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Theoretical study of the structure, energetics, and dynamics of silicon and carbon systems using tight-binding approaches

Description: Semiempirical interatomic potentials are developed for silicon and carbon by modeling the total energy of the system using tight-binding approaches. The parameters of the models were obtained by fitting to results from accurate first-principles Local Density Functional calculations. Applications to the computation of phonons as a function of volume for diamond-structured silicon and carbon and the thermal expansions for silicon and diamond yields results which agree well with experiment. The ph… more
Date: October 25, 1991
Creator: Xu, Chunhui.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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Laser diagnostics of H sup minus formation in a magnetic multicusp ion source

Description: The populations of ground electronic state atomic hydrogen and ground electronic state, vibrationally-rotationally excited hydrogen molecules in a negative hydrogen ion source discharge have been measured using vacuum ultraviolet (VUV) laser absorption spectroscopy. Vibrational states up to v=8 and rotational levels as high as J=15 have been measured. The measurements have been made under a range of discharge conditions. The complete vibrational population distribution for v=1-8, J=1 has been o… more
Date: May 1, 1991
Creator: Young, A. T.; Chen, P.; Kunkel, W. B.; Leung, K. N.; Li, C. Y. & Stutzin, G. C.
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Partner: UNT Libraries Government Documents Department
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Surface, interface and thin-film magnetism

Description: In the last quarter of the 20th century, with the information revolution and the ever growing need to acquire, store, and retrieve information, the science and technologies attached to magnetic recording have experienced an explosive growth. Central to those pursuits has been the materials science of magnetism as it applies to surfaces, interfaces, and thin films. This report discusses topics on thin-film magnetism such as: theory, physical effects, prospects, opportunities and future developme… more
Date: July 1, 1991
Creator: Falicov, L.M. (Lawrence Berkeley Lab., CA (United States) California Univ., Berkeley, CA (United States). Dept. of Physics)
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Partner: UNT Libraries Government Documents Department
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Spectral weight changes at the superconducting transition of Bi sub 2 Sr sub 2 CaCu sub 2 O sub 8+. delta

Description: An overview of our gap studies in high-{Tc} superconductors is presented for the workshop on Fermiology of high-{Tc}'s. The work is centered on the study of single crystal Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}. In a conventional BCS superconductor, a superconducting gap {Delta} is formed when the near Fermi edge electrons condense to form Cooper pairs at low temperatures. As the material goes superconducting the density of states is modified such that the spectral intensity in the regio… more
Date: January 1, 1991
Creator: Dessau, D. S.; Shen, Z. X.; Wells, B. O.; Spicer, W. E. (Stanford Univ., CA (United States). Stanford Electronics Labs.) & Arko, A. J. (Los Alamos National Lab., NM (United States))
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Partner: UNT Libraries Government Documents Department
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Binding and catalytic reduction of NO by transition metal aluminosilicates

Description: The objective of this research is to provide the scientific understanding of processes that actively and selectively reduce NO in dilute exhaust streams, as well as in concentrated streams, to N{sub 2}. Experimental studies of NO chemistry in transition metal-containing aluminosilicate catalysts are being carried out with the aim of determining the chemical rules for NO reduction on non-precious metals. The catalyst supports chosen for this investigation are A and Y zeolites, mordenite, and mon… more
Date: September 1, 1991
Creator: Klier, Kamil; Herman, Richard G. & Hou, Shaolie
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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Chemical activation of molecules by metals: Experimental studies of electron distributions and bonding

Description: The formal relationship between measured molecular ionization energies and thermodynamic bond dissociation energies has been developed into a single equation which unifies the treatment of covalent bonds, ionic bonds, and partially ionic bonds. This relationship has been used to clarify the fundamental thermodynamic information relating to metal-hydrogen, metal-alkyl, and metal-metal bond energies. We have been able to obtain a direct observation and measurement of the stabilization energy prov… more
Date: October 1991
Creator: Lichtenberger, Dennis L.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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First-principles calculations: The elemental transition metals and their compounds

Description: If done with sufficient care, present day a priori theory yields calculated enthalpies of formation whose agreement with experiment (when such data is available) is of the order of the experimental scatter. Comparisons will be made for the Pt-Ti systems for which such data exist and for which one crystal structure involves atomics sites of low symmetry. Two other cases will be considered for which there is no direct experimental heats data. The first of these will be the structural stabilities … more
Date: January 1, 1991
Creator: Watson, R. E.; Fernando, G. W.; Weinert, M. & Davenport, J. W.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Cluster expansion of fcc Pd-V intermetallics

