4PO7-50 STM Imaging of the Effects of Individual Zinc Impurity Atoms in Bi₂Sr₂CaCu₂O_8+d

J.C. Davis 1, S.H. Pan 2, E.W. Hudson 1, K.M. Lang 1, H. Eisaki 3, and S. Uchida 3. 1 Physics Dept., University of California, Berkeley, CA 94720. 2 Physics Dept., Boston University, Boston, MA. 3 Dept. of Superconductivity, Tokyo University, Tokyo, Japan.

Presenting Author: J.C. Seamus Davis

I will describe recent research, using a high resolution very low temperature STM on the high-T_c superconductor BiSrCaCuO [1], in which both single impurity atoms and quantized magnetic vortices are used to probe the structure of the order parameter at the atomic scale.

Single impurity atoms generate local quasiparticle 'clouds' at the atomic scale. Imaging of the 'clouds' associated with individual Zn dopant atoms shows direct real-space consequences of the d-wave nature of the superconducting order parameter. Quasiparticle scattering resonances [1] are observed by tunneling spectroscopy at these sites, and the spectra are in good agreement with theories of quasi-particle scattering in a d-wave superconductor. The destruction of superconductivity within 1 nm of the Zn sites is also directly demonstrated.

In addition we report new results which clearly demonstrate the existence of the expected localized quasiparticle states inside the magnetic vortex cores in BSCCO. We show directly, by simultaneous imaging of the vortices and the scattering centers, that the single impurity atom quasi-particle scatterers influence the vortex solid structure by pinning the vortices.

4PO7-51 Effect of Hg-substitution on the phase stabilisation and T_c in Bi-2212 cuprate

R.S. Thampi 1, D.G. Kuberkar 1, G.K. Bichile 2, M.R. Gonal 3, Ramprasad 3, R.G. Kulkarni 1. 1 Department of Physics, Saurashtra University, Rajkot-360 005, India. 2 Department of Physics, Dr, B.A. Marathwada University, Aurangabad-431 004, India. 3 Materials Processing Division, BARC, Trombay, Mumbai-400 005, India.

Presenting Author: R.S Thampi

Effect of divalent Hg-substitution at Ba and Sr-site on the structural and superconducting properties of Bi₂Sr₂Ca₁Cu₂O_z phase have been investigated in the context of possible hole doping and chemical pressure effect. Also the effect of Hg-substitution in stabilising the Bi-2212 phase is investigated in the series of samples Bi_2-xHg_xSr₂Ca₁Cu₂O_z (x=0.0-0.3) and Bi_2.3Sr_2-xHg_xCa₁Cu₂O_z (x=0.0-0.5) synthesised using solid state reaction(SSR) and nitrate solution method (NSM). It is observed that the Hg doping effect is more pronounced at Sr-site, showing an improvement in T_c and structure. Also the structural and superconducting properties of both the series of samples prepared by SSR and NSM methods show different behaviours. This study was carried out using XRD, SEM, iodometry, a.c. susceptibility and resistivity measurements.

4PO7-52 Effect of Y-Substitution on the Electrical Properties of Epitaxial Bi₂Sr₂Ca_1-xY_xCu₂O_8-d Thin Films

E. Baca 1, V. Holguin 1, P. Prieto 1, and W. Lopera 2. 1 Departamento de Física, Universidad del Valle, A.A. 25360, Cali, Colombia. 2 Departamento de Física, Universidad de Antioquia, A.A. 1226 Medellín, Colombia.

Presenting Author: E. Baca Miranda

We have performed a study on the preparation conditions and electrical transport properties of epitaxial Bi₂Sr₂Ca_1-xY_xCu₂O_8+d thin films. The deposition of the films was carried out by a high oxygen pressure dc-sputtering technique on (100) SrTiO₃ substrates. The structural properties were determined by x-ray diffraction. By increasing the yttrium content the temperature dependence of resistivity change from a metallic to an insulating behavior. For x<0.50, the samples showed a metalic behavior in the normal state and the transition temperature decrease with increasing yttrium concentration. Width superconductor transition and as function of the x value is analized. For x>0.50, the samples showed a semiconductor behavior what correspond to a variable-range-hopping conduction between localized states in according to a two-dimensional system given by . Ultra-thin Bi₂Sr₂Ca_1-xY_xCu₂O_8+d films have been used as artficial barriers in tunneling heterostructures with Bi₂Sr₂CaCu₂O_8+d superconducting electrodes.

