2PO9-60 Synthesis and electrical transport of Hg_1-xTl_xBa₂Ca₂Cu₃O_8+d

D.C. Kim, J.S. Kim, S.J. Joo, H.R. Kang, and Y.W. Park, Department of Physics and Condensed Matter Research Institute, Seoul National University, Seoul 151-742, Korea

Presenting Author: D.C. Kim

Hg_1-xTl_xBa₂Ca₂Cu₃O_8+d compounds with different Tl concentration, x, were synthesized by using the high-pressure-high-temperature technique in a belt type high pressure furnace. The applied pressure is 18kbar and the temperature is raised to about 800 ~ 900° C during the sample synthesis. We found that the cooling process in synthesizing the precursor has a large effect on the sample synthesis condition and sample quality. By using the furnace - cooled precursor in oxygen atmosphere, we could get the nearly single phase of Hg_1-xTl_xBa₂Ca₂Cu₃O_8+d with the two step annealing process. The effect of Tl substitution for Hg on the superconducting properties was investigated by measuring the resistivity and the thermoelectric power as a function of temperature.

2PO9-61 Dissipative behavior of high T_c thin films at high current density

S. Reymond, L. Antognazza, M. Decroux, E. Koller, P. Reinert, and Ø. Fischer, Department of Condensed Matter Physics, University of Geneva, Switzerland

Presenting Author: S. Reymond

In order to investigate the dissipative behavior of high T_c under very high currents, we have applied short current pulses ranging from 10 ms to 1s along the ab plane of patterned Nd-123 and Bi-2212 thin films. The measurements are carried under 0.1 T in order to have a homogeneous field profile in the film. In Nd-123, we observe, after a delay time t* from the beginning of the pulse, a sharp voltage increase up to a highly dissipative state. We present evidence that T(t*) is well below T_c, thus the transition is not due to a thermal runaway above T_c. We find a different situation in Bi-2212, where the transition to the highly dissipative state is preceded by a series of small voltage jumps. We will discuss how these effects depends on time, current and temperature.

2PO9-62 Upper critical magnetic field of ion-irradiated YBaCuO and NdCeCuO films

Vadim S. Nozdrin, Sergei I. Krasnosvobodtsev, Natalia P. Shabanova, Valerii A. Dravin, and Aleksandr I. Golovashkin, P.N. Lebedev Physics Institute, Russian Academy of Sciences, Leninsky Prospect 53, 117924, Moscow, Russia

Presenting Author: V.S. Nozdrin

In the high-energy-He-ion irradiated YBaCuO and NdCeCuO epitaxial films the absence of the classic increase of the slope of the upper critical magnetic field temperature dependence (-dH_c2/dT) has been found. The irradiation effect has been proved to be the same as at variations of oxygen content or ratio Nd/Ce. Because of high sensitivity of critical temperature to a deviation of the oxygen content from optimal, even the fluences, which result in superconductivity suppress, are too low to change significantly concentration of scattering centres. So the increase of the electron scattering, which usually rises H_c2 and resistivity, is not realised in the irradiated high-temperature superconductors. The resistivity rise in this case is determined by the electron concentration change.

2PO9-63 Fluctuation Conductivity and the Dynamical Universality Class of the Superconducting Transition in the High-T_c Cuprates

J. Roa-Rojas 1, A.R. Jurelo 1,2, R. Menegotto Costa 1, L.M. Ferreira 1, P. Pureur 1, M.T.D. Orlando 3, P. Prieto 4 and G. Nieva 5. 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 Departamento de Física, Universidade Federal do Espírito Santo, 29060-900 Vitória, Brazil. 4 Departamento de Física, Universidad del Valle, 25360 Cali, Colombia. 5 Centro Atómico Bariloche, CNEA, 8400 S.C. de Bariloche, Argentina.

Presenting Author: P Pureur

We report on fluctuation conductivity measurements in several samples of the high-T_c compounds RBa₂Cu₃O₇ (R = Y, Gd, Dy, Ho), Bi₂Sr₂CaCu₂O₈ and Hg_0.82Re_0.18Ba₂Ca₂Cu₃O₈. Thin films, single crystals and ceramics are studied. Results near the transition are analysed using a Kouvel-Fisher-type method which clearly demonstrates the occurrence of a narrow but genuine critical regime in all cases. The characteristic exponent reveals that the universality class of the superconducting transition in these systems is that of the 3D-XY model, with dynamics given by the model-E, which predicts z = 3/2. The critical region extends at most 1 K above T_c. In most cases we study the stability of the critical regime upon the application of small magnetic fields. We also present results in some clean YBa₂Cu₃O₇ crystals where a scaling beyond 3D-XY is observed. This new regime may be indicating an ultimate weakly first-order character of the superconducting transition in this system.

