2PO9-10 Thermoelectric power of YBa₂Cu₃O_y superconductor up to 950° C

Qing-rong Feng 1, Yu-min Hou 1, David P. Tunstall 2, and Shannon Brown 2. 1 Department of Physics, Peking University, Beijing 100871, P.R. China. 2 The School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY 16 9SS United Kingdom.

Presenting Author: Q.-R. Feng

The thermoelectric power of YBa₂Cu₃O_y superconductor samples were measured during the process of synthesis (up to 950° C) and annealed at different oxygen pressure. Measurement data indicated that the thermoelectric power values were affected by the quality of the sample. When measured at different points in the synthetic process (e.g. after the first re-grind, after the second re-grind etc) the sample presented different thermoelectric properties, and also different high temperature resistance values. But after the final stage of synthesis the thermoelectric power should steady values, with heating and cooling curves approximately equal. Some of the feathers obtained in thermogravimetric analysis (TGA) of such samples are echoed in these thermoelectric power curves. The mechanisms of the thermoelectric power in YBa₂Cu₃O_y superconductor are discussed.

*2PO9-11 Thermopower Data Analysis for the Mercury-based HTS: Band Structure Calculations within a Narrow-band Model

Marina V. Elizarova, Alexey O. Lukin, and Vitaliy E. Gasumyants, Dept. Semiconductor Physics and Nanoelectronics, St. Petersburg State Technical University, St. Petersburg 195251, Russia

Presenting Author: M.V. Elizarova

We have performed the analysis of the normal-state thermopower for HgBa₂Ca_n-1Cu_nO_2n+2+d (n=1,2,3) within a narrow-band model. This model was earlier shown to allow one to describe the normal-state transport properties for Y- and Bi-based HTS giving a possibility to determine the main band spectrum parameters (the band filling degree, the effective bandwidth and the degree of charge carrier localization) for samples of different composition. Here we apply the same approach to the Hg-based HTS with varying number of CuO₂ layers for the case of the optimal doping, as well as in the underdoped and overdoped regimes.

The results obtained have shown that a narrow-band model is applicable to the Hg-compounds. Our calculations show that in all the HgBa₂Ca_n-1Cu_nO_2n+2+d phases the band filling is close to 0.5 and the conduction band is slightly asymmetrical. The total effective bandwidth for optimally doped HgBa₂Ca_n-1Cu_nO_2n+2+d (n=1,2,3) is about 70-200 meV with a tendency to a band broadening with increasing n. We assume that this is related to forming by CuO₂ layers additional states near the Fermi level that results in a rising density-of-state peak.

The main tendencies in the band structure modification in the underdoped and overdoped regimes for HgBa₂Ca_n-1Cu_nO_2n+2+d (n=1,2,3) are discussed in comparison with other HTS systems.

2PO9-12 Studies of Dynamic Critical Behavior in YBa₂Cu₃O_7-d Single Crystals

Shun-Hui Han 1,2, Yuri Eltsev 1, Björn Lundqvist 1, Östen Rapp 1. 1 Department of Solid State Physics, Royal Institute of Technology, SE - 100 44, Stockholm, Sweden. 2 Institute of Physics, Chinese Academy of Science, P.O. Box 603, Beijing, CN - 100080, China.

Presenting Author: S.H. Han

Up to now, the value of the dynamic critical exponent z is still an open question. We have studied this exponent in the intermediate (XY) critical regime in zero field for a set of high quality YBa₂Cu₃O_7-d single crystals both by previously obtained critical exponents of the dc critical conductivity fluctuations and in terms of Fisher-Fisher-Huse (FFH) dynamic scaling. In this regime (i.e. the absence of Coulomb interactions), one would expect model-E dynamics (z=1.5) to apply with the conserved charge-density conjugate to the phase fluctuations. However, we find the dynamic critical exponent to be z=2, which is in agreement with the suggestion of FFH that due to the plasma fluctuation, fully relaxational dynamics, model A (z=2), should be appropriate.

2PO9-13 Resistive Transition and Fluctuation Conductivity of Polycrystalline Hg,Tl-1223

Shun-Hui Han 1,2, Johan Axnas 1, Ingrid Bryntse 3, Irina Safonova 1, and Östen Rapp 1. 1 Department of Solid State Physics, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. 2 Institute of Physics, Chinese Academy of Science, P.O. Box 603, Beijing, CN-100080, China. 3 Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.

Presenting Author: S.H. Han

Detailed resistivity measurements were made for polycrystalline Hg_1-xTl_xBa₂Ca₂Cu₃O_8+d with x = 0 and 0.2 in zero applied magnetic field and in the temperature region above the superconducting transition T_c. An analysis on the fluctuation conductivity Ds is presented. Such studies are successful for Y- and Bi-based materials, but are more rare for Hg- and Tl-based materials. Two temperatures, T* and TG, could be identified for each sample. T* corresponds to a transition from two to three dimensional fluctuations in the mean field region (MFR), and T_G corresponds to a cusp point from the MFR to the critical region. All results are in agreement with the Lawrence-Doniach model. The c-axis coherence length and the interlayer coupling factor are described and the pseudo-gap effect on fluctuation conductivity is briefly discussed.