4PO7-53 Phase Formation and Superconductivity in PIT-Type (Bi,Pb)-1212*

N.M. Murphy, S.E. Dorris, D.J. Miller, Z.P. Luo, H. Claus, and V.A. Maroni, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA

Presenting Author: N.M. Murphy

Compositions in the system (Bi_0.30Pb_0.50)Sr_2.13(Ca_xY_1-x)Cu_2.20O_z were studied to identify conditions under which the (Bi,Pb)-1212 phase can be formed in a silver sheath and to determine if the 1212 phase is a superconductor. It was shown via powder-in-tube (PIT) processing that the (Bi,Pb)-1212 phase readily forms inside a silver sheath at 920-940° C in air. Composition, oxygen pressure, time, and temperature were all varied to study their effects on (Bi,Pb)-1212 phase purity and superconductivity. A precursor with a Y:Ca ratio of 0.85:0.15 (i.e., x = 0.15) resulted in a majority-phase (>80%) (Bi,Pb)-1212 product in silver-sheathed form, but this composite did not exhibit superconductivity down to 5 K. A precursor with a Y:Ca ratio of 0.15:0.85 (i.e., x = 0.85) gave a product containing mostly (Bi,Pb)-2212 and nearly undetectable amounts of (Bi,Pb)-1212. The resulting composite had a magnetization T_c of 80 K in zero-field, but the existence of superconductivity in this sample was attributed to the (Bi,Pb)-2212 phase because the superconducting volume closely corresponded to the amount of (Bi,Pb)-2212 phase present.

4PO7-54 Thermopower in Bi₂Sr₂Ca_1-xGd_xCu₂O_8+d single crystals with various Gd and oxygen content

X. Zhao, X.F. Sun, L. Wang, Q.F. Zhou, and X.-G. Li, Structure Research Laboratory, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P.R. China

Presenting Author: X. Zhao

Thermopower measurements on Bi₂Sr₂Ca_1-xGd_xCu₂O_8+d (0 £ x £ 0.41) and Bi₂Sr₂CaCu₂O_8+d (0.233 £ delta £ 0.302) single crystals in the temperature range of 90-290K were carried out to get information about the characteristics of the band structure and charge transport. For all these superconducting crystals, the carrier concentration is found to systematically increase with oxygen content and decrease with the doping of Gd. The magnitude of the thermopower S decreases monotonically with the increase of carrier density. The S varies linearly in T in the high temperature region with a negative slope which is weakly doping dependent, and a significant broad maximum could be found at lower temperature region, which shifts to low temperature with the increase of carriers. The data can be well fitted by the two-band model S = AT + (Bl +CT)exp(-l /T). From the fitting parameters, one can see that the value of A confirms the variation of carrier concentration with doping. The semiconducting gap of Bi-O layers E_g = 2l /k_B can be estimated and its value decreases with increasing carrier density. The variation of E_g for Gd-doped system is larger than that for the O-doped crystals at the low carrier region.

4PO7-55 Polarised XAFS Study of Influence of Pr-doping in BSCCO(2212) Single Crystal

Bhavna Dalela 1, Saurabh Dalela 1, S. Venkatesh 1, J.H. Choy 2, R.K. Singhal 1, and K.B. Garg 1. 1 Condensed Matter Physics Lab, Physics Department, Rajasthan University, Jaipur 302004, India. 2 Department of Chemistry, Seoul, National University, Seoul 151-742, Korea.

Presenting Author: K.B. Garg

Pr-doping has been a very interesting subject for investigation for deciphering the mechanism of high temperature superconductivity ever since it was discovered first that it quenches the superconductivity in the YBCO(123) system. However, its influence in case of Tl(1212) and BSCCO(1212) system has not been found to be so destructive. Pr-cation appears to behave appreciably differently when sitting inside the lattice of different HTSC systems. Inspite of a lot of work done in this direction, there is lack of unanimity on how Pr doping quenches the superconductivity. In the present contribution we report the results of XANES & EXAFS measurements on the Pr L_III and Cu K edges in well characterised single crystals of Bi₂Sr₂Ca_(1-x)Pr_xCu₂O₈ system. A number of polarised X-ray absorption spectra have been recorded with E-vector parallel to ab-plane and perpendicular to it at the synchrotron radiation facility SRRC, Taiwan. The data shows some interesting behaviour in the XANES and EXAFS parts of the spectra with orientation. The results will be discussed in terms of valence states of doped Pr ion and Cu, site-geometry and near neighbour environment of Pr and localisation of holes.

4PO7-56 Texturing Studies on Sm-Substituted Bi-2212 High T_c Superconductor Grown by Floating Zone Technique

S. Uthayakumar 1, E. Srinivasan 1, D.P. Paul 2, D. Prabhakaran 1, R. Jayavel 1, C. Subramanian 1, and P. Ramasamy 1. 1 Crystal Growth Centre, Anna University, Chennai -25, India. 2 Dept. Of Physics, University of Chittagong 4331, Bangladesh.

Presenting Author: C. Subramanian

Bismuth based high T_c superconductors have been extensively studied due to their potential applications. Among the known three phases of Bi-compound, Bi₂Sr₂CaCu₂O₈ phase is more stable with transition temperature (T_c=85K) and it is relatively easy to grow in the form of bulk crystals. Because of the weak links and porosity in the polycrystalline bulk sample, the current carrying capacity (J_c) decreases. In order to improve the J_c, textured bulk crystals have been grown by different techniques. Floating Zone growth has been proved to be the best technique for the growth of textured crystals. To enhance the superconducting nature and phase purity of Bi-2212 alkaline and rare earth elements were substituted.

