2PO9-20 A Comparative Study of Transport Properties of Composites HTSC + MgTiO₃ and HTSC + NiTiO₃. The Effect of Paramagnetic NiTiO₃.

M.I. Petrov, D.A. Balaev, K.A. Shaihutdinov, and K.S. Aleksandrov, Kirensky Institute of Physics, 660036, Krasnoyarsk, Russia

Presenting Author: K.A. Shaihutdinov

Two-phases bulk composites Y_3/4Lu_1/4Ba₂Cu₃O₇+NiTiO₃ and Y_3/4Lu_1/4Ba₂Cu₃O₇+MgTiO₃ with insulator volume content 7.5% and 15% modelling a network of Superconductor-Insulator-Superconductor junctions have been prepared. The temperature dependences of resistivity R(T), critical current density J_c(T) and current-voltage characteristics (CVCs) are presented. R(T) and J_c(T) dependences of composites Y_3/4Lu_1/4Ba₂Cu₃O₇+MgTiO₃ are similar to that of recently studied composites HTSC + CuO [1] (CuO have been used as an insulator component of the composites). The anomalous behavior of R(T) of Y_3/4Lu_1/4Ba₂Cu₃O₇+NiTiO₃ composites manifesting as a kink on R(T) curves at some temperature Tm have been observed. In the temperature range Tm < T < T_c the dissipation is Ohmic while below Tm the CVCs are strongly non-linear. This peculiarity is interpreted as Abrikosov vortex lattice melting at Tm. Alike effect have been observed on single crystals in external magnetic field [2]. There is a knee on the J_c(T) of composite sample Y_3/4Lu_1/4Ba₂Cu₃O₇+NiTiO₃ correlating with Neel temperature (~22K) of antiferromagnetic NiTiO₃.

[1] Petrov M.I., et al. Fiz. Tv. Tela 41, 969 (1999) (English transl. Phys. Solid State 41, (1999)) .

[2] Charalambous M., et al. Phys. Rev. B 45, 5091 (1992).

2PO9-21 Effects of Incomplete Superconducting Condensation on the Electronic Thermal Conductivity at Very Low Temperatures

Ju H. Kim, Physics Dept., University of North Dakota, P.O. Box 7129, Grand Forks, ND 58202

Presenting Author: J.H. Kim

We have computed the electronic thermal conductivity of an isotropic s-wave superconductor with a finite range T* of pairing interaction energy containing non-magnetic impurities. Whe show that, when T* is comparable to T_c, the electronic thermal conductivity exhibits a T-linear behavior at very low temperatures T but changes to a T-square behavior with increasing T. These T dependences are due to the incomplete condensation of quasiparticles at T=0. When T* >> T_c, as in conventional superconductors, the electronic thermal conductivity recovers the activated T dependence as these uncondensed quasiparticles disappear at T=0.

2PO9-22 Electrothermal Conductivity in the Mixed State of BSCCO Superconducting Samples

Julio E. Rodríguez and A. Mariño, Department of Physics, Universidad Nacional de Colombia, Bogotá, Colombia

Presenting Author: J.E. Rodríguez

We study the influence of low magnetic fields 0=H=1.2T, near T_c on the electrothermal conductivity P, which measures the electrical current density produced by a thermal gradient. Polycrystalline (Bi-Pb)SrCaCuO superconducting samples with nominal composition 2223 and 2234 were analyzed. The experimental results for superconducting single phase samples indicate a behavior consistent with a gap parameter of d-wave type.

2PO9-23 Anisotropy of Thermal Conductivity in YBaCuO Single Crystals

Vasilii E. Gusakov 1, Andrzej Jezowski 2, Sergey Barilo 1, Nikolay Kalanda 1, and Alexandr Saiko 1. 1 Institute of Solid State and Semiconductor Physics, National Acad., P. Brovki 17, Minsk 220072, Belarus. 2 Institute for Low Temperature and Structure Research, Polish Acad. of Sci., 2 Okolna St., 50-422 Wroclaw, Poland.

Presenting Author: V.E. Gusakov

The anisotropy of thermal conductivity, K, in quality single crystals YBaCuO (x=7) have been studied. The temperature dependence of K in c-direction is well described by the lattice thermal conductivity (T_max=60 K, K_max= 4 W/m K; TD=140 K). The precise measurements reveal the hysteretic behavior of K in c-direction. In ab-direction the upturn in K for temperatures below T_c is observed and temperatures of thermal conductivity peaks in ab- and c-direction coincide. Theoretical analysis suggests a model in which the observed peak below T_c in thermal conductivity for ab-direction is ascribed to the quasiparticle contribution.

2PO9-25 Anisotropy of the electrical resistance in granular high-T_c superconductors under magnetic field

Dario Daghero 1, Piero Mazzetti 1, Aurelia Stepanescu 1, and Aldo Masoero 2. 1 Unita' INFM del Politecnico di Torino, Dipartimento di Fisica, c.so Duca degli Abruzzi 24,10129 Torino, Italy. 2 Unita' INFM del Politecnico di Torino, Dipartimento di Scienze e Tecnologie Avanzate dell 'Universita' P.O. "Amedeo Avogadro", c.so Borsalino 54, 10131 Alessandria, Italy.

