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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 8-12 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Number or density autocorrelation functions can sometimes be obtained in terms of electron emission current autocorrelation functions if the current fluctuations are related to the number fluctuations. This is the case in field emission where the fluctuations in adsorbate number in a small probed region, caused by surface diffusion, are essentially proportional to the field emission current fluctuations. The autocorrelation function of the current fluctuations then mirrors that of the adsorbate number fluctuations. When correlation intervals are very short they may contain an average number of electrons so small that the rms electron fluctuation resulting from adsorbate number fluctuations is less than 1 e− and thus shows up only infrequently. The electron fluctuation can then "miss'' most adsorbate fluctuations. We have examined this problem by Monte Carlo modeling and confirm the existence of this effect when the electron fluctuation number is truncated to integer values. Even in this case the electron fluctuation correlation images the adsorbate number correlation function quite well to values of rms electron fluctuations as low as 0.4 e− per correlation interval. When shot noise is included imaging is good to the lowest value tested, 0.18 e−, but the number of summations required to overcome noise goes up very steeply. The reason for the improvement in imaging is that shot noise circumvents the problem of electron fluctuation inadequacy, since very small changes in mean electron emission probability, corresponding to much less than 1 e− per correlation interval, are reflected in the Poisson distribution. Thus shot noise in a way "amplifies'' the fluctuations but at the price of vastly increased noise. This conclusion is probably not confined to the specific case examined here but seems generally valid.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7626-7635 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Chemical, jump, and tracer diffusion coefficients, D, Dj, and D*, respectively, for the case of nearest-neighbor repulsive interactions have been determined by Monte Carlo simulations on a fixed square lattice. D was determined by the fluctuation autocorrelation function method and from the Kubo–Green equation, using values of 〈(δN)2〉/〈N〉 determined from isotherms. It was found that the two methods give essential agreement over the entire coverage and temperature range examined, 0.73≤T/Tc≤7.33, 0.1≤θ≤0.8. The changes in activation energies for D, D*, and Dj as function of T and θ were determined and can be understood in terms of the phase diagram of the system.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7636-7640 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Chemical, jump, and tracer diffusion coefficients D, Dj, and D*, respectively, have been determined by Monte Carlo simulations on a square lattice for the case of nearest-neighbor attractive interactions. D was determined by the fluctuation autocorrelation method and by the Kubo–Green equation, using values of 〈(δN)2〉/〈N〉 determined from isotherms. 〈(δN)2〉/〈N〉 can only be determined above Tc and, for this situation, the two methods yield identical D values within the error of the simulations. Distinct high- and low-T regimes for D are observed, because of the changes in 〈(δN)2〉/〈N〉 with T, with much higher activation energies and prefactors for T/Tc〈1.4 than for T/Tc〉1.4. For D* and Dj much smaller effects are seen. The numerical values of activation energies suggest that diffusing atoms are either isolated atoms in the dilute lattice gas or those which become detached from corners of close-packed (1×1) islands below Tc, but have a higher nearest-neighbor complement in the dense lattice gas above Tc.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7641-7647 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Tracer, jump, and chemical diffusion coefficients D*, Dj, and D, respectively, were determined on a 50×50 square array of fixed sites by the fluctuation and Kubo–Green methods for the case of nearest-neighbor repulsions and numerically equal next-nearest-neighbor attractions. Both methods give the same D values except at θ=0.5 and T/Tc〈0.9, where c(2×2) dominates and simulations are extremely sensitive to very small changes in the small amount of disorder. Activation energies are, in all cases, controlled by next-nearest-neighbor attraction, i.e., are positive relative to the noninteracting case and peak at θ=0.5. However, D has a maximum at θ=0.5 because of a deep minimum in 〈(δN)2〉/〈N〉. D* and Dj are very similar to each other and have a minimum at θ=0.5 and low T, because of the maxima in activation energies at this coverage, caused by the necessity to deform the c(2×2) structure.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7648-7652 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Tracer, jump, and chemical diffusion coefficients have been determined by Monte Carlo simulations on a square lattice for the case of nearest-neighbor attraction and numerically equal next-nearest-neighbor repulsion. For this case only second-order θ vs μ/kBT isotherms were found, indicating that the system is in a single phase at all coverages. 〈(δN)2〉/〈N〉 varies only very slightly with temperature at all coverages indicating balance between repulsive and attractive adsorbate–adsorbate interactions. Tracer and jump diffusion coefficients also show only very slight temperature dependence, relative to the noninteractive case. The same is true for the chemical diffusion coefficient determined from the Kubo–Green equation as the product of jump diffusion coefficient and 〈N〉/〈(δN)2〉, DKG. For JNN/kBT〈1, there is reasonably good agreement between DKG and DF, the chemical diffusion coefficient determined by the fluctuation method. For JNN/kBT〉1, DF decreases markedly with decreasing T. It is hypothesized that this behavior results from the freezing in of enough long-range order for this particular set of interactions to make the fluctuation results differ from the Kubo–Green ones. The latter are more microscopic, and it may be that the fluctuation results depend on probe size.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 11234-11235 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We show that (i) the comment on our paper by I. Vattulainen, S. C. Ying, T. Ala-Nissila, and J. Merikoski [J. Chem. Phys. (submitted)] does not contain any new physics compared to that discussed earlier in our and their publications, and (ii) their attempts to criticize our choice of the dynamics for describing oxygen diffusion on the W(110) surface are senseless. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9320-9326 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Monte Carlo technique is used to model surface diffusion on energetically heterogeneous surfaces. Simulations are performed for the bivariate barrier model with a concentration cH of high barriers distributed at random. The variation of the chemical and tracer diffusion coefficients with particles coverage and with cH is discussed both for interacting and noninteracting particles. Two different regimes are found for cH below and above the critical concentration for bond percolation. Above percolation the diffusion coefficients are found to vary as cH−1/T, a result which is not predicted by effective medium approximation (EMA). © 1997 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 3197-3203 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Employing the scaling arguments, we show that the temperature dependence of the chemical diffusion coefficient near the points corresponding to continuous phase transitions has power-law or inverse logarithmic singularities, Dc∝|ΔT|α/(1−α) for α〉0, ∝|ΔT|−α for α〈0, or ∝1/|ln|ΔT|| for α=0, where α is the specific heat exponent. Monte Carlo simulations, executed with the parameters corresponding to the O/W(110) system, indicate that these singularities, occurring in a very narrow temperature interval, can be reproduced only if the lattice size is large (L〉500). Outside the critical region, the temperature dependence of Dc is regular and the deviations from the ideal Arrhenius behavior are relatively weak. In particular, at appreciable coverages, the variations of the activation energy for chemical diffusion are about 10 kJ/mol (this value amounts to (approximate)10% of the activation energy). © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Surface Science 259 (1991), S. 139-150 
    ISSN: 0039-6028
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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