Séminaires de l’année 2015

Séminaire du LPTMS: Florian Angeletti

Matrix-correlated random variables: a dialogue between statistical physics and signal processing

Florian Angeletti, National Institute for Theoretical Physics, Stellenbosch

Finding statistical descriptions of non-equilibrium stationary state is often an arduous task. For specific systems, like ASEP or 1D diffusion-reaction systems, stationary solutions have been characterized using matrix product representations. These representations generalize the product structure of independent random variable to matrices; the non-commutativity of matrices generates correlation while preserving many of the algebraic properties of expectation. Depending on the matrix structure, the correlation can vary from short-range to long-range correlation. Moreover, from a signal processing perspective, these matrix-correlated random variables can be recast as specific Hidden Markov Models. In this talk, we propose to investigate the general statistical properties of this mathematical framework, with the long-term hope to improve our understanding of related physical systems. In particular, we shall focus on the statistical properties of sums of such random variables. Do we have a large deviation principle for this sum? Can we find analogous of the law of large number or the central limit theorem?

Tri-séminaire de Physique statistique : Pascal Viot

 Stochastic flow models

Pascal Viot (LPTMC, UPMC)

Filtration, and flow in micro/nano-channels and traffic flow are examples of processes subject to blocking when the channel conveying the particles becomes too crowded. We investigate a concurrent flow model where particles enter a channel randomly. If at any time two particles are simultaneously present in the channel, failure occurs. We obtain the exact solution for the survival probability, the distribution of the number of particles that pass before failure, and the instantaneous flux exiting the channel. Several generalizations of this simple model are also studied. We also consider  a counterflow model with two opposing Poisson streams. There is no restriction on the number of particles passing in the same direction, but blockage occurs if, at any time, two opposing particles are simultaneously present in the passage. All relevant quantities have been obtained exactly.

[1] Non-Markovian Models of Blocking in Concurrent and Countercurrent Flows, A. Gabrielli, J. Talbot and P. Viot Phys. Rev. Lett,  110, 170601 (2013) [2] Stochastic model of single-file flow with reversible blockage, Chloé Barré, Julian Talbot and Pascal Viot EPL, 104 60005 (2013) [3] Irreversible Blocking in Single-File Concurrent and Countercurrent Particulate Flows, J. Talbot, A. Gabrielli  and P. Viot J Stat Mech  to be published (2015)

Physics-Biology interface seminar: Thomas Gibaud

Shape controlled filaments suspensions – rheology and dynamics

Thomas Gibaud (ENS Lyon)

The mechanical behavior of a suspension of rigid and semiflexible filaments has been studied in great detail. In comparison the effect of the filament geometry has been relatively unexplored. Here, we hijack flagellar filaments from their original purpose in order to develop a versatile model rod-like bio-colloid whose shape and length can be tuned. We present experimental results on the rheological behavior of suspensions of (1) straight, (2) curly and (3) semi-straight/semi-curly flagella with an identical average contour length. We find that (1) and (2) show an elastic behavior at intermediate time but that (3) remains elastic and does not flow at long times. Using fluorescence microscopy, we track individual filament and find that this elastic plateau is related to a cage in which the filament is trapped for a certain among of time. Taken together, this highlights the role of filament geometry in suspension mechanics.


Séminaire du LPTMS: Savvas Zafeiropoulos

Chiral Random Matrix Theories for Wilson Fermions

Savvas Zafeiropoulos, Institut für Theoretische Physik - Goethe-Universität Frankfurt

We introduce Random Matrix Models for the Wilson-Dirac operator of Quantum Chromodynamics (QCD) and QCD-like theories. We show that they are equivalent to the epsilon-limit of the chiral Lagrangian for Wilson chiral perturbation theory (WchPT), which is the low energy effective theory describing lattice QCD with Wilson fermions. Results are obtained for the Hermitian Dirac operator of two-color QCD with quarks in the fundamental representation of the color group as well as the non-Hermitian Dirac operator of SU(3) QCD. For the latter case we propose a novel way for the derivation of the Low Energy constants of WchPT. Comparisons of our analytical results with direct numerical simulations of random matrices are shown.

PhDs' days

PhDs' days

Séminaire du LPTMS: Davide Forcella

Electromagnetic properties of viscous charged fluids

Davide Forcella (Université Libre de Bruxelles)

Recently the gauge/gravity correspondence has provided new interesting applications to high energy strongly coupled plasmas and strongly correlated condensed matter systems. At the same time, in an apparently unrelated field, there have been enormous progresses in the ability to engineer electromagnetic devices with exotic properties: such as negative refraction (energy and phase velocity for the electromagnetic field in the medium are in opposite directions), cloaking, photonics black-holes, etc. In the talk, building on these two recent developments, I will discuss the electromagnetic properties of viscous charged fluids, consisting for example of electrons in certain solids or high energy physics plasmas. In particular I will show that finite viscosity leads to multiple modes of evanescent electromagnetic waves at a given frequency, one of which is characterised by a negative index of refraction. I will then underline the concept of electron viscosity in actual electron systems (rarely discussed in literature and not measured in experiments yet) in condensed matter and its consequences on the electromagnetic response. In particular I will discuss how optical spectroscopy can be used to probe the electron viscosity. Indeed finite viscosity has the effect to decrease the reflectivity of a metallic surface, and support the occurrence, in a half-infinite sample, of oscillations of the electromagnetic field intensity as a function of distance from the interface. I will conclude with some comments on future perspectives and applications.

Séminaire du LPTMS : Grigory Astrakharchik

Lieb's soliton-like excitations in harmonic traps

Grigory Astrakharchik (Universitat Politècnica de Catalunya, Barcelona)

We study the solitonic Lieb II branch of excitations the in one-dimensional Bose gas in homogeneous and trapped geometry. Using Bethe-ansatz Lieb's equations we calculate the "effective number of atoms'' and the "effective mass'' of the excitation. The equations of motion of the excitation are defined by the ratio of these quantities. The frequency of oscillations of the excitation in a harmonic trap is calculated. It changes continuously from its "soliton-like'' value $omega_h/sqrt{2}$ in the high density mean field regime to $omega_h$ in the low density Tonks-Girardeau regime with $omega_h$ the frequency of the harmonic trapping. Particular attention is paid to the effective mass of a soliton with velocity near the speed of sound.

