Séminaires de l’année 2016

Physics-Biology interface seminar: Ruxandra Gref

Nanoparticules de type « cage » : applications pour lutter contre le cancer et les infections résistantes au traitement

Ruxandra Gref (ISMO, Université Paris-Sud)

Deux exemples de vecteurs de médicaments élaborés par une « chimie douce » (sans solvant) seront présentés : i) les nanoparticules hybrides organiques-inorganiques (metal-organic frameworks ou MOFs), qui sont des assemblages supramoléculaires cristallins hautement poreux, et ii) les nanoparticules à base de cyclodextrines, molécules « cage ». Avantageusement, ces nanoparticules sont capables d’encapsuler des quantités importantes de molécules thérapeutiques de nature chimique variée par simple imprégnation dans un milieu aqueux. La surface de ces vecteurs a été modifiée avec des éléments de reconnaissance spécifiques afin de moduler l’interaction des nanoparticules avec le milieu vivant et d’accroitre l’internalisation de celles-ci dans les cellules cible (cellules cancéreuses ou cellules infectées avec le VIH ou des bactéries). Finalement, des exemples seront présentés ou la libération des molécules thérapeutiques se fait « sur demande » suite à l’application d’un stimulus externe. Les applications de ces travaux se situent dans le domaine du traitement du cancer et des infections résistantes au traitement.

Séminaire du LPTMS: Haggai Landa

Quasi-bound states in periodically driven scattering

Haggai Landa (LPTMS)

I will discuss the interaction of a cold atom co-trapped with an ion in a Paul trap, a platform which in recent years promises to make possible investigations of ion-neutral interactions in the ultracold regime. The main theoretical and experimental difficulty in this system is due to the time dependence of the ion trap, which is periodically modulated by an AC voltage. We present a general approach for obtaining eigenfunctions of periodically driven time-dependent Hamiltonians and derive an explicit expansion for scattering problems with mixed cylindrical and spherical symmetry [1]. Using this method we study quasi-bound states of a spherically-symmetric potential in three dimensions subject to an axial driving force. The analysis of the ion-atom system is completed by solving self-consistently the time-dependent wavefunctions of the atom together with the periodic force it exerts on the ion [2].

[1]     H. Landa, arXiv: 1506.08779

[2]     H. Landa and G. V. Shlyapnikov, in preparation

Soutenance HDR Dmitry Petrov et mini-workshop

10:00 - 12:00

Soutenance HDR Dmitry Petrov

"Developing and applying the zero-range approach for few-atom problems" 13:30 - 17:30

Workshop on Nuclei, atoms, molecules: Few-body problem and beyond

13:03 "Spin ordering in small atomic clouds'' Jean Dalibard (Collège de France) 14:05 "Few-Nucleon Systems: Universality and Lattice QCD'' Ubirajara van Kolck (Institut de Physique Nucléaire Groupe de Physique Théorique) 14:40 "Contact modeling of the effective range in 1D quantum gases and integrability issue" Ludovic Pricoupenko (LPTMC - Université Pierre et Marie Curie) 15:15 Coffee break 15:45 "Observation of quantum phase slips in ultracold gases?" Giovanni Modugno (Dipartimento di Fisica e Astronomia, Fiorentino) 16:20 "Confinement and broken scale-invariance with ultracold atoms'' Wilhelm Zwerger (Technische Universität München - Physik Department) 16:55 "Four-body long-range interactions between ultracold weakly-bound diatomic molecules" Olivier Dulieu (Laboratoire Aimé Cotton - Université Paris-Sud)

Séminaire du LPTMS: Beatriz Seoane Bartolome

One solid, two glasses

Beatriz Seoane Bartolome (LPT-ENS Paris)

Glasses (aka amorphous solids) exhibit various anomalies whencompared with crystals (aka ordered solids):  they display enhanced transport, activated slow dynamics across energy barriers, excess vibrational modes with respect to Debye’s theory (the so-called Boson Peak), and respond drastically to very small mechanical deformations. In this work, we identify the common, universal origin to these anomalies in a realistic, three-dimensional model of glasses. We show that in packed hard spheres, vibrations become highly correlated in space and time at a sharply defined threshold, which we call "Gardner threshold".  This work is deeply related with the last developments in the analytical theory of glasses, where the glass problem has been finally solved exactly in the artificial limit of infinite spatial dimensions. The analytical solution predicts the existence of a genuine phase transition (a Gardner phase transition) within the glass, separating the glass :and the jamming transitions. In this work we, not only establish the  relevance of the (remanent of the) Gardner transition for real glasses, but also characterize it using well-defined observables, including time-dependent quantities and spatial correlations, that should be experimentally measurable.

Physics-Biology interface seminar: Denis Grebenkov

Inferring anomalous diffusion from single particle trajectories

Denis Grebenkov (PMC, École polytechnique)


Transport of macromolecules, organelles and vesicles in living cells is a very complicated process that essentially determines and controls many biochemical reactions, growth and functioning of cells. The passive thermal diffusion through the overcrowded cytoplasm is combined with the active transport by motor proteins attached to microtubules. This intricate mechanism results in anomalous diffusions that found abundant experimental evidences but no consensus on the physical mechanism and the appropriate mathematical model is achieved so far. Single-particle tracking (SPT) experiments survey random trajectories of individual tracers inside living cells and can thus provide the missing information on the intracellular transport in order to discriminate between different physical mechanisms and to identify the appropriate theoretical model of anomalous diffusion. In SPT, an ensemble average of the quantities of interest (e.g., diffusivity, viscosity, first passage times, etc.) is often unavailable or even undesired, as tracers move in spatially heterogeneous and time evolving media such as living cells. One faces therefore a challenging problem of inferring dynamical, structural and functional properties of living cells from a limited (small) number of individual random realizations of an unknown stochastic process.

After a short introduction to theoretical aspects of the intracellular transport, we discuss the recent progress onto probing ergodicity of the tracer dynamics from a single particle trajectory. The proposed estimators are first investigated for several models of anomalous diffusion. In the case of nonergodic continuous time random walks, we show analytically that both estimators do not vanish even for infinitely long trajectories. The estimators are then applied to two sets of earlier published trajectories: mRNA molecules inside live E. coli cells and Kv2.1 potassium channels in the plasma membrane. These tests suggest that the former set exhibits ergodic behavior while the latter reveals both ergodic and nonergodic features.

Séminaire du LPTMS: Elisabeth Agoritsas

Revisiting mean-field elasto-plastic models at the mesoscopic scale

Elisabeth Agoritsas (Université Grenoble-Alpes)

Encompassing very dissimilar systems (such as foams, emulsions, or granular materials), amorphous materials are composed of constituents of different shapes and sizes, such as bubbles in a soap foam or sand grains in a sandpile, so that they exhibit a structural disorder that plays a determinant role in their mechanical properties, while challenging their very description. Several elasto-plastic models have been developed at the mesoscopic scale, in order to account for the plasticity in sheared amorphous materials, such as the Hébraud- Lequeux (HL) model [P. Hébraud & F. Lequeux, Phys.Rev.Lett. 81, 2934 (1998)]. They have proven to be rather successful in reproducing certain features observed in amorphous systems, but not all at once. Moreover, a consistent picture connecting them is still missing. Here we discuss the physical ingredients that are put in such mean-field models, distinguishing between thermal and mechanical noises in the mean-field dynamics of amorphous materials. We focus in particular on the role of structural disorder, implemented by means of a distribution of energy barriers for the system to overcome locally when an external constant shear rate is applied to the material, and discuss specifically its implications for a generalization of the HL model [E. Agoritsas et al., Eur.Phys.J.E 38, 71 (2015)].

Séminaire du LPTMS: Jean-Marie Stephan

Filling fraction quantum quenches and the arctic circle

Jean-Marie Stephan (PKS-MPG Dresden)

I consider a simple non-equilibrium problem, where a critical one-dimensional system is prepared in a state with two different densities on the left and on the right, and let evolve with a Hamiltonian that conserves the number of particles. A typical example would be a fermionic system prepared with different left/right chemical potentials. For free systems a lot can, and has been understood by making use of a semiclassical picture, in which particles carrying a momentum k propagate ballistically with velocity v(k). Generalization to interacting systems is very much an open problem. I will discuss attempts at understanding such dynamics using field theory. A possible strategy is to study the behavior in imaginary time, the real time dynamics being recovered by performing the Wick rotation. I will show that all degrees of freedom outside a certain region may freeze in imaginary time, contrary to naive expectations. This behavior is analogous to the celebrated "arctic circle" phenomenon found in the study of two-dimensional classical dimer or vertex models. I will also show that the fluctuating region is described by a massless field theory with a position-dependent metric, a field theory in curved space. Such imaginary time pictures can be used to make predictions about the behavior of correlation functions, entanglement entropies, or return probabilities after the quench.

Physics-Biology interface seminar: Paolo Pierobon

Mechanics of B cell response

Paolo Pierobon (Institut Curie, Paris)

B lymphocytes are the antibodies producing cells and therefore essential effectors of adaptive immunity. In vivo, their activation is mostly triggered by the engagement of their B cell receptor (BCR) with antigens exposed at the surface of neighbouring antigen presenting cells. This leads to the formation of a signalling platform, the immune synapse, where cytoskeleton rearrangement are essential for the antigen extraction, internalization and processing. While it has been shown that on a hard substrate the cell follows a dynamics of spreading and contraction, this has never been investigated on substrates with rigidity close to the physiological one. We measure for the first time the forces produced by B cells on deformable antigen coated surfaces (traction force microscopy) and show that these forces are contractile, specifically induced by BCR activation and Myosin II dependent. We characterize the contractile dynamics of the cell and argue that in generating pulling forces, Myosin II plays a crucial role in antigen gathering and internalization. These results open interesting perspectives on the role of mechanics in the acquisition of specific antigen and more generally on receptor internalization.

Séminaire du LPTMS: Manon Michel

Event-chain paradigm for Monte Carlo methods: Infinitesimal, irreversible and rejection-free Markov chains.

