4PO7-80 Antiferromagnetism and superconductivity in Hg-based cuprate compounds of (Hg_1-xPb_x)Ba₂Ca₂Cu₃O_8+d

H.M. Shao 1, S.A. Aruna 1, C.C. Lam 2, L.J. Shen 2, Y.M. Cai 1, Z. Wu 3, and S.M. Lee 2. 1 Department of Physics and National laboratory of Solid State Microstructures, Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing, 210093, P.R. China. 2 Department of Physics and Center of Materials Science, The City University of Hong Kong, Hong Kong, China. 3 Department of Physics and Center for Superconductivity, University of Houston, Houston, TX 77204-5932 USA.

Presenting Author: S.A. Aruna

The phase diagram of Hg-based compound of (Hg_1-xPb_x)Ba₂Ca₂Cu₃O_8+d has been constructed based on the data obtained from experimental measurement of their magnetic susceptibility, the neutron scattering technique, the X-ray photoelectron spectroscopy and the electron probe microanalyzer (EPMA). The relationships between the transition temperature for the antiferromagnetic phase and the superconducting phase with the doping concentration were traced out in the phase diagram. Due to the difficulty in measuring the transition from the insulating state to the metallic state in the phase diagram, the phase boundary for these two phases has not yet been determined. From the results of our phase diagram, so far this boundary is only a qualitative one and is represented by a dashed line.

4PO7-81 Synthesis and properties of (Hg,Pb)(Ba,Sr)-1223 superconductors

M. Kuznetsov, S.R. Lee, N.P. Kiryakov, and Yu.D. Tretyakov, Moscow State University, Moscow, Russia 119899

Presenting Author: M.S. Kuznetsov

The powder, bulk and single crystals of Hg-1223 phase with substitution of Ba by Sr and Hg by Pb were obtained by ampoule and high pressure methods. The highly homogenious precursor powders were prepared by spray drying technique. The influence of substiution degree and post annealing conditions on T_c and irreversibility line position (Hg,Pb)(Ba,Sr)-1223 was studied. The highly overdoped (Hg,Pb)(Ba,Sr)-1223 with T_c = 67 K was obtained by high pressure synthesis. The dependence of T_c on different post annealing types was studied. For compounds with different Sr substitutions irreversibility line position shifts to higher fields region with increasing of Sr content . For the single crystals of Sr - rich composition irreversibility line in the H-(1-T/T_c) phase field lies above among all known Hg-1223 phases with different substitutions. This single crystals reveals peak effect feature at the T=50-70 K and H=1.5-2 T.

4PO7-82 Properties of Polycrystalline Hg_(1-x)Pb_xBa₂Ca₂Cu₃O_y Superconductors

H.R. Khan 1, K. Leuders 2, K. Kajikawa 3, M. Baenitz 4, and C. Ecker 2,5. 1 FEM, Materials Physics Department, D-73525 Schwabisch Gmund, Germany. 2 Fachbereich Physik, Freie Universitat Berlin, D-14195 Berlin, Germany. 3 Research Institute of Superconductivity, Kyushu University, Fukuoka 812-8581, Japan. 4 Max-Planck-Inst. fur Chemische Physik fester Stoffe, F-01187 Dresden, Germany. 5 New address: Inst. fur Physik, Humboldt-Universitat zu Berlin, D-10115 Berlin, Germany.

Presenting Author: K. Lueders

Partial substitution of Hg²⁺ by ions of various radii and valency such as Pb²⁺, Bi³⁺ and Ag²⁺ etc. in Hg-based HTS materials affects the HTS phase formation, chemical stability and the superconducting properties. By substitution of Pb²⁺ on the Hg²⁺ site in Hg-1223 it was possible to synthesize the Hg_(1-x)Pb_xBa₂Ca₂Cu₃O_y (x=0.1,0.2 and 0.3) compounds by a direct, one steop synthesis using the metal oxides. The compounds were characterized by X-ray diffraction and show a HTS-tetragonal phase. Small amounts of non superconducting phases such as HgBaCaO, BaCuO, CaCuO and CaO were also observed. The volume of the HTS-tetragonal superconducting phase decreases with increasing Pb-concentration and the tetragonal cell volume increases with increasing Pb-concentration because of different ionic radii of Pb and Hg: Hg²⁺ (0.11 nm) and Pb²⁺ (0.12 nm). SEM and EDXA investigations show that Pb substitution homogenizes the Hg distribution in the grains and subsequently enhances the chemical stability. The T_c values which differ only slightly from each other and the superconducting volume fraction were determined from the temperature dependence of the AC susceptibility. An interesting feature is the imaginary part of the AC susceptibility which in some cases exhibits a double peak structure affecting the irreversibility line. A comparison with theoretical treatments will also be discussed.

