Laboratoire de Physique
Theorique d'Orsay

Accès / Find us

Bâtiment 210
Univ. Paris-Sud 11
91405 Orsay Cedex
France
T. 01 69 15 63 53
F. 01 69 15 82 87



CNRS U-PSUD Retour au menu Imprimer Contact Plan Crédits
 
 
Agenda > Séminaires / Seminars > Ph. Statistique Dernier ajout : jeudi 2 octobre 2014.

Séminaire commun LPT/LPTMS 2007-2008

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 : Jean-Michel Caillol et Silvio Franz.


-  Lundi 1er Octobre

Seminaire de la fédération (pas de séminaire LPT/LPTMS)


-  Jeudi 11 Octobre à 14h au LPTMS

Lenka Zdeborova (LPTMS)

Phase transitions in the coloring of random graphs

Average-case analysis of computational complexity presents strong affinities with the study of disordered systems in statistical mechanics. We consider the problem of coloring a large random graph with a given number of colors such that no adjacent vertexes have the same color. Using the cavity method, we present a detailed and systematic study of the phase space of the solutions for different connectivities and numbers of colors. We show that, for fixed number of colors and as the connectivity increases, the set of solutions undergoes different phase transitions similar to those happening in the mean field theory of glasses. First, the phase space decomposes into an exponential number of pure states, and subsequently condensates over the largest such states. Another transition takes place when a finite fraction of frozen variables appears inside almost all the dominant pure states. Eventually a connectivity is reached beyond which no solutions are available. Finally, we discuss the algorithmic perspectives of our results.


-  Vendredi 19 Octobre à 14h au LPT

ANNULE POUR CAUSE DE GREVE DES TRANSPORTS !!!


-  Jeudi 25 Octobre à 11h au LPTMS

Cristina Toninelli (LPMA, Univ.Paris VI-VII), Marco Tarzia (LPTMS, Orsay)

Group Testing with random pools : optimal algorithms and phase transitions

The problem of Group Testing (GT) is to identify defective items in a set of objects by testing groups of items via the query "Is there at least one defective in the pool ?". After recalling the numerous applications of GT, we discuss the construction of efficient algorithms both when all tests should be performed in parallel (one-stage) and when subsequent stages of parallel tests are allowed. Focusing on the regime of small defective probability and large number of items, we identify optimal two-stage algorithms based on proper random choices of the pools. On the other hand, for one-stage algorithms we identify a non-detection/detection phase transition. Then, we explain the connection of GT with the Hitting Set (HS) problem, a generalization of Vertex Covering. By using the cavity method we show that HS on random regular hyper-graphs can be either in a replica symmetric or in a 1RSB phase. Finally, we show that the cavity methods provides very efficient decimation based algorithms able to solve large individual instances of the HS.


-  Jeudi 8 Novembre

Seminaire de la fédération (pas de séminaire LPT/LPTMS)


-  Jeudi 15 Novembre à 15h au LPT

Pascal Viot (LPTMC - Univ. Pierre et Marie Curie)

Gaz granulaires : de la theorie cinetique a l’analyse d’experiences

La presence de dissipation dans les interactions (collisions) entre particules granulaires induit des proprietes tres differentes de celles observees dans les systemes microscopiques habituels. A faible densitee et a faible dissipation, les theories cinetiques de type Boltzmann permettent de mettre en evidence les differentes deviations des distributions de vitesse au comportement Gaussien ainsi que l’absence d’equipartition de l’energie. Apres une revue de ces proprietes, nous etudierons quelques proprietes additionnelles quand les particules sont non-spheriques.

Dans une seconde partie, nous nous interessons a des recents dispositifs experimentaux qui permettent de simuler des thermostats "ideaux". En particulier, par une etude de simulation de dynamique moleculaire sur une bicouche granulaire, nous obtenons une statistique gaussienne de la distribution des vitesses sur plusieurs decades conformement aux resultats experimentaux de Baxter et Olfasen pour des densitees allant au dela de la limite des gaz granulaires et pour une vaste gamme de parametres microscopiques ( rapport de masse des particules, coefficient de restitution,...).


-  Jeudi 22 Novembre à 14h au LPTMS

Ginestra Bianconi (ICTP Trieste)

Entropy of randomized network ensembles

Randomized network ensembles are the null models of real networks and are extensively used to compare a real system to a null hypothesis. In this talk we study network ensembles with the same degree distribution, the same degree-correlations and the same community structure of any given real network. We characterize these randomized network ensembles by their entropy, i.e. the normalized logarithm of the total number of networks which are part of these ensembles. We estimate the entropy of randomized ensembles starting from a large set of real directed and undirected networks. We propose entropy as an indicator to assess the role of each structural feature in a given real network.We observe that the ensembles with fixed scale-free degree distribution have smaller entropy than the ensembles with homogeneous degree distribution indicating a higher level of order in scale-free networks.