G. E. Astrakharchik and L. P. Pitaevskii Lieb’s soliton-like excitations in harmonic traps EPL, 102, 30004 (2013)

Physics-Biology interface seminar: Olivier Rivoire

How history shapes geometry in a model of protein evolution

Olivier Rivoire (Laboratoire interdisciplinaire de Physique, UJF Grenoble)

The interactions between amino acids in a protein are heterogeneous but not arbitrary: they enable proteins to perform specific biochemical "functions". Understanding these interactions may require, however, looking beyond current functional requirements, to the evolutionary history of proteins. I will illustrate this point with a simple statistical mechanics model, which I will motivate with observations and experiments on natural proteins. The model relates the parameters controlling the evolution of a protein to the organization of the interactions inside its structure.

Séminaire du LPTMS: Jonas Ranft

Dynamics of synaptic domains: scaffold-protein aggregation at the postsynaptic membrane

Jonas Ranft, Laboratoire de Physique Statistique de l'ENS

The dynamic nature of synaptic receptor populations poses the question of how the number of receptors can be reliably controlled in spite of their continuous dynamic reorganization. Increasing evidence points to the stabilizing role of submembranous scaffold clusters that are formed by oligomerization of specific proteins. However, it is not clear in turn how these scaffold clusters form, what controls their sizes and what is their lifetime as compared to the protein turnover they are themselves subject to. Here, we investigate the dynamics of synaptic domains by studying a simplified model of the scaffold dynamics at the postsynaptic neuronal membrane. We find by using particle-based simulations, as well as analytic calculations, that for all parameter regimes, the system eventually reaches a non-equilbrium stationary state with a characteristic distribution of cluster sizes. The typical cluster size scales with the turnover rate and depends on how the diffusive mobility of clusters varies with their sizes. Lateral desorption of scaffold molecules can give rise to a critical size below which clusters dissolve, thus favoring a bimodal distribution of synapse sizes. The results provide a basis for the stability of clusters in the face of molecular noise, which is a necessary requirement for long-term memory and learning.

Séminaire du LPTMS: Alexandre Lazarescu

Fluctuations of the current and optimal profiles in the open Asymmetric Simple Exclusion Process

Alexandre Lazarescu, Institute for Theoretical Physics, KU Leuven

The asymmetric simple exclusion process (ASEP), where particles perform biased random walks with hard core repulsion, is one of the most studied model in non-equilibrium statistical physics. It has the mathematical property of being integrable, which makes it a good candidate for in-depth exact calculations. The quantity of particular interest there is the current of particles that flows through the system due to the bias of the jumps. In this presentation, we will see how we can obtain information about the distribution of that current, through various techniques: integrability, macroscopic fluctuation theory, and asymptotic direct diagonalisation. This allows us to build the phase diagram for the large deviations of the current, and examine the corresponding density profiles in each of its five phases. We show that two situations arise: in most phases, the system can be described hydrodynamically, but in one phase, where the current is larger than the limit set by hydrodynamics, the system becomes highly correlated. If time allows it, we will also see how these techniques and results could be generalised to some other observables or models.

Séminaire exceptionnel du LPTMS

The Self-Journal of Science : an ethical and realistic alternative for scientific publishing

Michaël Bon, CEA Saclay

Today, the publication industry hurts Science : extremely high costs, unreliable peer review, Science output decided by self-righteous scientific authorities, addiction to impact factor, chaotic management of the ever-increasing number of papers... Unscientific stakes have a much too big importance in the evolution of Science and outsiders, i.e. younger generations of scientists, are the one who suffer mostly from this situation. I will defend the idea that all these flaws root in the pyramidal organization of scientific edition. I will present a solution which allows theoretically and overall practically to set the system right. It comes in the form a repository (like arXiv) equipped with novel ideas and tools allowing an horizontal self-management of Science by the whole scientific community, to make an optimal peer review as well as an objective and unfalsifiable evaluation of articles. Its social logic makes it always self-regulated towards scientific quality. This new platform, "The Self-Journal of Science" is readily available and just has to be used, as it is free and as its particular nature makes it viral. I therefore welcome all scientist concerned with the global quality and credibility of Science to attend this lecture. Le site du journal : http://sjscience.org/

Physics-Biology interface seminar: Andrew Callan-Jones

The Cytoskeleton as an Active Gel: Modelling Cell Polarization, Shape Change, and Migration

Andrew Callan-Jones (Université Paris-Diderot)

Cell polarization and shape change are required for large-scale movements during embryo development and cancer metastasis. I will present recent work to understand these phenomena by studying two model systems: zebrafish embryos during gastrulation and confined HeLa cells. In both cases, individual cells are observed to undergo a novel type of polarization and transformation to a motile state that is crucially dependent on elevated levels of contractility in the actomyosin cortex. Polarization of zebrafish cells in vitro can be triggered by stimulating myosin activity: initially quasi-spherical, immobile cells switch to a polarized state characterized by a high cortical density at the cell rear, persistent cortical actin flows, and a distinctive pear-like morphology. Compressing HeLa cells between two plates results in a transition from a well-spread, mesenchymal-type migration mode to a rounded-up one sustained by cortical flow, and displaying an actin rich uropod at the rear, reminiscent of zebrafish. We have modeled these cell mechanical responses using active gel theory, a continuum-level description of out-of-equilibrium behavior of the cytoskeleton. In this talk, I will first provide a summary of this theory, and will then show how it accounts for the principal features of contractility-based polarization: cortical flow and density changes, cell shape change, and migration.

Tri-séminaire de Physique Statistique : Camille Aron

Driven-dissipative phyics: the relaxing virtues of a bath

Camille Aron (Princeton University)

Despite being ubiquitous from nano to macroscale, far-from-equilibrium physics remains largely an uncharted territory in which the standard toolbox of equilibrium statistical mechanics is a priori unavailable. Besides its fundamental relevance, nonequilibrium physics can also be viewed as a resource to achieve a new type of control over matter. In this talk, I will review recent developments in our understanding of driven-dissipative physics and illustrate how "bath engineering" offers a promising route to realize exotic many-body states in a nonequilibrium fashion, such as large-scale long-lived quantum entanglement or high-temperature superconductivity.

Séminaire du LPTMS: Stanislao Gualdi

Tipping points and monetary policy in a stylized macroeconomic agent-based model

Stanislao Gualdi, Centrale Supélec

Traditional approaches in economics rely on the assumption that economic agents are identical, non-interacting and rational. Within this framework, economic instabilities would require large exogenous shocks, when in fact small local shocks can trigger large systemic effects when heterogeneities and interactions are taken into account. The need to include these effects motivate the development of agent-based models (ABMs), which are extremely versatile and allow to take into account more realistic behavioural rules. In this talk we introduce a simple ABM, explore the possible types of phenomena that it can reproduce and propose a methodology that characterizes a model through its phase diagram. We then generalize the model with the aim of investigating the role and efficacy of the monetary policy of a central bank. We show that the existence of different equilibrium states of the economy can cause the monetary policy itself to trigger instabilities and be counter-productive.