Manon Michel (LPS-ENS)

Monte Carlo methods, by sampling high-dimensional integrals through random walks, have revolutionized the understanding of complex systems. The traditional Metropolis local random walks induce however a high rate of rejections, making any simulations around a phase transition point too expensive. I explain how a consistent interpretation of the mathematical lifting concept and the factorized Metropolis filter yield a new paradigm of irreversible algorithms. This new class of rejection-free algorithms indeed break detailed balance yet fulfill the global one and display moves that are infinitesimal, instead of finite random local moves.

As an application, I exhibit how event-chain algorithms bring considerable speed-ups for general particles systems, but also for classical continuous spin model, including the notoriously difficult problem of spin glasses. In particular, recent work on Heisenberg spins system shows a qualitative reduction of the dynamical scaling exponent, leading to an infinite speed-up. The powerful event-chain algorithm is general yet easy to implement. Its infinite number of samples provide direct access to observables that could not be obtained directly and complex interactions can be factorized into simple components.

Séminaire du LPTMS: Giacomo Gradenigo

Effective thermodynamics for a driven athermal system with dry friction

Giacomo Gradenigo (Grenoble-Alpes)

Which are the situations where an effective thermodynamic theory works even in the lack of thermal equilibrium? Which is the meaning of "temperature" for an out-of-equilibrium system?
I present here the study of a driven athermal system, i.e. a one-dimensional chain of masses connected by harmonic springs and subject to Coulomb dry friction, where answers to the above questions can be provided. Our main result is the evidence that the configuration space of our system is visited under the unfluence of an external driving in an equilibrium-like manner: configurations are sampled according to their energy with a Boltzmann-like probability even in presence of a dissipative dynamics.
This evidence is obtained by comparing the exact calculation of the partition function, obtained by means of a transfer matrix approach, to numerical simulations of the driven dissipative dynamics. Interestingly enough, the existence of a critical point at infinite effective temperature is pointed out.
I conclude presenting different lines along which the spring-block model here introduced can be exploited to investigate the scope of validy of equilibrium statistical mechanics for a driven athermal systems.

Physics-Biology interface seminar: Frédéric Pain

Assessment of optimal parameters for deep optogenetic stimulations in non human primate

Frédéric Pain (IMNC, Université Paris-Sud)

SPECIAL LOCATION (next door from the usual one)

Optogenetics has become ubiquitous in fundamental neuroscience labs as a very powerful tool to unravel brain networks connectivity and cellular mechanisms. Yet, its clinical translation requires a careful assessment of the inocuity of repeated and sustained high power light stimulations. In a preliminary studies to translational research in the field deep brain stimulation for Parkinson's disease we have studied in vivo in anesthetized rats the potential damages and non-physiological effects produced by high power optical neurostimulation in typical optogenetics experiments.2D Maps of light distribution and temperature increase were recorded in wild type anesthetized rats brains for relevant optical stimulation protocols used in optogenetics. The spatial profile of light distribution and heat were correlated and demonstrated as expected a rapid attenuation with distance to the fiber. Temperature increase remains below physiological changes for stimulations up to 400mW/mm². I will present optogenetics issues in a clinical translational context.

Séminaire du LPTMS: Patrice Hello

GW150914 : la naissance de l'astronomie des ondes gravitationnelles

Patrice Hello (LAL, Université Paris Sud)

One century after the initial theoretical prediction, the first direct observation of gravitational waves has been announced in February the 11th by the LIGO and Virgo collaborations. This is an extraordinary accomplishment after decades of worldwide instrumental efforts. I will first recall the nature of gravitational waves and their effect on matter and then I will review the interesting astrophysical sources for LIGO and Virgo. I will then describe the interferometric detection of gravitational waves and the LIGO-Virgo network. I will finally discuss the details of GW150914, the first gravitational wave signal detected by the LIGO instruments and emitted by a Binary Black Hole merger.  

Soutenance HDR Raoul Santachiara

Conformal invariance in statistical and quantum physics

Physics-Biology interface seminar: Franck Jülicher

Shaping a fly wing

Franck Jülicher (MPI-PKS Dresden)

A fundamental question in Biology is to understand the morphogenetic processes by which an organism of complex shape forms from a fertilized egg. This morphogenesis involves the dynamic remodeling of tissues consisting of many cells that grow and divide. The fly wing is an important model system for the study of multicellular dynamics during tissue morphogenesis. During pupal stages, the early fly wing undergoes a spectacular dynamic reorganization that involves cell flows, cell divisions and cell shape changes. This dynamic process generates the final shape of the wing. We characterize tissue remodeling by the contributions of specific cellular processes such as cell shape changes and cell neighbor exchanges to macroscopic shear at different times. We discuss the dynamics and the mechanics of this tissue using theoretical approaches that capture the essential physics of tissue remodeling. Our work suggests that local tissue contraction together with anisotropic active processes drive tissue remodeling in the fly wing. We show that mechanical boundary conditions play a key role in determining the final tissue shape.

Séminaire du LPTMS: Neil Ribe

Acrobatics of liquid ropes

Neil Ribe (FAST, Université Paris-Sud)

A thin `rope' of viscous fluid falling from a sufficient height onto a surface forms a steadily rotating helical coil. Tabletop laboratory experiments in combination with a numerical model for slender fluid ropes reveal that finite-amplitude coiling can occur in four distinct regimes (viscous, gravitational, inertio-gravitational, and inertial) corresponding to different balances among the three principal forces acting on the rope. The model further shows that the onset of coiling has distinct viscous, gravitational and inertial modes that connect smoothly with the corresponding finite-amplitude regimes. I will also discuss some striking examples of non-stationary behavior of liquid ropes, including propagating spiral waves of air bubbles, supercoiling, and the `fluid mechanical sewing machine' wherein the rope leaves complex `stitch' patterns on a moving surface.

Séminaire du LPTMS: Thimothée Thiery

Integrable models of directed polymers on the square lattice

Thimothée Thiery (LPT ENS Paris)

  Motivated by the aim of gaining a better understanding of the KPZ universality class in 1+1 dimension (KPZUC), there has recently been a strong research activity focused on finding exact solutions of models in the KPZUC. Indeed, interesting properties such as the emergence of Tracy-Widom (TW) type fluctuations in such models have up to now only been understood in integrable models. In this talk I will focus on one class of models believed to be in the KPZUC, namely directed polymers (DP) on the square lattice, and on one technique used to find exact solutions: the coordinate Bethe Ansatz (CBA). After an introduction to the links between KPZ and DP, I will show how one can identify the underlying algebraic structure present in any CBA solvable model of DP, therefore allowing to classify all such models. This classification contains all previously known exactly solvable models of DP on the square lattice as well as a new one, the Inverse-Beta polymer. Finally, I will sketch how CBA solvability permits to show the KPZUC (critical exponents and TW fluctuations) of the Inverse-Beta polymer.

Séminaire du LPTMS: Sanjib Sabhapandit *** séminaire exceptionnel ***

Fluctuation theorem for entropy production of a partial system in the weak coupling limit

Sanjib Sabhapandit (Raman Research Institute, Bangalore, India)

Small systems in contact with a heat bath evolve by stochastic dynamics. Here we show that, when one such small system is weakly coupled to another one, it is possible to infer the presence of such weak coupling by observing the violation of the steady state fluctuation theorem for the partial entropy production of the observed system. We give a general mechanism due to which the violation of the fluctuation theorem can be significant, even for weak coupling. We analytically demonstrate on a realistic model system that this mechanism can be realized by applying an external random force to the system. In other words, we find a new fluctuation theorem for the entropy production of a partial system, in the limit of weak coupling

Séminaire du LPTMS: Ritam Sinha *** séminaire exceptionnel ***

Thermalization with Chemical Potentials, and Higher Spin Black Holes

Ritam Sinha (Tata Institute of Fundamental Research, Mumbai)

We study the long time behaviour of local observables following a quantum quench in 1+1 dimensional conformal field theories possessing additional conserved charges besidesthe energy. We show that the expectation value of an arbitrary string of local observablessupported on a finite interval exponentially approaches an equilibrium value. The equilibriumis characterized by a temperature and chemical potentials defined in terms of the quenchedstate. For an infinite number of commuting conserved charges, the equilibrium ensemble isa generalized Gibbs ensemble (GGE). We compute the thermalization rate in a systematicperturbation in the chemical potentials, using a new technique to sum over an infinite numberof Feynman diagrams. The above technique also allows us to compute relaxation times forthermal Green’s functions in the presence of an arbitrary number of chemical potentials. Inthe context of a higher spin (hs[λ]) holography, the partition function of the final equilibriumGGE is known to agree with that of a higher spin black hole. The thermalization rate fromthe CFT computed in our paper agrees with the quasinormal frequency of a scalar field inthis black hole.

Séminaire du LPTMS: Nizar Demni *** séminaire exceptionnel ***

Premier temps d'atteinte de la chambre de Weyl par le processus de Dunkl radial

Nizar Demni (Institut de Recherches Mathématiques de Rennes, Université de Rennes)

J'introduirai le processus de Dunkl radial et expliquerai le lien avec les valeurs propres de certains processus matriciels. Ensuite, je montrerai comment calculer la loi du premier temps d'atteinte de la chambre de Weyl par ce processus et donnerai des formules explicites dans le cas des angles diedraux.

Physics-Biology interface seminar: Seminar cancelled


Séminaire du LPTMS: Rémy Dubertrand

Scattering theory of walking droplets in the presence of obstacles

Rémy Dubertrand (Université de Liège, Belgique)

Walking droplets that are sustained on the surface of a vibrating liquid, have attracted considerable attention during the past decade due to their remarkable analogy with quantum wave-particle duality. This was initiated by the pioneering experiment by Y. Couder and E. Fort in 2006, which reported the observation of a diffraction pattern in the angular resolved profile of droplets that propagated across a single slit obstacle geometry. While the occurrence of this wave-like phenomenon can be qualitatively traced back to the coupling of the droplet with its associated surface wave, a quantitative framework for the description of the surface-wave-propelled motion of the droplet in the presence of confining boundaries and obstacles still represents a major challenge. This problem is all the more stimulating as several experiments have already reported clear effects of the geometry on the dynamics of walking droplets. Here we present a simple model inspired from quantum mechanics for the dynamics of a walking droplet in an arbitrary geometry. We propose to describe its trajectory using a Green function approach. The Green function is related to the Helmholtz equation with Neumann boundary conditions on the obstacle(s) and outgoing conditions at infinity. For a single slit geometry our model is exactly solvable and reproduces some of the features observed experimentally. It stands for a promising candidate to account for the presence of boundaries in the walker's dynamics.

