4PO6-20 Investigation the Relation Between Microstructural Parameters and Magnetic Properties of KClO₃ Doped YBCO HTSC Materials

Anjela Veneva 1, Michael R. Koblischka 2, and Masato Murakami 1. 1 Superconductivity Research Laboratory, ISTEC, 1-16-25 Shibaura, Minto-ku, Tokyo 105, Japan. 2 NST, Priorparken 685, Brondby 2605, Denmark.

Presenting Author: A.V. Veneva

4PO6-21 New modulation structures observed in Bi₂.₁₃Sr_1.87CuO_6+d

S.F. Cui 1, C.R. Li 1, Z.H. Mai 1, J.W. Xiong 2, Z.X. Zhao 2, Y.T. Wang 3, and P.D. Hatton 4. 1 Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China. 2 National Laboratory for Superconductivity, Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China. 3 Institute of Semiconductors, Chinese Academy of Sciences and National Optoelectronic Technology Center, Beijing 100083, China. 4 Department of Physics, University of Durham, South Road, Durham DH1 3LE, United Kindgom.

Presenting Author: S.F. Cui

A new two-dimensional structure modulation along c- and b-axis has been discovered in superconducting single crystals of Bi_2.13Sr_1.87CuO_6+d (Bi2201) by X-ray scattering. Such modulation structure does not exist in non-superconducting Bi2201 single crystals, but instead lattice distortions are observed in the a - b plane. This phenomenon may indicate that both strain relaxation and charge modulation in the a - b plane are important to the occurrence of the superconductivity in the copper oxides.

4PO6-22 Superlattice Structure in BSCCO Single Crystal Observed by High Resolution X-ray Diffraction

H. Zhang, L.L. Cheng, and K. Yang, Materials Physics Laboratory, State Key Laboratory for Artificial Microstructure and Mesoscope Physics, Department of Physics, Peking University, Beijing 100871, China

Presenting Author: H. Zhang

The single crystal of Bi₂Sr₂CaCu₂O_y was prepared by a traveling solvent floating-zone method. The structure of the sample was analyzed by X-ray diffraction. It demonstrated the sample was single crystal. The T_c of the sample is 80 K measured by means of the standard four-probe method. The high resolution x-ray analyses were performed using a Philips Materials Research Diffractometor equipped with a four crystal monochromator, delivering a pure Cu K a 1 line of wavelength, l =0.15406 nm. The instrument is capable of j -scan, pole figure measurement, rocking curve, reflectivity, and small angle scattering.

Considering the Bi₂Sr₂CaCu₂O_y like a multi-layer structure, the reflectivity measurement should indicate that its reflecting curve is similar to a multi-layer sample. The reflectivity of 00l reflection confirms it. The reflecting curve is similar to GaAs/GaAlAs/GaAs multi-layer sample, which shows that the crystal is of a multi-layer structure along the c-axis. After knowing the multi-layer structure of the crystal, we considered if the structure was a superlattice, or if it had a long period? To make this question clear, the small angle scattering was carried out. A peak at about 700 Å in the pattern indicates that the crystal is a superlattice.

An important question is if the multi-layer structure of the crystal has relationship with superconductivity? The answer is positive. After annealing in different atmospheres, it is observed that the reflecting curves and small angle scattering patterns are different with the different value of T_c. These facts tell us that so far we have not understood the structure of high T_c superconductors completely.

4PO6-23 Subtle Structural Changes in Different Blocks in Cuprates

H. Zhang, K. Yang, and L.L. Cheng, Materials Physics Laboratory, State Key Laboratory for Artificial Microstructure and Mesoscope Physics, Department of Physics, Peking University, Beijing 100871, China

Presenting Author: L.L. Cheng

In our former work, it was observed that there exist two subtle thermal transitions in Y-123 compound and the Bi-system compounds at about 300 ° C and 700 ° C, respectively. The unit cell of the sample is composed of two structural blocks: perovskite and rock salt. What should be made clear is that the two thermal and structural changes take place in the whole cell or from different blocks. This is of importance to know the characteristics of the Bi-system. In this work, Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_10+y was used as an example for our studying. Our analyses focus on making the above question clear. The bond lengths in the unit cell are analyzed carefully before and after two thermal and structural changes based on the results of high resolution x-ray diffraction. Careful analysis of the bond length in the samples demonstrates that the two jumps of the parameters are correlated with the two blocks. After the jump at about 300 ° C, the bond lengths in the rock salt block change more significant than in the perovskite block. For example, the Sr-O(3) and Bi-O(4) bonds which are located in the rock salt block change from 2.66 to 2.68 Å and 2.75 to 2.78 Å, respectively. But the bond lengths in the perovskite block are hardly changed. After the jump at about 700 ° C, the bond lengths in the perovskite block change more significant than in the rock salt block, the Cu(1)-O(1) and Cu(2)-O(2) bonds which are located in the perovskite block change from 1.92 to 1.95 Å and 1.94 to 1.97 Å, respectively. The Sr-O(3) and Bi-O(4) bonds change very little. The change of the bond lengths clearly shows that the different jump of the parameters takes place in different blocks.