Séminaire du LPTMS: Andrii Gudyma

Breathing modes of one-dimensional trapped BEC

Andrii Gudyma (LPTMS)

We calculate the breathing mode frequency in a one-dimensional Bose gas confined to a harmonic trap. We predict a smooth crossover from Thomas–Fermi Bose–Einstein condensate (BEC) regime to Gaussian BEC regime using Hartree approximation and sum rules approach, adopted to small system sizes. Local density approximation (LDA) correctly captures the crossover from Tonks-Girardeau to Thomas-Fermi BEC regimes. Hartree and LDA predictions can be continuously matched for N > 25 particles, providing a complete zero-temperature description for large number of particles and resulting in a non-monotonic reentrant behavior of the breathing frequency. For smaller number of particles (ranging from N = 2 to N = 25) we perform extensive diffusion Monte Carlo simulations. This permits us to obtain a complete picture, applicable to arbitrary number of particles and any repulsive interaction strength. We provide perturbative analysis for both weak and strong coupling regimes. We revisit Innsbrick group experiment [Science 325, 1224 (2009)], analyzing the measurements done in Thomas–Fermi Gaussian BEC crossover, and demonstrate that the breathing frequency follows a reentrant behavior as a function of the interaction strength in full agreement with our theory.

Séminaire du LPTMS: Iacopo Mastromatteo

Latent liquidity models: an universal mechanism for the anomalous response of financial markets

Iacopo Mastromatteo, CMAP Ecole polytechnique

Financial markets are remarkable information-processing systems: even though they are driven out-of-equilibrium by long-range correlated forces, they can swiftly remove predictability from their drive and output a diffusive price signal. As empirical results seem to indicate, this is due to anomalous response properties which are to a large extent universal. Yet, the details about the onset of such a peculiar response have not yet been modeled in a fully consistent way. I will first present a general framework in which such universality is justified on the basis of general principles (dimensionality and existence of a price). I will then characterize the response of market to trades (price impact) in a specific reaction-diffusion model which can account for several stylized facts characterizing the microstructure of financial markets.

Tri-séminaire de Physique Statistique: Vincent Hakim

Collective cell motion and statistical mechanics : global modes and fluctuations in confined cell assemblies.

Vincent Hakim (Laboratoire de Physique Statistique, ENS, Paris)

In different biological processes, cells move in a coordinated way. Several experiments have quantitatively investigated this phenomenon. I will describe experimental results obtained by the team of P Silberzan (Institut Curie, Paris) as well as a simple model of interacting persistent random walkers that we have developed to phenomenologically describe collective cell motion. The model helps to explain the observed dynamics of a confined cell assembly which displays stochastic reversals of global rotational motion and pulsatile collective modes.

Séminaire du LPTMS: Cesare Nardini

Non-Equilibrium long-range interacting systems : kinetic theory and large deviations

Cesare Nardini, Edinburgh University

Long-range interacting systems include gravitational systems, plasma in the low density limit, two-dimensional and geophysical fluid models. In many physical contexts, long-range interacting systems are found to be out of equilibrium because of external driving. Examples come from climate dynamics, plasma physics and, recently, experimental setups with cold atoms driven by laser light. Geophysical flows, for example, are characterised by their self-organisation into large-scale coherent structures such as jet-streams, cyclones and anti-cyclones. The description of these structures, of their evolution and of extreme events they undergo (such as the sudden switch of the system between multiple attractors) are outstanding problems at the interface between climate science and statistical mechanics. In order to address the description of driven long-range interacting systems in a theoretical way, we concentrate in this talk on models as simple as possible that still retain the following two main characteristics: non-local (i.e. long-range) nature of the interactions and broken detailed balance (i.e. non-equilibrium dynamics). We present results both for particle systems and quasi two-dimensional flows, and we show that their dynamics can be described very accurately in the limit where there is a separation of time scales between the evolution of the mean state and the evolution of the fluctuations around it. The main theoretical tools developed are kinetic theory and large deviations techniques: the accuracy of the results obtained will be compared to direct numerical simulations. Ongoing work and perspectives on a combination of kinetic theory and large deviations theory to describe multistability in a very performant way will also be described.

Séminaire du LPTMS: Sergej Moroz

Few- and many-body quantum physics of p-wave interacting fermions in two dimensions

Sergej Moroz, University of Colorado, Boulder

Due to the current search of Majorana fermions, the quantum physics of two-dimensional identical fermions with short-range p-wave interactions is of immediate interest. In the first part of the seminar, I will talk about the effective theory of a chiral p+ip fermionic superfluid at zero temperature. This theory naturally incorporates the parity and time reversal violating effects such as the Hall viscosity and the edge current. In the second part, I will concentrate on the few-body physics near a p-wave resonance and introduce the super Efimov effect- a new type of few-body universality manifesting itself by a tower of three-body bound states with a double-exponential scaling.

Physics-Biology interface seminar: Arezki Boudaoud

Stochasticity and robustness in growth and morphogenesis

Arezki Boudaoud (ENS Lyon)

How do organisms cope with natural variability to achieve well-defined morphologies and architectures? We addressed this question by combining experiments with live plants and analyses of stochastic models that integrate cell-cell communication and tissue mechanics. This led us to counterintuitive results on the role of noise in development, whereby noise is either filtered or enhanced according to the level at which it is acting.

Séminaire du LPTMS: Erik Sorensen

Dynamics at a Quantum Critical Point: Combining Quantum Monte Carlo and Holography

Erik Sorensen, LPT Toulouse

The real time dynamics near quantum critical points have proven very challenging to obtain both from a numerical and analytical perspective. Here we focus on the superfluid-insulator transition occurring for bosons on a lattice. New large-scale QMC results have made it possible to obtain very precise results for many quantities in particular the frequency dependent conductivity at imaginary frequencies.  Since the numerical results remain confined to imaginary times/frequencies additional tools are needed to extend the results to the rest of the complex plane. Here, recent insights from conformal field theory and holography have yielded a wealth of information that combined with the QMC results yield quantitative and experimentally testable results for the frequency-dependent conductivity near the quantum critical point.