2PO9-64 Longitudinal and Transverse Thermal Conductivity in Bi- and Y-Based High-T_c Superconductors

B. Zeini 1, A. Freimuth 1, B. Büchner 1, R. Gross 1, A.P. Kampf 2, M. Kläser 3, G. Müller-Vogt 3, and L. Winkler 4. 1 II. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany. 2 Theoretische Physik III, Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany. 3 Kristall- und Materiallabor, Universität Karlsruhe, 76128 Karlsruhe, Germany. 4 2. Physikalisches Institut, RWTH Aachen, 52056 Aachen, Germany.

Presenting Author: A. Freimuth

Measurements of the transverse (k_xx) and longitudinal (k_xy) thermal conductivity in high magnetic fields in Y- and Bi-based high-T_c superconductors are presented. Assuming that k_xy is purely electronic and using the magnetic field dependence of k_xx we separate the electronic thermal conductivity (k_xx^el) of the CuO₂-planes from the phononic thermal conductivity (k_xx^ph). In YBa₂Cu₃O_7-d (YBCO) kxxel shows a pronounced maximum in the superconducting state. The maximum in Bi₂Sr₂CaCu₂O_8+d (BSCCO) is much weaker, due to stronger impurity scattering. The maximum of k_xx is strongly suppressed by a magnetic field. An additional magnetic field independent contribution to the maximum of k_xx occurs in YBCO, reminiscent to the contribution of the CuO₂-chains, as measured from the anisotropy in untwinned single crystals. No indication for a maximum and a magnetic field dependence of k_xx^ph is found. Our data analysis reveals that as in the normal state a transport (t ) and Hall (t _H) relaxation time must be distinguished. Whereas the temperature dependent contribution to t is strongly enhanced in the superconducting state, t _H displays the same temperature dependence as above T_c.

2PO9-65 Systematic Evolution of the Magnetotransport Properties of La-doped Bi-2201 in a Wide Doping Range

Yoichi Ando 1,2, T. Murayama 1,2, and S. Ono 1. 1 CRIEPI, 2-11-1 Iwato-kita, Komae, Tokyo 201-8511, Japan. 2 Dept. of Physics, Science University of Tokyo, Tokyo 162-8601, Japan.

Presenting Author: Y. Ando

The transport properties of Bi-2201 single crystals have been believed to be notably "dirtier" than those of other cuprates. In this paper, we show it is possible to obtain a series of high-quality crystals of Bi₂Sr_2-xLa_xCuO₆ (La-doped Bi-2201) in a wide doping range, from heavily-underdoped to overdoped regions [cf. Y. Ando and T. Murayama, Phys. Rev. B 60, R6991 (1999)]. At optimum doping, the best crystals show the highest zero-resistivity T_c (38 K) and the smallest residual resistivity ever reported for Bi-2201. In those series of Bi-2201 crystals, the normal-state transport properties show systematics that can be considered to be "canonical" to the cuprates; for example, in the underdoped region the in-plane resistivity r _ab shows the pseudogap feature and in the overdoped region the T dependence of r _ab changes to Tⁿ with n>1. Therefore, with the relatively wide temperature range of the normal state and with the accessibility to a wide doping range, Bi-2201 system offers a good testing ground for the models of the normal state of cuprates. As an example, we discuss how the "two scattering times" that show up in the magnetotransport properties of cuprates are examined in our series of Bi-2201 crystals.

2PO9-67 Experimental Study of the Resistivity Tensor in BSCCO by Means of Multi-Terminal Measurements

Stefano Sarti 1, Marianna Esposito 1, Renato Fastampa 1, Maurizio Giura 1, Danilo Neri 2, and Enrico Silva 2. 1 Dip. di Fisica and Unita' INFM, Universita' degli Studi "La Sapienza", P.le A.Moro 2, 00185 Roma, Italy. 2 Dip. di Fisica "E. Amaldi" and Unita' INFM, Universita' degli Studi Roma Tre Via della Vasca Navale 84, 00146 Roma, Italy.