-  Jeudi 29 Novembre à 14h au LPT

Werner Krauth (LPS - ENS)

Time correlations and "exact" sampling

In statistical physics, dynamical-systems theory and hydrodynamics, but also in Monte Carlo simulations, time correlations describe the approach to thermal equilibrium. In the long-time limit of diffusive systems, time correlations are characterized through a single number, the correlation time. In this talk, we review the crucial concept of time correlations in physics and computation, and the tantalizing difficulty of estimating the correlation time in Monte Carlo simulations.

We then discuss an exciting reformulation of Monte Carlo algorithms, due to Propp and Wilson, called exact sampling, where one can prove rigorously that the Monte Carlo simulation has run sufficiently long to generate a configuration which is completely decorrelated from the initial condition. We will describe a general renormalization-group algorithm which implements exact sampling for two-dimensional Ising spin glasses on large lattices and report on the applications in three dimensions.

(C. Chanal and W. Krauth "Renormalization group approach to exact sampling" arXiv:0707.4117 (2007))


Lundi 3 Décembre

Seminaire de la fédération


-  Jeudi 6 Décembre à 14h au LPTMS

Francesco Zamponi (LPT-ENS)

A first-principle theory of amorphous states of hard spheres

Hard spheres are ubiquitous in condensed matter : they have been used as models for liquids, crystals, colloidal systems, granulars, and powders. Packings of hard spheres are of even wider interest, as they are related to important problems in information theory, such as digitalisation of signals, error correcting codes, and optimization problems. In three dimensions the densest packing of identical hard spheres has been proven to be the FCC lattice, and it is conjectured that the closest packing is ordered in low enough dimension. Still, amorphous packings have attracted a lot of interest as theoretical models for glasses, because for polydisperse colloids and granular materials the crystalline state is not obtained in experiments for kinetic reasons. I will discuss a theory of amorphous packings, and more generally glassy states, of hard spheres that is based on the replica method and gives predictions on the structure and thermodynamics of these states. In dimensions between two and six these predictions have been successfully compared with numerical simulations. I will also discuss the limit of large dimension where an exact solution seem to be possible. Finally, I will discuss some possible extensions of the theory to different systems.


-  Jeudi 13 Décembre à 14h au LPT

Gregory Schehr (LPT - Orsay)

Statistics of the number of zero crossings : from real roots of Kac’s polynomials to persistence probability for the diffusion equation

We consider a class of real random polynomials, indexed by an integer d, of large degree n and focus on the number of real roots of such random polynomials. The probability that such polynomials have no real root in the interval [0,1] decays as a power law n^-\theta(d) where \theta(d)>0. We show that \theta(d) is precisely the exponent associated to the decay of the persistence probability for the diffusion equation with random initial conditions in space dimension d. For n even, the probability that such polynomials have no root on the full real axis decays as n^-2(\theta(d) + \theta(2)). We further show that the probability that such polynomials have exactly k real roots in [0,1] has an unusual scaling form given by n^-\tilde \varphi(k/\log n) where \tilde \varphi(x) is a universal large deviation function.

(G. Schehr, S.N. Majumdar, Phys. Rev. Lett. 99, 060603 (2007), arXiv:0705.2648).


-  Jeudi 20 Décembre à 14h au LPTMS

Guilhem Semerjian (LPT - ENS)

On message passing guided algorithms for solving constraint satisfaction problems

The statistical mechanics studies of computer science optimization problems have had two main outcomes. On the one hand they proposed a rich picture of the phase diagram of random instances, and on the other they led to new efficient heuristics (Survey Propagation) for practically solving given instances. The theoretical understanding of these new algorithms is not yet completely satisfactory.

In this seminar I will review general ideas about message passing guided algorithms and present recent results on the analysis of a representant of this class of procedures (Belief Propagation).

Joint work with Andrea Montanari and Federico Ricci-Tersenghi.