Séminaire exceptionnel : Pierre Charles (UPMC)

Pensée antique et science contemporaine

Pierre Charles (UPMC)

La science contemporaine nous amène à reconsidérer certains textes anciens. La mécanique quantique, tout particulièrement, utilise de nombreux concepts que l’on retrouve dans les philosophies de Pythagore, Héraclite, Platon, Aristote et Carnéade. Les théories modernes du langage et de la signification, la nature des nombres et des propositions logiques, nous offrent l’opportunité d’une confrontation.

Séminaire du LPTMS: Vincent Michal

Many-body physics of bosons in (quasi)disorder

Vincent Michal (LPTMS)

Anderson's localization was introduced more than fifty years ago in the context of single-particle physics: the eigenstates of the Shrödinger equation with a random potential may be localized in space leading to the absence of particle transport. It was realized quite recently that the localization idea is actually much more general and applies to a variety of situations. This program was put forward to solve the problem of electron lifetime in a quantum dot and lead to the discovery of the many-body localization physics (Anderson localization in the many-body Fock space). In the solid state context this approach has proven to be very useful to tackle the long-standing problem of the transport of interacting localized single-particle states in the absence of phonons. This gave rise to the demonstration of the energy threshold between the insulating and metallic regimes [1]. The physics of interacting bosons [2] is also very interesting particularly in connection with ongoing experiments on cold atomic gases. In this seminar I would like to show to you very recent results on the finite-temperature fluid-insulator transition of bosons in one dimension (1D). On the one hand I will present the case of the quasiperiodic potential (superposition of two incommensurate periodic potentials) and give predictions regarding the transport phase diagram including the unexpected freezing with heating behaviour [3]. On the other hand I will talk about the transport of strongly-interacting 1D bosons in the random potential and I will show the reentrance of the insulating state at strong interaction, hence completing the transport phase diagram of interacting disordered bosons at finite temperature [4]. References: [1] D.M. Basko, I.L. Aleiner, and B.L. Altshuler, Metal–insulator transition in a weakly interacting many-electron system with localized single-particle states, Annals of Physics 321, 1126 (2006). See references therein. [2] I. L. Aleiner, B. L. Altshuler, and G. V. Shlyapnikov, A finite-temperature phase transition for disordered weakly interacting bosons in one dimension, Nature Physics 6, 900 (2010). [3] V.P. Michal, B.L. Altshuler and G.V. Shlyapnikov, Delocalization of Weakly Interacting Bosons in a 1D Quasiperiodic Potential, Phys. Rev. Lett. 113, 045304 (2014). [4] V.P. Michal, I.L. Aleiner, B.L. Altshuler, G.V. Shlyapnikov, Finite-Temperature Fluid-Insulator Transition of Strongly Interacting 1D Disordered Bosons, arXiv:1502.00282.

Soutenance d'HDR: Olivier Giraud

Critical systems and quantum multifractality

Séminaire du LPTMS : Victor Yakovenko

Economic inequality from statistical physics point of view

Victor Yakovenko (Department of Physics, University of Maryland, College Park, USA)

Similarly to the probability distribution of energy in physics, the probability distribution of money among the agents in a closed economic system is also expected to follow the exponential Boltzmann-Gibbs law, as a consequence of entropy maximization. Analysis of empirical data shows that income distributions in the USA, European Union, and other countries exhibit a well-defined two-class structure. The majority of the population (about 97%) belongs to the lower class characterized by the exponential ("thermal") distribution. The upper class (about 3% of the population) is characterized by the Pareto power-law ("superthermal") distribution, and its share of the total income expands and contracts dramatically during booms and busts in financial markets. Globally, data analysis of energy consumption per capita around the world shows decreasing inequality in the last 30 years and convergence toward the exponential probability distribution, in agreement with the maximal entropy principle. Similar results are found for the global probability distribution of CO2 emissions per capita. All papers are available at http://physics.umd.edu/~yakovenk/econophysics/. For recent coverage in Science magazine, see http://www.sciencemag.org/content/344/6186/828

Séminaire du LPTMS : Alexey Sossinsky

Energetic reduction of elastic knots to normal form: Gradient descent along the Euler functional

 Alexey Sossinsky, Independent University of Moscow, Laboratoire Poncelet

Simple physical experiments show that a deformed knotted resilient elastic wire always returns to its equilibrium shape (which we call the normal form of the wire knot). The aim of this research (joint work with S.Avvakumov and O.Karpenkov) is to construct a mathematical model of such physical knots. This is done by supplying the space of knots with an energy functional and performing gradient descent w.r.t. the functional. The energy functional that we use is the sum of the Euler functional (the integral along the curve defining the knot of the square of the curvature) and a simple repulsive functional (that forbids crossing changes). Using a discretized version of gradient descent along that functional, we construct an algorithm that brings the knot to the shape that minimizes its energy. The algorithm is implemented in a computer animation that shows the isotopy bringing the knot to normal form. We will show some of these animations and demonstrate a few of the physical experiments with wire knots, compare the results of physical and mathematical experiments, and outline how our algorithm can be used in practice to recognize knots and to unravel trivial knots.

Physics-Biology interface seminar: Gerald G. Fuller

The Dynamics of Two Biological Interfaces

Gerald G. Fuller (Stanford University)

Seminar co-hosted by Éric Raspaud—SPECIAL TIME

Biological systems are normally high-interface systems and these surfaces are laden with biological molecules and cells that render them mechanically complex. The resulting nonlinearities with response to surface stresses and strain are often essential to their proper function and these are explored using recently developed methods that reveal an intricate interplay between applied stress and dynamic response. Two applications are discussed.

1. Vascular endothelial cells are nature's "rheologists" and line the interior walls of our blood vessels and are sensitive to surface shear stresses. These stresses are known to affect the shape and orientation of endothelial cells. It is evident that the spatial homogeneity of flow can affect vascular health and it is well-documented that lesions form in regions of high curvature, bifurcations, and asperities in blood vessels. Experiments are described where stagnation point flows are used to create regions of well controlled flow stagnation and spatial variation of wall shear stresses. Live-cell imaging is used to monitor the fate of cells attached to surfaces experiencing flow impingement and it is revealed that endothelial cells migrate and orient in such flows to create remarkable patterns of orientation and cell densification. This response, termed "rheotaxis", is used to explore mechano-transduction pathways within these cells.

2. The tear film of the eye is a composite structure of an aqueous solution of protein and biomacromolecules. This thin layer is further covered by a film comprised of meibomian lipids excreted during each blink. The purpose of the meibum has been largely unexplained although one prevailing suggestion is that it suppresses evaporation. Recent measurements in our laboratory demonstrate that this layer is strongly viscoelastic and this property has dramatic effects on the dynamics of the moving contact line and stability against dewetting.

