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Theorique d'Orsay

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Agenda > Séminaires / Seminars > Ph. Statistique Dernier ajout : jeudi 2 octobre 2014.

Séminaire commun LPT/LPTMS 2009-2010

Les séminaires de Physique Statistique des Systèmes Complexes se tiennent le jeudi à 14 heures (sauf indication contraire), soit au LPT (bâtiment 210) en salle 114, soit au LPTMS (bâtiment 100A) en salle 201.

Contact pour les séminaires : Cecile Appert-Rolland et Silvio Franz.

-  Vendredi 25 Juin 2010 à 11h, Salle des Conseils de l’IPN, Bâtiment 100

Soutenance de thèse de Carlos Eduardo Alvarez Cabrera (co-tutelle : Université Paris-Sud 11 et Université Los Andes -Bogota, Colombie) : On Shape and Electrostatics : Statistical Mechanics Studies of Model Systems

This doctoral work presents a series of studies of systems in which the particles interact by means of spheroidal hard core and electrostatic interactions. We first consider a dipolar hard sphere bilayer, studied by means of Monte Carlo (MC) simulations. The pressure between the layers is found to vary as -1/h⁵ , where h is the distance between layers. We observed vortex like structures, frustrated by the finite size of the system. Next we obtained the analytical solution to the screened potential of charged spheroidal colloid particles in the Debye-Huckel regime for Neumann and Dirichlet boundary conditions. This latter result agrees with the solution of the Poisson-Boltzmann equation far from the colloid for strongly charged particles. We also perform MC simulations of spheroidal colloids with a point charge at the center and find, both analytically and in simulations, that the effective potential is stronger in the direction where the curvature of the colloid is higher. Finally we present two studies under progress. The first one deals with the effect of the addition of small spheres to spheroidal prolate particles in the nematic phase. We have seen that for an aspect ratio of 3 this effect is mild, but for an aspect ratio of 4, the nematic to isotropic transition is shifted to higher spheroid densities. In the second work, preliminary MC results for a size bidisperse spherical charged colloidal system are provided. This allows for a test of recently proposed mean-field approaches for polydisperse charged systems (cell model and renormalized jellium). In addition, we have found that as the size difference between the colloids is increased, the screening of the smaller species increases, while the opposite effect is observed for the larger species.

-  Jeudi 10 Juin 2010 à 14h, au LPTMS, Bâtiment 100, salle 201

Marco Cosentino Lagomarsino ( Génophysique - Univ. Pierre et Marie Curie )  : The constrained growth of functional and evolutionary partitioning of a genome.

The many available sequenced and annotated genomes can be partitioned in different classes related to gene function and evolution. These partitioning sometimes displays remarkably collective behavior. For example, genomes of the same size tend to produce the same number of protein families, regardless of specific evolutionary or environmental traits. Clearly, the trends bringing to this collective behavior must transcend at least some of the specificity of a genome. Rather, they must be the product of more "universal" aspects of genome growth.

I will discuss how scaling laws for two different genomic partitionings, related to protein domain topologies and their functional annotations, are related, and present minimal genome growth models that reproduce the scaling laws, and can be analyzed using simple tools from statistical mechanics.

-  Jeudi 3 Juin 2010 à 14h, au LPTMS, Bâtiment 100, salle 201

Petr Sulc ( Los Alamos National Laboratory )  : Belief propagation for graph partitioning

We study the belief propagation algorithm for the graph bi-partitioning problem, i.e. the ground state of the ferromagnetic Ising model at a fixed magnetization. Application of a message passing scheme to a model with a fixed global parameter is not banal and we show that the magnetization can in fact be fixed in a local way within the belief propagation equations. Our method provides the full phase diagram of the bi-partitioning problem on random graphs, as well as an efficient heuristic solver that we anticipate to be useful in a wide range of application of the partitioning problem. Work done in collaboration with L. Zdeborova

-  Jeudi 27 Mai 2010 à 14h, au LPT, Bâtiment 210, salle 114

Camille Aron ( Laboratoire de Physique Théorique et Hautes Énergies, Université Pierre & Marie Curie, Paris ) : Driven Quantum Coarsening

We solve the dynamics of an ensemble of disordered quantum rotors coupled to two leads at different chemical potential letting a current flow through the system and driving it out of equilibrium. We discuss the phase diagram in the temperature, voltage, strength of quantum fluctuations and coupling to the bath. We uncover the coarsening regime. In particular, the scaling functions of correlation and response at late epochs are proven to coincide with their classical counterparts.