Description: A cluster expansion is used to compute fcc ground states from first principles for the Pd-V system. Intermetallic structures are not assumed but derived rigorously by minimizing the configurational energy subject to linear constraints. A large number of concentration-independent interactions are calculated by the method of direct configurational averaging. Agreement with the fcc-based portion of the experimentally-determined Pd-V phase diagram is quite satisfactory. 25 refs., 2 figs.
Date: June 1, 1991
Creator: de Fontaine, D.; Wolverton, C.; Ceder, G. (Lawrence Berkeley Lab., CA (United States)) & Dreysse, H. (Nancy-1 Univ., 54 (France). Lab. de Physique du Solide)
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Ab initio cluster studies of La₂CuO₄

Description: In this paper we examine the properties of small cluster models of La{sub 2}CuO{sub 4}. In Section 2, the Madelung/Pauli background potential used to imbed the primary cluster and the basis sets used to expand the cluster wavefunction are discussed. Section 3 presents the results of calculations on CuO{sub 6} in which the optical absorption and the photoemission spectrum are examined. The calculation on CuO{sub 6} and our earlier work on larger clusters suggest that a single-band Pariser-Parr-P… more
Date: 1991~
Creator: Martin, Richard L.
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Partner: UNT Libraries Government Documents Department
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An outline of positron measurements of superconducting oxides

Description: Positron measurements on superconducting oxides have gone through an evolution from divergent results of low statistical precision on samples of suspect quality to convergent results of higher statistical precision on high quality samples. We outline the elements affecting the progress of these experiments and questions that can be addressed at our present state of the art. 11 refs.
Date: March 1, 1991
Creator: Howell, R.H. & Fluss, M.J.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Energy functionals for inhomogeneous many-electron systems

Description: The primary purpose of these lectures is to provide a pedagogical introduction Density Functional Theory (DF). We begin with the very early picture of Thomas and Fermi for the ground state energy of a neutral atom and proceed to the highly successful method of Kohn and Sham (KS) which focuses attention on the exchange-correlation contribution E{sub XC}(n) to the energy functional. We stress the importance of a systematic approach to the study of the subtle nonlocal structure of E{sub XC}(n) in … more
Date: October 1, 1991
Creator: Geldart, D. J. W. & Rasolt, M.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Radiation and photochemistry section

Description: The highlights of this past year in the Radiation and Photochemistry Section at Argonne include: (1) picosecond optical studies of radical cations and excited states produced in hydrocarbon radiolysis provided the first kinetic measurements of ion transformation and production of triplet and singlet excited states by ion recombination. (2) studies of radical cations of alkyl-substituted amines and sulfides provided insights into ion-molecule reactions of radical cations in the condensed phase. … more
Date: January 1, 1991
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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The t-J model at small t/j: Numerical, perturbative, and supersymmetric results

Description: We discuss some recent results for one- and two-hole states in the t-J model at small t/J. These include numerical results (bandwidth determinations and accurate t/J values for 4 {times} 4 lattice one-hole ground-state level crossings), hopping-parameter perturbation theory (which gives the small-t/J one-hole bandwidth in terms of the static-vacancy ground state), and results at the supersymmetric point t/J = 1/2 (exact results for energies and bandwidths.) The perturbative results leads us to … more
Date: February 1, 1991
Creator: Barnes, T.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Ab initio quantum chemistry in parallel-portable tools and applications

Description: In common with many of the computational sciences, ab initio chemistry faces computational constraints to which a partial solution is offered by the prospect of highly parallel computers. Ab initio codes are large and complex (O(10{sup 5}) lines of FORTRAN), representing a significant investment of communal effort. The often conflicting requirements of portability and efficiency have been successfully resolved on vector computers by reliance on matrix oriented kernels. This proves inadequate ev… more
Date: January 1, 1991
Creator: Harrison, R.J.; Shepard, R. (Argonne National Lab., IL (United States)) & Kendall, R.A. (Battelle Pacific Northwest Lab., Richland, WA (United States))
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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Potential energy surfaces and reaction dynamics of polyatomic molecules

Description: A simple empirical valence bond (EVB) model approach is suggested for constructing global potential energy surfaces for reactions of polyatomic molecular systems. This approach produces smooth and continuous potential surfaces which can be directly utilized in a dynamical study. Two types of reactions are of special interest, the unimolecular dissociation and the unimolecular isomerization. For the first type, the molecular dissociation dynamics of formaldehyde on the ground electronic surface … more
Date: November 1, 1991
Creator: Chang, Yan-Tyng.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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Positron 2D-ACAR experiments and electron-positron momentum density in YBa sub 2 Cu sub 3 O sub 7-x

Description: We discuss positron annihilation (2D-ACAR) measurements in the C- projection on an untwinned metallic single crystal of YBa{sub 2}Cu{sub 3}O{sub 7-x} as a function of temperature, for five temperatures ranging from 30K to 300K. The measured 2D-ACAR intensities are interpreted in terms of the electron-positron momentum density obtained within the KKR-band theory framework. The temperature dependence of the 2D-ACAR spectra is used to extract a background corrected'' experimental spectrum which is… more
Date: December 1, 1991
Creator: Smedskjaer, L. C.; Welp, U.; Fang, Y.; Bailey, K. G. & Bansil, A.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Probing fine magnetic particles with neutron scattering

