Physics

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Author: search.filters.author.Bartelt, N. C.

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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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    Diffusion of Monolayer Adatom and Vacancy Clusters: Langevin Analysis and Monte Carlo Simulations of their Brownian Motion
    (American Physical Society, 1995) Khare, S. V.; Einstein, Theodore L.; Bartelt, N. C.
    In recent observations of Brownian motion of islands of adsorbed atoms and of vacancies with mean radius R, the cluster diffusion constant varies as R1 and R2. From an analytical Langevin description of the cluster's steplike boundary, we find three cases, R1, R2, and R3, corresponding to the three microscopic surface mass-transport mechanisms of straight steps. We thereby provide a unified treatment of the dynamics of steps and of clusters. For corroboration, we perform Monte Carlo simulations of simple lattice gases and derive atomistic diffusion constants.
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    Dynamics of step doubling: simulations for a simple model and comparison with experiment
    (Elsevier, 1995) Khare, S. V.; Einstein, Theodore L.; Bartelt, N. C.
    To interpret recent experiments on the dynamics of step doubling, we have studied a simple model of this phase transition. With Monte Carlo, we compute the time-dependence of the order parameter in the limit of rapid diffusion across terraces. Analysis of the data shows that the limiting step is the time for adjacent steps to touch each other; subsequent "zipping" together happens rapidly. From this vantage we develop an analytic expression for short times that changes into a phenomenological one for later times. Using data from two physical systems, we compare this function and another based on naive assumptions with a third based on chemical rate theory. For the more recent data, our expression describes the data best. Finally, in the opposite limit in which atoms can only move along step edges, we show characteristic configurations and compute the structure factor.
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    Distribution of terrace widths on a vicinal surface within the one-dimensional free-fermion model
    (American Physical Society, 1991) Joos, B.; Einstein, Theodore L.; Bartelt, N. C.
    For the terrace-step-kink model of a stepped surface, the distribution P(L) of terrace widths L is calculated at low temperature by mapping the problem onto the one-dimensional free-fermion model. In this approximation, the only energetic interaction between steps is a hard-core repulsion. A skewed distribution with a parabolic rise and a Gaussian tail is found; the exact asymptotic forms are displayed. By plotting ?L?P(L) vs L/?L?, we obtain a ‘‘universal’’ curve nearly independent of the average terrace width ?L?. With use of this scaling property, analytic approximants are constructed and the role of correlations discussed. We present some results for steps with energetic interactions in two special cases.
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    First-order transitions between surface phases with different step structures
    (American Physical Society, 1991) Bartelt, N. C.; Einstein, Theodore L.; Rottman, Craig
    A Comment on the Letter by Alerhand et al. Phys. Rev. Lett. 64, 2406 (1990).
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    Step doubling and related transitions on vicinal surfaces
    (AIP, 1992) Einstein, Theodore L.; Jung, T. M.; Bartelt, N. C.; Williams, Ellen D.; Rottman, Craig
    We discuss two types of step?height doubling transitions on vicinal surfaces. In one type, exemplified by Ge(111)(121), a phase transition can occur since the symmetry between alternate terraces is broken at low temperatures. There is evidence in both experiment and in Monte Carlo simulations that this transition can have Ising character. In the second type, exemplified by Si(001)(110), one subset of terraces is always favored, so that no symmetry is broken. If an actual phase transition occurs (rather than a smooth crossover), it is expected to be first order, implying a coexistence region in a temperature?misorientation phase diagram. Finally, the coalescence of steps into steps of height more than two atomic layers is briefly considered.
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    Brownian motion of steps on Si(111)
    (American Physical Society, 1993) Bartelt, N. C.; Goldberg, J. L.; Einstein, Theodore L.; Williams, Ellen D.; Heyraud, J. C.; Metois, J. J.
    Step motion on surfaces can now be measured quantitatively. We present a formalism for analyzing equilibrium step fluctuations and apply it to real-time reflection electron microscope observations of step motion on Si(111). The time correlation functions of the step positions and of their Fourier components are compared with predictions from Langevin equations for two extreme mechanisms for step motion: edge diffusion and terrace exchange.
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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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    Terrace-width distributions on vicinal Si(111)
    (American Physical Society, 1991) Wang, X.-S.; Goldberg, J. L.; Bartelt, N. C.; Einstein, Theodore L.; Williams, Ellen D.
    Using scanning tunneling microscopy, we have quantitatively characterized the configurations of steps on vicinal Si(111) surfaces misoriented by 1.2 degrees and 2.3 degrees towards the (1- 1- 2) direction. The measured terrace-width distributions are strongly peaked, consistent with predictions for thermally wandering steps. However, the distributions are much narrower than predicted for the simple terrace-step-kink model, indicating that the steps interact with energetic short-range repulsions. The magnitude of this energetic repulsion is gauged from a Gaussian fit to the data. The width of the distribution scales with step density as expected for repulsions which decay as the inverse square of step separation.