L. T. Wille

Florida Atlantic University, Boca Raton, Florida, United States

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Publications (102)138.34 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Using a realistic tight-binding Hamiltonian we have calculated the effective cluster interactions that describe the ordering of adsorbed Ir atoms on the Ir(111) surface. We have determined the relative stability of adatoms in "bulk" and "surface" sites and also mapped out the barrier between them. This information is then used as input in Monte Carlo simulations which allow us to determine typical diffusion behavior at various temperatures.
    Surface Review and Letters 01/2012; 04(05). · 0.28 Impact Factor
  • Luc T. Wille
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    ABSTRACT: Pattern recognition is arguably the critical first step in intelligence - be it natural or artificial. Science could not exist if humans were not able to spot regularities. Only subsequently do we analyze and classify them, and ultimately come up with underlying descriptions, some of which eventually make it to the lofty status of natural laws. Of course evolution did NOT ENDOw humans (or animals for that matter) with the ability to recognize patterns so we could build grandiose scientific edifices, rather we need that ability at the most elementary level. Humans simply cannot function in everyday life without pattern recognition capabilities and when those capabilities are impaired, as in patients with Alzheimer's disease for example, the result is nothing less than tragic.
    SIGACT News. 01/2004; 35:13-17.
  • S Ouannasser, L.T Wille, H Dreyssé
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    ABSTRACT: The cluster expansion model for the internal energy in terms of effective cluster interactions is employed to study the thermodynamic properties of two semi-infinite binary alloy systems: MoRe and MoW. A realistic tight-binding Hamiltonian is used with configurational averages computed by the direct method. This framework is applied to calculate the equilibrium atomic compositions of the (0 0 1) surface of MoxRe1−x and MoxW1−x systems for different bulk concentrations. In all cases it is found that the Mo component segregates strongly to the surface plane, followed by a Mo depletion in the second plane. The major role played in the segregation process by the relevant quantities, namely, point energies and pair interactions, is emphasized. These results are in good agreement with available experimental data.
    Surface Science 01/2004; · 1.87 Impact Factor
  • Luc T. Wille
    Annals of Biomedical Engineering 11/2003; 31(11):1441-1442. · 3.23 Impact Factor
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    ABSTRACT: We have studied the initial stages of Ni adsorption on Pt(111). Using the tight-binding direct-configurational-averaging method we calculate various ordering interactions. These parameters are used in turn in Monte Carlo simulations to elucidate the expected equilibrium phases for various temperatures and coverages. We find preferential adsorption in hcp sites at low coverage, with a cross-over to fcc growth at increasing deposition. In addition, coexistence of c(2×4) and c(2×6) phases is observed for a wide range of temperatures and concentrations.
    Surface Science - SURFACE SCI. 01/2003; 532:53-57.
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    ABSTRACT: Using singular spectrum analysis (SSA), we model the realized volatility and logarithmic standard deviations of two important futures return series. The realized volatility and logarithmic standard deviations are constructed following the methodology of Andersen et al. [J. Am. Stat. Ass. 96 (2001) 42–55] using intra-day transaction data. We find that SSA decomposes the volatility series quite well and effectively captures both the market trend (accounting for about 34–38% of the total variance in the series) and, more importantly, a number of underlying market periodicities. Reliable identification of any periodicities is extremely important for options pricing and risk management and we believe that SSA can be a useful addition to the financial practitioners’ toolbox.
    Physica A: Statistical Mechanics and its Applications 09/2002; · 1.68 Impact Factor
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    K. Ivanova, L. T. Wille
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    ABSTRACT: The dynamics of the S&P500 price signal is studied using a moving average technique. Particular attention is paid to intersections of two moving averages with different time horizons. The distributions of the slopes and angle between two moving averages at intersections is analyzed, as well as that of the waiting times between intersections. In addition, the distribution of maxima and minima in the moving average signal is investigated. In all cases, persistent patterns are observed in these probability measures and it is suggested that such variables be considered for better analysis and possible prediction of the trends of the signal. Comment: 17 pages, 9 figures; to be published in Physica A
    Physica A: Statistical Mechanics and its Applications 05/2002; · 1.68 Impact Factor
