*2PO9-40 Thermal Expansion of the Superconducting Solid Solutions of 123-type in the Nd₂O₃-Sm₂O₃-BaO-SrO-CuO System

Andrew I. Clyndyuck and Gennady S. Petrov, Department of Physical and Colloid Chemistry, Belarussian State Technological University, Minsk 220050, Belarus

Presenting Author: A.I. Clyndyuck

We studied thermal expansion of Nd_1-xSm_xBa_2-ySr_yCu₃O_7-d (0£ x£ 1; 0£ y£ 1.25) (I) solid solutions at 293-1193 K in air by a quartz dilatometer. On the temperature dependences of the relative elongation and linear thermal expansion coefficient (LTEC, a ) (I) during heating and cooling for all samples we observed three anomalies at about 500, 800 and 1050 K respectively. The temperatures of these anomalies were decreased at substitution of Ba by Sr and were changed unmonotonously at substitution of Nd by Sm.

The first anomaly was connected with the orthorhombic (OR) I (0£ d £ 0.25) ® OR II (0.25£ d £ 0.50) phase transition (order-disorder type). The second anomaly was due to the OR II ® tetragonal (T) phase transition, accompanied by the rearrangement of oxygen sublattice and beginning of oxygen loss. The third anomaly was connected, probably, with the further rearrangement of oxygen sublattice at T (d » 0.50) ® high-temperature (d » 0.80-0.90) phase transition. Average LTEC values of (I) is about (1.0-1.7, 1.7-2.6, 1.8-2.2)·10^-5K^-1 for temperature intervals 293-700, 700-1000, and 1000-1193 K respectively. Obtained results are in a good accordance with those determined earlier by us for other RBa₂Cu₃O_7-b phases (R - Y, rare-earth elements) and their solid solutions.

2PO9-41 High-Temperature Electrical Conductivity of (Nd,Sm)(Ba,Sr)₂Cu₃O_7-b Solid Solutions

Alexander A. Savitsky 1, Andrew I. Clyndyuck 2, Gennady S. Petrov 2, and Leonid A. Bashkirov 2. 1 Department of Physical Chemistry, Belarussian State University, Minsk 220080, Belarus. 2 Department of Physical and Colloid Chemistry, Belarussian State Technological University, Minsk 220050, Belarus.

Presenting Author: A.I. Clyndyuck

We investigated DC electrical conductivity (s ) of (Nd,Sm)(Ba,Sr)₂Cu₃O_7-b (I) solid solutions at 293-1073 K at different oxygen partial pressures (pO2) by four-probe method.

We observed one anomaly a kink at about 750 K (T*) on the temperature dependences of s. The anomaly was due to the structural (orthorhombic-tetragonal) phase transition, accompanied by the beginning of oxygen loss and rearrangement of oxygen sublattice. At pO₂ above 0.21*10⁵ Pa the value of s practically did not depend on the temperature (below T*). At low pO₂ (for example, in argon) below T* this dependence had a semiconducting character. Above T* s =s (T) dependence had a metallic character at different pO2 studied. We calculated the values of apparent activation energy (E) of the samples electrical conductivity. For all samples studied E values were about 20-30 kJ/mol above T*. In argon below T* E values were .about 2-16 kJ/mol, they almost did not depend on x,y-values, but E significantly increased with decreasing of oxygen content in the samples.

2PO9-42 Low-Temperature Heat Capacity of EuBa₂Cu₃O_7-b and YBaSrCu₃O_7-b

Gennady S. Petrov 1, Andrew I. Clyndyuck 1, Alexander P. Krasulin 2, and Leonid A. Bashkirov 1. 1 Department of Physical and Colloid Chemistry, Belarussian State Technological University, Minsk 220050, Belarus. 2 Laboratory of Thermodynamics of Organic Materials, Institute of Physico-Chemical Problems, Belarussian State University, Minsk 220080, Belarus.

Presenting Author: G.S. Petrov

Heat capacity of EuBa₂Cu₃O_6.85 (I) and YBaSrCu₃O_6.81 (II) was measured by adiabatic calorimetry at 5-300 K. The samples were prepared by ceramic method from europium, yttrium and copper oxides and barium and strontium carbonates at 1173 K in air with further annealing in oxygen at 673-723 K. On the temperature dependences of heat capacity of (I) and (II) two anomalies were observed. The first anomaly, at 92 K for (I) and 94 K for (II), were connected with the phase transition into superconducting state. The second one was found at about 250-255 K for (I) and (II). The nature of this anomaly is not known yet, but it may be due to the structural changes in copper-oxygen sublattice of the samples, having magnetic nature.