Gerald Fuller is the Fletcher Jones Professor of Chemical Engineering at Stanford University. He joined Stanford in 1980 following his graduate work at Caltech where he acquire his MS and PhD degrees. His undergraduate education was obtained at the University of Calgary, Canada. Professor Fuller's interests lie in studies of rheology and interfacial fluid mechanics. His work has been recognized by receipt of the Bingham Medal of The Society of Rheology, membership in the National Academy of Engineering, and honorary doctorates from the Universities of Crete, Greece, and Leuven, Belgium.

Tri-séminaire de Physique Statistique : Amir Dembo

Matrix optimization under random external fields

Amir Dembo, Stanford University

Consider the problem of maximizing the quadratic form <x,Wx> + <h,x> over unit norm n-dimensional vectors x, where W is a Wigner matrix which is independent of the Gaussian vector h whose entries are independent and identically distributed. Two recent studies of the large n asymptotic probability of deviations of such maximum from its typical value, take very different approaches. Fyodorov and Le Doussal (2014) use the replica method of statistical physics, whereas Dembo and Zeitouni (2015) rely instead on the mathematical theory of large deviations. I will describe the main points of the latter, using this specific example also to illustrate some of the differences, strength and weaknesses of each approach.

Séminaire du LPTMS: Alfredo Ozorio de Almeida

Semiclassical evolution of correlations between observables

Alfredo Ozorio de Almeida, Centro brasileiro de pesquisas físicas

The kernel for the evolution of multiple quantum correlations between observables within the Weyl representation can be mapped onto the trace of a single continous family of compound unitary operators. Thus the semiclassical limit of this kernel can be ascribed to the periodic orbits of a corresponding family of classical compound canonical transformations, in the celebrated manner of Gutzwiller. Breaking these periodic orbits, leads to an initial value alternative that avoids both the search for orbits and singularities at caustics or bifurcations.

Physics-Biology interface seminar: Stephan Grill

Actomyosin Force Generation and Pattern Formation

Stephan Grill (MPI-CBG Dresden)

Morphogenesis is one of the great unknowns in Biology. Much is known about molecular mechanisms of regulation, but little is known about the physical mechanisms by which an unpatterned blob of cells develops into a fully structured and formed organism. The actomyosin cortex is a thin layer underneath the cellular membrane that can self contract, which drives many of the large-scale morphogenetic rearrangements that are observed during development. How this cortex reshapes and deforms, and how such morphogenetic processes couple to regulatory biochemical pathways is largely unclear. I will discuss two emergent physical activities of the actomyosin cytoskeleton, an active contractile tension and an active torque, both of which can serve to drive flows and large-scale chiral rotations of the actomyosin cytoskeleton. I will illustrate how active tension drive flows, how molecular constituents of the cortex affect flows, and how morphogenetic patterns can be formed by coupling regulatory biochemistry to active cortical mechanics. A particular focus will be the investigation of how compressive cortical flow drives the formation of an actin filament alignment pattern for generating a cleavage furrow for cytokinesis.

Tri-Séminaire de Physique Statistique : Balázs Kégel

Learning to discover: signal/background separation and the Higgs boson challenge

Balázs Kégel (Laboratoire de l'Accélérateur Linéaire, Univ. d'Orsay)

Classification algorithms have been routinely used since the 90s in high-energy physics to separate signal and background in particle detectors. The goal of the classifier is to maximize the sensitivity of a counting test in a selection region. It is similar in spirit but formally different from the classical objectives of minimizing misclassification error or maximizing AUC. We start the talk by motivating the problem on an ongoing example of detecting the Higgs boson in the tau-tau decay channel in the ATLAS detector of the LHC. We formalize the problem, then go on by describing the usual analysis chain, and explain some of the choices physicists make when designing a classifier for optimizing the discovery significance. We derive different surrogates that capture this goal and show some simple techniques to optimize them, raising some questions both on the statistical and on the algorithmic side. We end the talk by presenting a data challenge we organized to draw the attention of the machine learning and statistics communities to this important application and to improve the techniques used to optimize the discovery significance. With a PhD in computer science, Balázs Kégl has been a researcher in the Linear Accelerator Laboratory of the CNRS and the chair of the Center for Data Science of the Université Paris-Saclay since 2014. He has published more than hundred papers on unsupervised and supervised learning, large-scale Bayesian inference and optimization, and on various applications. At his current position he has been the head of the AppStat team working on machine learning and statistical inference problems motivated by applications in high-energy particle and astroparticle physics.

Séminaire du LPTMS: Antoine Sterdyniak

Realization of strongly interacting topological phases on lattices

Antoine Sterdyniak, Universität Innsbruck

While fractional quantum Hall effect (FQHE) was realized experimentally thirty years ago in semiconductor heterostructures, strongly interacting chiral topological phases are still at the center of an important research effort, both as they serve as building blocks of more exotic phases such as fractional topological insulators and as a realization outside of semi-conductor physics is still missing. In this talk, I will describe realizations of these phases in cold atoms gases and in frustrated spins systems. I will first introduce optical flux lattices, which are continuous models that exhibit topological flat bands with a tunable Chern number and host fractional states beyond the FQHE. Then, I will focus on chiral spin liquids whose emergence on the kagomé lattice using local Hamiltonians has been shown very recently. Unlike itinerant particle systems where FQHE can be understood as a consequence of interactions in a partially filled topological band, I will show that such a picture does not hold for this chiral spin liquid. However, it can be described by model states obtained using a slave boson approach.

Séminaire du LPTMS : Sandro Stringari

Broken symmetries in spin-orbit coupled Bose Einstein condensed gases

Sandro Stringari (University of Trento and BEC Center)

In the seminar I will focus on the symmetries characterizing the 1D spin-orbit coupled hamiltonian first realized experimentally by the team of Ian Spielman at Nist. The nature of the new quantum phases and their dynamic properties will be discussed. Special focus will be given on the recently observed rotonic excitation, which follows from the breaking of time reversal and parity symmetry, on the emergence of stripes which follows from the spontaneous breaking of translational invariance, and on the dynamic instability of large amplitude oscillations which is the consequence of the violation of Galilean invariance.

