results reveal that multiple structural transitions of Josephson vortex flow lattices occur with increasing the current, while comparisons between simulation results and theoretical analysis for the equation indicate that resonances between Josephson vortex flow states and excited Josephson plasma modes lead to their dynamical structural transitions. Among appearing multiple flow states, the most important and surprising state is in-phase rectangular lattice flow state. In the state, since the superconducting phase is completely in-phase over all junctions, power spectra for excited oscillating electromagnetic fields are very sharp and the power at the peak position is much larger than that of other states. These results means that single crystals of layered cuprate superconductors can be used as very effective submilimeter-wave generators. Furthermore, we show that the current core of the Josephson vortex contracts or expands depending on the dynamical flow states with increasing the applied current.

2PO1-99 The Surface Plasmon and the Impedance of Thin Superconducting Films

M.M. Doria 1, Flavio M.R. d'Almeida 2, F. Parage 3, and O. Buisson 3. 1 Instituto de Fisica, Universidade Federal do Rio de Janeiro, C.P. 68528 Rio de Janeiro 21945-970 RJ Brazil. 2 Departamento de Fisica, Pontificia Universidade Catolica do Rio de Janeiro, Rio de Janeiro 22452-970 RJ, Brazil. 3 Centre de Recherches sur les Tres Basses Temperatures, Laboratoire Associe a l'Universite Joseph Fourier, C.N.R.S., BP 166, 38042 Grenoble-Cedex 9, France.

Presenting Author: M.M. Doria

Charge oscillations, largely known in a variety of systems, such as plasmas, metals, semi-conductors, and 2D electron gases, were also observed in low dimensional superconductors , namely, thin films, layered materials (2D) and wires (1D). For thin superconducting films, charge oscillations between the interfaces to the exterior media and the interior of the film are responsible for these low frequency modes that occur much below the superconductor energy gap. O. Buisson, P. Xavier, and J. Richard (Phys. Rev. Lett. 73, 3153 (1994)) detected these oscillations using reflectivity coefficient measurements, that give a resonance, due to an energy transfer process, whenever a plasma mode is excited in the film. Here we calculate this reflectivity from a new theoretical model. Several properties of the plasma resonances are derived, among them are the resonant frequencies, given as a plasma mode dispersion relation law, and the strength and width of the resonant signal, which give useful information about the energy loss processes involved, including normal electron scattering and emitted electromagnetic radiation. Our theoretical approach is fully dynamical incorporating both charge and current screening.