9,708 research outputs found
Quasi-chemical approximation for polyatomic mixtures
The statistical thermodynamics of binary mixtures of polyatomic species was
developed on a generalization in the spirit of the lattice-gas model and the
quasi-chemical approximation (QCA). The new theoretical framework is obtained
by combining: (i) the exact analytical expression for the partition function of
non-interacting mixtures of linear -mers and -mers (species occupying
sites and sites, respectively) adsorbed in one dimension, and its extension
to higher dimensions; and (ii) a generalization of the classical QCA for
multicomponent adsorbates and multisite-occupancy adsorption. The process is
analyzed through the partial adsorption isotherms corresponding to both species
of the mixture. Comparisons with analytical data from Bragg-Williams
approximation (BWA) and Monte Carlo simulations are performed in order to test
the validity of the theoretical model. Even though a good fitting is obtained
from BWA, it is found that QCA provides a more accurate description of the
phenomenon of adsorption of interacting polyatomic mixtures.Comment: 27 pages, 8 figure
Percolation in Hierarchical Scale-Free Nets
We study the percolation phase transition in hierarchical scale-free nets.
Depending on the method of construction, the nets can be fractal or small-world
(the diameter grows either algebraically or logarithmically with the net size),
assortative or disassortative (a measure of the tendency of like-degree nodes
to be connected to one another), or possess various degrees of clustering. The
percolation phase transition can be analyzed exactly in all these cases, due to
the self-similar structure of the hierarchical nets. We find different types of
criticality, illustrating the crucial effect of other structural properties
besides the scale-free degree distribution of the nets.Comment: 9 Pages, 11 figures. References added and minor corrections to
manuscript. In pres
Epidemic dynamics in finite size scale-free networks
Many real networks present a bounded scale-free behavior with a connectivity
cut-off due to physical constraints or a finite network size. We study epidemic
dynamics in bounded scale-free networks with soft and hard connectivity
cut-offs. The finite size effects introduced by the cut-off induce an epidemic
threshold that approaches zero at increasing sizes. The induced epidemic
threshold is very small even at a relatively small cut-off, showing that the
neglection of connectivity fluctuations in bounded scale-free networks leads to
a strong over-estimation of the epidemic threshold. We provide the expression
for the infection prevalence and discuss its finite size corrections. The
present work shows that the highly heterogeneous nature of scale-free networks
does not allow the use of homogeneous approximations even for systems of a
relatively small number of nodes.Comment: 4 pages, 2 eps figure
Velocity and hierarchical spread of epidemic outbreaks in scale-free networks
We study the effect of the connectivity pattern of complex networks on the
propagation dynamics of epidemics. The growth time scale of outbreaks is
inversely proportional to the network degree fluctuations, signaling that
epidemics spread almost instantaneously in networks with scale-free degree
distributions. This feature is associated with an epidemic propagation that
follows a precise hierarchical dynamics. Once the highly connected hubs are
reached, the infection pervades the network in a progressive cascade across
smaller degree classes. The present results are relevant for the development of
adaptive containment strategies.Comment: 4 pages, 4 figures, final versio
InAs/InP single quantum wire formation and emission at 1.5 microns
Isolated InAs/InP self-assembled quantum wires have been grown using in situ
accumulated stress measurements to adjust the optimal InAs thickness. Atomic
force microscopy imaging shows highly asymmetric nanostructures with average
length exceeding more than ten times their width. High resolution optical
investigation of as-grown samples reveals strong photoluminescence from
individual quantum wires at 1.5 microns. Additional sharp features are related
to monolayer fluctuations of the two dimensional InAs layer present during the
early stages of the quantum wire self-assembling process.Comment: 4 pages and 3 figures submitted to Applied Physics Letter
Computational complexity arising from degree correlations in networks
We apply a Bethe-Peierls approach to statistical-mechanics models defined on
random networks of arbitrary degree distribution and arbitrary correlations
between the degrees of neighboring vertices. Using the NP-hard optimization
problem of finding minimal vertex covers on these graphs, we show that such
correlations may lead to a qualitatively different solution structure as
compared to uncorrelated networks. This results in a higher complexity of the
network in a computational sense: Simple heuristic algorithms fail to find a
minimal vertex cover in the highly correlated case, whereas uncorrelated
networks seem to be simple from the point of view of combinatorial
optimization.Comment: 4 pages, 1 figure, accepted in Phys. Rev.