2PO9-14 Transport properties and Hall effects in ab-plane of the high T_c HgBa₂CaCu₂O_6+y

Y. Sun, J.D. Gua and G.C. Xiong, Department of Physics, Peking University, Beijing 100871, P.R. China

Presenting Author: Y. Sun

We have fabricated the HgBa₂CaCu₂O_6+y (Hg-1212) epitaxial films with zero resistance temperature (T_c) of 123K and critical current density (J_c) of 1MA/cm² at 77K and 0.3MA/cm² at 100K in zero magnetic field, respectively. By use of these high quality films the transport properties and the normal state Hall effects in ab-plane have been studied. The resistivity and Hall coefficients indicate that the Hg-1212 films are under-doped superconductors. In post annealing experiments we found that the Hg element can diffuse out from and into the films at 300 -350° C depending on the Hg vapor pressure. The Hg deficient films shown degraded superconductivities. Through annealing them in Hg atmosphere we recovered completely their superconductivities. Using these Hg-1212 films with different Hg content the relationship between T_c and carrier concentration (NH=1/eRH) was studied, which shown interesting behaviors. The curves of NH versus temperature were found to have two linear ranges. We also found that this phenomenon exists in the under-doped YBa₂Cu₃O_7-y compound. This discovery may lead to a new insight for understanding the high T_c cuprates.

2PO9-15 Dragging Excitation Characteristics from Thermoelectric Power in Bi₂(Sr_2-yLa_y)CuO_6+d Single Crystals

Yves Dumont 1,2, Claude Ayache 1, and Gaston Collin 3. 1 CEA, Service de Physique de l'Etat Condensé, C.E. Saclay, 91191 Gif sur Yvette, France. 2 Département de Physique, Université deVersailles St-Quentin, 45, av. des Etats-Unis 78035 Versailles, France. 3 Laboratoire Léon Brillouin, CEA-CNRS, C.E. Saclay, 91191 Gif sur Yvette, France.

Presenting Author: Y. Dumont

The ab-plane thermoelectric power S of Bi-2201 single crystals is used to establish the superconducting phase diagram which shows two branches, one matching the general curve for cuprates and a singular one, apparently linked to structural disorder. The low maximum T_c of 18 K allows to follow the sequence S(T)-curves over a large absolute temperature range in the normal state for both the under- and over-doped regions. The results support a drag mechanism, in which the dragging excitations appear related to the phase diagram through their effective number as well as their spectral shift and rearrangement.

2PO9-18 Magnetoresistance and thermoelectric power of Sr_1-xK_xBiO₃: a second-family of BiO based superconductors

D.C. Kim 1, J.S. Kim 1, S.J. Joo 1, C. Bougerol-Chaillout 2, S.M. Kazakov 3, J.S. Pshirkov 3, E.V. Antipov 3, and Y.W. Park 1. 1 Department of Physics and Condensed Matter Research Institute., Seoul National University., Seoul 151-742, Korea. 2 Laboratoire de Cristallographie, CNRS, BP166, 38042 Grenoble Cedex 9, France. 3 Department of Chemistry, Moscow State University, Moscow 119899, Russia.

Presenting Author: Y.W. Park

We have measured the temperature dependence of magnetoresistance and thermoelectric power (TEP) for the Sr_1-xK_xBiO₃ superconductor (x=0.45-0.6). The onset temperature of superconductivity in resistivity measurement is 12.5 K and the resistivity goes to zero at 10.2 K. The upper critical field (H(0)), and the coherence length, x (0) determined by the criterion r (H=H_c2) = 0.9r _n, where r _n is the normal state resistivity, are estimated to be 5.09 Tesla and 79.1A, respectively. TEP at room temperature has a small negative value (S=-1.96m V/K) and its temperature dependence shows two distinct features. The sign of TEP changes from negative to positive at 210K. As lowering the temperature, the negative slope of TEP (dS/dT) decreases and becomes rather flat at around 160K, which is a similar feature found in other BiO based superconductors.

2PO9-19 Thermoelectric Power studies of mercury doped Bi-2223 HTSC materials

S. Neeleshwar 1, D. Rama Sita 2, and P.Venugopal Reddy 1. 1 Department of Physics, Osmania University, Hyderabad-7, India. 2 School of Physics, University of Hyderabad, Hyderabad, India.

Presenting Author: P.V. Reddy

Thermopower studies of mercury doped HTSC materials with varying Hg concentration, were undertaken over a temperature range 80- 300K. It has been observed that the room temperature values( S-300) are small and negative and found remain almost constant with dopant concentration.

Futher, it has also been observed that the thermopower(S)values are found to increase continuously with decreasing temperature reaching a maximum value at temperature very close to superconducting transition temperature (T_c).

On decreasing the temperature further, S values are found to decrease rapidly reaching a lowest value at the transition temperature.

Behaviour of the normal state thermopower of all the materials has been discussed using the existing theoretical models in the literature, Nagosa-Lee, Model and two -band model and two band model with an extra linear term.