Physics

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Start date: 1980End date: 1989

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    Triangular lattice gas with first- and second-neighbor exclusions: Continuous transition in the four-state Potts universality class
    (American Physical Society, 1984) Bartelt, N. C.; Einstein, Theodore L.
    Using phenomenological renormalization (transfer-matrix scaling), we have reexamined the phase transition of a triangular lattice gas with particles having both nearest- and second-nearest-neighbor exclusions. Widely accepted classical studies indicated that disordering of the ordered (p(2x2)) state is first order. In contradiction, we show that the transition is second order; its exponents are consistent with the four-state Potts model universality class, in accord with its Landau-Ginzburg-Wilson Hamiltonian classification.
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    Optimization of data end points and taper width in extended absorption fine-structure analysis
    (American Physical Society, 1984) Hershfield, Selman P.; Einstein, Theodore L.
    The dependence of deduced interatomic spacing and inner potential on the lower and upper limits of integration, k1 and k2, as well as the taper width D, was studied for extended absorption fine structure using sinusoidal model data. The optimal values are achieved when 2(k1+k2)R or 2(k2?k1?D)R equals an odd half-integral multiple of pi (rather than when 2k1R and 2k2R are integral multiples of pi), where R is the spacing. Analytic approximations are used to elucidate numerical computations. Optimization of D is also discussed.
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    Critical Exponents of a Four-State Potts Chemisorbed Overlayer: p(2x2) Oxygen on Ni(111)
    (American Physical Society, 1981) Roelofs, L. D.; Kortan, A. R.; Einstein, Theodore L.; Park, Robert L.
    We report the first determination of critical exponents for a chemisorbed overlayer, using low-energy electron diffraction. We examine the order-disorder transition of p(2x2) oxygen on the (111) surface of nickel. This study is the first of critical behavior of a two-dimensional system in the four-state Potts universality class. Discussion of disparity between our results and predicted exponents considers several possibilities, including logarithmic corrections.
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    Extended Appearance-Potential Fine-Structure Analysis: Oxygen on Al(100)
    (American Physical Society, 1980) Laramore, G.E.; Einstein, Theodore L.; Roelofs, L. D.; Park, Robert L.
    To measure O-Al separation at Al(100) surfaces disordered (low-energy electron-diffraction beams extinguished) by reaction with oxygen, the extended appearance-potential fine structure was analyzed above the threshold for electron-bombardment excitation of the O 1s core. Calculation shows that the outgoing electron has angular momentum l=0, allowing simple Fourier inversion of the fine structure. The separation, 1.9+-0.05 A, suggests that oxygen lies under the top layer, a result undetectable in extended-x-ray-absorption fine-structure measurements on thicker films.
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    Effect of the central atom potential on the extended fine structure above appearance potential thresholds
    (American Physical Society, 1980) Laramore, G.E.; Einstein, Theodore L.; Roelofs, L. D.; Park, Robert L.
    The formalism previously given for describing the extended fine structure above appearance-potential-spectroscopy (APS) thresholds is extended by incorporating the effects of the excited "central" atom potential in an exact manner. The excitation-matrix elements are expressed in terms of the exact wave functions of the central atom potential. This introduces a "phase renormalization" into the excitation-matrix elements and eliminates a previously noted "phase difference" between single- and multiple-scattering calculations employing a plane-wave basis set. A series of approximations is then made which leads to an expression for the APS extended fine structure in terms of sinusoidal functions and hence provides a rationale for a Fourier-transform analysis. Simple model calculations assuming a constant "bare" excitation-matrix element, a spherically symmetric electronic density of states, and only S-wave scattering from the atomic cores are performed for a cluster of atoms having the atomic geometry of bulk vanadium. These calculations display the major predictions of the formalism and indicate that for a given system there may be some optimal energy range for data analysis. The problem of electron characteristic losses is considered, and it is pointed out that in the small-momentum-transfer limit the simple dipole selection rules appropriate to a photon-excitation process again apply. This may obviate many of the problems introduced by multiple angular momentum final states in the APS process.
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    Electron-induced extended-fine-structure measurements of thin-film growth and reaction
    (American Physical Society, 1987) Idzerda, Y. U.; Williams, Ellen D.; Einstein, Theodore L.; Park, R. L.
    The application of two electron-beam-induced extended-fine-structure (EFS) techniques (surface extended energy-loss fine structure (SEELFS) and extended appearance-potential fine structure (EAPFS)) to the study of thin films has been demonstrated by measurements on three well-characterized compositional phases of titanium deposited on Si(111).
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    Proposed decorated lattice-gas model of H/Pd(100)
    (American Physical Society, 1987) Bartelt, N. C.; Einstein, Theodore L.
    A Comment on the Letter by D. Tomanek, S. G. Louie, and C.-T. Chan, Phys. Rev. Lett. 57, 2594 (1986); 58, 287(E) (1987).
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    Angular momentum branching ratios for electron-induced ionization: Atomic and model calculations
    (American Physical Society, 1987) Mehl, M. J.; Einstein, Theodore L.
    We present calculations of the matrix elements for electron-induced ionization of core electrons of atoms. We use both self-consistent atomic potentials for accuracy and model potentials to gain physical insight. We pay particular attention to the angular momentum distribution of the two final-state electrons, especially when one of them lies near what would be the Fermi energy in a solid (i.e., as in an absorption fine-structure experiment). For nodeless core wave functions, in the dominant channel both final-state electrons have angular momentum one greater than that of the initial core state. For sufficiently deeply bound states, this first approximate selection rule holds until the incident electron energy exceeds the ionization threshold by at least 500 eV, i.e., over the experimentally relevant range. It is also possible to determine the angular momentum distribution of the final-state electron. The EXAFS-like electron tends to have angular momentum one greater than that of the initial core state, even in some cases where the first approximate selection rule does not hold. (EXAFS is extended x-ray-absorption fine structure.) The strongest trend is that the dipole component in a partial-wave expansion of the Coulomb interaction dominates the matrix element. In these studies, careful treatment of not just the core state but also the unbound states is crucial; we show that the conventional orthogonalized plane-wave approximation is inadequate, giving incorrect ordering of the channels. For model potentials with an adjustable screening length, low-lying bound resonances are found to play an important role.