Redefining the role of obstacles in pedestrian evacuation
The placement of obstacles in front of doors is believed to be an effective strategy to increase the flow of pedestrians, hence improving the evacuation process. Since it was first suggested, this counterintuitive feature is considered a hallmark of pedestrian flows through bottlenecks. Indeed, despite the little experimental evidence, the placement of an obstacle has been hailed as the panacea for solving evacuation problems. In this work, we challenge this idea and experimentally demonstrate that the pedestrians flow rate is not necessarily altered by the presence of an obstacle. This result - which is at odds with recent demonstrations on its suitability for the cases of granular media, sheep and mice - differs from the outcomes of most of existing numerical models, and warns about the risks of carelessly extrapolating animal behaviour to humans. Our experimental findings also reveal an unnoticed phenomenon in relation with the crowd movement in front of the exit: in competitive evacuations, an obstacle attenuates the development of collective transversal rushes, which are hazardous as they might cause falls.Fil: Garcimartín, A.. Universidad de Navarra; EspañaFil: Maza, D.. Universidad de Navarra; EspañaFil: Pastor, J. M.. Focke Meler Gluing Solutions S.A.; EspañaFil: Parisi, Daniel Ricardo. Instituto Tecnológico de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Martín Gómez, C.. Universidad de Navarra; EspañaFil: Zuriguel, I.. Universidad de Navarra; Españ
Velocity fluctuations and hydrodynamic diffusion in sedimentation
We study non-equilibrium velocity fluctuations in a model for the
sedimentation of non-Brownian particles experiencing long-range hydrodynamic
interactions. The complex behavior of these fluctuations, the outcome of the
collective dynamics of the particles, exhibits many of the features observed in
sedimentation experiments. In addition, our model predicts a final relaxation
to an anisotropic (hydrodynamic) diffusive state that could be observed in
experiments performed over longer time ranges.Comment: 7 pages, 5 EPS figures, EPL styl
Entropy-driven phase transition in a system of long rods on a square lattice
The isotropic-nematic (I-N) phase transition in a system of long straight
rigid rods of length k on square lattices is studied by combining Monte Carlo
simulations and theoretical analysis. The process is analyzed by comparing the
configurational entropy of the system with the corresponding to a fully aligned
system, whose calculation reduces to the 1D case. The results obtained (1)
allow to estimate the minimum value of k which leads to the formation of a
nematic phase and provide an interesting interpretation of this critical value;
(2) provide numerical evidence on the existence of a second phase transition
(from a nematic to a non-nematic state) occurring at density close to 1 and (3)
allow to test the predictions of the main theoretical models developed to treat
the polymers adsorption problem.Comment: 14 pages, 6 figures. Accepted for publication in JSTA
Multicomponent reaction-diffusion processes on complex networks
We study the reaction-diffusion process on uncorrelated
scale-free networks analytically. By a mean-field ansatz we derive analytical
expressions for the particle pair-correlations and the particle density.
Expressing the time evolution of the particle density in terms of the
instantaneous particle pair-correlations, we determine analytically the
`jamming' effect which arises in the case of multicomponent, pair-wise
reactions. Comparing the relevant terms within the differential equation for
the particle density, we find that the `jamming' effect diminishes in the
long-time, low-density limit. This even holds true for the hubs of the network,
despite that the hubs dynamically attract the particles.Comment: 8 pages, 6 figure
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