-  Jeudi 6 Mai 2010 à 14h, au LPT, Bâtiment 210, salle 114

Royce Zia ( VirginiaTech, Department of Physics, USA ) : Convection cells without shear or gravity : Surprises from a simple model in non-equilibrium statistical mechanics

A clear signature of far-from-equilibrium systems, convection cells are ubiquitous in nature. Typically, they are driven by external forces, like shear or gravity in combination with temperature gradients. Here, we show the existence of such cells in a simple (possibly the simplest) system involving only a temperature gradient. In particular, we study a 2-D Ising lattice gas in contact with two thermal reservoirs. For simplicity, we impose a sharply localized temperature gradient. With the higher T maintained at "infinity" while setting the lower T below the Onsager temperature, convection cells emerge. Other motivations, further surprises, and some theoretical considerations, will also be presented.


-  Mardi 13 avril 2010, 14h, Salle 114, Bâtiment 210

Peter Forrester (University of Melbourne) : A finale for spacing distribution in random matrix theory

Recently a numerical analyst Folkmar Bornemann has shown that the most efficient way to compute spacing distributions in classical random matrix theory is to use Fredholm determinant formulas. I will review these developments, including some contributions which come from my own work.

-  Jeudi 25 Mars 2010 à 14h, au LPTMS, Bâtiment 100, salle 201 David Lacoste (Laboratoire PCT, ESPCI, Paris) :

Relations de fluctuations pour les moteurs moléculaires

Les moteurs moléculaires sont des molécules capables de convertir l’énergie chimique issue de l’hydrolyse de l’ATP en travail mécanique. Ces systèmes opèrent typiquement loin de l’équilibre, dans un régime dans lequel les approches thermodynamiques classiques ne s’appliquent pas. Sur des modèles simples de moteurs moléculaires, issus des modèles de type ratchet, on présentera certains aspects de cette conversion d’énergie, et on discutera l’existence de relations de fluctuations très générales, valables arbitrairement loin de l’équilibre comme la relation de Gallavotti-Cohen en particulier.

-  Vendredi 19 Février 2010 à 14h, au LPTMS, Bâtiment 100, salle 201


Ivan Junier (Institut des Systemes Complexes Paris Île-de-France (ISC-PIF) rue Lhomond 75005 Paris, France ) :

Insights into genetic regulation : confronting polymer physics and genomics

There is increasing evidence that the overall regulation of genetic expression relies on a proper spatio-temporal organization of chromosomes inside the cells. This can be seen in particular from the transcription machinery : in some eukaryotes and bacteria, transcription of highly active (co-functional or co-regulated) genes occurs within discrete foci called transcription factories, where RNA polymerases, transcription factors and their target genes co-localize.

I will show that the spatial coordination of co-functional genes relies on two complementary types of genomic organization whose use depends on the number of co-functional genes to coordinate. Then, using single chain polymer theory, I will present a possible mechanism for the formation of transcription factories, which allows to understand the importance of genomic organization in the process.

In a second part, I will show that DNA inter-gene distances can be used to efficiently work out functional links between genes. In particular, I will present a network — the closome — that reflects the tendency for COGs (cluster of orthologous genes) to be close to each other along the DNA. It has been obtained by analyzing the genomic organization of more than 800 different bacteria.

-  Jeudi 25 Février 2010 à 14h, au LPTMS, Bâtiment 100, salle 201

Kirsten Martens (Laboratoire de Physique de la Matière Condensée et des Nanostructures, Université Lyon 1) : Dependence of the Fluctuation-Dissipation Temperature on the Choice of Observable

On general grounds, a nonequilibrium temperature can be consistently defined from generalized fluctuation-dissipation relations only if it is independent of the observable considered. In this talk I will argue that the dependence on the choice of observable generically occurs when the phase-space probability distribution is nonuniform on constant energy shells. I will relate quantitatively this observable dependence to a fundamental characteristics of nonequilibrium systems, namely, the Shannon entropy difference with respect to the equilibrium state with the same energy. The results will be illustrated on various mean-field models.

-  Jeudi 18 Février à 14h, au LPT, Bâtiment 210, salle 114

Paolo Pierobon (Inserm U932, Institut Curie, Paris) : Intracellular traffic models and single molecule beyond the membranes

Intracellular transport along microtubules or actin filaments, powered by molecular motors such as kinesins, dyneins or myosins, has been recently modeled using one-dimensional driven lattice gases. I will discuss some generalizations of these models, that include extended particles and defects. I will also present the most recent experiments on single molecule study inside the live cell, where I measured the velocity, processivity and step size of Myosin V. Finally I will attempt a connection between theory and experiments.