Séminaire du LPTMS: Serguei Brazovskii ***séminaire exceptionnel ***

Modeling of dynamical phase transitions in electronic systems induced by ultra-fast optical pumping

Serguei Brazovskii (LPTMS)

I shall report the recent studies on phenomenological theory of dynamical phase transformations in cooperative electronic systems achieved by means of a femto-second optical pumping. 1. Experiments on charge density waves recovered coherent unharmonic undulations of the order parameter, critical slowing down of the collective mode, and evolution of the particles spectra. The numerical modeling reproduced the dynamical phase transition, and the waves emitted by “earthquakes” from in depth annihilation events of domain walls. 2. The special case of resonance optical pumping to excitons is realized in systems with a neutral-ionic ferroelectric transition. The modeling of the quantum-coherent quasi-condensate of excitons interacting with the order parameter recovers the dynamical realization of the “excitonic insulator” state and spacio-temporal patterns with self-focusing, domains segregation, and local dynamical phase transitions.

Séminaire du LPTMS: Yasar ATAS *** séminaire exceptionnel ***

Exact many-body dynamics of a harmonically quenched Tonks-Giradeau gas at finite temperature

Yasar ATAS (University of Queensland, Brisbane, Australia)

Ultracold quantum gases offer a fascinating and remarkably insightful playground for exploring and understanding the fundamentals of out-of-equilibrium behaviour of quantum matter. Experimental systems whose non-equilibrium dynamics can be described by exactly solvable models of quantum many-body theory play a particularly important role.  A paradigmatic example in this realm is the Tonks-Girardeau (TG) gas  of strongly interacting bosons, previously treated only for zero-temperature dynamics.

In this talk, I will present an exact finite temperature dynamical theory of a harmonically trapped TG gas, and apply it to the problem of breathing-mode oscillations after a sudden confinement quench.  The method is based on the Fredholm determinant expression of the density matrix at finite temperature and provides a straightforward and efficient technique to compute finite temperature properties of TG. I will identify physical regimes for observing a phenomenon of frequency doubling in the oscillations of the momentum distribution of the gas.  Exact results will be compared to a finite-temperature hydrodynamic approach based on local density approximation with the frequency quasi-doubling phenomena surprisingly well captured. The exact dynamical theory developed opens the way to solving finite-temperature problems involving anharmonic traps, as well as arbitrary quench protocols of the trapping potential.


Séminaire du LPTMS: Jean-Noël Fuchs

Magnetic field properties of itinerant electrons in a quasicrystal

Jean-Noël Fuch (LPS, Univeristé Paris-Sud & LPTMC, Université Paris 6)

Quasicrystals are solids with long range order but without periodicity. Here we study the properties of electrons moving in a 2D quasicrystal in the presence of a perpendicular magnetic field. The motion of electrons on a two-dimensional isometric Rauzy tiling is described by a tight-binding model. A magnetic field perpendicular to the plane is applied, which couples to the orbital motion of electrons via a Peierls phase in the hopping amplitudes and to the spin via a Zeeman coupling. For several approximants to the quasicrystal, the energy spectrum in a magnetic field is computed numerically with open or periodic boundary conditions. The resulting Hofstadter butterfly is studied in detail (Landau levels, labeling of gaps, etc). The grand potential is then obtained at a given temperature and chemical potential and its derivatives allow us to study the orbital magnetic susceptibility as well as the spin susceptibility of the Rauzy tiling. The spin susceptibility is a temperature-smoothed version of the zero-field density of states. The orbital susceptibility has a complicated structure - it changes sign several times - as a function of the chemical potential except near the band edges where it is approximatively described by an effective mass tensor.

Séminaire du LPTMS: Nicolas Laflorencie *** séminaire exceptionnel ***

Recent numerical insights on the many-body localization problem

Nicolas Laflorencie (Laboratoire de Physique Théorique, Université Paul Sabatier, Toulouse)

In this talk, I will discuss the problem of the many-body localization (MBL) in one-dimensional (1D) quantum interacting systems.
After an introduction to this quite popular field, I will focus on recent numerical results obtained in Toulouse on the s=1/2 random-field Heisenberg chain. State-of-the-art exact diagonalization techniques have allowed us to unveal several features of MBL physics in 1D, in particular the existence of a mobility edge, and an unexpected slow entanglement spreading in the ergodic regime.

Séminaire du LPTMS: Marie Chupeau

Random walks in confined geometry: convex hull and cover times

Marie Chupeau (LPTMC, UPMC, Paris)

What is the impact of a confinement on random walks? To answer this question, I will first focus on a partial confinement, an infinite reflecting wall, and study how the space taken by a planar random walk is affected by the presence of the wall. The space explored by a random walk in the plane can be characterized by its convex hull, defined as the minimum convex polygon enclosing the trajectory, and in particular by its perimeter. After having studied a one-dimensional version of the problem, I will show that the mean perimeter of the convex hull of a planar random walk exhibits a non-trivial dependance on the initial distance of the walker to the wall, in sharp contrast with the results in one dimension. Second, I will consider a random walk in total confinement and study the time it takes to visit all sites of a given domain. This time, known as the cover time, is a key observable to quantify the efficiency of exhaustive searches, which require a complete exploration of an area and not only the  discovery of a single target. Despite its broad relevance, in biology, ecology, or robotics, the cover time has remained elusive and so far, explicit results have been scarce and mostly limited to regular random walks. I will show how to determine the distribution of the cover time for a broad range of random search processes, and highlight its universal features. I will conclude with an application of this result and discuss how the cover time can be optimized for some research strategies.

Séminaire du LPTMS: David Mukamel

Mixed order phase transitions: from DNA denaturation to jamming processes

David Mukamel (Weizmann Institute of Science, Israël)

Phase transitions of mixed nature, which on the one hand exhibit a diverging correlation length as in second order transitions and on the other hand display a discontinuous order parameter as in first order transitions have been observed in a diverse classes of physical systems. Examples include DNA denaturation, models of wetting, glass and jamming transitions, rewiring networks  and some one-dimensional models with long-range interactions. An exactly soluble Ising model which provides a link between some of these rather distinct classes of systems is introduced. Renormalization group analysis which provides a common framework for studying some of these systems, elucidating the relation between them will be discussed as well as the extreme value statistics of the locally ordered domains that characterize the various phases.

Séminaire du LPTMS: Giovanna Morigi *** séminaire exceptionnel ***

Dissipation-assisted prethermalization in long-range interacting atomic ensembles

Giovanna Morigi (Universität des Saarlandes, Allemagne)

We theoretically characterize the semiclassical dynamics of an ensemble of atoms after a sudden quench across a driven-dissipative second-order phase transition. The atoms are driven by a laser and interact via conservative and dissipative long-range forces mediated by the photons of a single- mode cavity. These forces can cool the motion and, above a threshold value of the laser intensity, induce spatial ordering. The relaxation dynamics exhibits a long prethermalizing behaviour which is first solely dominated by coherent long-range forces, and then by their interplay with dissipation. Remarkably, this final stage is orders of magnitude slower and exhibits correlations which are established by spatially-correlated noise and which go beyond mean-field. This behaviour challenges the common understanding of the role of dissipation in the relaxation dynamics. It further implies that cavity cooling of an atomic ensemble into the selforganized phase can require longer time scales than the typical duration of an experiment.

Séminaire du LPTMS: Cécile Monthus

Many Body Localization Transition : level statistics and entanglement entropy

Cécile Monthus (IPhT, CEA, Saclay)

At the Many-Body-Localization (MBL) Transition,  the statistics of energy levels and the entanglement entropy of eigenstates change: the Many-Body-Localized phase is characterized by the Poisson level statistics and an area-law for the entanglement, while the Delocalized phase is characterized by the Wigner-Dyson level statistics and a volume-law for the entanglement. In the first part of the talk, the Dyson Brownian Motion approach for quantum spin Hamiltonians with random fields will be described: the statistics of energy levels can be studied via Langevin and Fokker-Planck equations, and one obtains the level repulsion exponent $beta$ in terms of the Edwards-Anderson matrix elements. In the second part of the talk, we will consider the strong disorder limit of the MBL transition, where the critical level statistics is close to the Poisson statistics, in order to determine the statistical properties of the rare extensive resonances that are needed to escape from the area-law entanglement of the Localized phase. At criticality, the entanglement entropy can grow with an exponent $0< alpha < 1$ anywhere between the area law $alpha=0$ and the volume law $alpha=1$, as a function of the resonances properties. In addition, the correlation length exponent takes the simple value $nu=1$. Independently of this strong disorder limit, we will explain why for the Many-Body-Localization transition concerning individual eigenstates, the correlation length exponent $nu$ is not constrained by the usual Harris inequality $nugeq 2/d$.  

Physics-Biology interface seminar: Madan Rao

Active Composite Cell Surface - local control of clustering and sorting

Madan Rao (Raman Research Institute & National Centre for Biological Sciences-TIFR, Bangalore)

The surface of a living cell needs to regulate and control its local composition in a variety of contexts such as endocytosis and signalling. We have shown that many cell surface molecules are organised at multiple scales by their coupling to a thin cortical actomyosin layer, which actively drives local cell membrane composition and shape. This includes both transmembrane proteins and lipid anchored proteins, such as GPI-anchored proteins. Crucial to this engagement is the spontaneous emergence of localized contractile platforms in the 2dim active cortical fluid. Both the nanoscale clustering and the mesoscale segregation display the unique signatures of activity. We have recently recapitulated many of these effects in a minimal reconstituted system. I will end by discussing potential implications of this active composite cell surface for the processing of cellular information.