Presenting Author: D. Daghero

The electrical resistance of granular high-T_c superconductors submitted to a magnetic field and crossed by a supercritical current shows a marked anisotropy, in the sense that it depends on the angle between magnetic field and current density.

In monocrystalline samples, in which the dissipation arises from the current-driven vortex motion, this effect may be simply explained in terms of the angle-dependent coupling between fluxoids and current.

In granular superconductors, where the electrical resistance is mainly determined by the resistive transition of weak links, the anisotropy has a more complicated origin. Actually, in the absence of flux penetration within grains, the local magnetic field and the local current density remain mutually perpendicular in each resistive weak link, independently of the directions of the corresponding macroscopic quantities.

In this paper the origin of the anisotropy of the electrical resistance is explained in terms of the distributions of local magnetic field intensities and local current densities in the intergrain region, due to the magnetic screening effect of the superconducting grains and to the angular distribution of weak links' surfaces.

A quantitative model is then developed, whose results are compared to the experimental ones obtained on a YBCO specimen under different physical conditions.

2PO9-26

Y. Tsuchiya 1, K. Iwaya 1, A. Maeda 1, H. Kitano 1, T. Hanaguri 2, J. Takeya 3, K. Nakamura 3, and Y. Ando 3. 1 Dept. Basic Science, The Univ. Tokyo. 2 Dept. Advanced Materials Science. 3 CRIEPI.

Presenting Author: A. Maeda

A sharp break at the characteristic field Hk and subsequent plateau observed in the field dependence of thermal conductivity k(H) in Bi₂Sr₂CaCu₂O_y(BSCCO) by Krishana et al, were interpreted as the phase transition from the d_x2-y2 state to the fully-gapped state. If such a sudden change takes place in the electronic state, we expect that a similar anomaly shows up in the electronic conductivity.

We measured both the thermal and microwave impedance in the quite same peaces of crystal, and compared the data. In sample A, where the plateau was observed in k(H), we could not find any anomaly at Hk. This result indicates that the origin of the plateau in k(H) is not a drastic phase transition but is rather somewhat gradual crossover.

We also found that there were no qualitative difference between sample A and sample B, where the plateau was not observed in k(H).

We are continuing this kind of comparison between k(H) and microwave impedance in crystals with various degree of disorder, as those with Zn doped. The systematic in the result will be presented at the conference.

2PO9-27 Anisotropy in the superconducting state and C-axis resistivity of precisely oxygen controlled Bi₂Sr₂CaCu₂O_8+d

Takenori Fujii 1, Takao Watanabe 2, and Azusa Matsuda 1,2. 1 Department of Applied Physics, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, Japan. 2 NTT Basic Research Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa, Japan.

Presenting Author: T. Fujii

We have measured the anisotropy of magneto resistivity near T_c for pure Bi₂Sr₂CaCu₂O_8+d single crystals with various oxygen contents. Blutter's scaling rule was succesfully applied to the magneto resistivity and enabled us to estimate the anisotropy of the superconducting coherence length above T_c. The anisotropy increases with reducing the oxygen content from a slightly over-doped state (T_c» 84K) to an optimum-doped state (T_c» 88K), and with reducing temperature. Due to the anisotropic pinning (intrinsic pinning), Blutter's scaling rule is no longer valid below T_c for narrow angle ranges centered at an H//ab configuration. Excluding this region, we can again estimate the anisotropy by using the scaling method. The obtained anisotropy decreases with decreasing temperature. The entire temperature dependence of the anisotropy shows a sharp peak-like structure with its maximum at T_c. The maximum anisotropy for each doping level is proportional to the square root of the maximum r _c (just above T_c), indicating that the Lawrence-Doniach model is valid if inter-layer coupling is proportional to 1/r _c. The reduction of the anisotropy below T_c can be explained by the increase in the interlayer critical current. However, we do not have a definite explanation for the anisotropy reduction above T_c.

2PO9-28 Experimental fitting to the bipolaronic model of the normal-state resistance of Bi₂Sr₂CaCu₂O₈ single crystals

Weimin Chen, J. Jung, and J.P. Franck, Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1.

Presenting Author: J. Franck

The experimental normal-state resistance data from Bi₂Sr₂CaCu₂O₈ single crystals were fitted to the (bi)polaronic conduction model, R = R₀ (T + aT²)/(1 + bT), with satisfactory agreement over a wide temperature range. One significant result is the alternative interpretation of the usual fluctuating conduction region (within about 100 K above T_c). This region is consistently accounted for in terms of (bi)polaronic conduction, showing a largely reduced or suppressed fluctuating effect for a charged Bose-gas. We estimate the effective polaron mass to be ~ 4me.