LABORATOIRE DE PHYSIQUE THEORIQUE DE LA MATIERE CONDENSEE

ACTUALITES

Dans cette rubrique sont regroupées quelques faits marquants qui se sont déroulés dernièrement au laboratoire (séminaires, publications importantes, parution de livres, colloques...).

Les archives sont également accessibles ici .

 

Comment les 2 mètres d'ADN qui se trouvent dans le noyau de chacune de nos cellules sont ils repliés ? Comment cette organisation permet-elle le fonctionnement correct de nos chromosomes ? Pour répondre à ces questions, des biologistes moléculaires ont mis au point dans les années 2000 la technique de Capture de Conformation des Chromosomes (3C). Elle consiste à fixer chimiquement les chromosomes, à couper la molécule d'ADN en petits fragments puis à relier entre eux les fragments qui se trouvent à proximité. Ces fragments sont ensuite séquencés, ce qui permet de dresser une carte de contacts du génome complet (voir figure) ; carte qui reflète ainsi le repliement des chromosomes. Mais comment lire cette carte ?

La théorie des bandes permet de classer les cristaux en isolants ou conducteurs électriques en fonction de leur spectre en énergie – organisé en bandes d'énergie permise séparées par des bandes interdites – et de son remplissage en électrons. Si le niveau de Fermi, qui caractérise ce remplissage, est dans une bande permise, c'est un métal ; s'il tombe dans un gap (une bande interdite), c'est un isolant. Un des enseignements principaux de la découverte récente du graphène (2004) et des isolants topologiques est que le spectre des bandes d'énergie n'est pas suffisant pour décrire complètement les propriétés électroniques d'un cristal. En effet la structure de bande complète se compose du spectre en énergie et des fonctions d'onde. Une information essentielle (typiquement, une phase géométrique) est contenue dans les fonctions d'ondes, qui ne transparaît pas dans le spectre. Cette information supplémentaire permet de raffiner la théorie des bandes en distinguant les isolants triviaux (phase géométrique nulle) des isolants topologiques (phase géométrique non-nulle).  

The glass transition (from a liquid to a solid disordered phase) is an ubiquitous phenomenon in condensed matter physics, still lacking  a complete description. A main step in the analysis of a standard phase transition consists of identifying the correct order parameter, allowing  then to construct a Landau functional, first analyzed in term of a mean-field description, and then, eventually, promoted to a truly fluctuating field in order to get a full-fledged theoretical description. Based on the "random first-order transition" (RFOT) theory, the physical order parameter for the glass transition has been identified as the similarity, also called "overlap", between equilibrium configurations (consider this sentence as a simplified summary for a complex situation) . In this description, the liquid to glass transition corresponds to such an overlap jumping from a (nearly) vanishing value (in the liquid phase) to a large value at the transition.

In the last few years a lot of effort has been devoted to develop an effective field theory of glass-forming systems directly formulated in terms of an overlap field. It leads naturally to a scalar field theory in presence of quenched disorder, which can be studied using standard tools of statistical physics, such as the Non Perturbative Renomalization Group.

In this context, we have studied the critical point that terminates the transition line in an extended phase diagram where one introduces a coupling between liquid configurations. We have shown that, in agreement with other recent results, the long-distance physics in the vicinity of this critical point is in the same universality class as that of a paradigmatic disordered model: the random-field Ising model (RFIM). One motivation for studying this specific region of parameters stems from recent numerical works that have directly focused on the behavior of supercooled liquids in the presence of such an attractive coupling and have provided evidence for a first-order transition line and a terminal critical point. In consequence, our predictions are prone to direct tests in the future.

Ref : "Random-Field-like Criticality in Glass-Forming Liquids" G. Biroli, C. Cammarota, G. Tarjus, and M. Tarzia Phys. Rev. Lett. 112, 175701 (2014).

Description du livre

Paolo Maiuri, Jean-Francois Rupprecht, Stefan Wieser, Verena Ruprecht, Olivier Benichou, Nicolas Carpi, Mathieu Coppey, Simon De Beco, Nir Gov, Carl-Philipp Heisenberg, Carolina Lage Crespo, Franziska Lautenschlaeger, Mael Le Berre, Ana-Maria Lennon-Dumenil, Matthew Raab, Hawa-Racine Thiam, Matthieu Piel, Michael Sixt and Raphael Voituriez.  Cell(2015), in press. Available online at: http://www.cell.com/cell/abstract/S0092-8674(15)00180-4

Eukaryotic cell migration is essential for a large set of biological processes, from development to immunity or  cancer. Assessing quantitatively the exploratory efficiency of cell trajectories is therefore crucial. In the absence of external guidance, cell movement can be described as a random motion, and proposed models have ranged from simple Brownian motion to persistent random walks, Levy walks, or composite processes such as intermittent random walks. Such models differ in the cell persistence, which quantifies the ability of a cell to maintain its direction of motion. The variety of behaviors, observed even along a single cell trajectory, stems from the fact that, as opposed to a passive tracer in a medium at thermal equilibrium, which performs a classical Brownian motion, a cell is self-propelled, and as such, belongs to the class of active Brownian particles. This class of processes is extremely vast and needs to be restricted to have some predicting power.

 In this work, we show on the basis of experimental data in vitro and in vivo that cell persistence is robustly coupled to cell migration speed. We suggest that this universal coupling constitutes a generic law of cell migration, which originates in the advection of polarity cues by an actin cytoskeleton undergoing flows at the cellular scale. Our analysis relies on a theoretical model that we validate by measuring the persistence of cells upon modulation of actin flow speeds. Beyond the quantitative prediction of the coupling, the model yields a generic phase diagram of cellular trajectories, which recapitulates the full range of observed migration patterns.