Description: Because thermal neutrons are scattered both by nuclei and by unpaired electrons, they provide an ideal probe for studying the atomic and magnetic structures of fine-grained magnetic materials, including nanocrystalline solids, thin epitaxial layers, and colloidal suspensions of magnetic particles, known as ferrofluids. Diffraction, surface reflection, and small angle neutron scattering (SANS) are the techniques used. With the exception of surface reflection, these methods are described in this … more
Date: January 1, 1991
Creator: Pynn, R.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Ground state searches in fcc intermetallics

Description: A cluster expansion is used to predict the fcc ground states, i.e., the stable phases at zero Kelvin as a function of composition, for alloy systems. The intermetallic structures are not assumed, but derived regorously by minimizing the configurational energy subject to linear constraints. This ground state search includes pair and multiplet interactions which spatially extend to fourth nearest neighbor. A large number of these concentration-independent interactions are computed by the method o… more
Date: December 1, 1991
Creator: Wolverton, C.; de Fontaine, D. (Lawrence Berkeley Lab., CA (United States)); Ceder, G. (Massachusetts Inst. of Tech., Cambridge, MA (United States)) & Dreysse, H. (Nancy-1 Univ., 54 (France). Lab. de Physique du Solide)
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Partner: UNT Libraries Government Documents Department
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Fermi surface of YBCO by DHVA

Description: These proceedings demonstrate how far scientist have come in the last four years of high temperature superconductivity. Knowledge of the energy bands and Fermi surfaces from experiment has come rather late. Photoemission, first showed proof of the validity of the energy band calculations. Positron annihilation, presented by West, after a rough start, is now giving evidence of the Fermi surface. Both of these techniques involve electronic excitations and hence, although they show the Fermi surfa… more
Date: January 1, 1991
Creator: Smith, J. L.; Fowler, C. M.; Freeman, B. L.; Hults, W. L.; King, J. C. & Mueller, F. M.
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Partner: UNT Libraries Government Documents Department
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Quasi-molecular processes in dense plasmas

Description: Quasi-molecular phenomena occur in dense plasmas when the interatomic spacing is comparable to the characteristic wavelength of the electrons. If the electronic states are bound, covalent orbitals arise with different excitation energies, radiative rates, and collisional rates than for isolated ions. For continuum electrons, charge localization near transient clusters of nuclei can influence many scattering and transport processes. We identify several novel consequences of quasi-molecular pheno… more
Date: January 1, 1991
Creator: Younger, S.M.
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Partner: UNT Libraries Government Documents Department
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Direct, experimental evidence of the Fermi surface in YBa sub 2 Cu sub 3 O sub 7-x

Description: We report new measurements of the electron-positron momentum spectra of YBa{sub 2}Cu{sub 3}O{sub 7-x} performed with ultra-high statistical precision. These data differ from previous results in two significant respects: They show the D{sub 2} symmetry appropriate for untwinned crystals and, more importantly, they show unmistakable, statistically significant, discontinuities that are evidence of a major Fermi surface section. These results provide a partial answer to a question of special signif… more
Date: April 29, 1991
Creator: Haghighi, H.; Kaiser, J. H.; Rayner, S. L.; West, R. N.; Liu, J. Z.; Shelton, R. et al.
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Partner: UNT Libraries Government Documents Department
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Theoretical studies on the electronic structure and properties of complex ceramic crystals and glasses

Description: This progress report summarizes the research activities for the period July 1, 1990--June 30, 1991, the first year of a three-year renewal. The major accomplishments for the current year are: study of electronic structures and optical properties of several important ceramic crystals such as Y{sub 2}O{sub 3}, Al{sub 2}O{sub 3}, MgO, MgAl{sub 2}O{sub 4}, ZrO{sub 2},V{sub 2}O{sub 3}, V{sub 2}O{sub 5} and all polycrystalline forms of SiO{sub 2}; theoretical study on superconducting oxides; first-pr… more
Date: January 10, 1991
Creator: Ching, Wai-Yim.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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A two-dimensional ACAR study of untwinned YBa sub 2 Cu sub 3 O sub 7-x

Description: We have carried out 2D-ACAR measurements on an untwinned single crystal of YBa{sub 2}Cu{sub 3}O{sub 7-x} as a function of temperature, for five temperatures ranging from 30K to 300K. We show that these temperature dependent 2D-ACAR spectra can be described to a good approximation as a superposition of two temperature independent spectra with temperature dependent weighting factors. We show further how the data can be used to correct for the background'' in the experimental spectrum. Such a back… more
Date: December 1, 1991
Creator: Smedskjaer, L.C. (Argonne National Lab., IL (United States)) & Bansil, A. (Northwestern Univ., Evanston, IL (United States). Dept. of Physics)
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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