Séminaire du LPTMS: Dmitry Savin

Probing eigenfunction nonorthogonality by parametric shifts of resonance widths

Dmitry Savin (Brunel University, Uxbridge, UK)

Resonances feature themselves in the energy-dependent S-matrix as its poles in the complex energy plane. They can be analytically described as the complex eigenvalues of an effective non-Hermitian Hamiltonian. Notably, the associated resonance wavefunctions are known to be nonorthogonal, which has many important applications ranging from nuclear physics to quantum optics and solid state. This talk will consider an open (scattering) quantum system under the action of a perturbation of its interior. It is demonstrated that the resulting change of resonance widths is a sensitive indicator of the nonorthogonality of resonance wavefunctions, being zero only if those were orthogonal. Focusing further on chaotic systems, we will introduce a new type of parametric statistics in open systems, and derive (within random matrix theory) the distribution of the resonance width shifts in the regime of weakly open system. Applications and recent data for microwave cavities will be also discussed.

Physics-Biology interface seminar: Simon Scheuring

High-Speed Atomic Force Microscopy: The dawn of dynamic structural biochemistry

Simon Scheuring (INSERM & Aix-Marseille Université)

The advent of high-speed atomic force microscopy (HS-AFM [1]) has opened a novel research field for the dynamic analysis of single bio-molecules: Molecular motor dynamics [2,3] membrane protein diffusion [4], assembly [5] and conformational changes [6] could be directly visualized. Further developments for buffer exchange [7] and temperature control [8] during HS-AFM operation provide breakthroughs towards the performance of dynamic structural biochemistry using HS-AFM.

[1] Ando et al., Chem Rev. 2014 Mar 26;114(6):3120-88.
[2] Kodera et al., Nature. 2010 Nov 4;468(7320):72-6.
[3] Uchihashi et al., Science. 2011 Aug 5;333(6043):755-8.
[4] Casuso et al., Nat Nanotechnol. 2012 Aug;7(8):525-9.
[5] Chiaruttini et al., Cell. 2015 Nov 5;163(4):866-79.
[6] Ruan et al., 2016, in preparation
[7] Miyagi et al., Nat Nanotechnol. 2016, in press
[8] Takahashi et al., 2016, in preparation

Séminaire du LPTMS: Grégory Schehr

Finite temperature free fermions and the Kardar-Parisi-Zhang equation at finite time

Grégory Schehr (LPTMS, Université Paris-Sud)

I will consider a system of N one-dimensional free fermions confined by a harmonic well. At zero temperature (T=0), it is well known that this system is intimately connected to random matrices belonging to the Gaussian Unitary Ensemble. In particular, the density of fermions has, for large N, a finite support and it is given by the Wigner semi-circular law. Besides, close to the edges of the support, the quantum fluctuations are described by the so-called Airy-Kernel (which plays an important role in random matrix theory). What happens at finite temperature T ? I will show that at finite but low temperature, the fluctuations close to the edge, are described by a generalization of the Airy kernel, which depends continuously on temperature. Remarkably, exactly the same kernel arises in the exact solution of the Kardar-Parisi-Zhang (KPZ) equation in 1+1 dimensions at finite time. I will also discuss recent results for fermions in higher dimensions.    

Physics-Biology interface seminar: David Pastré

Probing protein interactions on a microtubule bench by fluorescence microscopy: Application to YB-1, a mRNA-binding protein

David Pastré (Université d'Évry)

A typical procedure adopted by biologists to analyze protein interactions is to take advantage of the high throughput capability of the two hybrid system, generally in yeast, or that of the combination of affinity purification with mass spectrometry to obtain an exhaustive list of potential partners for a protein of interest. However, these assays require cell lysis, antibodies and adsorption onto non physiological substrates leading to false positives and false negatives. There is therefore a need to control the relevance of the proposed interactions in a context closer to native conditions, such as in living mammalian cells. To that end, novel methods are currently developed to provide a better view on protein interactions. In line with this, we have proposed a novel technology to detect and quantify direct or indirect protein interactions by fluorescence microscopy in living mammalian cells using microtubules as platforms. Microtubules are micrometer-long rigid cylinders of about 25 nm in diameter that are present in the cytoplasm of all eukaryotic cells. Due to their geometry, they provide an ideal surface to probe molecular interactions by fluorescence microscopy. As a proof of concept, we used microtubules to probe the interactions between mRNA-binding proteins like YB-1 in the cytoplasm.

Soutenance de thèse : Pierre Ronceray

Active contraction in biological fiber networks

Large-scale force generation is essential for biological functions such as cell motility, embryonic development, wound healing and muscle contraction. In these processes, forces generated at the molecular level by motor proteins are transmitted by disordered fiber networks, resulting in large-scale active stresses.  While fiber networks are well characterized macroscopically, this stress generation by microscopic active units is not well understood. In this Thesis, I present a comprehensive theoretical and numerical study of force transmission in elastic fiber networks. I show that the linear, small-force response of the networks is remarkably simple, as the macroscopic active stress depends only on the geometry of the force-exerting unit. In contrast, as non-linear buckling occurs around these units, local active forces are rectified towards isotropic contraction, making the local geometry of force exertion irrelevant. This emergent contractility is amplified by non-linear force transmission through the network. This stress amplification is reinforced by the networks' disordered nature, but saturates for high densities of active units. Our predictions are quantitatively consistent with experiments on reconstituted tissues and actomyosin networks, and that they shed light on the role of the network microstructure in shaping active stresses in cells and tissue.

Séminaire du LPTMS: Jean-Louis Barrat

Elastoplastic models for amorphous plasticity

Jean-Louis Barrat (Université Joseph Fourier, Grenoble)

I will present our current understanding, based on microscopic simulations and experiments, of the elementary mechanisms that govern the deformation and flow of amorphous materials. I will show how these mechanisms can be incorporated into  simple lattice models governed by long range elastic interactions, discuss the mean field analysis of these models as well as fluctuation related issues such as the  statistics of avalanche like events.


  • Modeling deformation and flow of disordered materials, J-L. Barrat, J.J. de Pablo, MRS Bulletin 32, 941 (2007)
  • Driving rate dependence of avalanches statistics and shapes at the yielding transition, C. Liu, E. Ferrero, F. Puosi, J-L. Barrat, K. Martens, PRL 116 065201 (2016)
  • On the relevance of disorder in athermal amorphous materials under shear, E. Agoritsas, E. Bertin, K. Martens, J-L. Barrat EPJE 38 15071 (2015)
  • Rheology of athermal amorphous solids: Revisiting simplified scenarios and the concept of mechanical noise temperature, A. Nicolas, K. Martens, J-L. Barrat EPL 107 44003 (2014)

Séminaire du LPTMS: Purushottam Dixit

Metastability transition in genome evolution and population structure of bacterial species

Purushottam Dixit (Columbia University, US)

Two processes govern diversification of bacterial genomes. While point mutations always increase population diversity, horizontal gene transfers act as a cohesive force at the population level. Currently, a detailed quantitative understanding of how these two opposing forces shape bacterial evolution at the level of individual genes, genomes, and populations is lacking. In a theoretical model, we identify two qualitatively distinct phases in the dynamics of genome evolution, characterized by the second eigenvalue of the Markov process describing evolution.  In the  divergent phase the cohesion due to recombination is not sufficient to overcome mutational drift. As a consequence both individual genes and the entire genomes within the same species keep diverging from each other in the course of evolution.  At the population level, transient clusters of sub-populations are continuously formed and dissolved. In contrast, in the metastable phase,  the recombination has the upper hand.  In this phase, genomes of descendants of a pair of sister cells remain close to each other for long periods of time but eventually escape the pull of recombination and diverge indefinitely.  The population of the entire species remains genetically cohesive and stable over time and does not fragment into sexually isolated sub-populations. Real bacterial examples are discussed as well.

Séminaire du LPTMS: Vladimir Kravtsov *** séminaire exceptionnel ***

Ergodic transition on disordered random regular graph

Vladimir Kravtsov (ICTP, Trieste, Italy)

We consider statistics of eigenfunctions on the random regular graph with on-site disorder. The results of numerical diagonalization show a  transition in the statistics of eigenfunctions deeply inside the region of extended states that is interpreted as a transition from extended ergodic to extended  non-ergodic (multifractal) states. A comparison with the Rosenzweig-Porter random matrix theory and a generalized population dynamics calculations is discussed.

Physics-Biology interface seminar: Anne-Florence Bitbol

Inferring interaction partners from protein sequences

Anne-Florence Bitbol (Laboratoire Jean Perrin, UPMC)

SPECIAL LOCATION (due to renovation work at LPS)

Specific protein-protein interactions play crucial roles in the stability of multi-protein complexes and in signal transduction. Thus, mapping these interactions is key to a systems-level understanding of cells. However, systematic experimental identification of protein interaction partners is still challenging, while a large and rapidly growing amount of sequence data is now available. Is it possible to identify which proteins interact just from their sequences? We propose an approach based on sequence covariation, building on methods used with success to predict the three-dimensional structures of proteins from sequences alone. Our method identifies specific interaction partners with high accuracy among the members of two ubiquitous prokaryotic protein families, and paves the way to identifying novel protein-protein interactions directly from sequence data.

Séminaire du LPTMS: duo Frédéric Abergel-Damien Chalet

Mathematical modelling of limit order books

Frédéric Abergel (Laboratoire MICS, Ecole Centrale de Paris)

The limit order book is at the core of any modern, electronic financial markets. In this talk, I will present some results pertaining to their statistical and mathematical properties. Questions such as ergodicity and stationarity, dependencies, relation betwen time scales... will be addressed and sometimes answered too.


Nonparametric statistics from cumulative sample sums

Damien Chalet (Laboratoire MICS, Ecole Centrale de Paris)

Building on recent results on record statistics of random walks, a novel family of nonparametric statistics is introduced. Single-sample statistics are shown to provide location tests, while two-sample statistics correspond to distribution equality tests. The power and efficiency of such statistics are analyzed.

Séminaire du LPTMS: Pierre Ronceray & Stefano Spigler *** séminaire exceptionnel ***

1) Stefano Spigler : "Distribution of avalanches in perturbed disordered systems"

Disordered systems under a perturbation usually show a stepwise response, due to the existence of many, sample dependent, local energy minima. In this talk I will show that the distribution of the jumps (static avalanches) of the total energy, under some generic assumptions, depends on the specific system only through its (replica symmetry breaking) order parameter.

