Structural and Superconducting Properties of High-Tc Superconductors

Thomas Frello

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    Abstract

    The structural and superconducting properties of high-Tc compounds are investigated by use of hard x-ray diffraction and neutron diffraction, magnetooptical imaging, susceptibility measurements and gas volumetric sample preparation. The systems studied are single crystal samples of ReBa2Cu3O6+x and La2−x−yReySrxCuO4 (Re=Rare Earth element), and Ag-clad Bi2Sr2Ca2Cu3O10+x tapes for power transmission applications. The four major topics covered are:

    1. Oxygen ordering in the ReBa2Cu3O6+x system
    The ordering of the oxygen atoms in the basal plane of ReBa2Cu3O6+x has profound influence on the superconducting properties. The work presented in this thesis represents the most systematic study performed so far of the oxygen superstructure formation in YBa2Cu3O6+x (YBCO) as function of oxygen concentration x and temperature. High-purity single crystals have been prepared to oxygen concentrations in the superconducting orthorhombic distorted state. Dopings from 0.35 ≥ x ≥ 0.87 are investigated. The dynamic behavior of the dominant ortho-II superstructure phase is studied over time periods ranging from seconds to months, and we demonstrate that dynamical scaling applies to the superstructure formation. A highly unusual YBCO sample with x = 0.62 and an anomalous tetragonal symmetry is characterized by x-ray diffraction and magneto-optical imaging, and we find that it is bulk superconducting. Oxygen ordering has also been investigated for other systems with Re=La,Nd. For nonsuperconducting NdBa2Cu3O6+x with x = 0.5 we observe a mixture of several oxygen superstructures coexisting in the same sample. The configurations are of a nature that cannot give rise to charge transfer and the observations are consistent with the different Tc(x) dependence for YBa2Cu3O6+x and NdBa2Cu3O6+x.

    2. Flux turbulence in ReBa2Cu3O6+x
    By magneto-optical imaging turbulent features of the boundary zone dividing
    flux of opposite polarity has been observed in high-quality single crystals of YBCO. We demonstrate the existence of flux turbulence in underdoped YBCO and near-optimally doped NdBa2Cu3O6+x single crystals. It is shown that the temperature for which the turbulence is maximum scales with Tc and that the chemical purity is important for the turbulent behavior. A number of materials parameters like critical current density, electrical anisotropy and oxygen ordering have only little or no influence on the turbulence. There are indications that a sample in the turbulent state displays incomplete flux shielding behavior.
    3. Stripe phases in nickelates and cuprates
    Anomalous suppression of superconductivity occurs in La2−xBaxCuO4 and La2−x−yNdySrxCuO4 when the hole doping (Ba,Sr content) is close to 1/8. Neutron scattering experiments indicate that at low temperatures charges and spins order in stripes similar to what is seen in nickelates. We have investigated the scattering associated with charge ordering with hard x-rays, and in La1.775Sr0.225NiO4 and La1.6−xNd0.4SrxCuO4 with x = 0.12 we find reflections consistent with the neutron data. Weak charge scattering is also observed in La1.6−xNd0.4SrxCuO4 with x = 0.15, and for the cuprates additional information on the stripe symmetry transverse to the CuO2 planes is obtained, and next- nearest-neighbor-plane correlations are observed. In Eu-doped La1.68−xEu0.2Sr0.12CuO4 surprisingly strong superstructure reflections are observed at the positions expected for charge ordering peaks. The reflections are essentially temperature-independent and of unknown origin.
    4. Phase development and texture formation in BSCCO/Ag tapes
    The solid state chemical reactions and microstructural changes taking place in the ceramic core of Ag-clad BSCCO tapes during annealing are monitored in-situ using hard x-ray diffraction. We observe that the texture of the Bi-2212 phase begins to improve already during the initial heating and that the final texture of the 2212 and 2223 phases are identical. The phase transformation involves the secondary phases (Ca,Sr)2PbO4, (Ca,S r)2CuO3, the 3321 and 2201 phase and a liquid phase which appears during annealing. We observe no finite-size broadening of the 2212 phase reflections during the transformation process, and this is discussed in relation to various transformation mechanisms. Magneto-optical imaging is combined with hard x-ray diffraction to investigate the influence of the thermomechanical processing steps on the overall tape performance. The magneto-optical results show how defect structures play a major role as a current limiting factor. The major importance of additional thermomechanical processing steps is to improve the grain connectivity.

    The scope of this thesis has been rather broad, dealing with studies of selected
    aspects in a variety of systems rather than focusing on exhaustive investigations of one single system. The thesis consists of a basic text and eight papers attached at the end. This division does not represent a difference in involvement of the different subjects, but merely that the topics described only in the text part have been studied so recently that they have not yet been written in a form suitable for publication. I recommend that text and papers are not read separately but that a paper is read the first time it is mentioned in the main text. Especially the information in papers 1,3,5 and 6 is a prerequisite to appreciate the content following in the main text.
    Original languageEnglish
    Place of PublicationRoskilde
    PublisherRisø National Laboratory
    Number of pages97
    ISBN (Print)87-550-2476-9
    ISBN (Electronic)87-550-2477-7
    Publication statusPublished - 2000
    SeriesDenmark. Forskningscenter Risoe. Risoe-R
    Number1086(EN)
    ISSN0106-2840

    Keywords

    • Risø-R-1086
    • Risø-R-1086(EN)

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