Biomedical subjects

Hugues Chaté

Publications and source records attributed to Hugues Chaté.

14 recordsLinked to original sources

PubMed2006

Simple model for active nematics: quasi-long-range order and giant fluctuations.

We propose a simple microscopic model for active nematic particles similar in spirit to the Vicsek model for self-propelled polar particles. In two dimensions, we show that this model exhibits a Kosterlitz-Thouless-like transition to quasi-long-range orientational order and that in this nonequilibrium context, the ordered phase is characterized by giant density fluctuations, in agreement with the predictions of Ramaswamy et al.

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PubMed2005

Nonperturbative fixed point in a nonequilibrium phase transition.

We apply the nonperturbative renormalization group method to a class of out-of-equilibrium phase transitions (usually called "parity-conserving" or, more properly, "generalized voter" class) which is out of the reach of perturbative approaches. We show the existence of a genuinely nonperturbative fixed point, i.e., a critical point that does not seem to be Gaussian in any dimension.

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PubMed2005

Langevin description of critical phenomena with two symmetric absorbing states.

On the basis of general considerations, we propose a Langevin equation accounting for critical phenomena occurring in the presence of two symmetric absorbing states. We study its phase diagram by mean-field arguments and direct numerical integration in physical dimensions. Our findings fully account for and clarify the intricate picture known so far from the aggregation of partial results obtained with microscopic models. We argue that the direct transition from disorder to one of two absorbing states is best described as a (generalized) voter critical point and show that it can be split into an Ising and a directed percolation transition in dimensions larger than one.

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PubMed2005

Integration of Langevin equations with multiplicative noise and the viability of field theories for absorbing phase transitions.

Efficient and accurate integration of stochastic (partial) differential equations with multiplicative noise can be obtained through a split-step scheme, which separates the integration of the deterministic part from that of the stochastic part, the latter being performed by sampling exactly the solution of the associated Fokker-Planck equation. We demonstrate the computational power of this method by applying it to the most absorbing phase transitions for which Langevin equations have been proposed. This provides precise estimates of the associated scaling exponents, clarifying the classification of these nonequilibrium problems, and confirms or refutes some existing theories.

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PubMed2004

From ballistic to brownian vortex motion in complex oscillatory media.

We show that the breaking of the rotation symmetry of spiral waves in two-dimensional complex (period-doubled or chaotic) oscillatory media by synchronization defect lines (SDLs) is accompanied by an intrinsic drift of the pattern. Single vortex motion changes from ballistic flights at a well-defined angle from the SDLs to Brownian-like diffusion when the turbulent character of the medium increases. It gives rise, in nonturbulent multispiral regimes, to a novel "vortex liquid."

Biological Clocks↗

PubMed2004

Quantitative phase diagrams of branching and annihilating random walks.

We demonstrate the full power of nonperturbative renormalization group methods for nonequilibrium situations by calculating the quantitative phase diagrams of simple branching and annihilating random walks and checking these results against careful numerical simulations. Specifically, we show, for the [see text] case, that an absorbing phase transition exists in dimensions d=1 to 6 and argue that mean-field theory is restored not in d=3, as suggested by previous analyses, but only in the limit d--> infinity.

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PubMed2004

Noise-induced macroscopic bifurcations in globally coupled chaotic units.

Large populations of globally coupled identical maps subjected to independent additive noise are shown to undergo qualitative changes as the features of the stochastic process are varied. We show that, for strong coupling, the collective dynamics can be described in terms of a few effective macroscopic degrees of freedom, whose deterministic equations of motion are systematically derived through an order parameter expansion.

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PubMed2004

Cellular automata on high-dimensional hypercubes.

The emergence of nontrivial collective behavior is studied in large families of cellular automata rules implemented on high-dimensional hypercubes. Evidence is found that the region of rule space where such macroscopic dynamics exists is well-defined in the infinite-dimension limit.

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PubMed2004

Onset of collective and cohesive motion.

We study the onset of collective motion, with and without cohesion, of groups of noisy self-propelled particles interacting locally. We find that this phase transition, in two space dimensions, is always discontinuous, including for the minimal model of Vicsek et al. [Phys. Rev. Lett. 75, 1226 (1995)]] for which a nontrivial critical point was previously advocated. We also show that cohesion is always lost near onset, as a result of the interplay of density, velocity, and shape fluctuations.

Animals↗

PubMed2003

Nominal thermodynamic temperature in nonequilibrium kinetic Ising models.

We show that a nominal temperature can be consistently and uniquely defined everywhere in the phase diagram of large classes of nonequilibrium kinetic Ising spin models. In addition, we confirm that, at critical points, the large-time "fluctuation-dissipation ratio" X( infinity ) is a universal amplitude ratio, and find in particular X( infinity ) approximately 0.33(1) and X( infinity )=1 / 2 for the magnetization in, respectively, the two-dimensional Ising and voter universality classes.

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PubMed2003

Absorbing phase transitions of branching-annihilating random walks.

The phase transitions to absorbing states of the branching-annihilating reaction-diffusion processes mA-->(m+k)A, nA-->(n-l)A are studied systematically in one space dimension within a new family of models. Four universality classes of nontrivial critical behavior are found. This provides, in particular, the first evidence of universal scaling laws for pair and triplet processes.

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PubMed2003

Vortex glass and vortex liquid in oscillatory media.

We study the disordered, multispiral solutions of two-dimensional oscillatory media for parameter values at which the single-spiral/vortex solution is fully stable. Using the complex Ginzburg-Landau (CGLE) equation, we show that these states, heretofore believed to be static, actually evolve extremely slowly. This is achieved via a reduction of the CGLE to the evolution of the sole vortex coordinates. This true defect-mediated turbulence occurs in two distinct phases, a vortex liquid characterized by normal diffusion of spirals, and a slowly relaxing, intermittent, "vortex glass."

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PubMed2002

Large-scale finite-wavelength modulation within turbulent shear flows.

We show that turbulent "spirals" and "spots" observed in Taylor-Couette and plane Couette flow correspond to a turbulence-intensity modulated finite-wavelength pattern which in every respect fits the phenomenology of coupled noisy Ginzburg-Landau (amplitude) equations with noise. This suggests the existence of a long-wavelength instability of the homogeneous turbulence regime.

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PubMed2002

Comment on "Deterministic equations of motion and phase ordering dynamics".

Zheng [Phys. Rev. E 61, 153 (2000)] claims that phase ordering dynamics in the microcanonical phi(4) model displays unusual scaling laws. We show here, performing more careful numerical investigations, that Zheng only observed transient dynamics mostly due to the corrections to scaling introduced by lattice effects, and that Ising-like (model A) phase ordering actually takes place at late times. Moreover, we argue that energy conservation manifests itself in different corrections to scaling.

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