-  Jeudi 10 Janvier à 14h au LPT

Gerald Kneller (Centre de Biophysique Moléculaire - Orleans)

Dynamique fonctionnelle des protéines et dynamique vitreuse - études par simulation moléculaire

Différentes techniques expérimentales ont montré que la dynamique de relaxation des protéines est caractérisée par un spectre de temps de relaxation vaste, allant de la picoseconde à la seconde, voire à l’heure. La dynamique fonctionnelle qui peut être observée par spectroscopie de corrélation de fluorescence et par des études cinétiques couvre la partie « lente » du spectre, et le processus stochastique sous-jacent montre le comportement caractéristique de la dynamique brownienne fractionnaire (DBF). Par études de simulation moléculaire nous avons récemment mis en évidence que de tels processus de diffusion non-markoviens et auto-similaires dans le temps dominent également la dynamique interne des protéines à des échelles de temps beaucoup plus courtes, de l’ordre de 0.1 ps à 1 ns. Dans cet exposé seront présentés des éléments qui indiquent « l’universalité » du modèle DBF pour la dynamique de relaxation des systèmes complexes en général et des applications à la modélisation de données expérimentales.


-  Jeudi 17 Janvier à 14h au LPT

Sabine Klapp (Stranski-Laboratorium for Physical und Theoretical Chemistry and Institute of Theoretical Physics Technische Universitaet Berlin)

Dipolar ordering between two and three dimensions

We present computer simulation results for dipolar fluid films under strongly coupled conditions where the bulk fluid and films of mesoscopic thicknesses display ferromagnetic ordering. We demonstrate that the ordering persists down to nanoscopic wall separations where the system consists of only three monolayers. For smaller thicknesses we observe stripe-like defects (domains) and finally the breakdown of ferromagnetic ordering for systems close to the two-dimensional limit. We also discuss the influence of wall boundary conditions and additional external magnetic fields. Our results are relevant for systems of magnetic colloids but also for the ordering behavior of thin solid-like magnetic films.


-  Jeudi 24 Janvier à 14h au LPTMS

Raoul Santachiara

Interfaces critiques dans des modèles de spins : évolutions stochastiques de Loewner-Schramm et théories conformes.

L’ évolution de Loewner-Schramm (SLE) est un processus stochastique de croissance décrivant des courbes invariantes conformes qui caractérisent les systèmes critiques en dimension d=2. Cette approche représente un outil important pour la description géométrique des phases critiques. Nous présenterons les concepts principaux sous-jacents a cette approche et le lien entre les SLEs et les théories conformes . Nous décrirons les interfaces SLE dans une famille de modèles de spins qui comprend le modèle d’Ising et le modèle de Potts a trois états.


Attention, jour, heure et salle inhabituels

-  Mercredi 30 Janvier au LPTMS a 11h en salle des conseils de l’IPN

Thierry Giamarchi

Creep and Depinning of domain walls

Many seemingly different macroscopic systems (magnets, ferroelectrics, spintronic materials) have their properties controlled by the propagation of domain walls. For the domain walls, the competition between elasticity and disorder leads to glassy effects quite analogous to more usual glasses, but with also important differences. Understanding both their static and dynamics thus poses challenging problems both from the point of view of fundamental physics but also in view of practical applications. Despite important progress many questions remain. I will give some examples and will discuss in details the question of the depinning of the walls when subjected to an external force. One important and still unresolved question is the effect of the temperature on this depinning transition.


-  Jeudi 7 Fevrier à 14h au LPTMS

Alexander Hartmann (Computational Theoretical Physics, Institut für Physik, Universität Oldenburg, Germany)

Solution space structure of optimization problems

We study numerically the solution-space clustering properties of random ensembles of three glassy optimization problems originating in computational complexity. Namely we investigate the vertex-cover problem, the number-partitioning problem and the satisfiability problem. We use branch-and-bound type algorithms to obtain exact solutions of these problems for moderate system sizes. For the satisfiability problem, we also use heuristics, which allow to obtain solutions in the "solvable part" of phase space for large systems.

Using two methods, direct neighborhood-based clustering and hierarchical clustering, we investigate the structure of the solution space. The main result is that the correspondence between solution-space structure and the phase diagrams of the problems is not unique. Namely, for vertex cover we observe a drastic change of the solution space from large single clusters (in the replica-symmetric phase, as obtained by previous analytical studies) to multiple nested levels of clusters (in the replica-symmetry broken phase). In contrast, for the number-partitioning problem, the phase space looks always very simple, similar to a random distribution of the lowest-energy configurations. This holds in the ``easy’’/solvable phase as well as in the ``hard’’/unsolvable phase. Finally, for the satisfiability problem, we observed several changes of the solution landscape, regarding the number of clusters, their relative sizes and regarding the fraction of frozen variables.