Séminaire du LPTMS : Anthony Perret et Caterina De Bacco

Statistique d’extrêmes de variables aléatoires fortement corrélées

Anthony Perret

La statistique des valeurs extrêmes est une question majeure dans divers domaines des sciences. Dans ce contexte, une question naturelle qui se pose alors est la suivante: ces valeurs extrêmes sont-elles isolées, loin des autres variables ou bien au contraire existe-t-il un grand nombre d'autres variables proches de ces valeurs extrêmes ? Cette question a suscité l'étude de la densité d'état de ces événements quasi-extrêmes. Il existe pour cette quantité peu de résultats pour des variables fortement corrélées, qui pourtant est le cas dans de nombreux modèles d'intérêt en physique statistique. Deux pistes de modèles physiques de variables fortement corrélées pouvant être étudiés analytiquement se démarquent : les positions d’une marche aléatoire et les valeurs propres de matrice aléatoire. Ce sont les deux modèles que j'ai étudiés dans ma thèse. Après avoir très brièvement discuté le cas des marches aléatoires, je me concentrerai dans cet exposé au cas où la collection de variables aléatoires est l'ensemble des valeurs propres d'une matrice aléatoire gaussienne hermitienne de grande taille. Je discuterai plus particulièrement la distribution de l’écart entre les deux plus grandes valeurs propres pour laquelle j’ai obtenu une formule faisant intervenir des fonctions transcendantes de Painlevé. Les comportements asymptotiques de cette formule permettent par exemple de trouver des nouveaux régimes intéressants dans le modèle de Sherrington-Kirkpatrick sphérique.

The cavity method in routing optimization problems on networks.

Caterina De Bacco

 In recent years the cavity method, or message-passing algorithm, has been successfully exploited by statistical physicists to solve combinatorial optimization problems such as the K-satisfability (K-SAT). In this talk we introduce this method and outline how it is applied to solve routing problems on communication or traffic networks. In this context the typical situation is where many users want to communicate at the same time over a given network. One then wants to optimize the overall routing by minimizing communication path length and the traffic overlap either at nodes or at edges given a set of constraints. We will show how the cavity method provides the algorithmic tools to tackle these types of optimization problems efficiently.

Physics-Biology interface seminar: Martine Ben Amar

Growth of living fibrous tissues: from biofilms to fibrosis

Martine Ben Amar (ENS Paris)

Morphologies of soft materials in growth, swelling or drying have been extensively studied recently. Shape modifications occur as the size varies transforming ordinary spheres, cylinders and thin plates into more or less complex objects. Existence of fibers exacerbates this complexity, giving anisotropy to the growth process itself. The growth is coupled to the environment, for bacteria the substrate, in pathology the healthy tissue. In pathological situations such as wound-healing or desmoplastic tumor growth, the immune system reacts with a battery of morphogenetic gradients, making a new tissue full of collagene and eventually sending active cells. All these factors contribute to a high level of compressive stress at the origin of patterns and deformity. I will show how we can predict quantitatively these patterns on the simple drop geometry of the biofilms and on the spherical shape of tumors.

For the pathological cases, it turns out that the wrinkling process dominates the growth, deforming the tissues and exacerbating the immune system which reacts via passive (fibroblasts) and active cells (myo-fibroblasts). I will show that the consequence is a huge increase of the stiffness, which stops spontaneously when the healing is achieved but not in case of implants or tumors. Naive estimations can be given explaining difficulties encounted in drug treatments, for example.

Joint work with Min Wu.

Séminaire du LPTMS: Thomas Quella

Topological aspects of SU(N) magnetism and its cold atom realization

Thomas Quella, Universität Köln

SU(N) spin systems exhibit a number of novel topological quantum states, including specifically a variety of Haldane phases (in 1D) and abelian and non-abelian spin liquid phases (in 2D). In this talk we will review the classification of Haldane phases and their characterization in terms of fractionalized boundary spins and non-local string order parameters. Moreover, we will address the possible realization of these phases in ultra-cold gases of alkaline-earth fermions. If time permits we will also briefly comment on the systematic design of critical and topological long-range SU(N) spin models in one and two dimensions which are based on infinite dimensional matrix product states associated with SU(N) WZW models.

Séminaire exceptionnel du LPTMS : Daniel Cabra

Skyrmion Lattices in Antiferromagnetic Systems

Daniel C. Cabra  (University of La Plata)

Ordering of the frustrated classical Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions on the antiferromagnetic triangular lattice is studied under a magnetic field by means of semiclassical calculations and large-scale Monte Carlo simulations. We show that even a small DM interaction induces the formation of anAntiferromagneticSkyrmion crystal (AF-SkX) state. Unlike what is observed in ferromagnetic materials, we show that the AF-SkX state consists of three interpenetrating usual Skyrmion crystals (one by sublattice), and most importantly, the AF-SkX state survives in the limit of zero temperature. To characterize the phase diagram we compute the average of the topological order parameter, the chirality, which measures the numberof Skyrmions. As themagneticfieldincreases, this parameter presents a series of discrete jumps that may indicate transitions between different topological states. Increasing the magnetic field the model exhibits a first-order transition from a spiral phase into a three-sublattice Skyrmion structure where multiple Bragg peaks coexist in the spin structure factor.

Séminaire quantique informel du LPTMS : Alberto Rosso

Dynamic Nuclear Polarization and the paradox of Quantum Thermalization

Alberto Rosso (LPTMS)

Dynamic Nuclear Polarization (DNP) is to date the most  effective technique to increase the nuclear polarization up to a factor 100,000 opening disruptive perspectives for medical applications. In DNP, the nuclear spins are driven to an - out of equilibrium - hyperpolarized state by microwave saturation of the electron spins in interaction with them. Here we show that the electron dipolar interactions compete with the local magnetic fields resulting in two distinct dynamical phases: for strong interactions the electron spins equilibrate to an extremely low effective temperature that boosts  DNP  efficiency. For weak interaction this spin temperature is not defined and the polarization profile has an 'hole burning' shape characteristic of the non interacting case. The study of the many-body eigenstates reveals that these two phases are intimately related to the problem of thermalization in closed quantum systems where breaking of ergodicity is expected varying the strength of the interactions.

Séminaire du LPTMS: Vicente Garzo

Instabilities in granular fluids at moderate densities

Vicente Garzo, Departamento de Fisica, Universidad de Extremadura

It is well established that when granular materials are externally excited (rapid flow conditions) they behave like a fluid. In this regime, binary collisions prevail and hence, kinetic theory may be considered as a quite useful tool to describe the dynamics of grains. The main difference with respect to ordinary or molecular fluids is that granular systems are constituted by macroscopic grains that collide inelastically so that the energy decreases with time when the system evolves freely (homogeneous cooling state, HCS). One of the most characteristic features of granular fluids, as compared with normal fluids, is the spontaneous formation of velocity vortices and density clus- ters in the HCS. These instabilities can be well described through a linear stability analysis of the hydrodynamic equations and follow from the presence of a dissipation or sink term in the equation for the balance of energy. Apart from its fundamental interest, the determination of the critical length scale Lc for the onset of instabilities in freely cooling flows offers one of the best opportu- nities to assess the theoretical predictions of granular hydrodynamics (which are based on kinetic theory calculations) when one compares those predictions with those independently obtained by molecular dynamics (MD) simulations. In this talk I will give some insight into the above problem by determining the critical length scale L c for granular fluids at moderate densities. The comparison between kinetic theory and MD simulations shows in general an excellent agreement for the onset of velocity vortices, indicating the applicability of hydrodynamics to monodisperse and polydisperse granular flows even for strong inelasticity and finite density.

