Séminaires de l’année 2013

Séminaire de la Fédération PHYSTAT-SUD : Ludwik Leibler


Ludwik Leibler, Matière Molle et Chimie, ESPCI, Paris

Glass-workers shape marvelous objects without using moulds or precise temperature control because glass is a unique material that transforms from liquid to solid in a very progressive way. Can we imagine other compounds that offer similar opportunities? I will present the concept of solidification by molecular-networks topology freezing and introduce vitrimers, organic materials that behave just like glass. Beside opening new perspectives in chemistry and physics of glass formation and catalytically controlled exchange reactions, discovery of vitrimers could profoundly affect many industries that rely on polymers and composites.

Séminaire du LPTMS: J. Bonart

Impurties in trapped one-dimensional quantum liquids

Julius Bonart, LPTHE

Recent experiments with cold atoms on impurity dynamics immersed in trapped 1D quantum liquids have revealed an interesting interplay between the dynamic polaronic impurity mass shift and the renormalization of the optical potential. We show that the influence of the external trap on the Bose gas leads to a steeper effective potential for the impurity. We propose a framework in which this potential renormalization and the mass shift can be quantitatively understood by using a semi-classical theory of density wave excitations in the Luttinger liquid. Then we present more rigorous results on the impurity dynamics which are obtained via the non equilibrium formalism of a quantum Brownian particle. We show that the obtained theoretical results reproduce well recent experimental data.

Soutenance HDR : Alberto Rosso

La physique des extrêmes : de la diffusion anormale aux systèmes désordonnés

Alberto Rosso


Physics-Biology interface seminar : Jean-François Joanny

Properties of the actin cortex and dynamics of cytokinesis

Jean-François Joanny (Institut Curie - Paris)

In this talk I discuss first the properties of the actin cortical layer in contact with the plasma membrane of a cell using the hydrodynamic theory of active gels: within this framework, the cortical layer can be viewed as an active prewetting layer.

I then discuss cytokinesis which is last stage of cell division when the two daughter cells separate. Cytokinesis proceeds by the formation of an actin contractile ring at the equator of teh cell and by contraction of this ring. The ring formation induces tension gradients in the cortical layer and a cortical flow. Using active gel theory, it is shown numerically that cytokinesis can only be completed if the activity gradient is large enough and that the radius of the contractile ring decreases linearly with time in agreement with experiments.

Séminaire du LPTMS: Christian Hagendorf

Spin chains with dynamical lattice supersymmetry

Christian Hagendorf, Université de Genève

Starting from simple observations about the spectrum of the spin-1/2 XXZ and XYZ Heisenberg chains at particular anisotropy I will show how to construct a representation of the N=(2,2) supersymmetry algebra on the lattice. The supercharges are dynamical: they change the number of sites. The construction generalises to higher spin. In particular, it allows to identify lattice precursors of the N=(2,2) superconformal minimal models. The supersymmetry allows to prove the existence of zero-energy ground states which have often rich properties related to enumerative combinatorics. I will discuss in detail the case of the Fateev-Zamolodchikov chain at spin one whose ground-state components are related to the weighted enumeration of alternating sign matrices.

Séminaire exceptionnel du LPTMS : David Saad

David Saad (Aston University)

Polymer physics for route optimisation on the London underground

Optimizing paths on networks is crucial for many applications, from subway traffic to Internet communication. As global path optimization that takes account of all path-choices simultaneously is computationally hard, most existing routing algorithms optimise paths individually, thus providing sub-optimal solutions. This work includes two different aspects of routing. In the first [1] we employ the cavity approach to study analytically the routing of nodes on a graph of given topology to predefined network routers and devise the corresponding distributive optimisation algorithm. In the second [2] we employ the physics of interacting polymers and disordered systems (the replica method) to analyse macroscopic properties of generic path-optimisation problems between arbitrarily selected communicating pairs; we also derive a simple, principled, generic and distributive routing algorithm capable of considering simultaneously all individual path choices.

Two types of nonlinear interactions are considered with different objectives: 1) alleviate traffic congestion at both cyber and real space and provide better route planning; and 2) save resources by powering down non-essential and redundant routers/stations at minimal cost. This saves energy and man-power, and alleviates the need for investment in infrastructure. We show that routing becomes more difficult as the number of communicating nodes increases and exhibits interesting physical phenomena such as ergodicity breaking. The ground state of such systems reveals non-monotonic complex behaviours in average path-length and algorithmic convergence, depending on the network topology, and densities of communicating nodes and routers.

We demonstrate the efficacy of the new algorithm [2] by applying it to: (i) random graphs resembling Internet overlay networks; (ii) travel on the London underground network based on Oyster-card data; and (iii) the global airport network. Analytically derived macroscopic properties give rise to insightful new routing phenomena, including phase transitions and scaling laws, which facilitate better understanding of the appropriate operational regimes and their limitations that are difficult to obtain otherwise.

[1] C. H. Yeung, D. Saad, The Competition for Shortest Paths on Sparse Graphs, Phys. Rev. Lett., 108, 208701 (2012).

[2] C. H. Yeung, D. Saad and K. Y. M. Wong, From the Physics of Interacting Polymers to Optimizing Routes on the London Underground, submitted (2012).

Séminaire du LPTMS: A. de Luca

Insights into the many-body localization transition from the structure of wave functions.

Andrea de Luca, LPT ENS

The possibility of combining interactions and disorder in a quantum system has recently become more concrete under the name of many-body localization transition. With a combination of old and new techniques, we explore the two sides of the transition from the point of view of the wave function statistics, pointing out analogies and differences with the usual  Anderson transition. The many-body localization seems to be characterized more by ergodicity breaking than by concrete localization in the Fock space.

Physics-Biology interface seminar : Carsten Janke

Encoding functional information into the microtubule cytoskeleton

Carsten Janke Institut Curie - Orsay

Our team is interested in the functions of tubulin posttranslational modifications. Tubulin, the building block of the microtubules, is modified by a range of very special posttranslational modifications, such as enzymatic generation of glutamate or glycine side chains, enzymatic removal and re-addition of a C-terminal tyrosine. These modifications take place once the tubulin has polymerized into microtubules, gs energy apipt> -psud.fr/seminars brtione into the micrurity. Wenformation transmolecularity. Wenpx;fwillpropritransmol cytoskeletonh2>