On the basis of the experimental data we calculated thermodynamic properties, the values of Debye temperature (Q ) at low temperatures, the coefficients of lattice and electronic terms of heat capacity (b and g ) and dependences Q =Q (T) at 5-300 K for (I) and (II). For (II) the quantitative thermodynamic data were obtained for the first time.

Our results were discussed and compared with the literature data for other 123 HTS phases.

2PO9-43 In-Plane Anisotropy and Chain Contribution Studies from Magnetoresistivity Measurements of Untwinned YBa₂Cu₃O_7-d Single Crystals

T. Björnängen, J. Axnas, Yu. Eltsev, and O. Rapp, Department of State Physics, Kungliga Tekniska Högskolan, SE-100 44 Stockholm, Sweden

Presenting Author: T. Björnängen

The magnetoresistivity of untwinned YBa₂Cu₃O_7-d single crystals was measured above T_c in the range T_c + 2 K to T_c + 84 K for magnetic fields up to 12 T applied both perpendicular to the planes and parallel to the a or b axes directions in the planes. The current was parallel to either the a or the b axis. A fluctuation theory (by Maki and Thompson) that allows for an in-plane coherence length anisotropy (x _{a¹ x b}), as well as extended calculations of the usual Aronov-Hikami-Larkin theory (by Dorin et al.) that do not, were used in the analyses. The results suggest a coherence length anisotropy close to one. The magnetoconductivity, D s , was compared for the four possible combinations of B and I in the plane, to determine the role of the CuO-chains in the magnetoconductivity. A different field direction dependence was found for I½ ½ a and I½ ½ b (D s _{B÷ ÷ I÷} » ½ D s _{B^ I½} for I½ ½ a, but (D s _{B÷ ÷ I÷} ‹½ D s _{B^ I½} for I½ ½ b), indicating a chain contribution to D s .

2PO9-44 Spin gap in the normal state of YBa₂Cu₃O_y with La doping and oxygen-deficiency

X.S. Wu 1,2 and J. Gao 1. 1 Department of Physics, The University of Hong Kong, Hong Kong. 2 Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, P.R. China.

Presenting Author: X.S. Wu

We have synthesized the La doped YBa_2-xLa_xCu₀O_y ceramic cuprates with x<0.30 by the solid state reaction technique under the quenching temperature range of 350-650° X-ray diffraction (XRD), energy dispersive spectrum (EDS), and scanning electron microscopy (SEM) studies show that all the synthesized samples contain a single phase, La is replacing for Ba at Ba-sites in YBa₂Cu₃O_y (YBCO). The electronic transport properties were measuere by the standard four-probe method. For the completely oxidized samples, the zeroresistance temperature, T_c0, is almost unchanged with x<0.20 whilst T_c0 decreases with the increase of La content for the quenching samples. The spin gap temperature, T_s, for all the quenching and annealing samples were determined from the measured temperature dependence of resistance relationship curves. T_s increases with the decrease of T_c0, which indicate that T_s relate to the carrier concentration in the La-doped YB_2-xLa_xCu₀O_y superconductors.

2PO9-45 Transport Properties of (Pb₂Cu)Sr₂Dy_xCe_n-x-d Cu₂O_2n+6: n=3, 5 epitaxial films

Sumio Ikegawa and Yuichi Motoi, Advanced Materials & Devices Laboratory, Corporate Research & Development Center, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210-8582, Japan

Presenting Author: S. Ikegawa

In the layered cuprate, (Pb₂Cu)Sr₂Dy_xCe_n-x-d Cu₂O_2n+6 (Pb-32 n2 phase), the distance, d, between a pair of CuO₂ planes can be arbitrarily chosen. We have studied the in-plane resistivity and thermopower for Pb-32 n2 epitaxial films with n=3,5. The samples were grown by sequential deposition using the MBE technique. After optimization of the growth conditions, we succeeded in making the hole concentration p=0.089 for the Pb-3252 sample. This value is within the range for the occurrence of superconductivity in layered cuprates, although resistivity of the Pb-3252 sample shows weak localization behavior. Here, p was estimated from thermopower. The increase in p by the substitution of Dy³⁺ for Ce⁴⁺ is lower than expected from the simple calculation of the valence. The cation deficiency in the fluorite block is supposed to be one of the reasons for the low efficiency of carrier doping and localization behavior. The electrical conduction within a CuO₂ plane does not deteriorate with the increase of d from 0.87 nm (n=3) to 1.41 nm (n=5). Thus, the Pb-32 n2 phases with n=3-8 have the potential to control the Josephson coupling along the c-axis.

