1PO2-70 Resistive irreversibility line of Bi2223/Ag tape in high magnetic fields

Emil Babic 1, Ivica Kuseviae 1, John R. Cooper 2, Louis Jansen 3, Wei Guo Wang 4, Hua Kun Liu 4, and Shi Xue Dou 4. 1 Department of Physics, Faculty of Science, University of Zagreb, Bijenièka 32, 10000 Zagreb, Croatia. 2 Interdisciplinary Research Centre in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 OHE, United Kingdom. 3 High Magnetic Field Laboratory, 25, Avenue des Martyrs, B.P. 166, F 38042 Grenoble Cedex 9, France. 4 Centre for Superconducting and Electronic Materials, University of Wollongong, Wollongong NSW 2522, Australia.

Presenting Author: E. Babic

The magnetoresistance of a (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+y Ag sheathed tape was measured for temperature range 15 K£ T£ 120 K and magnetic field B£ 23 T applied perpendicular to the current and broader surface of tape. Throughout the explored field range, the onset of resistivity resistivity shows thermally activated behavior rµexp(–U*(1–T/T_cs)/k_BTB^0.5) where U* is the sample dependent constant, and T_cs is the temperature somewhat higher than the temperature of the resistivity onset. Such behavior of the resistance can be attributed to a thermally activated flux flow in a highly viscous vortex-liquid regime characterized with a plastic deformation of vortices. Experimentally determined resistive irreversibility line B_irr vs. T_irr agrees with the power-law dependence B_irr(T_irr)µ (T_cs/T_irr–1)² for all applied fields and it can be derived from the exponential behavior of resistivity. Our B_irr(T_irr) data can also be fitted to the Josephson coupling model of Gray et al. (Phys. Rev. B 45 (1992) 10071). This fit provides very reasonable values for B_c2, B_c1, B_c, x _ab, l _ab and anisotropy constant g of Bi2223 compound, and removes the unphysical divergence of B_irr(T® 0) resulting from the above power-law fit. The variation of B_irr at the lowest temperatures fits also well a (-square root of T_irr/T_o) dependence previously observed in several similar compounds.

1PO2-71 Quantum theory for the Josephson plasma in high-T_c superconductors

Tomio Koyama, Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 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-mechanically 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 suppression 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.

1PO2-74 Josephson Plasma Resonance as a Probe of the Superconducting State of Bi:2212 and Bi:2201

Yuji Matsuda 1, M.B. Gaifullin 1, N. Chikumoto 2, J. Shimoyama 3,4, and K. Kishio 3,4. 1 Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106-8666, Japan. 2 Superconductivity Research Laboratory, ISTEC, Shibaura, Minato-ku, Tokyo 105, Japan. 3 Dept. of Superconductivity, Univ. of Tokyo, Bunkyo-ku, Tokyo 113, Japan. 4 Institute of Material Science, Univ. of Tsukuba, Tsukuba, Ibaraki 305, Japan.

Presenting Author: Y. Matsuda

The Josephson plasma is a collective Cooper pair oscillation mode through the insulating layers. By sweeping the microwave frequency continuously from 20 GHz to 200 GHz, we have measured the Josephson plasma resonance (JPR) of Bi:2212 and Bi:2201. The JPR is a novel tool that provides important information on both the superfluid and the low-energy excitations out of the condensate. We show that the c-axis superfluid response shows an unusual doping dependence. The JPR is also a powerful means of investigating the properties of the vortex state, enabling us to determine the magnitude of the interlayer phase coherence quantitatively. We discuss the JPR in the vortex liquid, Bragg glass and vortex glass.

1PO2-76 Josephson Plasma Mode in Fields Parallel to Layers of Bi₂Sr₂CaCu₂O_8+d

Itsuhiro Kakeya, Kazuo Kadowaki, Tomoyuki Wada, and Ryo Nakamura, Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

Presenting Author: I. Kakeya

Josephson plasma resonance (JPR) in under- and optimally-doped Bi₂Sr₂CaCu₂O_8+d single crystals has been investigated in magnetic field orientation parallel to the ab plane at microwave frequencies between 20 and 50 GHz.

In under-doped sample, external field (H // ab) dependence of the resonance mode above 20 GHz splits into two branches: one of them appearing at low temperatures shifts to even lower temperature at higher frequency. The higher temperature branch shows unique field dependence that the resonance temperature once decrease as the field increases but turns to increase, and finally exceeds the resonance temperature at zero field. Between two temperature regions, a gap in temperatures appears, in which no resonance is observed. Such splitting and the field dependence of higher temperature branch cannot be explained by present theory of JPR for the vortex pancakes and seem to be related to a strong coupling effect between Josephson plasma mode and Josephson vortex oscillation.