-  Jeudi 14 Fevrier à 14h au LPTMS

Koji Hukushima (Univ. Tokyo)

Monte Carlo study of some Potts (glass) models

Potts model is a generalization of the Ising model with two states to an arbitrary number $q$ of the discrete states. Since a mean-field theory for a Potts spin glass model has revealed the existence of a peculiar 1-step replica-symmetry-breaking (RSB) transition and a formal similarity to structural glass, much interest has shifted toward Potts glass models on finite-dimensional lattices and/or on sparse random graphs. The latter at zero temperature corresponds the coloring problem. We studied statistical-mechanical properties of some Potts (glass) models by using finite-temperature Monte Carlo simulations. One of the models we studied is the three-state +/-J Potts glass model in three dimensions. We obtained a phase diagram of the model as functions of temperature and the fraction $p$ of the antiferromagnetic bonds. We also show some results for the coloring problem and a mathematical puzzle, SUDOKU.


Séminaire exceptionnel organisé par le LPT

-  Mardi 19 Fevrier à 11h au LPT

Constantino Tsallis (CBPF, Bresil)

Mecanique statistique non extensive et generalisation du theoreme de la limite centrale

Il est bien connu que l’ entropie de Boltzmann-Gibbs-von Neumann-Shannon (BG) et la mecanique statistique associee ont un domaine de validite extremement large. Mais il n’ est pas universel. Par exemple, beaucoup de systemes interessants violent l’ hypothese d’ ergodicite dans des etats stationnaires ou quasi-stationnaires. Dans le but d’ elargir quelque peu l’ applicabilite des methodes de la mecanique statistique, une entropie plus generale, Sq, a ete propose en 1988. Pour q=1, elle reproduit l’ expression de BG. L’ entropie de BG est additive (donc extensive pour des systemes sans correlations, ou avec des correlations generiques de courte portee). L’ entropie Sq est, par contre, nonadditive pour q different de l’ unite. Neanmoins, pour une valeur speciale de q (qui depend de quelques characteristiques assez universelles du systeme), elle peut etre extensive, donc epousant adequatement la thermodynamique classique dans la limite de systemes tres larges. Cette situation sera illustre avec des examples physiques, incluant des systemes fortement enchevetres quantiquement. Les fortes correlations qui demandent l’ utilisation de Sq sont generiquement invariantes par echelle dans le regime asymptotique, et donnent lieu a des q-generalisations des theoremes centraux de la limite (standard et celle de Levy-Gnedenko). Des applications (e.g., pour des atomes froids dans des reseaux optiques dissipatifs, et pour des systemes Hamiltonians avec des interactions a longue portee) seront brievement presentees.


-  Jeudi 13 Mars à 14h au LPT

Henri Beresticky (CAMS)

Une espece biologique peut-elle suivre un changement climatique ?


-  Jeudi 20 Mars à 14h au LPTMS

Julien Tailleur

(Department of Physics & Astronomy, University of Edinburgh)

Grandes déviations hors de l’équilibre : retour à l’équilibre via un changement de variables

La théorie des grandes déviations a pour objet l’étude asymptotique d’évènements rares. Si elle permet une reformulation naturelle de la physique statistique d’équilibre, elle prend tout son intérêt hors de l’équilibre, où les fonctions de grandes déviations permettent une extension du concept de potentiels thermodynamiques. Une méthode permettant le calcul de ces fonctions est donc un des buts majeurs de la physique statistique hors de l’équilibre. Je présenterai un formalisme analogue à celui introduit récemment par Bertini et collaborateurs (PRL 87, 040601, (2001)) visant à remplir ce rôle.

En me basant sur un exemple traité par cette théorie, un processus d’exclusion simple dans la limite hydrodynamique, plongé hors de l’équilibre via un contact avec deux réservoirs de densités différentes, je montrerai qu’une transformation des densités et des courants ramènent ce système à l’équilibre. Ce retour à l’équilibre explique la possibilité de calculer explicitement la fonction de grandes déviations grâce à des changements de variables élémentaires. Ce traitement s’applique également aux autres cas résolus par la ’Théorie des fluctuations macroscopiques’.


-  Jeudi 27 Mars à 14h au LPT

Stephane Zaleski

(Institut d’Alembert)

La fragmentation des masses liquides dans les jets d’atomiseurs


-  jeudi 3 Avril à 14h au LPTMS

Sergei Nechaev

(LPTMS)

On spectral density of some random hierarchical matrices

We propose the new ensemble of of hierarchical random matrices with power-law tails in the spectral density. The construction is based on randomization of the Parisi-type hierarchical block matrix $T$. We show that if the distribution on matrix elements of $T$ is Gaussian with zero mean and exponentially decreasing variance, $\sigma^2_\gamma=2^-\beta \gamma$ ($\gamma$ is the hierarchical level of $T$), then the tail of the spectral density, $\rho(\lambda)$, of Gaussian ensemble of randomized Parisi matrices, is $\rho(\lambda) \sim |\lambda|^(2-\beta)/(1-\beta)$ for $\lambda\to\infty$ and $0<\beta<1$ (in the limit $N\to \infty$). For $\beta>1$ one has termination of the the power-law behavior. We conjecture that the similar asymptotic behavior of $\rho(\lambda)$ remains for hierarchical random graphs with Bernoulli-type distribution on matrix coefficients in the $N\to\infty$ limit.