2) Pierre Ronceray : "Suppression of crystalline order by competing liquid structures"

Upon cooling, a molecular liquid will eventually either crystallize, or continuously arrest in a amorphous state known as a glass.  What features of the liquid determine this fate? Why is crystallization avoided in glass-formers?  As the liquid is cooled down, the local arrangements of its constituent molecules will increasingly correspond to low-energy configurations.  It is likely that the local structure characterizing the crystalline ground state will be found among these stable arrangements, and one can thus expect that it will accumulate on cooling. However, other non-crystalline structures can also be stable in the liquid, and compete with the accumulation of crystalline order. On what conditions can such a competition stabilize the supercooled liquid to the point of dynamical arrest? We tackle this question using a lattice model where both the crystalline order and the non-crystalline structures are explicitly specified in terms of local configuration of binary spins.  We show that in order to efficiently suppress crystallization, the non-crystalline structure should be energetically and entropically favoured. Importantly, we demonstrate that it must also geometrically antagonize crystalline order. We quantify this effect in terms of overlap of structures: crystal-''agonist'' structures with good overlap with the crystal tend to facilitate crystallization, while crystal-antagonist ones tend to impede it and dramatically increase the crystal nucleation time.  

Séminaire du LPTMS: Alessio Recati

Coherently driven 2-component Bose gases

Alessio Recati (Università di Trento, Italia)

In the present talk I will give an overview on the physics of 2-component Bose gases in presence of an spin exchange - Rabi - term. The Rabi term reduces the gauge symmetry of the system  and a gap opens for relative phase excitations. The latter has many consequences. In particular it  leads to the formation of vortex dimers coupled  through sine-Gordon solitons. Results on the dynamics of the vortex dimer are presented. The system exhibits also a second order (quantum) phase transition between a neutral (paramagnetic) and a polarised (ferromagnetic) state, provided the interspecies interaction is large enough. I will discuss some phenomena related to the phase transition and to the  two phases  and the interplay  between Rabi coupling and density-density interaction:

(i) at a mean-field level for a continuous model - which well described atoms in a shallow trap.

(ii) for a one-dimensional single band Hubbard model - which should properly describe atoms  confined in an one-dimensional optical lattice. 

Séminaire du LPTMS: Steven Tomsovic *** séminaire exceptionnel ***

Universality in the Entanglement and Localization of Strongly Chaotic Subsystems

Steven Tomsovic (Washington State University, USA)

The entanglement and localization in eigenstates of bipartite systems with strongly chaotic subsystems are studied as a function of interaction strength.  Excellent measures for this purpose are the von-Neumann entropy, Havrda-Charvát-Tsallis entropies, and the inverse participation ratio.  All the entropies are shown to follow a remarkably simple exponential form, which describes a universal and rapid transition to nearly maximal entanglement for increasing interaction strength. An unexpectedly exact relationship between the subsystem averaged inverse participation ratio and purity (linear entropy) is derived that infers the transition in the localization as well.

Physics-Biology interface seminar: Hélène Berthoumieux

Fluctuations in in vivo reactive systems.

Hélène Berthoumieux (LPTMC - Université Pierre & Marie Curie, Paris)

SPECIAL LOCATION (due to renovation work at LPS)

For a chemist, a living cell is a reactive system in which the concentrations of biomolecules are not determined by the thermodynamics but are controlled by energy sources maintaining the system in an out-of-equilibrium state. Chemical reaction networks are thus perturbed by a thermal noise and the fluctuations of these energy sources. Theoretical and experimental studies have shown that the fluctuations of in vivo systems break the fluctuation-dissipation theorem, which is a result of statistical physics at equilibrium. One can thus ask what information is contained in the correlation functions of protein concentrations and how they relate to the response of the reactive network to a perturbation. Answers to these questions are of prime importance to extract meaningful parameters from the in vivo fluorescence correlation spectroscopy data. Here, we present a theoretical study of the fluctuations of the concentration of a reactive species involved in a cyclic network that is in a non-equilibrium steady state perturbed by a noisy force, taking into account both the breaking of detailed balance and extrinsic noises that are known to be important in a cell.

Séminaire du LPTMS: Giuseppe Policastro

AdS/CFT and condensed matter systems

Giuseppe Policastro (LPT-ENS, Paris)

 In this talk I will give an introduction to the idea of the holographic correspondence and some of its applications. In recent years holography has been shown to be a very useful tool to get theoretical insight into systems that can not be studied by more conventional methods, typically because there is no small expansion parameter; such systems include the QCD quark/gluon plasma, high-temperature superconductors, condensed matter systems in the vicinity of a quantum critical point, and strongly correlated electron systems (non-Fermi liquids). I will discuss how, while it is usually not possible to match any given realistic system with a corresponding holographic description, the method is useful for deriving results of a general nature. I will highlight the main successes of the method and some of the open questions.

Séminaire du LPTMS: Olivier Giraud

Weinberg representation, coherent and anticoherent spin states

Olivier Giraud (LPTMS)

We introduce a new representation for density matrices of symmetric quantum states in finite-dimensional Hilbert spaces. This representation is particularly suited to the description of spin-j states in terms of linear combinations of spin coherent states. We describe several applications of such a representation: identification of the classical state closest to a given quantum state, construction of anticoherent spin states, classicality and separability criteria.

Physics-Biology interface seminar: Markus Basan

Tradeoffs between fast growth and adaptability shape microbial phenotypes

Markus Basan (ETH Zurich, Switzerland & Harvard University, USA)

SPECIAL LOCATION (due to renovation work at LPS)

Microorganisms exhibit a striking diversity of phenotypes in different conditions. Changes in growth rates are accompanied by large variations in metabolic strategies, gene expression and cell size. However, the molecular basis and underlying rationale of many of these complex patterns remains poorly understood. We illustrate how a quantitative approach, based on establishing empirical relations between cellular phenotypes, can help to elucidate such questions by focusing on three long-standing biological problems: the origin of overflow metabolism, the control of cell size and finally we provide an outlook on the emergence of severe, multi-hour lag phases. Coarse-grained models yield a quantitative and predictive understanding of phenotypic patterns under environmental as well as genetic perturbations and can even shed light on underlying molecular mechanisms. A common theme that emerges from these seemingly diverse questions is the existence of fundamental tradeoffs between fast growth and the ability to swiftly adapt to environmental changes or stress conditions.

Séminaire du LPTMS: Leonardo Mazza

Interacting gases with a synthetic gauge field in the synthetic dimension

Leonardo Mazza (Ecole Normale Supérieure / PSL Research University, CNRS)

Synthetic ladders realized with one-dimensional alkaline-earth(-like) fermionic gases and subject to a gauge field represent a promising environment for the investigation of quantum Hall physics with ultracold atoms. We unveil the existence of a hierarchy of fractional insulating and conducting states by means of both analytical bosonization techniques and numerical methods based on the density-matrix renormalization group algorithm. We show that such states can be exploited for constructing a topological Thouless pump where the charge transported after one cycle is quantized to fractional values and demonstrate this behavior with a full many-body time-dependent calculation.

1 - S. Barbarino, L. Taddia, D. Rossini, L. Mazza, R. Fazio, Nat. Commun. 6, 8134 (2015)

2 - S. Barbarino, L. Taddia, D. Rossini, L. Mazza, R. Fazio, New J. Phys. 18, 035010 (2016)

3 - L. Taddia, E. Cornfeld, D. Rossini, L. Mazza, E. Sela, R. Fazio, in preparation

Séminaire du LPTMS: Mustansir Barma *** séminaire exceptionnel ***

Ordered Phases in Coupled Driven Systems

Mustansir Barma (TIFR Centre for Interdisciplinary Sciences, Tata Institute of Fundamental Research, Hyderabad, India)

Instabilities in the hydrodynamic equations for coupled, driven diffusive systems can herald macroscopic phase separation. We study the resulting ordered phases in a lattice model of light and heavy particles sliding under gravity on a fluctuating surface in 1D or 2D.   We find strong phase separation with fluctuationless phases for particles, but a different organization for the landscape, leading to intricate steady state dynamics of interfaces between phases.

Soutenance de thèse: Joël Bun


Applications of Random Matrix Theory for high-dimensional statistics.


In the present era of Big Data, new statistical methods are needed to decipher large dimensional data sets that are now routinely generated in almost all fields – physics, image analysis, genomics, epidemiology, engineering and finance, to quote only a few. It is very natural to try to identify common causes that explain the joint dynamics of a large number of quantities. The primary aim of this thesis is to understand theoretically the so-called curse of dimensionality that describe phenomena which arise in high-dimensional space using Random Matrix Theory. Special care is devoted to the statistics of the eigenvectors of large noisy matrices, which turn out to be crucial for many applications. Moreover, I will present how to build reliable estimators that are consistent with the dimension of the problem. In the case of correlation matrices, the estimator we obtain provide better performance than all previously proposed methods for real-world applications within financial markets.

Séminaire du LPTMS: Christian Hagendorf *** séminaire exceptionnel ***

The spin one XXZ chain and symmetry classes of alternating sign matrices

Christian Hagendorf (Université Catholique de Louvain, Belgique)

The spin 1/2 XXZ Heisenberg chain is a prototype model in the statistical mechanics of magnetism. It describes the interaction of localised spins on a one-dimensional chain with anisotropy. In 2000, Razumov and Stroganov observed for a particular anisotropy a surprising appearance of integer sequences in the spin-chain ground state. These sequences enumerate so-called alternating sign matrices. They are well known to combinatorialists through rather different problems such as determinant calculus or the enumeration of plane partitions. In this talk, I give an overview of the relation between spin chains, models of statistical mechanics and alternating sign matrices. Furthermore, I discuss the spin one XXZ chain whose ground state displays for any anisotropy connections to enumeration problems of alternating sign matrices with symmetries. The proof of these connections uses tools from quantum integrability, in particular the algebraic Bethe ansatz and the quantum separation of variables method. (Joint work with Alexi Morin-Duchèsne)

Séminaire informel du LPTMS: Ricardo Mendonça

Some new problems on random walks and exclusion processes

Ricardo Mendonça (Uni. de Sao Paolo)

In this informal talk I am going to present two problems related with random walks and exclusion processes: the longest-increasing subsequence (LIS) problem for random walks and the symmetric simple exclusion process in discrete time over graphs. The first problem corresponds to a new incarnation of the LIS problem for discrete i.i.d. variables (random permutations), but now with correlated variables, while the second problem touches on many interesting (and hard) problems in combinatorics, computer science, and graph theory. So far I have more questions than answers, so your attendance, comments and contributions are welcome!