In optimally-doped samples, the resonance mode, which has been observed as a single line below 34 GHz, splits into two at 44 GHz, and the shape of the resonance mode is quite similar to one observed in the under-doped sample at 20 GHz. This implies that the threshold frequency above which the splitting occurs depends on anisotropy of samples, that is, the coupled mode is observed at lower frequency in sample with lower inherent Josephson plasma frequency.

1PO2-77 c-Axis Josephson Plasma Edge in T* Phase Superconductor

T. Kakeshita 1, K.M. Kojima 1, S. Uchida 1, S. Adachi 2, and S.Tajima 2. 1 Department of Superconductivity, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan. 2 Superconductivty Research Laboratory, ISTEC, Shinonome 1-10-13, Koto-ku, Tokyo, 135, Japan.

Presenting Author: T. Kakeshita

Charge transport properties and optical reflectivity spectra are studied for single crystals of T* system, Nd_2-x-ySr_xCe_yCuO_4-d and SmLa_1-xSr_xCuO_4-d , which are composed of alternating a rocksalt-type and a fluorite-type blocks, and are known to be a hole-doped superconductor. The crystals are grown by TSFZ (traveling solvent floating zone) method and annealed under high-pressure oxygen (~150atm, 600ºC). Sphere resonance measurement by Shibata and Yamada on T*- SmLa_1-xSr_xCuO_4-d (Phys. Rev. Lett. 81, 3519(1998)) suggests a presence of two Josephson plasma frequencies arising from the two different junctions. We are aiming to confirm whether two plasma edges are really observed in the c-axis reflectivity spectrum and whether or not a transverse optical plasmon exists between he two frequencies.

1PO2-78 Vortex State in the Mesa-structured Bi-2212 Single Crystal with the Josephson Current along the c-axis

Kazuto Hirata, Ooi Shuuichi, and Takashi Mochiku, National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan

Presenting Author: K. Hirata

Recent progress in understanding the vortex states of high T_c superconductors (HTSCs) has provided an interesting phase transition in vortex physics, when the magnetic field is applied parallel to the c-axis. First order melting transition of the vortex lattice into liquid state has been confirmed at first experimentally. Computer simulations have reproduced it well and shown the physical meanings of the vortex lattice melting. They have also enhanced stimulated discussions on the peak effect in HTSCs from the point of the first order transition, assuming the point defects.

To understand the vortex states and the interlayer coupling of Bi-2212 in the presence of Josephson current, we have performed the current-voltage (I-V) measurements along the c-axis on the mesa-structured Bi-2212. Pronounced effects of the magnetic fields to the I-V characteristics appear in the multiple-branching behaviors in the vortex glass phase, and in the Josephson critical current at the transition between the Bragg glass and the vortex soft glass phase, respectively. These behaviors are discussed as a consequence of missing a long-range order of the correlation of the vortex lines along the c-axis and in terms of the size effect and the chirality of the vortices in the Lorenz-force free configuration.

1PO2-79 New method of studying the anisotropy of lower critical fields in HTSC

V.A. Finkel and V.V. Derevyanko, National Science Center Kharkov Institute of Physics & Technology,1st Academicheskaya St., UA 310108 Kharkov, Ukraine

Presenting Author: V.A. Finkel

The modern theory of magnetic properties of anisotropic type II superconductors makes it possible to study the anisotropy of the field H_c1 on the basis of simple experiments of polycrystalline textured objects. For layered HTSC H_c1(g ) = H^c_c1(cos^2g + m_c/m_ab sin^2g )^-1/2. (g is the angle between the magnetic anisotropy axis c and the external field H).

The idea of the new method of determining the anisotropy of lower critical field of HTSC lies in the following. If we rotate a magnetic field H relatively a certain directions in the laboratory system of coordinates, the H forms average angles a ₁, b _i, and g _i, respectively, with the magnetic anisotropy axis c. We may receive three or more equations for H_c1 (g ) . In consequence of the decision of the system of equations we determine the H^ab_c1, H^c_c1, m_c/m_ab and g values for HTSC crystal.

Experiments at the liquid nitrogen temperature were made on textured samples of YBa₂Cu₃O_7-d . Experiments involved precision measurements of critical currents I_c(0) of HTSC in zero magnetic field as a function of a magnitude and direction of the "treatment field" H_tr (0 £ H_tr £ 1000 Oe). In consequence of measurements we obtained the H^ab_c1, H^c_c1 and m_c/m_ab values closed to parameters of YBa₂Cu₃O_7-d single crystals.