-  Jeudi 10 Avril à 14h au LPTMS

á preciser


Attention, heure exceptionnelle

-  Jeudi 17 Avril à 11h au LPT

Jorge Kurchan

(ESPCI)

Jamming versus Glass transition

Jamming and glass transitions are two phenomena whose mutual relation is not quite clear. The `jamming transiton’ happens when a system of, say, hard frictionles spheres is rapidly compressed and reaches an infinite pressure point. This point is characterised by a diverging length and soft modes, just as a critical phenomenon. At the bottom of this criticallity is the fact that when the system jams it does so under globally isostatic mechanical equilibrium.

On the other hand, the glass transition is also characterised by diverging dynamic (and presumably also static) lengths. It may happen at finite temperatures and pressures, and for soft potentials.

The question as to what, if any, is the relation between the two transitions has been difficult to answer due to a lack of situations where both sets of theories apply simultaneously. I shall discuss a family of models that does precisely that : it posseses on the one hand a jamming transition with diveging lengths and soft modes, and on the other an ideal glass transition.


-  Jeudi 24 Avril au LPTMS

Dr. Bartlomiej Waclaw

(Institut fuer Theoretische Physik, Leipzig, Germany)

Finite-size effects in growing networks and counting of metastable states in Ising spin glasses on arbitrary graphs

Many real-world networks like e.g. citation networks can be approximated by a model where new nodes and links are consecutively added to the network without rearranging old connections. The best known example is the Barabasi-Albert model and its generalizations. Such networks have a power-law distribution of degrees, that is the number of nearest neighbors. However, for any finite network the power law cannot extend to infinity and must have a cut-off. Since the cut-off has a direct impact on many processes on the network like disease spreading or percolation, it is important to know how it scales with the network size. I will demonstrate a method which allows one to calculate these finite-size corrections and show that the scaling exponent depends only on the exponent in the power law.

The second part of the talk will be devoted to the problem of counting one-spin stable states in Ising spin glasses, where couplings between spins are taken at random from some prescribed distribution. One-spin stable states are local energy minima such that a flip of any single spin increases the energy. Their number N_s is known to grow exponentially with the system size, but the rate of growth depends strongly on both the distribution of coupling constants and the graph. Here I will assume that couplings have a gaussian distribution with zero mean and fixed variance. I will develop a systematic method which allows to compute N_s for arbitrary graphs and demonstrate it on several examples, including 2D and 3D regular lattice and their randomized versions. The results indicate that the rate of growth is mainly determined by local properties of the graph, on which the spin glass is defined.


-  Jeudi 1er et 8 Mai

Férié (pas de séminaire)


Attention, jour et heure exceptionnels !!!

-  Mardi 13 Mai à 11h au LPTMS

Alvaro Dominguez (Física Teórica, Universidad de Sevilla)

Two-dimensional structures of colloids at fluid interfaces

I will briefly review the formation of structures in colloids composed by micrometer—sized particles trapped at a fluid interface. Emphasis will be placed on the investigation of the effective interaction between the particles.

Recent experiments have estimulated the interest in the theoretical research of the effective interactions which can be caused by electric and magnetic forces, by elastic stresses (e.g., if a fluid is in a nematic phase) and by capillary forces. I will discuss the current status of the research and its relevance for the interpretation of the experiments, which is actually a source of polemic.


-  Jeudi 15 Mai à 14h au LPT

Nicolas Brunel

(Laboratory of Neurophysics and Physiology, Universite Paris Descartes)

Connectivite synaptique optimisant le stockage en memoire

Une des hypotheses majeures en neuroscience est que l’apprentissage et la memoire sont dus a des modifications synaptiques dans les reseaux de neurones du cerveau. Cette hypothese a conduit a un enorme effort experimental pour determiner les conditions dans lesquelles les synapses peuvent etre modifiees par l’activite des neurones pre- et post-synaptiques, et en parallele a un effort theorique important pour comprendre comment des `regles d’apprentissage’ deduites de ces donnees experimentales permettent le stockage en memoire dans des reseaux de neurones. Malgre ces efforts combines, il n’existe cependant a l’heure actuelle que peu de comparaisons detaillees entre theorie et experience.