Physics-Biology interface seminar: Ulrich Schwarz

Physics of active contractile matter

Ulrich S. Schwarz (Heidelberg University)

Biological systems such as cells and tissue use non-equilibrium processes to actively generate mechanical stress, movement and growth. Some of these processes can actually be reconstituted in biomimetic experiments with active soft matter. In this talk, we will first discuss why and how contractile forces are generated by biological systems and how they can be measured with soft elastic substrates ("traction force microscopy"). Because kilo-Pascal stresses are typically transmitted through micrometer-sized contacts, the relevant force scale is nano-Newton. We then introduce different theoretical approaches to understand and model the contractility of biological matter. Because closed systems have to conserve momentum, the most central concept here is the one of a "force dipole", similarly as for the theoretical description of microswimmers, but now coupled to a mechanical rather than to a hydrodynamic environment. We present a stochastic theory for the biologically most relevant example of a contractile force dipole, namely the "myosin II minifilament". We then explain why on the large length scale of cells and tissue, the mechanical properties of these systems are dominated by tensions rather than by their elastic modulus, with dramatic consequences for their shape and force transmission to the environment.

Séminaire du LPTMS: Jean Wolff

Budding of domains in mixed bilayer membranes

J. Wolff,  Institut Charles Sadron (UPR CNRS), Strasbourg

We propose a model that accounts for budding behavior of domains in lipid bilayers, where each of the bilayer leaflets has a coupling between its local curvature and local lipid composition. The compositional asymmetry between the two monolayers leads to an overall spontaneous curvature. The membrane free energy contains three contributions: bending energy, line tension, and a Landau free energy for a lateral phase separation. Within a mean-field treatment, we obtain various phase diagrams which contain fully-budded, dimpled and flat states. In particular, for some range of membrane parameters, the phase diagrams exhibit a tri-critical behavior as well as three-phase coexistence region. The global phase diagrams can be divided into three types and are analyzed in terms of the curvature-composition coupling parameter and domain size.   wolff Ref.: J.Wolff, S. Komura, D. Andelman, PRE 91, 012708 (2015)

Séminaire exceptionnel du LPTMS: Ines de Vega

Open quantum systems: hierarchical structures, chain mappings and thermofield dynamics

Ines de Vega, LMU Munich

Open quantum systems (OQS) cannot always be described with the Markov approximation, which requires a large separation of system and environment time scales. In this talk I will give an overview of some recent advances to tackle the dynamics of an OQS beyond the Markov approximation, with special emphasis in hierarchy-based [1,2] and chain mapping-based [3] approaches. In the latter context, I will discuss the use of a thermofield transformation to describe thermal environments [4].
[1] Y. Tanimura. PRA 41, 6676–6687 (1990)
[2] I. de Vega, J. Phys. A: Math. Theor. 48, 145202 (2015)
[3] A. W. Chin, J. Prior, S. F. Huelga, and M. B. Plenio, Phys. Rev. Lett. 107, 160601, 1102.1483 (2011).
[4] I. de Vega and M.C. Bañuls, arXiv:1504.07228

Séminaire du LPTMS: Françoise Cornu

First passage fluctuation relations rules by cycle affinities

Françoise Cornu, LPT Orsay

For a non-equilibrium stationary state  described by a Markovian process it is well known that the entropy production rate can be expressed in terms of the affinity associated with every transition  in the graph representation of the master equation. We exhibit the invariance of cycle affinities in finite state Markov processes under various natural probabilistic constructions : for instance under conditioning and under a new combinatorial construction that we call ``drag and drop''. For semi-markovian processes whose corresponding graph is made of a single cycle, we establish that the cycle current obeys a fluctuation relation for first passage times at integer winding numbers, which is dual to the fluctuation relation for the cycle current at fixed time : contrarily to seminal fluctuation relations about the probabilities for measuring a random cumulative exchange quantity or its opposite value during a given time, the latter fluctuation relations deal with the probabilities for the random time needed for one cycle to be  performed  in one sense or in the opposite one  with a given winding number.   Reference :  M. Bauer and F. Cornu,  J. Stat. Phys. 155 (2014) 703-736.

Physics-Biology interface seminar: Xavier Trepat

Control of collective cell dynamics by adhesion, tension and fracking

Xavier Trepat (ICREA @ Institute for Bioengineering of Catalonia (IBEC), Barcelona)

A broad range of biological processes such as morphogenesis, tissue regeneration, and cancer invasion depend on the collective dynamics of epithelial cells. Such dynamics are determined by an exquisite balance between intercellular adhesion, cytoskeletal tension, and intracellular pressure. To study this balance in a fully quantitative manner I will present new techniques to map physical forces between and within cells. Using these techniques we studied how cellular forces are regulated and transmitted by the proteins that comprise intercellular adhesion complexes. To do so, we perturbed the main molecular players of the intercellular adhesome using RNAi and studied how these perturbations impact physical forces and cellular velocities in epithelial cell collectives. We found that perturbations targeting adherens junctions, but also tight junctions, gap junctions, and desmosomes have a significant impact on cell velocities, cell deformations, cell-matrix traction forces, and cell-cell forces. We developed a cross-validation analysis to show that concentrations of cell-cell adhesion proteins are significant predictors of cell-cell forces. Finally, I will discuss the determinants of epithelial integrity in the presence of stretching and transepithelial pressure.

Tri-séminaire de Physique Statistique : Abhishek Dhar

Waiting times for entropic fluctuations in non-equilibrium processes

Abhishek Dhar (International centre for theoretical sciences, ICTS, Bangalore)

Nonequilibrium processes generate entropy. The total entropy generated in a fixed  finite time interval is a fluctuating variable. A  lot of recent work have looked at the probability of observing entropic fluctuations of different sizes in a fixed time interval. These distributions have been discussed in the context of fluctuation relations and large deviation functions. Here, we  ask the question: how long does one have to wait to see an entropic fluctuation of a specified size ? We present some results on the waiting time distribution,  in the context of some simple examples of non-equilibrium processes.