The present investigation is aimed at the texturing studies on Sm-substituted Bi-2212 crystals grown by floating zone technique. Preparation of feed rods was carried out using ceramic technique according to the ratio of Bi₂Sr₂Ca_1-xSm_xCu₂O_8+d (x=0.0 to 0.5). After calcinating the mixed powders, feed rods of size 40 x 5 x 5 mm3 were fabricated. The sintered rods were predensified at a faster pulling rate of 10mm/hr. The final growth was carried out at a slower pulling rate of 1mm/hr.

The optical microscope and SEM studies reveal step-like growth patterns on the outer surface of the crystal. The formation of step-like pattern is attributed to the fast cooling rate. Phase purity was confirmed by the powder XRD studies. Sm substitution in the Bi-2212 system increases the T_c value (for low concentrations of Sm) and influences on the texturing nature of the crystal. The incorporation of Sm into the Bi-2212 crystal system has been confirmed by ICP analysis. The superconductivity studies have been carried out by the AC susceptibility technique. The texturing ratio as calculated from the XRD studies decreases as Sm concentration increases.

*4PO7-57 EDX analysis, microstructure and magnetic properties of CuS doped Bi-2212 superconductors

J. Albino Aguiar, C.L.S. Lima, Y.P. Yadava, J.M. Ferreira, and E. Montarroyos, Departamento de Física, Universidade Federal de Pernambuco, 50690-901 Recife, PE, Brazil

Presenting Author: J. Albino Aguiar

In high temperature superconductors substitution and doping studies are of immense importance for basic as well as applied aspects. Due to existence of essentially one kind of Cu site, Bi₂O₃-SrO-CaO-CuO (BSCCO) superconductor system is a simpler and better candidate for studying effect of the substitution on the superconducting transition temperature T_c. In high temperature ceramic superconductors, one normally use metal oxides for the substitution and doping studies, in the present work we have studied effect of CuS doping on Bi-2212 (nominal composition Bi_2+ySr_3-y-nCa_n[(CuO)_1-x(CuS)_x]₂O_6+d , (0 < x < 0.1, 0 < y < 0.1 and 0.8 < n < 1) phase. XRD studies and magnetization measurements show stabilization of superconducting Bi-2212 phase in these materials with an increase in T_c of the samples. All CuS doped samples exhibited a higher T_c than that of the undoped Bi-2212 superconductors. EDX analysis show incorporation of S in the Bi-2212 lattice. SEM images show that the microstructure of the samples consist of unreacted copper oxide and Bi-2212 phase. In view of above observations, most likely improvement in T_c of the CuS doped Bi-2212 superconductors is due to modifications in the oxygen stoichiometry of the samples with a consequent improvement in hole concentration.

4PO7-58 Effect of alloying Ag-sheath and doping oxide core in Bi-Pb-Sr-Ca-Cu-O superconductor composites

Mustafa Yavuz, Peter M. McIntyre, Rainer Soika, and Donald Naugle, TAMU, Physics Dept., College Station, TX 77843-4242

Presenting Author: M. Yavuz

In this work the doping effect of the core of the Bi-based superconductor composites by the transition elements and alloying Ag-sheath in terms of electrical and mechanical properties were investigated. It was found that the transition element addition to Bi-2223 phase reasonably increases the critical current densities to a certain amount above which it acts adversely. The solid solubility of those elements in Bi-2212 and Bi-2223 phases were also investigated to find the its pinning effect in the 2212 and 2223 matrices separately. The alloying of the silver sheath by transition elements was found to be relatively promising in terms of the mechanical and electrical properties.

4PO7-59 Effect of Ag Based Alloy Additions on Superconducting Properties of BSCCO Materials

Michail S. Tselyevskii, Natalia F. Ladutko, Sergey N. Ustinovich, Zhanna M. Tomilo, and Nina A. Prytkova, Laboratory of Superconducting Materials Physics, Institute of Solid State and Semiconductor Physics, National Academy of Sciences of Belarrus, Minsk, 220072, Belarus

Presenting Author: M.S. Tselyevskii

In this paper the study of influence of Ag and Ag based alloy (Ag-Cu-Zn) additions on phase composition and superconducting characteristics of BiSrCaCu₂O_y superconductors had been carried out. The samples contained various amounts of additions (x =0, 0.5, 1.0 and 2.0 wt.%) had been prepared by melting with subsequent annealing. Material with 1.0 wt.% Ag and Ag-Cu-Zn additions showed higher superconducting transition temperature as compared with materials without additions.

The results of phase analysis and superconducting characteristics of BSCCO/Ag and BSCCO/Ag-M composites are presented.

The work was supported by the Fundamental Investigation Fund of Belarus (Grant F 97M-067).