-  Jeudi 21 Janvier à 14h, au LPT, Bâtiment 210, salle 114

Laetitia Gauvin (Laboratoire de Physique Statistique, ENS, Paris) : Analysis of socio-economic sytems from a statistical physics point of view : case of Schelling segregation model

The Schelling’s segregation model introduced in the 1970’s has attracted much attention in social sciences and more recently in statistical physics. Here, we study the collective behavior of this socio-economic model making used of methods borrowed from statistical physics following an approach whose relevance was not conspicuous. The main model parameter is the "tolerance" which quantities how much an agent is willing to accept neighbors with different socio-economic characteristics. The dynamics of the model may lead to segregated states, i.e. to large domains composed of agents with similar characteristics. To study this dynamics through numerical simulations, we introduce a measure of segregation based on percolation theory. We argue that this measure and the tolerance parameter play roles similar respectively to the order parameter and the temperature in spin models. Based on this analogy, we introduce the associated specific heat and susceptibility. This physical approach allows to distinguish quantitatively several regimes and to characterize the transitions between them, leading to the building of a phase diagram. Some of the transitions evoke empirical sudden ethnic turnovers. We exhibit a dynamically frozen phase reminiscent of glassy or kinetically constrained systems. We also establish links with spin1 models in physics, in particular the Blume-Emery-Griffiths and Blume-Capel models for binary mixtures and alloys in the presence of vacancies. Although the model studied can be considered as a toy model in comparison with the real social systems, our approach provides generic tools to analyze the dynamics of more complex socio-economic systems.

-  Jeudi 14 Janvier 2010, 14h00, Salle 114, Bâtiment 210

Konrad Hinsen (Centre de Biophysique Moléculaire, Orléans) : Les cristaux de protéines : un pont entre les mondes microscopiques et macroscopiques

Un grand nombre de cristaux de protéines sont fabriqués tous les jours dans les laboratoires de recherche dans le but d’observer la structure atomique de leurs constituants par cristallographie. Mais ces cristaux peuvent aussi être étudiés à l’échelle macroscopique en tant que matériau. Pour faire le lien entre ces deux échelles, il est nécessaire d’avoir un modèle mécanique du cristal entier à l’échelle microscopique. Comme premier pas dans le développement d’un tel modèle, je montrerai comment un modèle simple pour la dynamique des protéines (le modèle du réseau élastique) peut être étendu à un cristal entier. Il permet de calculer des quantités microscopiques (les fluctuations des positions atomiques) et macroscopiques (la vitesse de propagation du son dans le cristal) accessibles toutes les deux à l’expérience.

-  Jeudi 17 Décembre 2009, 14h00, Salle 114, Bâtiment 210

Raphael Chetrite (Physics of Complex Systems, Weizmann Institute of Science) : Fluctuations d’un système markovien : du théorème de fluctuation-dissipation au niveau 2,5 des grandes déviations.

Je présenterai durant ce séminaire divers éléments de la physique d’un système Markovien hors d’équilibre. Dans une première partie, je montrerai comment obtenir une cinématique, une vitesse typique de ces processus dont la trajectoire est en général non différentiable. Je montrerai alors que dans le référentiel Lagrangien associé à cette vitesse typique, le système devient un processus d’équilibre si le système initial était un processus markovien sans saut. Dans une seconde partie, je présenterai d’abord un rapide survol de l’ensemble des relations de fluctuations, puis je me restreindrai au cas d’une dynamique homogène en temps (stationnaire). Je montrerai alors que la relation de Gallavotti-Cohen qui est une symétrie de la fonction de grande déviation de la production d’entropie (niveau 1 des grandes déviations = pour les observables) peut être généralisée en une symétrie du niveau 2,5 des grandes déviations (pour le temps local et les densités empiriques qui décrivent une trajectoire).

-  Jeudi 10 Décembre à 14h, au LPTMS, Bâtiment 100, salle 201

Herve Isambert (Institut Curie, Centre de Recherche, CNRS-UMR168, Paris) : Evolution of biomolecular networks under duplication-divergence processes

Genomic duplication-divergence processes are the primary source of new protein functions and thereby contribute to the evolutionary expansion of functional molecular networks. Yet, it is still unclear to what extent such duplication-divergence processes also restrict by construction the emerging properties of molecular networks, regardless of any specific cellular functions.