La connectivite synaptique a ete etudiee en detail depuis le debut des annees 90 par des enregistrements intracellulaires de neurones multiples in vitro dans de nombreuses aires du cerveau (en particulier le neocortex, le cervelet et l’hippocampe). Ces enregistrements revelent deux caracteristiques de la connectivite qui semblent universelles : (1) deux neurones `proches’ sont connectes avec une probabilite de l’ordre de 10%, alors que la geometrie de leurs axones/dendrites indique que cette probabilite pourrait en principe etre proche de 1 ; (2) les `efficacites synaptiques’ strictement positives ont une distribution similaire dans toutes ces regions. Apres avoir presente ces donnees experimentales, je decrirai des etudes theoriques recentes utilisant des techniques de physique statistique qui permettent d’expliquer ces deux observations par un critere d’optimalite de stockage en memoire en presence de bruit. Ces etudes theoriques permettent aussi de reproduire la statistique des `motifs synaptiques’ de paires et triplets de neurones dans le cortex cerebral.


-  Jeudi 22 Mai à 14h au LPTMS

A. De Martino

Cellular metabolism as a constrained optimization problem

Understanding the organization of reaction fluxes in metabolic networks from the underlying stoichiometry is a central issue in systems biology. Methods based on mass-balance conditions coupled with local optimization rules are able to reproduce experimental findings for relatively simple organisms in specific conditions. Here we define and study a constraint-based model of metabolic networks where neither mass balance nor flux stationarity are postulated. The relevant flux configurations optimize the global growth of the system. Such solutions provide the correct statistics of fluxes for the bacterium E.coli in different environments. Comparison with random metabolic networks enables us to clarify the role of conserved pools of metabolites in determining growth rate and flux variability in natural networks. Finally, we are able to connect phenomenological gene essentiality with fluxes with smaller allowed variability in E.coli metabolism.


 !!!ATTENTION heure inhabituelle !!!!!!

-  Jeudi 5 Juin à 14h au LPTMS

David Declercq

(ETIS/ENSEA/UCP CNRS UMR-8051)

Quelques aspects des codes correcteurs d’erreur modernes

Tout d’abord, je rappellerai les principes fondateurs des codes correcteurs d’erreurs, ainsi que leur application à la transmission ou au stockage de l’information. Le cadre mathématique donné par Claude Shannon en 1948 fixe les limites à la correction d’erreur, mais sans proposer de fonction de codage atteignant cette limite. Les codes dits ’modernes’ peuvent s’approcher de cette limite dans le régime asymptotique, c’est à dire quand la taille des mots de code tend vers l’infini. Je rappelerai brièvement les structures des turbo-codes et des codes LDPC, et surtout leur décodeur par propagation de croyance, dont les équations de mise à jour sont issues de l’approximation de Bethe de l’énergie. Ensuite, je parlerai de deux sujets de recherche liées aux codes LDPC, à savoir :
-  Dynamique de décodeurs quantifiés et topologies de graphes,
-  Estimation de la distance minimale des codes par la méthode impulsionnelle.


 !!!ATTENTION : JOUR ET HEURE EXCEPTIONNELS !!!

-  Lundi 9 Juin à 11h au LPTMS

Idetoshi Nishimori

Lee-Yang zeros and Griffiths singularities in 2d and 3d spin glasses

The distribution of zeros of the partition function is studied for the two- and three-dimensional Ising spin glasses on the complex field plane, the Lee-Yang zeros. We estimate the density of zeros on the imaginary-field axis by an importance-sampling Monte Carlo algorithm, which enables us to sample extremely rare events. Our results in 2d clearly suggest that the density has an essential singularity at the origin, giving the first evidence for Griffiths singularities in spin glasses in equilibrium.


-  Jeudi 12 Juin à 14h au LPT

Vincent Russier

(Institut de Chimie et des Materiaux de Paris-Est, Vitry-sur-Seine)

Interface entre liquides simples non miscibles : étude par dynamique moléculaire et fonctionnelle de la densité.

L’interface entre deux liquides partiellement ou non miscibles intervient dans un grand nombre de situations réelles et/ou d’applications : séparation liquide/liquide ; membranes biologiques ; électrochimie entre électrolytes liquides. Il s’agit en général de processus de transfert soit ionique soit d’espèces neutres, pour lesquels la structure est un paramètre important. Du point de vue fondamental ce systeme fait appel à un ensemble de problèmes types de la physique des liquides aux interfaces : mouillage, ondes capillaires, coexistence de phases et présente donc un grand intérêt en soi.