This study was supported by Special Coordination Funds of the Science and Technology Agency of the Japanese government.

2PO9-46 Transport Properties of Some Superconducting Multilayers

H.C. Yang 1, L.M. Wang 2, and H.E. Horng 3. 1 Department of Physics, National Taiwan University, Taipei 106, Taiwan, R.O.C. 2 Department of Electrical Engineering, Dah-Yah University, Chang-Hwa, 515, Taiwan, R.O.C. 3 Department of Physics, National Taiwan Normal University, Taipei 117, Taiwan, R.O.C.

Presenting Author: H.C. Yang

We present the transport measurements of (YBa₂Cu₃O_y/Nd_1-xSr_xMnO₃) (YBCO/NSMO) multilayers. The YBCO and the NSMO layers were prepared by the off-axis magnetron sputtering. The colossal magneto-resistance in the normal state was observed for the NSCO above a critical thickness for a fixed YBCO layer. A lower activation energy associated with a diminishing of the sign reversal implied a decrease of the flux pinning. The Hall coefficients were presented and the results were compared with our previous results in YBa₂Cu₃O_y/Y_1-xPr_xBa₂Cu₃O_y superlattices.

2PO9-47 Influence of weakly bounded oxygen on physical properties of HTSC YBa₂Cu₃O_7-x ceramics.

A.A. Blinkin, V.M. Arzhavitin, V.V. Derevyanko, Yu. Razdovskii, and V.A. Finkel, National Science Centre Kharkov Institute of Physics & Technology, 310108 Kharkov, Ukraine

Presenting Author: A.A. Blinkin

Results are reported from studies of crystal structure, superconductivity, electrical resistance and internal friction of high-T_c YBa₂Cu₃O_7-x ceramics (6.05 £ (7-x) £ 6.95). The temperature dependence of internal friction Q^-1(T) exhibits the peaks at 200-400C, which are due to the diffusion of oxygen atom at O4 and O5 sites. Ortho-tetra and orthoI-orthoII phase transition as well as the jumps of elastic moduli at 6.3 £ (7-x) £ 6.5, associated with phase transitions, have been observed.

2PO9-48 Magnetic Field Induced Superconductor-Insulator Transition and Abnormal Hall Effects in Y_0.5Pr_0.5Ba₂Cu₃O_7-d Thin Films

Zhao Hao 1, B.Y. Zhu 1, B.R. Zhao 1, Z.X. Zhao 1, Y.M. Ni 1, V.V. Moshchalkov 2, and J. Vanacken 2. 1 National Laboratory for Superconductivity, P.O. Box 69, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. 2 Laboratorium voor Vaste-Stoffysica en Manetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium.

Presenting Author: Z. Hao

A magnetic field induced superconductor-insulator transition (SIT) has been investigated in the Y_0.5Pr_0.5Ba₂Cu₃O_7-d (YPBCO) thin films in the pulsed magnetic fields up to 50 T. We found that the conductivity follows a power law, sigma T² a with 2a =0.35 at 49 Tesla, in the crossover temperatures from ~ 9 K to ~ 40 K, due to the high magnetic field induced localization.

A similar power-law dependence was also observed in the ultrathin YPBCO films (nonsuperconducting) from ~ 2 K to ~ 40 K. Furthermore, we observed an electrical current driven delocalization in the localized YPBCO films in static 5.5 T magnetic field. The currents destroy the superconductivity at low temperatures, and delocalize the insulating state into a metal-like state within the crossover temperature range. These are discussed based on the intercell (inter-liquid) tunneling effects and the current induced depinning of the vortex scattering centers, respectively.

2PO9-49 The Low-Temperature Anomaly of YBa₂Cu₃O_7-d Films from the Microwave Penetration-depth Measurement

S.H. Tsai, H.T. Luei, and J.T. Luei, Department of Physics, National Tsing Hua University, Hinchu, 30043, Taiwan, Republic of China

Presenting Author: S.H. Tsai

We use the microwave dielectric resonator to measure the temperature and magnetic field dependencies of the penetration depth for the YBCO films under different treatments, such as ion milling, thermal oxygen exposure and the coverage of other materials, such as Au, Nb or Co for investigating the surface effects. The upturn behaviors of l (T) was observed at lower temperature and the occurring temperature of upturn dependants on the surface properties. The model of surface proximity effect is used to elucidate this anomaly.