4PO6-24 Stress Between Perovskite And Rock Salt Blocks In YBa₂Cu₃O_7-d

H. Zhang, X. Du, L.L. Cheng, and Z.Z. Gan, Materials Physics Laboratory, State Key Laboratory for Artificial Microstructure and Mesoscope Physics, Department of Physics, Peking University, Beijing, 100871, China

Presenting Author: Z.Z. Gan

We developed a model based on the fact that in high T_c superconductors, the unit cell of the superconductors can be divided into two different blocks: one is perovskite, the other rock salt. We suggested that the interaction between the two different blocks is very closely related with the superconductivity. And then we developed a computer program to calculate the stress between the two blocks. The stress between the perovskite and rock salt blocks in Y-system superconductors (YBa₂Cu₃O_7-d , d =0.07~0.62) was calculated. A clear correlation among the superconducting transition temperature and the stress between the perovskite and rock salt blocks is established. And the correlation between the static mechanical properties and the superconducting transition temperature are discussed. We suggest that the perovskite and rock salt blocks are somewhat independent structural units in physical properties, and the interaction between the two blocks is important in determining the superconducting properties.

4PO6-25 Normal State Transport Properties of YBa₂Cu₃O_7-d /PrBa₂Cu_2.8Ga_0.2O_7-d superlattices

Jerome Dumont 1, Arnaud Defossez 1, Brigitte Leridon 2, and Jean-Pierre Countour 2. 1 Laboratoire de Physique du Solide, ESPCI, 10, rue Vauquelin, 75231 Paris cedex 05, France. 2 Unite Mixte de Physique, Thomson-csf/CNRS, Domaine de Corbeville, 91404 Orsay cedex, France.

Presenting Author: B.C. Leridon

YBa₂Cu₃O_7-d /PrBa₂Cu_2.8Ga_0.2O_7-d superlattices have been extensively studied around the transition temperature T_c and their resistivity behavior in this range of temperature has often been depicted as governed by a Kosterlitz-Thouless transition. However, the lowering of T_c with respect to pure YBCO has never been explained in these structures. By studying carefully the normal state resistivity of different superlattices from T_c to 325 K, and making comparison with other oxygen deficient or Cobalt substituted materials we demonstrated an underdoping effect in these structures. This allows us to explain the variations of T_c and the normal state resistivity as a function of the respective superconducting or insulating thicknesses. The origin of this underdoping effect is discussed. This result sheds a new light on the interpretation of physical measurements made on YBa₂Cu₃O_7-d -based superconductor/insulator superlattices.

4PO6-26 Scanning tunneling microscope studies on the atomic structures in NdBa₂Cu₃O_7-d Single Crystals

M. Nishiyama 1, N. Chikumoto 2, M. Murakami 2,3 and K. Ogawa 1. 1 Graduate School of Integrated Science, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, 236-0027, Japan. 2 Superconductivity Research Laboratory, International Superconductivity Technology Center (ISTEC-SRL), 1-16-25 Shibaura, Minato-ku, Tokyo, 105-0023, Japan. 3 Iwate University, 4-3-5 Ueda, Morioka, Iwate, 020-8551, Japan.

Presenting Author: M. Nishiyama

NdBa₂Cu₃O_7-x (Nd123) single crystals grown by the oxygen-controlled-melt-growth process are known to have a large critical current density J_c noticeably in a high magnetic field. To clarify atom structures of the strong pinning centers not identified yet but which must be dispersed in Nd123, ultrahigh-vacuum scanning tunneling microscope/spectroscopy studies have been performed. As-grown surfaces of the Nd123 single crystals are found to be atomistically clean and stable. We have observed two characteristic surface structures, i.e., island and striped structures and three types of scanning tunneling spectra depending on the phases examined, each with different surface structures. The spectra are found to range from metallic to semiconducting with a large band gap. The Nd123 crystal with high J_c is concluded to consist of finely dispersed phases with a variety of band structure, i.e., from metals to insulators.

4PO6-27 Josephson phenomenology and microstructure in 45° c-axis tilt YBCO grain boundaries on exact and vicinal cut substrates

Filomena Lombardi 1, Francesco Tafuri 2, Franco Carillo 1, Fabio Miletto 1, Fabrizio Ricci 1, and Umberto Scotti 1. 1 Physics Dept., University of Napoli "Federico II", 80125 Napoli, Italy. 2 Engeneering Dept., IIa University of Napoli, Aversa (CE), Italy.