4PO7-83 Effect of pressure on T_c of Tl₂Ba₂CuO_x single crystals

T.G. Togonidze, O.M. Vyaselev, and N.N. Kolesnikov, Instute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, Russia

Presenting Author: T.G. Togonidze

The mesurements of critical temperature, T_c, of high quality Tl:2201 single crystals under quasihydrostatic pressure up to P<12kbar, exhibited a sign-reversal dependence of dT_c/dP on the oxygen content: dT_c/dP=-0.34 K/kbar and 0.12 K/kbar for the crystals with T_c @ 40K and 90K, respectively. The T_c dependence on the pressure may be explained by the changes of carrier concentration under pressure and/or the existance of the local poaramagnetic centres in samples with lower T_c.

4PO7-84 Replacement study of Thallium by Zn & Ni in Tl-1223 superconductor phase

R. Awad 1, N.S. Ali 2, I.H. Ibrahim 1, A.I. Abou-Aly 1, and A.I. Saad 2. 1 Physics Dept., Faculty of Science, Alexandria University, Egypt. 2 Physics & Chemistry Dept., Faculty of Education, Alexandria University, Egypt.

Presenting Author: I.H. Ibrahim

Superconducting samples of type (Tl_1-xM_x)Ba₂Ca₂Cu₃O_9-d , where M=Zn and Ni , with 0 £ x £ 0.6 are prepared using a single process of solid state reactions. The characterization and the investigation of the superconductivity for the prepared samples are preformed. X-ray data show that the tetragonal structure of Tl-1223 does not change by the substitution of Zn or Ni but the lattice parameters "a" and "c" are varied. The transition temperature "T_c", determined from electrical resistance measurements, increases up to x=0.2 and then decreases for Zn substitution samples whereas it decreases with increasing Ni-content.

4PO7-85 XANES and EXAFS Study of Site Geometry of Doped Sn Cations in the Tl-Pb(1223) Superconducting System

Saurabh Dalela 1, Bhavna Dalela 1, R.K. Singhal 1, D.M. Baggio-Saitovich 2, and K.B. Garg 1. 1 Condensed Matter Physics Lab, Physics Department, Rajasthan University, Jaipur 302004, India. 2 CBPF, Rua Dr. Xavier Sigaud, 150 Urca, 22290-180, Rio de Janerio-RJ, Brazil.

Presenting Author: K.B. Garg

Addition of Sn to Tl-Pb(1223) [Tl_0.5Pb_0.5Sn_xSr_1.6Ca_2.4Cu₃O_y] has been reported to lead to a remarkable enhancement of hysteresis magnetisation and a tenfold increase in the intragranular critical current density. Sn was reported to be primarily in the minority impurity phase. Later measurements with Mossbauer and AES (Auger Emission Spectroscopy), on the other hand, showed that most of Sn was in the lattice and a little on the grain boundaries. Thus Sn appears to occupy two sites with weights differing but the intensity of one site inside of lattice corresponds to tetravelent state and increases with Sn concentration. Both contributions thus appear to come from the main phase with the contribution from the minority phase, if at all, overlapping with them i.e. the major part of the Sn resides inside bulk of the crystal instead of grain boundaries as evidenced by these investigations. Therefore, it was of interest to further probe the site geometry of the Sn cations. This investigation has been done using XAFS technique. XANES and EXAFS measurements made at Cu K edge using SR source at LURE, Orsay are analysed. The data is analysed and discussed in terms of the valence of the Cu cation and the changes in near-neighbour co-ordination and distances.

