5,115 research outputs found

    The Reaction-Diffusion Front for A+B→∅A+B \to\emptyset in One Dimension

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    We study theoretically and numerically the steady state diffusion controlled reaction A+B→∅A+B\rightarrow\emptyset, where currents JJ of AA and BB particles are applied at opposite boundaries. For a reaction rate λ\lambda, and equal diffusion constants DD, we find that when λJ−1/2D−1/2â‰Ș1\lambda J^{-1/2} D^{-1/2}\ll 1 the reaction front is well described by mean field theory. However, for λJ−1/2D−1/2≫1\lambda J^{-1/2} D^{-1/2}\gg 1, the front acquires a Gaussian profile - a result of noise induced wandering of the reaction front center. We make a theoretical prediction for this profile which is in good agreement with simulation. Finally, we investigate the intrinsic (non-wandering) front width and find results consistent with scaling and field theoretic predictions.Comment: 11 pages, revtex, 4 separate PostScript figure

    Scale-dependent variation in coral community similarity across sites, islands, and island groups

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    Community similarity is the proportion of species richness in a region that is shared on average among communities within that region. The slope of local richness (α diversity) regressed on regional richness (Îł diversity) can serve as an index of community similarity across regions with different regional richness. We examined community similarity in corals at three spatial scales (among transects at a site, sites on an island, and islands within an island group) across a 10 000-km longitudinal diversity gradient in the west-central Pacific Ocean. When α diversity was regressed on Îł diversity, the slopes, and thus community similarity, increased with scale (0.085, 0.261, and 0.407, respectively) because a greater proportion of Îł diversity was subsumed within α diversity as scale increased. Using standard randomization methods, we also examined how community similarity differed between observed and randomized assemblages and how this difference was affected by spatial separation of species within habitat types and specialization of species to three habitat types (reef flats, crests, and slopes). If spatial separation within habitat types and/or habitat specialization (i.e., underdispersion) occurs, fewer species are shared among assemblages than the random expectation. When the locations of individual coral colonies were randomized within and among habitat types, community similarity was 46–47% higher than that for observed assemblages at all three scales. We predicted that spatial separation of coral species within habitat types should increase with scale due to dispersal/extinction dynamics in this insular system, but that specialization of species to different habitat types should not change because habitat differences do not change with scale. However, neither habitat specialization nor spatial separation within habitat types differed among scales. At the two larger scales, each accounted for 22–24% of the difference in community similarity between observed and randomized assemblages. At the smallest scale (transect–site), neither spatial separation within habitat types nor habitat specialization had significant effects on community similarity, probably due to the small size of transect samples. The results suggest that coral species can disperse among islands in an island group as easily as they can among sites on an island over time scales that are relevant to their establishment and persistence on reefs

    Annotated Computer Output for Analyses of Unbalanced Data: SAS GLM

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    41 pages, 1 article*Annotated Computer Output for Analyses of Unbalanced Data: SAS GLM* (Searle, S. R.; Henderson, H. V.) 41 page

    On Deriving the Inverse of a Sum of Matrices

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    21 pages, 1 article*On Deriving the Inverse of a Sum of Matrices* (Henderson, H. V.; Searle, S. R.) 21 page

    Coarsening in a Driven Ising Chain with Conserved Dynamics

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    We study the low-temperature coarsening of an Ising chain subject to spin-exchange dynamics and a small driving force. This dynamical system reduces to a domain diffusion process, in which entire domains undergo nearest-neighbor hopping, except for the shortest domains -- dimers -- which undergo long-range hopping. This system is characterized by two independent length scales: the average domain length L(t)~t^{1/2} and the average dimer hopping distance l(t)~ t^{1/4}. As a consequence of these two scales, the density C_k(t) of domains of length k does not obey scaling. This breakdown of scaling also leads to the density of short domains decaying as t^{-5/4}, instead of the t^{-3/2} decay that would arise from pure domain diffusion.Comment: 7 pages, 9 figures, revtex 2-column forma