Séminaire du LPTMS: Alexandre Nicolas

Fluctuations in the flow of disordered solids and complex fluids: the ups and downs of simple models

Alexandre Nicolas (LPTMS)

Disordered solids, such as foams, concentrated emulsions and metallic glasses, are examples of complex systems made of complex entities. However, when describing or modelling their macroscopic response, most of the complexity of the entities under study can often be left aside. This applies in particular to the fluctuations in their flow. In this talk, I will present some successes and caveats associated with simple descriptions of these fluctuations.

A very broad class of materials, disordered solids span all damping regimes, from overdamped to underdamped. This fact calls for a clarification of the effect of inertia on their flow. The first part of my talk will be dedicated to showing that, macroscopically, the effect of inertia is identical to that of simple (but shear-rate-dependent) thermal fluctuatiions.

But this does not imply that any fluctuations can be modelled as thermal fluctuations. Accordingly, I will then insist on a caveat associated with these simple models, by questioning the popular description of the mechanical noise (i.e., the stress fluctuations induced local microruptures in the deformed material) as an effective activation temperature.

Finally, if time allows, I will very briefly touch upon fluctuations in a very different type of complex systems, namely, crowds of pedestrians, and the statistical fluctuations in evacuation times.

Séminaire du LPTMS: Pierre Illien

Active transport in complex environments

Pierre Illien (R. Peierls Centre for Theor. Phys., Oxford University, UK)

Understanding the dynamics of an active or a biased particle in a host medium which hinders its motion is an ubiquitous problem of nonequilibrium statistical physics with important applications, from transport in biological systems to active microrheology. Going beyond the usual effective descriptions of the environment of the active tracer, we propose a model which takes explicitly into account the correlations between the dynamics of the tracer and the response of the bath. Relying on paradigmatic models of statistical mechanics, we determine analytically exact and approximate solutions to this out-of-equilibrium problem. They reveal many striking effects, such as superdiffusion in the high-density limit, velocity anomalies or negative differential mobility.

Séminaire du LPTMS: Isabelle Guyon

Network Reconstruction: the Contribution of Challenges in Machine Learning

Isabelle Guyon (Laboratoire de Recherche en Informatique, Université Paris-Sud)

Networks of influence are found at all levels of physical, biological, and societal systems: climate networks, gene networks, neural networks, and social networks are a few examples. These networks are not just descriptive of the “State of Nature”, they allow us to make predictions such as forecasting disruptive weather patterns, evaluating the possible effect of a drug, locating the focus of a neural seizure, and predicting the propagation of epidemics. This, in turns, allows us to device adequate interventions or change in policies to obtain desired outcomes: evacuate people before a region is hit by a hurricane, administer treatment, vaccinate, etc. But knowing the network structure is a prerequisite, and this structure may be very hard and costly to obtain with traditional means. For example, the medical community relies on clinical trials, which cost millions of dollars; the neuroscience community engages in connection tracing with election microscopy, which take years before establishing the connectivity of 100 neurons (the brain contains billions). This presentation will review recent progresses that have been made in network reconstruction methods based solely on observational data. Great advances have been recently made using machine learning. We will analyze the results of several challenges we organized, which point us to new simple and practical methodologies.   La reconstruction du réseau: la contribution des défis en Machine Learning : Les réseaux d'influence se retrouvent à tous les niveaux des systèmes physiques, biologiques et sociaux: réseaux climatiques, réseaux de gènes, réseaux de neurones, et réseaux sociaux en sont quelques exemples. Ces réseaux ne sont pas seulement descriptif d’un «état de la nature», ils nous permettent de faire des prédictions telles que la prévision des conditions météorologiques, l'évaluation de l'effet possible d'un médicament, la localisation d’un foyer épileptique, et la prévision de la propagation des épidémies. Ceci nous permet alors d'intervenir pour obtenir les résultats souhaités: évacuer les populations d'une région avant qu’elle soit frappée par un ouragan, administrer un traitement, vacciner, etc. Mais la connaissance de la structure du réseau est une condition préalable à l’application de ces méthodes, et cette structure peut être très difficile et coûteuse à obtenir avec des moyens traditionnels. Par exemple, la communauté médicale s’appuie sur les essais cliniques, qui coûtent des millions de dollars; la communauté des neurosciences analyse des images de microscopie électorale, ce qui prendra des années avant d'établir la connectivité de 100 neurones (alors que le cerveau en contient des milliards). Cette présentation examinera les récents progrès qui ont été faits dans les méthodes de reconstruction de réseaux fondées uniquement sur des données d'observation. De grands progrès ont été récemment réalisés grâce à l'apprentissage des machines (machine learning). Nous allons analyser les résultats de plusieurs défis que nous avons organisées, qui nous pointent vers de nouvelles méthodes simples et pratiques.

Séminaire du LPTMS: David Mukamel

Long-range correlations and anomalous heat transport in momentum conserving models

David Mukamel (Weizmann Institute of Science, Israël)

  • Anupam Kundu, Ori Hirschberg and & Mukamel Long-range correlations in stochastic transport with energy and momentum conservation, J.Stat.Mech.: Theor. Exp., 033108 (2016)
  • Julien Cividini, Anupam Kundu, A. Miron & David Mukamel, Temperature profile and boundary conditions in an anomalous heat transport model,  preprint cond-mat arXiv:1609.06614.

Physics-Biology interface seminar: Daan Noordermeer

CTCF mediates allele-specific 3D domain structure at paternally imprinted gene loci

Daan Noordermeer (I2BC, Gif-sur-Yvette, France)

NOTE THE NEW LOCATION (due to renovation work at LPS)

Imprinted genes are mammalian genes where only one copy (allele) is active, depending on whether it is inherited from the mother or the father. This selective activity is determined by allele-specific DNA methylation at defined sites in the genome, so-called Imprinting Control Regions (ICRs). Recent microscopy studies by the group of Robert Feil (IGM-Montpellier, France) have revealed that imprinted genes are differently organized in the cell nucleus, depending on their parental origin (Kota et al., 2014). We have used high-resolution 4C-seq studies (Circular Chromosome Conformation Capture) to dissect the mechanisms and dynamics of this differential organization at the Dlk1-Dio3 and Igf2-H19 loci.

Recent studies have revealed that mammalian genomes are organized into Topologically Associating Domains (TADs) that demarcate ‘gene regulatory neighborhoods’ (Dixon et al., 2012). These physical domains are formed through a mechanism of ‘loop extrusion’ of the DNA fiber, with borders that are demarcated by opposing binding sites of the architectural CTCF protein (Fudenberg et al., 2016). I will show that the imprinted Dlk1-Dio3 and Igf2-H19 loci are organized into different DNA domains, determined by allele specific CTCF binding.

Both the Dlk1-Dio3 and Igf2-H19 loci are contained within large, invariant Topologically Associating Domains (TADs). The presence of the CTCF protein at the unmethylated ICRs on the maternal allele allows the establishment of new loops within the TADs. This result in the formation of a domain that acts like a cage, thereby shielding regulatory elements from nearby genes.

At the paternal alleles, DNA methylation at the ICR inhibits CTCF binding. As a result, the paternal Igf2-H19 allele displays little specific organization within the TAD. In contrast, at the paternal Dlk1-Dio3 locus, loops are formed between more distant unmethylated CTCF sites. The paternal allele therefore forms a much larger subdomain that is contained within the TAD.

Our work, for the first time, shows that constitutive TADs can have a markedly different allele-specific internal domain organization. Moreover, it shows that methylation-dependent DNA binding of the CTCF protein at ICRs guides the process of loop extrusion, thereby change the 3D structure of chromatin domains. We speculate that the imprinted patterns of gene expression at these loci are mostly imposed by the maternal 3D architecture, supporting previous genetic studies.

Séminaire du LPTMS: Armita Nourmohammad *** séminaire exceptionnel ***

Statistical physics of molecular evolution: from gene regulation to immune system

Armita Nourmohammad (Lewis Sigler Institute, Princeton University)

Molecular phenotypes, such as gene expression or protein binding affinities are important targets of natural selection, and are often subject to time-dependent pressure form the environment. However, the map between encoding DNA sequences and molecular phenotypes is often too difficult to quantify. In this talk, I will show that universality is an emerging property of complex phenotypes, which are encoded by multiple genomic loci. I will introduce a non-equilibrium framework for adaptive dynamics of such phenotypes in time-dependent environments, and between co-evolving populations. In time-dependent environments, changes in the environment drive the evolution of the species, but not vice versa. As an example, I will present strong evidence that adaptation dominates the evolution of gene expression levels in Drosophila. Co-evolving populations reciprocally affect the fitness of each other, acting as time-dependent environments with feedback. As an example, I will show evidence of co-adaptation between interacting cellular populations of HIV viruses and the antibody repertoire of a patient over the course of an infection. In particular, I discuss the conditions for emergence of broadly neutralizing antibodies, which are recognized as critical for designing an effective vaccine against HIV.

Séminaire du LPTMS : Mikhail Zvonarev

Quantum motion of an impurity through a medium at zero temperature in one spatial dimension

Mikhail Zvonarev (LPTMS, Université Paris-Sud)

I will report on a theoretical and experimental progress in understanding the dynamics of an impurity injected into a one-dimensional quantum liquid. I will show that the momentum distribution of the impurity subject to a constant external force exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. As a consequence, the impurity exhibits periodic dynamics that is interpreted as Bloch oscillations, which arise even though the quantum liquid is translationally invariant. I will also discuss a quantum flutter phenomenon, whose essence is that the impurity injected into a liquid with some initial momentum sheds only a part of it to the background gas, and forms a correlated state that no longer decays in time; furthermore, if the initial momentum is large enough, the impurity undergoes long-lived oscillations.  