We have addressed this question motivated by the evolution of protein-protein interaction networks, transcription networks and signal transduction networks. Our approach relies on a general duplication-divergence model, based on the statistically necessary deletions of physical or regulatory interactions, arising from stochastic duplications at various genomic scales, from single gene to whole genome duplications.

Major evolutionary scenarios are shown to depend on two global parameters only : i) a protein conservation index (M), that controls the evolutionary history of biomolecular networks, and ii) a distinct topology index (M’) controlling their resulting structure. We then demonstrate that conserved, non-dense networks, which are of prime biological relevance, are also necessary scale-free by construction, irrespective of any evolutionary variations or fluctuations of the model parameters. This turns out to result from a fundamental linkage between individual protein conservation and network topology under general duplication-divergence evolution.

By contrast, the conservation of network motifs including at least one interaction cannot be indefinitely preserved under general duplication-divergence evolution (independently from any network rewiring dynamics), in broad agreement with empirical evidences between phylogenetically distant species. This affects, in particular, the resolution of regulatory conflicts in duplicated transcription networks, as well as, the decoupling of signaling pathways in duplicated signal transduction networks.

All in all, these evolutionary constraints, inherent to duplication-divergence processes, appear to have largely controlled the overall topology and scale-dependent conservation of large biomolecular networks.

-  Jeudi 26 Novembre à 14h, au LPT, Bâtiment 210, salle 114

Nicolas Dupuis (Laboratoire de Physique Théorique de la Matière Condensée, Université Pierre et Marie Curie, Paris) : Comportement infrarouge et fonction spectrale de bosons en interaction

Nous montrons que le groupe de renormalisation non-perturbatif (NPRG) apporte une solution simple au problème des divergences infrarouges qui apparaissent dans la théorie de la suprafluidité lorsqu’on pousse l’approche perturbative au-delà de la théorie de Bogoliubov. Le NPRG montre qu’en dimension d\leq 3, la théorie de Bogoliubov n’est plus valable pour des échelles de longueur supérieures à une longueur caractéristique 1/k_G, elle-même supérieure à la longueur de "réparation" (healing length). Dans le régime infrarouge (p\ll k_G) , le comportement de la fonction de Green à une particule est entièrement déterminé par les symétries (identités de Ward) et la divergence de la susceptibilité longitudinale induite par le couplage entre fluctuations transverses et longitudinales. Le mode de Bogoliubov (mode de Goldstone de la phase suprafluide) coexiste avec un continuum d’excitations, en accord avec les prédictions de l’approche hydrodynamique de Popov. Nous soulignons le lien entre ces résultats et le comportement critique à la transition suprafluide-isolant de Mott observée expérimentalement dans les gaz bosoniques ultrafroids en présence d’un réseau optique.

-  Jeudi 19 Novembre à 14h, au LPTMS, Bâtiment 100, salle 201

Thierry Mora (Lewis-Sigler Institute for Integrative Genomics, Princeton University) : Statistical physics modeling of the bacterial flagellar motor

Many bacteria like E. coli swim by virtue of small rotary motors that drive rotation of helical flagella. Each motor is powered by a transmembrane proton flux passing through the motor. This flux is converted into torque with near-perfect efficiency by a mechanism whose details remain largely unknown. First I will describe the important biophysical properties of the motor, as measured in experiments, including the recent observation of a stepping behaviour at low speeds. I will then present a simple physical model that allows us to explain most of these data, but also to make new predictions. In particular, I will show how steps can be interpreted as barrier-crossing events in a corrugated energy landscape. Then I will show how to use our model to calculate the effect of shot noise (due to the discrete nature of the energy source—the protons) on motor diffusivity, and thus propose experiments to measure the proton cooperativity in the torque generation process.

-  Vendredi 23 Octobre 2009, 14h00, Salle 114, Bâtiment 210

Lenka Zdeborova (Los Alamos National Laboratory, U. S. A.) : Long time dynamics in mean field glassy models

We introduce a generalization of the cavity method that allows to follow Gibbs states in mean field systems when an external parameter, e.g. the temperature, is tuned. Our method yields a static way to study long time dynamics in mean field models of glass formers, spin glasses, or random constraint satisfaction problems. For the optimization problems the method gives limits of performance for the simulated annealing algorithm. It also gives new results on stability of the non-equilibrium states, the spinodal point upon heating, we also demonstrate the presence of temperature chaos, and study consequences of the shape of the energy landscape for algorithmic hardness. Reference : Krzakala, Zdeborova ; arXiv:0909.3820v1.