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    ABSTRACT: Thin film composed of magnetic clusters deposited on various substrates has attracted strong attention both as a new manufacturing technique to realize high density magnetic recording media [1] and to create systems with unique magnetic properties. Since the features of films are influenced by the cluster formation process during the flight path, the relevant physical scale to be studied is as large as centimeters. Moreover, since the behavior of segregation processes on the substrate is not negligible, the relevant smallest physical scale is of the order of 1 nm. In this paper a new model of cluster growth and deposition processes based on a combination of Direct Simulation Monte Carlo (DSMC) [2] and Monte Carlo (MC) methods is introduced to examine the effects of cluster deposition conditions on film formation by an adiabatic expansion process. Several types of size distributions of generated clusters under various conditions are obtained by the present model. The results of the simulations show that the size distribution is strongly related to the experimental conditions and in turn the resulting film properties are varied. [1] H. Mizuseki, M. Ishihara, X. Hu, Y. Kawazoe, and N. Ohta, IEEE Trans. Magn. (1996) 4335. [2] H. Mizuseki, Y. Jin, Y. Kawazoe, and Luc T. Wille, J. Appl. Phys., (2000) 6561.
    03/2002;
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    ABSTRACT: A cluster growth and deposition model based on hybrid modeling is introduced to examine the experimental conditions of the cluster growth process in vacuum chamber and deposition process on substrate. This hybrid model is a simulation method including physical length and time scale characteristics of macro and microscale. We simulated the behavior of the cluster during the flight path by direct simulation Monte Carlo (DSMC) method and the deposition behavior on the substrate by a simple MC model. Several size distributions of the clusters and various morphologies of deposited film were obtained, and the relationship between macroscopic and microscopic physical phenomena during deposition process was examined.
    Computational Materials Science 01/2002; 24:88-92. · 1.88 Impact Factor
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    ABSTRACT: We propose a hybrid model to simulate the cluster deposition process. This model consists of the direct simulation Monte Carlo and Monte Carlo methods that combine the macro with the nano scale. We investigate the effects of certain experimental parameters in cluster-formation process on the film morphology, including packing density. The model reveals that the morphology is strongly affected by the size distribution of the incident clusters. These results indicate the possibility of designing unique thin-film structures and properties.
    Journal of Crystal Growth 01/2002; 236(1–3):429 - 433. · 1.55 Impact Factor
  • P.S. Das, L.T. Wille
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    ABSTRACT: The results of finite element simulations of carbon nanotubes under pressure are presented and compared with atomistic studies. The dominant failure mode is shell buckling, characterized by diamond shaped bulges. Critical pressures are calculated for two types of boundary conditions.
    Computational Materials Science 01/2002; 24:159-162. · 1.88 Impact Factor
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    ABSTRACT: A functional scheme is described to parallelize computer simulations of grid-based ecological landscape models. The method is implemented using the Message Passing Interface protocol and is applied to the Everglades Landscape Vegetation Model. On a two-processor system, the speed-up is satisfactory and the overall performance of the program is competitive with traditional parallelization techniques such as geometrical decomposition. The method is discussed, timing information is provided for three different parallel machines, and some further developments are indicated.
    Ecological Modelling 10/2001; · 2.07 Impact Factor
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    Scripta Materialia 05/2001; 44(8):1911-1914. · 2.82 Impact Factor
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    H. Mizuseki, Y. Jin, Y. Kawazoe, L.T. Wille
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    ABSTRACT: Thin films obtained by cluster deposition have attracted strong attention both as a new manufacturing technique to realize high-density magnetic recording media and to create systems with unique magnetic properties. Because the film’s features are influenced by the cluster properties during the flight path, the relevant physical scale to be studied is as large as centimeters. In this paper, a new model of cluster growth processes based on a combination of the Direct Simulation Monte Carlo (DSMC) method and the cluster growth model is introduced to examine the effects of experimental conditions on cluster growth by an adiabatic expansion process. From the macroscopic viewpoint, we simulate the behavior of clusters and inert gas in the flight path under different experimental conditions. The internal energy of the cluster, which consists of rotational and vibrational energies, is limited by the binding energy which depends on the cluster size. These internal and binding energies are used as criteria of the cluster growth. The binding energy is estimated by surface and volume terms. Several types of size distribution of generated clusters under various conditions are obtained by the present model. The results of the present numerical simulations reveal that the size distribution is strongly related to the experimental conditions and can be controlled.