Dans le contexte d’un modèle simple de liquides (Lennard- Jones) non miscibles, la structure à l’interface entre les liquides est décrite à partir de deux approches complémentaires : la simulation numérique par dynamique moléculaire et la théorie de la fonctionnelle de la densité. On montre que la structure à travers l’interface est en partie conditionnée par la transition de "drying" que l’on observe au delà d’une température Tdw le long de la courbe de coexistence liquide/liquide/ vapeur des liquides en présence. Le caractère de premier ordre de cette transition est mis en évidence à partir du comportement de l’adsorption (nombre d’excès de molécules en surface) et des tensions superficielles.


 !!!ATTENTION JOUR ET HEURE EXCEPTIONNELS !!!

-  Lundi 16 Juin à 11h au LPT

Henk van Beijeren

(Institute for Theoretical Physics, Utrecht University)

Green-Kubo formalism for solids

Resume : The Green-Kubo formalism yields macroscopic transport equations on the basis of the microscopic equations of motion, by postulating the existence of a set of slowly varying microscopic phase functions, whose dynamics allow for a closed macroscopic description. These functions always include the long-wavelength Fourier components of energy, momentum and mass densities. In systems with broken symmetries the order parameters describing these have to be added. In solids these primarily are displacement fields, describing the displacements of the atoms from their equilibrium positions. The resulting macroscopic equations are the elastic equations describing propagation and damping of sound and Fourier’s law of heat conduction. The Green-Kubo formalism expresses the transport coefficients and damping constants occurring in these equations in terms of time integrals of current-current time correlation functions. I will present the general structure of these equations together with the Green-Kubo expressions for transport and damping coefficients. If time allows I will consider mode-coupling predictions for the long time behavior of the correlation functions relevant for heat diffusion. I will discuss divergences of transport coefficients and their finite-size renormalization.


 !!!ATTENTION : JOUR ET HEURE EXCEPTIONNELS !!!

-  Mercredi 18 Juin à 17h au LPTMS

Martin WEIGT

(ISI Torino)

Inference of spatial details in protein-protein interactions from multi-species sequence data

Resume :

Protein-protein interactions are at the basis of most biological processes, and understanding the molecular basis of such interactions is one of the outstanding challenges in systems biology. Experimental approaches to elucidate details of protein-protein interactions normally involve the generation of co-crystall structures ; the latter are hard to obtain for transient interactions (interaction energies of few kT). The rapidly growing number of sequenced genomes (ca. 500 bacterial species) is starting to offer an alternative approach : The genetic variability of the amino-acid sequence of structurally equivalent interacting protein pairs inside and across species allows to identify universal molecular mechanisms. More specifically, a mutation in the interaction surface of one protein has most likely a deleterious effect on the interaction, and has to compensated for by an adequate mutation in the interaction partner. Residue pairs in contact are thus expected to be correlated between proteins.

In our work, a message passing-approach to infer global statistical models for the amino-acid sequences of interacting protein pairs (in statistical-physics language a disordered 21-states Potts model) is proposed. Based purely on sequence information, this approach is able to detect strong direct couplings of residues across proteins. For the case of proteins involved in two-component signal transduction (where co-crystal structures are known), the approach unveils a network of coupled residues which, compared to the actual co-crystal structure, are found to define with impressive accuracy the interaction surface. Besides for structure prediction, the approach can be used to predict sequences of unknown interaction partners up to amino-acid resolution.

This work is a collaboration with R.A. White, T. Hwa (UCSD), H. Szurmant and J.A. Hoch (Scripps).


-  Jeudi 19 Juin à 14h au LPTMS

Vaclav JANIS

(Institute of Physics of the Academy of Sciences of the Czech Republic, Praha, Czech Republic)

Replica-symmetry breaking in the mean-field theory of spin glasses : discrete vs. continuous schemes

Resume : F. Guerra and M. Talagrand proved that the replica-symmetry breaking construction of Parisi leads to the exact free energy of the Sherrington-Kirkpatrick model. The proof, however, does not specify whether hierarchies of replicas should be distributed discretely or continuously. In our talk we discuss the genesis of both, discrete and continuous schemes within a thermodynamic approach with enforcing thermodynamic homogeneity of free energy. We show that hierarchic replications of the phase space are introduced to test and reach thermodynamic homogeneity and the number of hierarchies is fixed by stability conditions. We demonstrate that the continuous distribution can be obtained from the limit to infinite-many hierarchical levels of the discrete scheme. In addition to this, we construct the Parisi solution without demanding an infinite number of hierarchies needed for thermodynamic stability. A natural question is then raised. How does the Parisi continuous solution behave in situations with a stable equilibrium state described by only a finite number of replica hierarchies (1RSB) ?


 !!!ATTENTION : JOUR ET HEURE EXCEPTIONNELS !!!