Séminaire du LPTMS: Peter Karpov *** séminaire exceptionnel ***

Modeling of phase transitions and patterns formation in ensembles of solitons

Peter Karpov (National University of Science and technology "MISiS", Moscow, Russia)

Vulnerability of broken symmetries of cooperative electronic states gives rise to topological defects like electronic vortices, walls, stripes. A typical quasi-one-dimensional architecture brings these objects to the microscopic scale giving rise to solitons as elementary particles taking from electrons their major roles as carriers of charge or spin. Because of the long-range ordering, the solitons experience unusual super-long-range forces leading to a sequence of phase transitions in their ensembles: the higher-temperature transition of the confinement and the lower one of aggregation into macroscopic walls. In this talk I shall review the available analytical theory and present the recent results of an extensive numerical modeling for ensembles of both neutral and charged solitons in two- and three-dimensional systems. We suggest a specific Monte Carlo algorithm preserving the number of solitons, which substantially facilitates the calculations, allows to extend them to the three-dimensional case and to include the important long-range Coulomb interactions. The results confirm the first confinement transition, except for a very strong Coulomb repulsion, and demonstrate a pattern formation at the second transition of aggregation.   Reference: P. Karpov and S. Brazovskii, PRB 94, 125108 (2016).

Séminaire du LPTMS: Friedrich Wagner *** commun avec l'IPN ***

Optimized and self optimized magnetic confinement

Friedrich Wagner (Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald)

Magnetic confinement in toroidal fusion systems has been developed to a degree, which allows building the first fusion reactor, ITER. The goal of ITER is the operation of a Deuterium-Tritium plasma under conditions, which allow the production of 500 MW fusion power with a power amplification of Q=10. But ITER is still an experiment and it will solve for its basic concept – the tokamak – many fundamental research issues. As the tokamak concept has drawbacks – prone to current driven instabilities and intrinsically pulsed – alternatives are pursued in parallel. The stellarator is the second most advanced toroidal magnetic confinement concept whereat the largest and most advanced device of this line, Wendelstein 7-X, has just started operation in Greifswald, Germany. Wendelstein 7-X is an experiment for basic research, which nevertheless should demonstrate the reactor viability of this concept. The major difference between tokamak and stellarator is the way the poloidal field, essential for confinement, is produced. In case of the tokamak, it is generated by a strong toroidal current induced within the toroidal plasma ring. In stellarators, it is produced externally by currents within strongly shaped coils.
The system optimization aims i.a. at the quality of the thermal insulation of the plasma to reach high core temperatures by economic means. The tokamak benefits from the reduced system symmetry with continuous symmetry in toroidal direction. The stellarator plasma is unavoidably 3-dimensional and toroidally periodic with the corollary of insufficient collisional confinement. But with a rigorous system optimization those field properties which are decisive for the “laminar” radial transport can be made 2-dimensional. Systems with these features are called quasi-symmetric and they should have similar collisional transport like truly symmetric systems, be more stable and intrinsically suitable for steady-state operation.
On the other hand, fusion plasmas are thermodynamically open systems where turbulent processes govern radial transport in symmetric and quasi-symmetric systems. Like in other open systems also, self-organized processes can determine important and relevant plasma characteristics. Under high plasma pressure giving rise to a large turbulence level, the turbulent eddies can develop a coherence where ultimately the energy of the turbulence field is transformed into the kinetic energy of a macroscopic flow. This flow turns out to be sheared acting back onto its original drive, the turbulence field. Within a sub-millisecond time scale, the turbulence is quenched in a highly non-linear process and a quiescent regime with good confinement emerges. This regime is called high-confinement mode (H-mode) and is the basis to reach the goals of ITER. Some of these mechanisms play also a role in other open systems with classical turbulence.
In this seminar the physics of plasma optimization and self-optimization will be presented.

Physics-Biology interface seminar: Aleksandra Walczak

Optimal immune systems

Aleksandra Walczak (LPT-ENS, Paris)

NOTE THE NEW LOCATION (due to renovation work at LPS)

Biological organisms have evolved a wide range of complex strategies to defend themselves against pathogens. I will present a common evolutionary framework that balances the benefits and costs involved in protection against pathogenic environments to maximize the long growth rate of populations. I will show that such a general evolutionary perspective recovers the basic forms of known immunity. I will then focus on adaptive immunity which is based on a combinatorically encoded repertoire of receptors that protects organisms from a diverse set of pathogens. A well-adapted repertoire should be tuned to the pathogenic environment to reduce the cost of infections. I will discuss a general approach for predicting the optimal repertoire that minimizes the cost of infections contracted from a given distribution of pathogens.

Séminaire du LPTMS: Mikhail Zubkov

Momentum space topology, anomalous quantum Hall effect, and the absence of equilibrium static chiral magnetic effect

Mikhail A. Zubkov (ITEP, Moscow, Russia & LMPT, Université François Rabelais, Tours)

Using derivative expansion applied to the Wigner transform of the two - point Green function we analyze the anomalous quantum Hall effect (AQHE), and the chiral magnetic effect (CME). The corresponding currents are proportional to the momentum space topological invariants. We reproduce the conventional expression for the Hall conductivity in the ideal tight – binding models of (2+1) D topological insulators. At the same time using this method we prove, that in the equilibrium (3+1) D theory the static CME is absent in a certain class of solids, as well as in the properly regularized relativistic quantum field theory. References:
  • Absence of equilibrium chiral magnetic effect”, Phys. Rev. D 93 (2016) no.10, 105036, doi:10.1103/PhysRevD.93.105036, arXiv:1605.08724 [hep-ph], by M. A. Zubkov
  • Wigner transformation, momentum space topology, and anomalous transport”, Annals Phys. 373 (2016) 298-324, doi:10.1016/j.aop.2016.07.011,  arXiv:1603.03665 [cond-mat.mes-hall],  by M. A. Zubkov

Séminaire du LPTMS: Daniel Braun **** commun avec le LPS ****

Ultimate limits of quantum measurements

Daniel Braun (Institut für Theoretische Physik, Universität Tübingen)

Quantum enhanced measurements aim at measuring physical quantities with increased precision by using quantum mechanical effects. At the heart of most theoretical efforts in the field lies quantum parameter estimation theory (q-pet), which provides a powerful and general framework for analysing the performance of measurements in the quantum realm. Most of the work has gone into the investigation of the benefits of entangled states, which are, however, limited in the presence of decoherence. In this talk I will give a wider perspective of quantum enhanced measurements, and explore in particular possibilities of quantum enhancement without the use of entanglement. The examples discussed range from the sensitivity of artificial noses, over the measurement of chemical potentials, to measurements of the speed of light. The latter example goes well beyond q-pet, and I will show that the ultimate precision of speed-of-light measurements is set by both quantum and general relativistic effects, offering a new perspective on possible quantum fluctuations of space-time geometry. ***** ATTENTION : le séminaire "théorie de la matière condensée sur le plateau" a lieu au Parc Club, RdC du bât.18-20  ***** Plan d'accès : http://www.leparcorsayuniversite.com/uploads/plan-masse-2012.jpg

Séminaire du LPTMS: Jacek Grela *** séminaire exceptionnel ***

Dynamical non-Hermitian random matrices - Hydrodynamics and relevance of eigenvectors

Jacek Grela (LPTMS, Université Paris-Sud)

I present a study of non-Hermitian dynamical random matrices with special emphasis put on identifying relevant degrees of freedom. Hermitian case will be revisited and compared to and the formalism of characteristic polynomials and Green's function formalism discussed. We will present how a hydrodynamical picture of Burgers equations arises and what a special role eigenvectors play in it. Examples and interpretation of results in a couple of statistical physics models will be delivered. References :  

Séminaire du LPTMS: Frédéric van Wijland

How far from equilibrium is active matter?

Frédéric van Wijland (Laboratoire Matière et Systèmes Complexes, Université Paris 7 Denis Diderot)

Active Matter in 2016 encompasses such a broad range of systems that it is almost a challenge to define what's behind this terminology. One of the unifying characteristics to all active systems is that individual constituents convert an external continuous supply of energy into random motion. On a representative model of an active system, we will explain how, by looking at the movie of its time evolution, one can tell whether, and how far, the system lies away from equilibrium. We will also provide a microscopics based explanation of some of the macroscopic effects that are specific to active particle systems. Reference : É. Fodor, C. Nardini, M. E. Cates, J. Tailleur, P. Visco and F. van Wijland, "How far from equilibrium is active matter?", Phys. Rev. Lett. 117, 038103 (2016)

Séminaire LPTMS: Jakub Otwinowski *** séminaire exceptionnel ***

The diversity of evolutionary rates

Jakub Otwinowski (Department of Biology, University of Pennsylvania, Philadelphia)

We have evidence of evolution happening over billions of years, yet we have little understanding of how long it takes for anything to evolve. In other words, we often do not understand quantitatively what affects the rate of evolution over long time-scales. I will show how spatial structure can impede the rate of evolution with a model related to surface growth physics. The fitness landscape is also an important factor in the rate of evolution, and I will discuss how it can be approximated from high-throughput sequence-function experiments. Finally, I show how the rate of evolution can be defined in coevolving populations with non-equilibrium measures of adaptation.