    Renormalization Group Study of the A+B->0 Diffusion-Limited Reaction

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    The A+B→0A + B\to 0 diffusion-limited reaction, with equal initial densities a(0)=b(0)=n0a(0) = b(0) = n_0, is studied by means of a field-theoretic renormalization group formulation of the problem. For dimension d>2d > 2 an effective theory is derived, from which the density and correlation functions can be calculated. We find the density decays in time as a,b \sim C\sqrt{\D}(Dt)^{-d/4} for d<4d < 4, with \D = n_0-C^\prime n_0^{d/2} + \dots, where CC is a universal constant, and Câ€ČC^\prime is non-universal. The calculation is extended to the case of unequal diffusion constants DA≠DBD_A \neq D_B, resulting in a new amplitude but the same exponent. For d≀2d \le 2 a controlled calculation is not possible, but a heuristic argument is presented that the results above give at least the leading term in an Ï”=2−d\epsilon = 2-d expansion. Finally, we address reaction zones formed in the steady-state by opposing currents of AA and BB particles, and derive scaling properties.Comment: 17 pages, REVTeX, 13 compressed figures, included with epsf. Eq. (6.12) corrected, and a moderate rewriting of the introduction. Accepted for publication in J. Stat. Phy

    TeV-scale electron Compton scattering in the Randall-Sundrum scenario

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    The spin-2 graviton excitations in the Randall-Sundrum gravity model provides a t-channel contribution to electron Compton scattering which competes favourably with the standard QED contributions. The phenomenological implications of these contributions to the unpolarized and polarized cross-sections are evaluated.Comment: 11 pages, 5 figure

    Anomalous self-diffusion in the ferromagnetic Ising chain with Kawasaki dynamics

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    We investigate the motion of a tagged spin in a ferromagnetic Ising chain evolving under Kawasaki dynamics. At equilibrium, the displacement is Gaussian, with a variance growing as At1/2A t^{1/2}. The temperature dependence of the prefactor AA is derived exactly. At low temperature, where the static correlation length Ο\xi is large, the mean square displacement grows as (t/Ο2)2/3(t/\xi^2)^{2/3} in the coarsening regime, i.e., as a finite fraction of the mean square domain length. The case of totally asymmetric dynamics, where (+)(+) (resp. (−)(-)) spins move only to the right (resp. to the left), is also considered. In the steady state, the displacement variance grows as Bt2/3B t^{2/3}. The temperature dependence of the prefactor BB is derived exactly, using the Kardar-Parisi-Zhang theory. At low temperature, the displacement variance grows as t/Ο2t/\xi^2 in the coarsening regime, again proportionally to the mean square domain length.Comment: 22 pages, 8 figures. A few minor changes and update

    Large magnetic anisotropy in Ferrihydrite nanoparticles synthesized from reverse micelles

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    Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of reverse micelles, used as nanoreactors to obtain average particle sizes ≈\approx 2 to 4 nm. The blocking temperatures TBmT_B^m extracted from magnetization data increased from ≈10\approx 10 to 20 K for increasing particle size. Low-temperature \MOS measurements allowed to observe the onset of differentiated contributions from particle core and surface as the particle size increases. The magnetic properties measured in the liquid state of the original emulsion showed that the \FH phase is not present in the liquid precursor, but precipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibility \chi_{ac}(\emph{f},T) measurements showed the dependence of the effective magnetic anisotropy energies EaE_{a} with particle volume, and yielded an effective anisotropy value of Keff=312±10K_{eff} = 312\pm10 kJ/m3^3.Comment: 8 pages, 10 figures. Nanotechnology, v17 (Nov. 2006) In pres

    Production/maintenance cooperative scheduling using multi-agents and fuzzy logic

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    Within companies, production is directly concerned with the manufacturing schedule, but other services like sales, maintenance, purchasing or workforce management should also have an influence on this schedule. These services often have together a hierarchical relationship, i.e. the leading function (most of the time sales or production) generates constraints defining the framework within which the other functions have to satisfy their own objectives. We show how the multi-agent paradigm, often used in scheduling for its ability to distribute decision-making, can also provide a framework for making several functions cooperate in the schedule performance. Production and maintenance have been chosen as an example: having common resources (the machines), their activities are actually often conflicting. We show how to use a fuzzy logic in order to model the temporal degrees of freedom of the two functions, and show that this approach may allow one to obtain a schedule that provides a better compromise between the satisfaction of the respective objectives of the two functions
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