18 research outputs found
Reaction, Levy Flights, and Quenched Disorder
We consider the A + A --> emptyset reaction, where the transport of the
particles is given by Levy flights in a quenched random potential. With a
common literature model of the disorder, the random potential can only increase
the rate of reaction. With a model of the disorder that obeys detailed balance,
however, the rate of reaction initially increases and then decreases as a
function of the disorder strength. The physical behavior obtained with this
second model is in accord with that for reactive turbulent flow, indicating
that Levy flight statistics can model aspects of turbulent fluid transport.Comment: 6 pages, 5 pages. Phys. Rev. E. 65 (2002) 011109--1-
Phase diagram of an Ising model with long-range frustrating interactions: a theoretical analysis
We present a theoretical study of the phase diagram of a frustrated Ising
model with nearest-neighbor ferromagnetic interactions and long-range
(Coulombic) antiferromagnetic interactions. For nonzero frustration, long-range
ferromagnetic order is forbidden, and the ground-state of the system consists
of phases characterized by periodically modulated structures. At finite
temperatures, the phase diagram is calculated within the mean-field
approximation. Below the transition line that separates the disordered and the
ordered phases, the frustration-temperature phase diagram displays an infinite
number of ``flowers'', each flower being made by an infinite number of
modulated phases generated by structure combination branching processes. The
specificities introduced by the long-range nature of the frustrating
interaction and the limitation of the mean-field approach are finally
discussed.Comment: 32 pages, 7 figure
From Network Structure to Dynamics and Back Again: Relating dynamical stability and connection topology in biological complex systems
The recent discovery of universal principles underlying many complex networks
occurring across a wide range of length scales in the biological world has
spurred physicists in trying to understand such features using techniques from
statistical physics and non-linear dynamics. In this paper, we look at a few
examples of biological networks to see how similar questions can come up in
very different contexts. We review some of our recent work that looks at how
network structure (e.g., its connection topology) can dictate the nature of its
dynamics, and conversely, how dynamical considerations constrain the network
structure. We also see how networks occurring in nature can evolve to modular
configurations as a result of simultaneously trying to satisfy multiple
structural and dynamical constraints. The resulting optimal networks possess
hubs and have heterogeneous degree distribution similar to those seen in
biological systems.Comment: 15 pages, 6 figures, to appear in Proceedings of "Dynamics On and Of
Complex Networks", ECSS'07 Satellite Workshop, Dresden, Oct 1-5, 200
Reaction Diffusion Models in One Dimension with Disorder
We study a large class of 1D reaction diffusion models with quenched disorder
using a real space renormalization group method (RSRG) which yields exact
results at large time. Particles (e.g. of several species) undergo diffusion
with random local bias (Sinai model) and react upon meeting. We obtain the
large time decay of the density of each specie, their associated universal
amplitudes, and the spatial distribution of particles. We also derive the
spectrum of exponents which characterize the convergence towards the asymptotic
states. For reactions with several asymptotic states, we analyze the dynamical
phase diagram and obtain the critical exponents at the transitions. We also
study persistence properties for single particles and for patterns. We compute
the decay exponents for the probability of no crossing of a given point by,
respectively, the single particle trajectories () or the thermally
averaged packets (). The generalized persistence exponents
associated to n crossings are also obtained. Specifying to the process or A with probabilities , we compute exactly the exponents
and characterizing the survival up to time t of a domain
without any merging or with mergings respectively, and and
characterizing the survival up to time t of a particle A without
any coalescence or with coalescences respectively.
obey hypergeometric equations and are numerically surprisingly close to pure
system exponents (though associated to a completely different diffusion
length). Additional disorder in the reaction rates, as well as some open
questions, are also discussed.Comment: 54 pages, Late
Kinetic phase transitions in a reversible unimolecular/bimolecular surface reaction scheme
Surface Science2761-399-108SUSC
Mercados educacionais e a comunidade Education markets and the community
Os defensores de abordagens baseadas no mercado para a educação invocam, às vezes, a necessidade de "empoderamento" (empowerment) das comunidades nas tomadas de decisão educacionais para justificar a reforma dos sistemas educacionais públicos convencionais. Com base em exemplos da Austrália, da Inglaterra e do País de Gales, da Nova Zelândia e dos eua, este artigo explora as maneiras complexas e contraditórias como as políticas educacionais contemporâneas usam o conceito de envolvimento das comunidades. Ele demonstra mais ainda que o papel e o impacto do envolvimento das comunidades nas escolas nada têm de simples. O artigo conclui que, se o envolvimento das comunidades quer ter efeitos progressistas, ele deve ser articulado com uma política de educação democrática mais ampla.<br>The proponents of market-based approaches to education have sometimes invoked the need for community empowerment in educational decision-making to justify the reform of conventional state educational systems. Drawing upon examples from Australia, England and Wales, New Zealand and the usa, this paper explores the complex and contradictory ways in which the concept of community involvement is used in contemporary education policy. It further demonstrates that the role and impact of community involvement in schools are by no means straightforward. The paper concludes that, if community involvement is to be progressive in its effects, it will need to be articulated with a broader democratic politics of education