528 research outputs found
Jamming and percolation in anisotropic random sequential adsorption of straight rigid rods on a two-dimensional triangular lattice
Percolation of linear k-mers (also known as rods or needles) is studied through Monte Carlo simulationsand finite size scaling, in the case of anisotropic random sequential adsorption on the triangular latticeof LxL and periodic boundary conditions. The efects on the percolation threshold of both the size k ofthe species and the anisotropy degree have been studied. Extensive numerical work enables theconfirmation of a non-monotonic size dependence of the threshold in the isotropic case, which becomesmonotonic in the nematic limit. Finally, a complete analysis of critical exponents and universality hasbeen done, showing that the percolation phase transition involved in the system is not affected, havingthe same universality class of the ordinary random percolation.Fil: Perino, Ernesto JesĂșs. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - San Luis. Instituto de FĂsica Aplicada "Dr. Jorge AndrĂ©s Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias FĂsico MatemĂĄticas y Naturales. Instituto de FĂsica Aplicada "Dr. Jorge AndrĂ©s Zgrablich"; ArgentinaFil: Matoz Fernandez, D.A.. University of Dundee; Reino UnidoFil: Pasinetti, Pedro Marcelo. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - San Luis. Instituto de FĂsica Aplicada "Dr. Jorge AndrĂ©s Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias FĂsico MatemĂĄticas y Naturales. Instituto de FĂsica Aplicada "Dr. Jorge AndrĂ©s Zgrablich"; ArgentinaFil: Ramirez Pastor, Antonio Jose. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - San Luis. Instituto de FĂsica Aplicada "Dr. Jorge AndrĂ©s Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias FĂsico MatemĂĄticas y Naturales. Instituto de FĂsica Aplicada "Dr. Jorge AndrĂ©s Zgrablich"; ArgentinaXVII Taller Regional de FĂsica EstadĂstica y Aplicaciones a la Materia CondensadaSan LuisArgentinaUniversidad Nacional de San Luis. Facultad de Ciencias FĂsico MatemĂĄticas y Naturales. Instituto de FĂsica Aplicada "Dr. Jorge AndrĂ©s Zgrablich"
The spread of epidemic disease on networks
The study of social networks, and in particular the spread of disease on
networks, has attracted considerable recent attention in the physics community.
In this paper, we show that a large class of standard epidemiological models,
the so-called susceptible/infective/removed (SIR) models can be solved exactly
on a wide variety of networks. In addition to the standard but unrealistic case
of fixed infectiveness time and fixed and uncorrelated probability of
transmission between all pairs of individuals, we solve cases in which times
and probabilities are non-uniform and correlated. We also consider one simple
case of an epidemic in a structured population, that of a sexually transmitted
disease in a population divided into men and women. We confirm the correctness
of our exact solutions with numerical simulations of SIR epidemics on networks.Comment: 12 pages, 3 figure
Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: influence of the surface and of the inter-particle interactions
We study the magnetic properties of spherical Co clusters with diameters
between 0.8 nm and 5.4 nm (25 to 7500$ atoms) prepared by sequential sputtering
of Co and Al2O3. The particle size distribution has been determined from the
equilibrium susceptibility and magnetization data and it is compared to
previous structural characterizations. The distribution of activation energies
was independently obtained from a scaling plot of the ac susceptibility.
Combining these two distributions we have accurately determined the effective
anisotropy constant Keff. We find that Keff is enhanced with respect to the
bulk value and that it is dominated by a strong anisotropy induced at the
surface of the clusters. Interactions between the magnetic moments of adjacent
layers are shown to increase the effective activation energy barrier for the
reversal of the magnetic moments. Finally, this reversal is shown to proceed
classically down to the lowest temperature investigated (1.8 K).Comment: 13 figures submitted to Phys. Rev.
Cosmological Implications of Neutrinos
The lectures describe several cosmological effects produced by neutrinos.
Upper and lower cosmological limits on neutrino mass are derived. The role that
neutrinos may play in formation of large scale structure of the universe is
described and neutrino mass limits are presented. Effects of neutrinos on
cosmological background radiation and on big bang nucleosynthesis are
discussed. Limits on the number of neutrino flavors and mass/mixing are given.Comment: 41 page, 7 figures; lectures presented at ITEP Winter School,
February, 2002; to be published in the Proceeding
Edge overload breakdown in evolving networks
We investigate growing networks based on Barabasi and Albert's algorithm for
generating scale-free networks, but with edges sensitive to overload breakdown.
the load is defined through edge betweenness centrality. We focus on the
situation where the average number of connections per vertex is, as the number
of vertices, linearly increasing in time. After an initial stage of growth, the
network undergoes avalanching breakdowns to a fragmented state from which it
never recovers. This breakdown is much less violent if the growth is by random
rather than preferential attachment (as defines the Barabasi and Albert model).
We briefly discuss the case where the average number of connections per vertex
is constant. In this case no breakdown avalanches occur. Implications to the
growth of real-world communication networks are discussed.Comment: To appear in Phys. Rev.
Correlations in Scale-Free Networks: Tomography and Percolation
We discuss three related models of scale-free networks with the same degree
distribution but different correlation properties. Starting from the
Barabasi-Albert construction based on growth and preferential attachment we
discuss two other networks emerging when randomizing it with respect to links
or nodes. We point out that the Barabasi-Albert model displays dissortative
behavior with respect to the nodes' degrees, while the node-randomized network
shows assortative mixing. These kinds of correlations are visualized by
discussig the shell structure of the networks around their arbitrary node. In
spite of different correlation behavior, all three constructions exhibit
similar percolation properties.Comment: 6 pages, 2 figures; added reference
A suberized exodermis is required for tomato drought tolerance.
Plant roots integrate environmental signals with development using exquisite spatiotemporal control. This is apparent in the deposition of suberin, an apoplastic diffusion barrier, which regulates flow of water, solutes and gases, and is environmentally plastic. Suberin is considered a hallmark of endodermal differentiation but is absent in the tomato endodermis. Instead, suberin is present in the exodermis, a cell type that is absent in the model organism Arabidopsis thaliana. Here we demonstrate that the suberin regulatory network has the same parts driving suberin production in the tomato exodermis and the Arabidopsis endodermis. Despite this co-option of network components, the network has undergone rewiring to drive distinct spatial expression and with distinct contributions of specific genes. Functional genetic analyses of the tomato MYB92 transcription factor and ASFT enzyme demonstrate the importance of exodermal suberin for a plant water-deficit response and that the exodermal barrier serves an equivalent function to that of the endodermis and can act in its place
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
Oscillation effects on neutrino decoupling in the early universe
In the early universe, neutrinos decouple from equilibrium with the
electromagnetic plasma at a temperature which is only slightly higher than the
temperature where electrons and positrons annihilate. Therefore neutrinos to
some extent share in the entropy transfer from e^+e^- to other species, and
their final temperature is slightly higher than the canonical value T_nu =
(4/11)^{1/3} T_gamma. We study neutrino decoupling in the early universe with
effects of neutrino oscillations included, and find that the change in neutrino
energy density from e^+ e^- annihilations can be about 2-3% higher if
oscillation are included. The primordial helium abundance can be changed by as
much as 1.5 x 10^-4 by neutrino oscillations.Comment: minor changes, matches version to appear in PR
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
- âŠ