    Applied Physics A 01/2001; 73(6):731-735. · 1.69 Impact Factor
  • Article: End notes
    Luc T. Wille, George E. Dieter
    JOM: the journal of the Minerals, Metals & Materials Society 10/2000; 52(11):55-55. · 0.99 Impact Factor
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    ABSTRACT: The stability of Ir domains on Ir(111) is investigated theoretically by computing the relevant interaction energies. The study is based on a representation of the total energy in terms of a generalized Ising model containing effective cluster interactions which are determined from the electronic structure. A simple but realistic tight-binding Hamiltonian is used with configurational averages computed by a direct method. A transition from hcp (hexagonal close-packed) growth to fcc (face-centred cubic) growth as a function of adlayer concentration is observed and compact cluster formation is favoured, in precise agreement with experiment.
    Journal of Physics Condensed Matter 07/2000; 12(31):7005. · 2.22 Impact Factor
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    ABSTRACT: Film formation with deposited magnetic clusters has attracted strong attention as a new manufacturing technique to realize high-density magnetic recording media and to create materials with unique magnetic properties. Such clusters are typically obtained by adiabatic expansion of a metal vapor. It is therefore important to clarify the growth mechanism since this has a profound effect on the cluster magnetic moment. In this article a new simulation method based on a combination of Direct Simulation Monte Carlo (DSMC) and a cluster collision model is introduced to examine the effect of experimental conditions in cluster beam growth. We simulate the behavior of clusters and inert gas atoms in the flight path under different experimental conditions. In particular, we find a bimodal size distribution curve and a decreasing average moment as a function of flight path length. © 2000 American Institute of Physics.
    Journal of Applied Physics 04/2000; 87(9):6561-6563. · 2.21 Impact Factor
  • C. F. Cornwell, L. T. Wille
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    ABSTRACT: A new method is described that permits the efficient execution of parallel molecular dynamics simulations for irregular problems with several thousands of atoms on Single-Instruction Multiple-Data computers. The approach is based on a data-parallel atomic decomposition scheme and has overall time-complexity O( N ) , where N is the size of the system. The method has been implemented on a MasPar MP-1 computer and was found to obey theoretical timing estimates. The methodology has wide applicability and can be ported to other architectures in a straightforward manner.
    Computer Physics Communications 01/2000; 128:477-491. · 2.41 Impact Factor
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    L.T Wille
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    ABSTRACT: Low-energy configurations of atomic clusters held together by Lennard-Jones (LJ) interactions are determined by simulated annealing. Using a combination of quenches and annealings at various cooling rates the topology of the potential energy surface is elucidated and the probability of convergence to the global minimum is analyzed. An understanding of this phenomenon is relevant to glass formation, protein folding, and other problems with competing minima.
    Computational Materials Science 01/2000; · 1.88 Impact Factor
  • L.T. Wille, H. Dreyssé
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    ABSTRACT: The problem of epitaxial growth in binary metallic systems is reviewed and related to microscopic driving forces. A formal connection between these quantities and surface segregation observable is established. This methodology is applied to two systems of great current interest: Ag–Rh and Fe–Cr. The effects of the morphology on the magnetic properties of these two systems are also discussed.
    Computational Materials Science 06/1999; · 1.88 Impact Factor

Publication Stats

907 Citations
138.34 Total Impact Points

Institutions

  • 1988–2012
    • Florida Atlantic University
      • • Department of Physics
      • • Department of Ocean and Mechanical Engineering
      Boca Raton, Florida, United States
    • University of California, Berkeley
      • Department of Materials Science and Engineering
      Berkeley, MO, United States
  • 1998
    • University of New South Wales
      • School of Physics
      Kensington, New South Wales, Australia
  • 1995
    • University of Strasbourg
      Strasburg, Alsace, France
  • 1985
    • Ghent University
      Gand, Flanders, Belgium