-  Lundi 23 Juin à 14h au LPTMS

Srikanth Sastry

(JNC Bangalore)

Growing length scales, configurational entropy and dynamics in glass forming liquids

Resume :

The relationship between dynamics, a growing length scale associated with cooperatively rearranging regions, and configurational entropy underlie the Adam-Gibbs theory of dynamics in glass forming liquids. While the Adam-Gibbs relation between relaxation times and configurational entropy has been widely employed to rationalize experimental and computer simulation data, attempts to directly study the length scales associated with cooperatively rearranging regions have been relatively few. Fresh impetus in this direction comes from recent studies of spatially heterogeneous dynamics, wherein analysis of spatial correlations of the mobility of particles allows estimation of a dynamical length. Finite size scaling is employed in the present work to evaluate a dynamical length scale in a model glass forming liquid, whose relationship to relaxation times and configurational entropy are examined. Comparison with theoretical predictions reveal partial agreement, and yield surprises that require further theoretical analysis to resolve. The configurational entropy is found to determine relaxation times for all temperatures and system sizes studied through the Adam-Gibbs relation, but the configurational entropy of cooperatively rearranging regions grows as the temperature is lowered, contrary to the assumptions of Adam-Gibbs theory.


 !!!ATTENTION : SOUTENANCE DE THESE !!!

-  Vendredi 11 Juillet à 14h30 au LPT, Amphi I

Aurelien GAUTREAU

(LPT, ORSAY)

Dynamique sur reseaux complexes et dynamique des reseaux complexes

Resume :

Depuis une dizaine d’annees, les reseaux complexes font l’objet d’etudes intensives. La caracterisation des proprietes des reseaux statiques est desormais bien etablie. Les reseaux sont le support de processus dynamiques. Nous en etudierons un en particulier : la propagation d’epidemie. Nous nous interesserons aux temps d’arrivee de la maladie dans differentes villes dans le cadre d’un modele de propagation mondial, dit modele de metapopulation. Ensuite, nous decrirons quelques methodes pour caracteriser empiriquement l’evolution temporelle d’un reseau. Ces mesures nouvelles debouchent sur la proposition d’un modele d’evolution gouvernee par le trafic pour les reseaux peses. Enfin, dans un cas limite et dans le cadre d’un modele issu des problemes d’opinion, nous etudierons l’interaction entre les processus dynamique sur un reseau et la topologie du reseau lui-même (reseaux adaptifs).


-  Jeudi 25 Septembre à 14h au LPTMS

S. Cocco (LPS de l’Ecole Normale Superieure)

Approches inverses pour la reconstruction d’interactions effectives entre neurones retiniens

Resume :

Les enregistrament multi-electrodes donnent acces a l’activite simultanee d’un grand nombre de neurones pendant plusieurs heures. Deduire des informations sur le cablage de la retine a partir de ces donnees est un probleme important en neurobiologie. Je presenterai deux approches inverse qui permettent d’adresser ce probleme et d’inferer des interactions effectives entre neurones. J’ analyserai avec ces methodes des enregistrement de l’activites des cellules ganglionaires dans la retine de salamandre et discuterai les resultats obtenus.


-  Jeudi 02 Octobre à 14h au LPT

Andrea GAMBASSI

(Max Planck Institut für Metallforschung, Stuttgart)

The Casimir effect : from quantum to critical fluctuations.

Resume :

The Casimir effect in quantum electrodynamics (QED) is perhaps the best-known example of fluctuation-induced long-ranged force acting on objects (conducting plates) immersed in a fluctuating medium (quantum electromagnetic field in vacuum). A similar effect emerges in statistical physics, where, e.g., colloidal particles immersed in a binary liquid mixture experience an additional force due to the classical thermal fluctuations occurring in the surrounding medium. This Casimir-like force acquires universal features upon approaching a critical point of the medium and becomes long-ranged at criticality. In turn, this universality allows the theoretical investigation of the force via representative models and also a stringent experimental test of the corresponding predictions. In contrast to QED, the strength and sign of the Casimir force resulting from critical fluctuations can be easily tailored by surface treatments and temperature control. The talk reviews some recent advances in the theoretical study of the universal properties of the critical Casimir force arising in thin films. Our predictions compare very well with the experimental results obtained for wetting layers of fluids. We discuss how the Casimir force between a colloidal particle and a planar wall immersed in a binary liquid mixture has been measured with femto-Newton accuracy, comparing these experimental results with the corresponding theoretical predictions.


-  Jeudi 09 Octobre à 14h au LPT

Ihor Mryglod

(Institute od Condensed Matter Physics, L’viv, Ukraine)

Resume :

Title not yet decided