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Post-annealed thin films comprised of mixed YBa2Cu3O7-x (123) and Y2Ba4Cu8O16 (248) phases with both faulted and unfaulted microstructures have been characterized by planar and cross-section transmission electron microscopy. The influence of 248-type faults on the 123 structure, possible mechanisms for the higher Tc's of faulted films, and observations of a fourfold ordered structure are discussed.
Part of the work described below was delivered as an invited talk by one of us (A.K.) at the special symposium to honour Dr. Ben Abeles for his 65th birthday. During the questions part of my talk, I was asked by Ben Abeles: ‘what about magnetoresistance…’. I answered that this is a complicated issue and I don't have time to discuss this matter. After my talk Ben explained that he always asks this question because his thesis was on magnetoresistance measurements. I remembered this question and decided that this paper would be an excellent place give to the answer. Hence section IV.2 is specialy dedicated to Dr. Ben Abeles.
We report optical studies of predominantly c-axis oriented high-Tc superconducting films before and after ion milling. A significant increase in midinfrared reflectivity resulted from ion milling of a few thousand Å Features associated with α-axis texture vanished after ion milling, resulting in pure c-axis films. This suggests that α-axis grains reside mostly near the surface in post-annealed high-Tc superconducting films. Therefore, even after annealing (for the purpose of raising Tc), highly oriented c-axis films can be retained by ion milling. This may be important for device applications where optically smooth surfaces and interfaces are to be routinely prepared.
The near-EF electronic structure and Fermi surface of Bi2212 has been mapped out with ARPES. A key feature of our measured bandstructure is the existence of an extended region of flat CuO2-derived bands at EF. Comparative analysis of this data with that from NdCeCuO and YBCO7 suggests that many of the anomalous (normal) physical properties of Bi2212 and YBCO7 (NdCeCuO) may be related to the existence (absence) of such bands at EF. Superconducting gap anisotropy at least an order of magnitude larger than that of the conventional superconductors has been observed in the a-b plane of Bi2212 in ARPES. For samples with Tc of 88K, the gap size reaches a maximum of approximately 20 meV along the Cu-O bond direction, and a minimum of much smaller or vanishing magnitude 45° away. The experimental data is discussed within the context of various theoretical models. In particular, a detailed comparison with what is expected from a superconductor with a dx2-y2 order parameter is carried out, yielding a consistent picture.
The relationship between the magnetic and crystalline microstructure of SrRu03 thin films is analyzed using transmission electron microscopy. Regions with a stripe magnetic domain structure in different orientations are observed in Lorentz imaging mode when the specimens are zero-field-cooled through the ferromagnetic transition temperature, Tc ≈ 150K. The different orientations of the stripe regions correspond to different crystallographic domains as determined by electron diffraction and magnetic image contrast; all of the six possible orientations of the orthorhombic SrRuO3 structure grown epitaxially on a SrTiO3 cubic substrate are identified. The results show that the uniaxial anisotropy indicated for these multi(crystal)domain films is the same as that determined for single crystal films by bulk magnetization measurements, and is therefore primarily magnetocrystalline in nature.
A transmission electron microscopy study of a post-annealed YBa2Cu3O7−x thin film shows that extra Cu–O planes within the structure can aggregate as stacking faults to form a defect microstructure rather than forming the well-ordered Y2Ba4Cu8O16 phase. Interaction of the stacking faults with the surrounding matrix results in strain effects and microstructural variations which may hinder ordering as well as influencing superconducting properties if occurring in higher concentration. When viewed normal to the plane of the film, the boundaries of the stacking faults can be imaged as dislocation-like defects, indicating the size and shape of the stacking faults and their relationship to other defects such as twins and second phase precipitates.
The successful synthesis of high-Tc YBa2Cu3O7 films by means of electron-beam codeposition are reported. Several important growth parameters have been surveyed in a preliminary way. The substrates investigated include Al2O3, ZrO2, MgO, and SrTiO3, The films were characterized by resistivity measurements, x-ray diffraction, microprobe, and Rutherford backscattering analysis. Some TEM and critical current density studies were also carried out. The best results to date have been obtained on SrTiO3 substrates with which polycrystalline epitaxial growth has been achieved. Resistive superconducting transitions with zero resistance at 89.5 K and a 2 K width have been observed in these films.
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