Presenting Author: F. Lombardi

YBaCuO_7-d (YBCO) artificial grain boundary Josephson junctions have been fabricated employing a recently implemented biepitaxial technique. MgO seed layer on exact (110) SrTiO₃ substrates and SrTiO₃ seed layer on vicinal cut MgO substrates have been used to obtain a misalignement of the YBCO c-axis (45° c-axis tilt). Junctions based on these grain boundaries exhibit good Josephson properties. High values of the I_cR_N product and a Fraunhofer-like dependence of the critical current on the magnetic field, differently from traditional biepitaxial junctions have been obtained. The correlation between transport properties and microstructures has been investigate by Trasmission Electron Microscopy on exact substrates. The presence of atomically clean basal plane faced (BPF) tilt grain boundaries, among other type of interfaces has been shown. Pure BPF grain boundaries have been selected by using SrTiO₃ buffer layer on vicinal cut MgO substrates. Transport properties have shown the dielectric nature of these kind of grain boundaries. A comparison with different angle c-axis tilt grain boundaries is reported. The possibility of employing these junctions to explore the symmetry of the order parameter is also discussed.

*4PO6-28 Atomic Force Microscopy with Conducting Tips: Correlation Studies Between Microstructure and Electrical Properties of YBaCuO Thin Films

Annick F. Dégardin, Olivier Schneegans, Frédéric Houze, Pascal Chretien, Eleonor Caristan, and Alain J. Kreisler, Laboratoire de génie électrique de Paris, SUPÉLEC, Universités Paris 6 et 11, CNRS UMR 8507, Plateau de Moulon, 91192 Gif-sur-Yvette Cedex, France

Presenting Author: A.F. Dégardin

High quality YBaCuO thin films are of significant interest for such applications as microwave devices or radiation detectors. To test this quality, superconducting film surfaces have been studied with an original laboratory-made system associated with a commercial atomic force microscope. Using a doped silicon probe coated with doped diamond, topographical and local electrical contact resistance surface images could be obtained simultaneously within a given area of the sample. YBaCuO films sputtered on MgO and SrTiO₃ single crystals have been observed and results are discussed in relation with superconducting film properties.

For films grown on MgO substrates, island-shaped grains exhibiting terraces of one unit cell height could be observed. The electrical connection areas between grains were clearly visible on the electrical images, that could be correlated to the electrical transport properties of the films. The effect of passivation layer at the film surface has been evidenced.

For films deposited on SrTiO₃ substrates, the observed grain structure clearly exhibited a spiral shape. Moreover, the currently admitted resistance anisotropy of YBaCuO has been exhibited.

Electrical images will be also discussed in relation with transmission electron microscopy. Finally, to establish comparisons, YBaCuO laser ablated film surfaces will be studied with our technique.

4PO6-29 Effects of Substrate Preparation on Properties of YBaCuO Thin Films

Annick F. Degardin 1, Éléonor Caristan 1, Alain J. Kreisler 1, Xavier Castel 2, and Andre Perrin 2. 1 Laboratoire de génie électrique de Paris, Supélec, Universités Paris 6 et 11, CNRS UMR 8507, Plateau de Moulon, 91192 Gif-sur-Yvette Cedex, France. 2 Laboratoire de chimie du solide inorganique et moléculaire, CNRS UMR 6511, Université Rennes 1, Avenue du Général Leclerc, 35042 Rennes Cedex, France.

Presenting Author: A.F. Dégardin

Substrate preparation is a major step for the elaboration of good quality superconducting films. In this work, the influence of substrate annealing has been studied by considering structural and physico-chemical properties of YBaCuO thin films sputtered on (100) MgO single crystals. These latter were selected for their good dielectric properties interesting for microwave applications. In fact, substrate thermal treatment has been reported to favor the formation of steps on the MgO surface and hence promote YBaCuO growth.

Various characterization techniques have been used to discuss properties of the bare substrates and the films deposited on them. Atomic force microscopy has allowed to determine surface microstructure. X-ray diffraction and phi-scan measurements have allowed to identify crystalline phases and determine film orientation in the growth plane. Secondary ion mass spectrometry analysis was used to identify impurity diffusion from substrate to film. Moreover, critical temperature measurements by ac susceptibility and microwave effective surface resistance measurements were also performed on the films.

It was shown that structural properties as well as ac and microwave performances were significantly improved for the films deposited on annealed substrates. Besides, results related to electrical transport properties of the films (e.g. critical current density) will be also discussed.