Séminaire du LPTMS: Giulia De Rosi

Collective oscillations of trapped atomic gases in low dimensions: a tool for the investigation of collisional processes

Giulia De Rosi (Università di Trento, Italia)

Since 20 years, both theoretical and experimental investigation of collective oscillations has been carried out in trapped quantum gases. Fermionic and bosonic gases at different interaction, temperature, dimensions and geometrical configurations have been studied. We show a unified description of collective modes for all above atomic gases. All collective frequencies have been calculated by solving a single equation for the flow velocity derived starting from the hydrodynamic equations. Moreover, by using the sum- rule approach, we predict a different excitation signal at high temperature for the dipole compression mode in the hydrodynamic (single frequency) and collisionless (beating of 2 frequencies) regime for a one dimension (1D) harmonically trapped Bose gas. This theoretical prediction opens promising perspectives for the experimental investigation of collisional effects in 1D.
  • G. De Rosi and S. Stringari, Collective oscillations of a trapped quantum gas in low dimensions, Phys. Rev. A 92 , 053617 (2015).
  • G. De Rosi and S. Stringari, Hydrodynamic VS collisionless dynamics of a 1D harmonically trapped Bose gas, to appear in Phys. Rev. A, preprint arXiv:1608.08417

Physics-Biology interface seminar: Sandrine Morlot

Deficient ribosome biogenesis is an early marker of cellular senescence

Sandrine Morlot (IGBMC, Strasbourg)

NOTE THE NEW LOCATION (due to renovation work at LPS)

Saccharomyces cerevisiae is a powerful model organism to study replicative aging as asymmetric division gives rise to an aging mother cell and a rejuvenated daughter cell [Mortimer and Johnston 1959, Egilmez and Jazwinski 1989]. However the cellular mechanisms controlling replicative lifespan and the rejuvenation process are still poorly understood partly due to the technical limitations of following individual cells from birth to death. In this context, we have developed a high-throughput microfluidic device to follow up to 3200 single cells in parallel throughout their lifespan under the microscope. Thanks to this technology, we have established a timeline of events occurring successively during cellular aging. We have observed that cells experience a sharp transition into senescence. Indeed yeasts divide regularly every 90 minutes until a senescence entry point (SEP) which occurs after 20 generations. After this point, cell cycles strongly slow down until death [Fehrmann et al. Cell Reports 2013]. Furthermore we have measured that the SEP is preceded by an abrupt increase in the nuclear volume and more specifically in the size of nucleolus. The nucleolus is the nuclear compartment where ribosome biogenesis is initiated. This age-dependent nuclear defect is retained by the mother only, as daughter cells recover a normal nucleus and nucleolus, in agreement with the daughter cell rejuvenation paradigm. We have characterized that pre-ribosome particles accumulate in the nucleolus approximately 10 hours before entering into senescence. Our analysis suggests that this deficiency in ribosome biogenesis triggers cellular senescence.

Séminaire du LPTMS: Nicolas Rougerie

Emergent anyons in quantum Hall physics

Nicolas Rougerie (LPMMC, Université de Grenoble)

Anyons are hitherto hypothetical particles with quantum statistics different from those of bosons and fermions. They can occur only in low dimensions, 2D being the case of interest for this talk. A convenient description of 2D anyons is to treat them as ordinary bosons or fermions coupled to magnetic flux tubes. This leads to a model in terms of an effective Hamiltonian, which contains rather peculiar two- and three-body interactions. Recently, we have considered the case of tracer particles immersed in a so-called Laughlin liquid. We argued that, under certain circumstances, these become anyons, as made manifest by the emergence of the aforementioned Hamiltonian in an effective description of their motion. The anyon Hamiltonian is notoriously hard to solve even in simple cases, and well-controled simplifications are highly desirable. I will discuss a possible mean-field approximation, that we have rigorously derived in a limit corresponding to "almost bosonic anyons".   Anyons émergents dans le cadre de l'effet Hall quantique : Les anyons sont des particules, jusqu'ici hypothétiques, avec des statistiques quantiques différentes de celles des bosons et des fermions. Elles ne sont possibles qu'en basse dimension, 2D étant le cas considéré dans cet exposé. Une description pratique des anyons 2D est de les voir comme des bosons ou des fermions couplés à des tubes de flux magnétique, avec le Hamiltonien associé. Récemment, nous avons considéré le cas de particules traceuses immergées dans un liquide de Laughlin et avons montré que, dans certaines circonstances, les particules traceuses sont décrites par ce Hamiltonien effectif. Ce dernier est notoirement difficile à étudier, et des simplifications bien contrôlées sont hautement désirables. Je décrirais une possible approximation de champ moyen, que nous avons rigoureusement dérivée dans une certaine limite correspondant à des anyons "quasi-bosoniques". Référence :

Séminaire du LPTMS: Julien Tailleur

The Physics of Active Matter

Julien Tailleur (Laboratoire Matière et Systèmes Complexes, Université Denis-Diderot)

Over the past ten years, there has been a growing interest among physicists for 'active matter', a codename that encompasses systems in which energy is taken from the environment to generate self-propulsion at the single particle level. Active particles, such as run-and-tumble bacteria, self-diffusiophoretic colloids or actin filaments in motility assays, are strongly out-of-equilibrium and exhibit much richer behaviours that their passive counterpart. In this talk I will review recent progresses regarding the physics of active particles. I will show how simple concepts like pressure, the force density exerted by assemblies of particles on their container, play a new role for active systems because of the lack of equation of state. I will also show how new collective phenomena emerge, from the transition to collective motion to the existence of cohesive matter without cohesive forces, that have no counterpart in thermal equilibrium.

Physics-Biology interface seminar: David Head

High-fidelity computational modelling of biofilms and fibre networks

David Head (University of Leeds, UK)

NOTE THE NEW LOCATION (due to renovation work at LPS)

Mathematical models of complex systems can help develop fundamental insight and accelerate the development of novel materials and therapeutic treatments, but the simplifications that must necessarily be made reduce both validity and predictability, even for complex models that are solved numerically. Here I will describe recent results in two research streams aimed at developing and validating high-fidelity in silico models for biofilms and fibre networks, aiming to reduce the realism gap to experimental systems while maintaining sufficient performance to permit solution for times and system sizes of interest. The first is a bespoke model for biofilms in which mechanical relaxation between cells replaces the "pushing" rules typically used, and has been applied to the archetypal biofilm of dental plaque, revealing insight into long-term ecological dynamics not easily assayed experimentally. Secondly, I will discuss dynamic simulations of peptide gels and collagen hydrogel scaffolds, both having important applications as novel biomaterials but where network formation must also be simulated to better approximate the experiments; reaching realistic time scales is a substantial challenge.

Séminaire du LPTMS: Andrei Bernevig

Topological insulators and semimetals

Andrei Bernevig (Princeton University, USA)

I will review topological states of matter with specific emphasis on the symmetry-protected topological insulators in different dimensions. I will show how Majorana states can be realized at the end of topological chains with fermionic parity, as well as in vortex cores in 2D chiral superconductors. I will then show how some of the simplest tensor Braided categories come out of simple, intuitive physical arguments about braiding vortices. I will present a slew of topological insulators protected by time reversal, symmorphic, and then nonsymmorphic symmetries. At then end, I will present new work aiming to provide a complete and comprehensive classification of all topological insulators existent in crystals.

Séminaire du LPTMS: duo Thibault Congy/Aurélien Grabsch

Nonlinear polarization waves in two-component Bose-Einstein condensates

Thibault Congy (LPTMS, Université Paris-Sud)

T. Congy, A. M. Kamchatnov & N. Pavloff, Dispersive hydrodynamics of nonlinear polarization waves in two-component Bose-Einstein condensates, preprint cond-mat.quant-gas arXiv:1607.08760

Truncated linear statistics associated with the top eigenvalues of random matrices

Aurélien Grabsch (LPTMS, Université Paris-Sud)

A. Grabsch, S. N. Majumdar & C. Texier, Truncated linear statistics associated with the top eigenvalues of random matrices, preprint cond-mat.stat-mech arXiv:1609.08296

Physics-Biology interface seminar: Atef Asnacios

Mechanosensing: insights from experimental physics at the single-cell scale

Atef Asnacios (MSC, Université Paris-Diderot, France)

NOTE THE NEW LOCATION (due to renovation work at LPS)

As part of their physiological functions, most cells need to adapt to their mechanical environment. In particular, the rigidity of the extracellular matrix was shown to control cell traction forces, shape, and ultimately cell differentiation. In this context, most studies focused on the role of biochemical regulation in rigidity sensing.

In contrast to this biochemical signaling-centered approach, our aim is to reveal the physical/mechanical phenomena involved in mechanosensing. To this end, we have developed original single-cell techniques combining mechanical measurements (traction force, mechanical power…) with monitoring of cell structures (evanescent wave microscopy). In particular, we have designed a unique protocol allowing us to change the effective stiffness felt by a single cell in real time (~0.1 second), thus allowing us to show that an early purely mechanical response of single cell to stiffness indeed does exist.

We will present the results of experiments combining single-cell traction force measurements, dynamic control of stiffness, and monitoring of adhesion complexes, and will discuss how cell shape (~ contact angle) and mechanical adaptation to rigidity (~ impedance matching) could control cell fate.

Séminaire du LPTMS: Vladimir Kravtsov *** séminaire exceptionnel ***

Phases and phase transitions on disordered Bethe lattice: analytical treatment by one-step replica symmetry breaking

Vladimir Kravtsov (ICTP, Trieste, Italy)

We apply the replica trick to the problem of localization on disordered  Bethe lattice. We show that the formulation of the problem in terms of  the one-step replica symmetry breaking leads naturally to existence of  the multifractal phase and to phase transitions between this phase and  the localized and fully extended (ergodic) delocalized phase. We prove  the symmetry m→1-m of the problem with respect to the replica symmetry  breaking parameter m and use this symmetry to derive exact expressions  for the transition points in terms of the Lyapuniov exponents and the  branching number K. We also suggest a simple approximation which allows  to compute the fractal dimension D1 in the multifractal phase and the  critical values of disorder at the phase transitions with the best known  so far accuracy. Reference :
  • B. L. Altshuler, L. B. Ioffe and V. E. Kravtsov, Multifractal states in self-consistent theory of localization: analytical solution, preprint arXiv:1610.00758

Séminaire du LPTMS: Andrey Lokhov ***séminaire exceptionnel***

Optimal structure and parameter learning of Ising models and calibration of the D-Wave quantum computer

Andrey Lokhov, Los Alamos National Laboratory

Reconstruction of structure and parameters of a graphical model from binary samples is a problem of practical importance in a variety of disciplines, ranging from statistical physics and computational biology to image processing and machine learning . The focus of the research community shifted towards developing universal reconstruction algorithms which are both computationally efficient and require the minimal amount of expensive data. In this talk, we introduce a new method, Interaction Screening, which accurately estimates the model parameters using local optimization problems. We provide mathematical guarantees that the algorithm achieves perfect graph structure recovery with an information-theoretically optimal number of samples and outperforms state of the art techniques, especially in the low-temperature regime which is known to be the hardest for learning. As an application, we show how the method can be used for an efficient calibration of the D-Wave quantum computer.