Presenting Author: S. Sarti

We present measurements of the in-plane (r _ab) and out-of-plane (r _c) resistivity in BSCCO single crystals. The resistivities are measured simultaneously on the same sample by means of a multi-terminal technique. Results are presented in the normal state (up to 250 K) and in the superconducting state, both in presence and in absence of an applied magnetic field. The technique is also able to test whether the dynamic is local or not: we present an accurate experimental analysis of the temperature and magnetic field ranges where the dynamic is local or non local. We also show that the ratio r _c/r _ab decreases continuously as the zero-resistance is approached, showing that the in-plane superconductivity sets in simultaneously with the interplanar superconducting coupling. Finally, we observe and discuss an anomalous increase of the current flowing across the planes in a narrow temperature range around T_c. This effects results to be extremely dependent on the applied magnetic field, disappearing as soon as a very small (of order 10 G) field is applied parallel to the c-axis of the sample.

2PO9-68 Resistive upper critical magnetic field of a superconducting Bi₂Sr_2-xLa_xCuO_6+d thin film grown on a vicinal substrate

Y.Z. Zhang 1,2, R. Deltour 1, J.-F. de Marneffe 1, Y.L. Qin 2, L. Li 2, Z.X. Zhao 2, L. Jansen 3, and P. Wyder 3. 1 Universite Libre de Bruxelles, Physique des Solides, CP 233, B-1050, Bruxelles, Belgium. 2 National Laboratory for Superconductivity, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China. 3 Grenoble High Magnetic Field Laboratory, MPI-CNRS, 25, Avenue Des Martyrs, BP 166, 38042 Grenoble Cedex 9, France.

Presenting Author: Y.Z. Zhang

Transport properties have been measured for a superconducting Bi₂Sr_2-xLa_xCuO_6+d thin film grown on a vicinal SrTiO₃ single crystal substrate in magnetic field up to 23.5 T. The thin film was patterned into crossed bridges for studying the in-plane and out-of-plane resistive transitions and upper critical magnetic field H_c2(T). In normal state, metallic resistivity is observed along the x-axis (parallel to the rotating axis for the tilted substrate), while non-metallic resistivity is observed along the y-axis. For analysing H_c2, both the resistive transition curves, r _x(T,H) and r _y(T,H), are taken into account. Different resistive criteria have been adopted to study the H_c2(T) along the x-direction and show upward curvatures. In contrast, resistivity peaks show nearly linear curvature for magnetic field higher than 4 T in y-direction. Numerical analyses of our data and previews published data lead to a firm conclusion that upward curvature of H_c2(T) does not imply a divergent H_c2(T) value at T = 0 K.

2PO9-69 Composition and Thickness Dependence on SI transition in Bi2212/Bi2201 Mixed Crystal Thin Film

Yang Qi, Kazuo Sakai, Hironaru Murakami, and Toshimichi Ito, Dept. of Electrical Eng., Osaka Univ., 2-1 Yamada-oka, Suita-shi, Osaka565-0871, Japan

Presenting Author: Y. Qi

The authors have already reported that superconductor/insulator (SI) transition takes place at about 50mol% as the Bi2212 molar fraction (f) in the Bi2212/Bi2201 mixed crystal thin film (MCTF)[1] and moreover, that it has a close relationship to the thin film thickness in the 70mol% MCTF [2]. It is well-known that SI transition is brought about by a microscopic effect. However, it is found that the Kosterlitz-Thouless (KT) transition participates in the SI transition by transferring from the formation of the two dimensional vortex and anti-vortex pair to that of the three dimensional vortex-string and anti-vortex-string pair or vortex ring. This is interpreted to be because the three-dimensionality relaxes the phase fluctuation in Ginzburg-Landan (GL) mean field theory generated inside two-dimensional layer. Thus, the appearance regime of the SI transition is studied as function of the thickness in the Bi 2212/Bi2201 MCTF with the different fs owing to finding out the validity of this interpretation. Therefore, the sheet resistance and Hall coefficient are examined in the thin films fabricated every 100Å in the thickness lower than 1000Å with f=50 and 60mol%. In similarity, the MCTF with 80, 90mol% will be also examined.

[1] H. Ota et al., Physica C, 317-318(1999) 661.

[2] Y. Qi et al., J. Low Temp.Phys (accepted).