*4PO7-86 Tuning of the Ferromagnetic and Superconducting Transitions by Tin-doping in RuSr₂GdCu₂O₈

Abbie C. McLaughline and J. Paul Attfield, Interdisciplinary Research Centre in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, England and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, England

Presenting Author: A.C. McLaughlin

Recently a remarkable new ruthenocuprate RuSr₂GdCu₂O₈, in which superconductivity and ferromagnetism coexist at low temperatures, has been synthesised and its structure characterised by powder X-ray synchrotron diffraction data [1,2]. RuSr₂GdCu₂O₈ has a Curie temperature T_M = 132 K and a superconducting transition temperature T_c = 0-39 K, depending on the synthesis conditions. Solid solutions of formula Ru_1-xSn_xSr₂GdCu₂O₈ have been prepared. Powder synchrotron X-ray diffraction and physical measurements show that the Sn⁴⁺ substitutes only at the Ru sites in this layered material. The substitution of diamagnetic Sn⁴⁺ for Ru suppresses the ferromagnetic moment in the ruthenate layers and a Curie temperature T_M = 80 K has been recorded for the highest doped sample. The conductivity of the CuO₂ layers is also enhanced with increasing concentrations of Sn, and the onset of superconductivity increases from 36 K to 48 K. This may reflect the increase in hole transfer to the CuO₂ planes and the reduction of ferromagnetic order within the ruthenate layers.

[1] J.L. Tallon, C. Bernhard, M.E. Bowden, P.W. Gilberd, T.M. Stoto and D.J. Pringle, IEEE Trans. Appl. Superconductivity, 9, 1696 (1999).

[2] A.C. Mclaughlin, W. Zhou, J.P. Attfield, A.N. Fitch and J.L. Tallon, Phys. Rev. B, 60, 7512 (1999).

4PO7-87 Electronic Structure, Transport and Kinetic Studies of Doped BPSCC (2223) system

M.N. Khan 1, M. Khizar 1, M.M. Mansoor 1, and B.N. Mukashev 2. 1 GIK Institute of Engineering Sciences and Technology, Topi, NWFP, Pakistan. 2 Institute of Physics and Technology, Ministry of Sciences, Academy of Sciences Republic of Kazakhstan.

Presenting Author: M.N. Khan

High-T_c Superconductors with a nominal composition of Bi_1.72Pb_0.34Sr_1.83Ca_1.97Cu_3.13O_Y (long length wires and tapes) with a variety of dopants (M=Ag, Sm, B, Gd and Y) were prepared by the heat treatment of rapidly quenched glass precursors. The optimum heat-treatment temperature TOpt was related to the temperature at which a partial melting of the specimen occurred as determined by differential thermal analysis. Activation energies and frequency factors, employing different models were evaluated. It was observed that both peritectic transition and reaction rates were dependent on the ambient atmosphere. The resistivity measurements revealed that the critical temperature decreases with increasing Y, Gd, Sm, B and Ag concentration. The critical current density J_c measured at 77 K from I-V data shows an increase with silver addition. These results revealed that silver addition does not destroy the superconductivity and at the same time enhances the critical current density. X-ray diffraction results showed that the volume fraction of the high-T_c (2223) phase decreases and that of low-T_c (2212) phase increases with increase of these rare-earth ion contents.

These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the of magnetic nature of the substituted ion in the composition.

4PO7-88 Doping Dependence of the O.P. Symmetry in Y_1-xCa_xBa₂Cu₃O_7-d

Guy Deutscher, Amir Kohen, and Yoram Dagan, School of Physics and Astronomy Tel-Aviv University, Tel-Aviv 69978, Israel

Presenting Author: G. Deutscher

We have studied Andreev reflections and the zero bias anomaly in Y_1-xCa_xBa₂Cu₃O_7-d films having the [110] (or [103]) orientation. The films were grown using the template method on [110] SrTiO₃ substrates. In overdoped samples (x=0.2), having a T_c of 60 to 70 K, point contact measurements using an Au tip show a clear Andreev characteristic, with a conductance maximum at 10 mV. Indium tunneling contacts on the same samples do not show any zero bias conductance peak, but a sub-gap of about 3 meV and a gap like feature at 10 mV. The absence of bound states indicates that the order parameter in the overdoped samples cannot have the d_x2-y2 symmetry. The usual zero bias conductance peak characteristic of bound states is observed in optimally doped samples (x=0) with a T_c of 87 K.

4PO7-89 Field Induced Pinning in Zn Doped Melt Textured YBCO

G. Krabbes (1), G. Fuchs (1), P. Schaetzle (1), S. Gruss (1), R. Hayn (1), S. - L. Drechsler (1), J. W. Park (2), F. Hardinghaus (2)

Presenting Author: G. Krabbes

The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by appropriate oxidizing treatment of the as grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density j_c in the j_c vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides of high quality of the bulk YBCO, the peak effect is the reason is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.