TcSUH
Bi-Weekly Seminar
Nanoscale Modulations in Optimally Doped and Underdoped YBCO: X-ray Evidence and Theory for Atomic Displacements in the Higher-Order Ortho Phases
Date: Friday April 30, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
High intensity x-ray measurements at the undulator line of the Advanced Photon Source have shown a distinct pattern of diffuse satellites about the normal Bragg peaks which possess a characteristic asymmetry and have a correlation range of only 4-5 unit cells in all directions. Measurements made at 7K show an intensity distribution about the Bragg peaks which strongly resemble those found in totally disordered tetragonal YBCO doped with Al, indicating substantial disorder within these modulated regions. An intensity ?t for the optimally doped crystal reveals a pattern of modulations along the a-axis with a strong c-axis component of the Ba atoms. In the underdoped crystal, an ab initio calculation of the relaxed atoms in the modulated Ortho-V phase gives nearly perfect agreement with former experiments, leading to the unavoidable conclusion that these crystals show essentially diffusion-limited formation of imperfect Ortho phases predicted some years ago by de Fontaine and co-workers. They are thus of static origin, and have little to do with stripes, CDW’s, or soft optic phonons.
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Bi-Weekly Seminar
Higher Jc at Reduced Pinning Potential
Date: Friday April 16, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In a decade of study it has been frequently noted that continuous columnar pinning centers provide the largest pinning potential, Upin. It has been assumed therefore that such pinning, e.g., created by ions, provides the highest values of Jc, and pinned field, Bpin. Generally, studies of columnar pinning are discontinued at pinning center densities beyond which Jc is observed to decrease. It has also been broadly noted that such decrease is due to degradation of the order parameter and the critical current, Tc. We previously reported theoretical findings that Jc and Bpin were limited by the pinning center microstructure, not Tc, and could be markedly improved by reducing the damage incurred in the formation of pinning centers, even though Upin is reduced in the process. We now report experimental results. Spatially discontinuous melting occurs for ions having Se dE/dx 0.7 ke. This becomes continuous for Se 3.5 ke. We studied the interval 1.4 Se < 4.0 ke, using parallel ions (no splay). We find Jc is higher by a factor of 6 at Se ~ 2.0 ke (intermittent columns) than at Se ~ 3.5 ke (continuous columns), despite the fact that Upin is reduced by ~ 75% at the lower Se. In addition, pinnable field is increased by factors > 10 at the lower Se. Future basic studies and application to current problems of interest will be discussed.
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Bi-Weekly Seminar
Oxygen Ion Conductivity and Electronic Conductivity in the Bi-Ru-Alkaline Earth Oxides
Date: Friday March 26, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Bismuth-alkaline earth oxides include a rhombohedral solid solution that is a very good oxygen ion conductor perpendicular to the c-axis at relatively low temperatures (1 S cm-1 at 600-700°C). There is complete mutual on-site solubility between Ba, Sr, and Ca. The Bi content ranges from <10 to 30 cation %. The conductivity mechanism, the variation of conductivity and other properties of the oxide with temperature, and the phase relations of this phase will be discussed. The rhombohedral oxide dissolves many materials potentially used as electrodes. We have synthesized a new rhombohedral oxide saturated in Ru. It has the same oxide conductivity as the Ru-free phase. Phase relations in the Bi-Ru-Sr-Ba-O system will be discussed. The rhombohedral phase is in equilibrium with an alkaline earth ruthenate (not ruthenium oxide) under oxidizing conditions. Alternative electrode materials include Bi-Ru pyrochlores.
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Bi-Weekly Seminar
Rutherford Scattering of Identical Ions-A Test of Quantum Mechanics at MeV Energies with Massive Particles
by: Ki B. Ma
Date: Friday February 20, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Elastic Rutherford scattering between positively charged ions is mediated by the repulsive Coulomb potential. Its differential cross section according to classical mechanics comes out to be the same as the exact quantum mechanical result. Its characteristic 1/E2 energy dependence and cosec4(/2) angular dependence have been well established since 1911. Not predicted by Rutherford then, and missed in Goldberger and Watson2 even as late as 1964, these results break down when the scattering ions are identical, and oscillations in the differential cross section as a function of the scattering angle appear instead of the familiar smooth dependence. This behavior is thoroughly discussed in Landau and Liftshiz3 (1965), but the oscillations were summarily declared to vanish “on averaging over even a small range of values of. This oscillatory behavior is a hallmark of quantum mechanical interference. Dictated by the exchange symmetry of the scattering particles and without a classical counterpart, it constitutes a test for the validity of quantum mechanics versus classical mechanics, a test that can be conducted using massive particles at MeV energies, at room temperature. Here, we report preliminary results on the observation of quantum interference in the Rutherford scattering of carbon from carbon at incident energies between 1 and 4 MeV.
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Bi-Weekly Seminar
Interface and Processing Effects on Oxide Thin Film Epitaxy
Date: Friday November 21, 2003
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Understanding the epitaxial mechanisms, interface structures, and processing condition effects has become a technically important issue in oxide thin film epitaxy and device fabrication. We have systematically investigated the effects from strains, substrate surface step terraces, misfit dislocations, and processing partial pressures and temperatures on the epitaxial behavior and physical properties of various as-grown ferroelectric (Ba,Sr)TiO3, ferromagnetic (La,Ca)MnO3, and highly ionic conductive oxide Gd:CeO2 thin films. Many interesting phenomena have been observed in these systems. For instance, misfit edge dislocation spacings in the ferroelectric Ba0.6Sr0.4TiO3 thin films on (001) LaAlO3 can be adjusted by processing oxygen partial pressures. Strain energy in the highly ionic conductive Gd:CeO2 thin films on (001) LaAlO3 can be released by unique directionally aligned nanoparticles. Models of the surface-step-terrace that induce the formations of conservative and non-conservative nano domain boundary structures have been developed to understand the microstructure-property relationships. Details will be presented in the talk.
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