Reference: K. Saito and A. Dhar, arXiv:1504.02187

Séminaire du LPTMS: Gernot Akemann

Universal Lyapunov Exponents from Products of Random Matrices

Gernot Akemann, Universität Bielefeld

Random Matrices find many applications in Physics and Mathematics. In particular products of random matrices can be used as a simple model for linear time evolution. In this talk I will review recent progress on an exact solution of this model for Gaussian random matrices. Both the spectral statistic of its complex eigenvalues and its real positive singular values are given by a determinantal point process with an underlying integrable kernel. In the limit of an infinite product the Lyapunov exponents become deterministic and we compute their variances and higher order cumulants. Remarkably the moduli of the complex eigenvalues and the singular values become identical in this limit. We will also discuss universality and possible extensions.

Soutenance HDR : Martin Lenz

On the microstructure of active cellular processes

Eukaryotic cells use a multitude of protein machines to regulate their own structure. In this thesis, we study how the geometrical arrangement of these interacting microscopic active elements sculpt the cell's own internal microstructure and its membrane enclosure.

We first focus on the mechanisms generating actomyosin contractility, a crucial driver of cell motion and organization. We question the current position of highly organized, sarcomeric contractility as the only possible mechanism to drive contractility. We propose an alternative mechanism, and show that only it can account for the observed contractility of disordered actomyosin assemblies. It moreover yields qualitatively new effects in intracellular force transmission, including stress reversal and amplification, consistent with experimentally observations in fiber networks.

We next elucidate some of the mechanisms through which the cell deforms and cuts its own membrane, thus enabling exchanges with the extracellular medium as well as between its internal compartments. We find that the function of the proteins responsible for this remodeling is strongly influenced by the mechanics of the membrane, and use these effects to elucidate the modes of operation of proteins clathrin and dynamin, as well as of protein complex ESCRT-III.

Soutenance de thèse : Anthony Perret

Statistique d’extrêmes de variables aléatoires fortement corrélées

Anthony Perret

La statistique des valeurs extrêmes est une question majeure dans divers contexte scientifiques. Cependant, la description de la statistique d'un extremum global est certainement une caractéristique importante mais celle-ci ne décrit la fluctuation que d'une seule variable, parmi un grand nombre de variables aléatoires. Une question naturelle qui se pose alors est la suivante: ces valeurs extrêmes sont-elles isolées, loin des autres variables ou bien au contraire existe-t-il un grand nombre d'autres variables proches de ces valeurs extrêmes ? Ces questions ont suscité l'étude de la densité d'état de ces événements quasi-extrêmes. Il existe pour cette quantité peu de résultats pour des variables fortement corrélées, qui pourtant est le cas de nombreux modèles fondamentaux. Deux pistes de modèles physiques de variables fortement corrélées pouvant être étudiés analytiquement se démarquent alors: les positions d’une marche aléatoire et les valeurs propres de matrice aléatoire. Ce sont les deux modèles que j'ai étudiés dans ma thèse et que je vais présenter durant cette soutenance.

Séminaire commun LPTMS-IPN : Jacques Treiner

Pourquoi l’objectif des 2°C appartient, hélas, au passé ?

Jacques Treiner (Paris VI et Sciences Po)

Depuis la conférence de Copenhague de 2009, la référence de 2°C comme augmentation de la température moyenne de la Terre apparait comme un objectif à ne pas dépasser. Cette valeur, choisie à la suite de négociations extrêmement difficiles entre 26 pays seulement (notamment sans l’UE) n’a pas de caractère contraignant, mais c’est le seul objectif chiffré servant depuis lors de référence internationale. C’est asn doute la raison pour laquelle ce chiffre sert de référence pour la COP21 à Paris en décembre 2015. Mais il n’est pas difficile de se rendre compte que, hélas, les conditions requises pour que ce scénario se réalise sont devenue impossibles à remplir. Quelles conséquences en tirer ?

Physics-Biology interface seminar: Alexis Peaucelle

Looking for the mechanical control of growth in plants. Is there a simple law?

Alexis Peaucelle (INRA Versailles/Cambridge University)

Plants are strikingly good at math, especially geometry. One could find parts or full plants shaped as spheres, circles, straight lines, and flat surfaces, golden and right angles and all sorts of exotic and pretty combination of shapes. These shapes are generated through complex tissue growth. We want to understand this puzzling beauty by focusing on the biophysical properties of the cell wall and its related biochemistry. We will present some of our results on dark grown hypocotyl and pavement cells demonstrating that pectin methylesterification status change is necessary for cell and tissue differentiation, growth and is related to changes in cell wall elasticity. Then we will expose puzzling results showing that cell growth rates are proportionate to the elastic stretching of the cell wall (Pressure divided by the Young modulus) and not plastic properties of the cell wall components. Finally, we will present preliminary experiments that could explain this paradox as well as some others such as microtubule partial correlation with oriented growth, and sound-induced plant growth.

Séminaire du LPTMS: Emmanuel Trizac et Christophe Texier

Shortcutting adiabaticity : how to change rapidly the state of a trapped gas ?

Emmanuel Trizac, LPTMS

Inspired from H-theorem requirements, a novel class of exact solutions to the quantum or classical Boltzmann equation is uncovered. These solutions, valid for arbitrary collision laws, hold for time-dependent confinement. We exploit them, in a reverse-engineering perspective, to work out a protocol that shortcuts any adiabatic transformation between two equilibrium states in an arbitrarily short time span, for an interacting system. Particle simulations corroborate the analytical predictions.  

Topological phase transitions in multichannel disordered wires in the chiral  symmetry classes

Christophe Texier, LPTMS

We have considered the one-dimensional multichannel Dirac equation where the mass is a random NxN matrix with elements uncorrelated in space. In order to analyze the spectral properties of the model, we have introduced a matricial random process (Riccati matrix), related to the scattering matrix describing the reflection on the semi-infinite medium. We have found that the stationary distribution of the Riccati matrix corresponds to a generalisation of the Laguerre ensemble of random matrices. The knowledge of this distribution has allowed us to derive explicit determinantal representations for the density of states (DoS), which is shown to present a power law behaviour at low energy. Varying the mass over disorder ratio allows to drive N phase transitions where the DoS exponent vanishes and which are shown to be of topological nature as they correspond to the change of a topological quantum number (Witten index). Reference :Aurélien Grabsch and Christophe Texier, Topological phase transitions and superuniversality in the 1D multichannel Dirac equation with random mass, preprint cond-mat arXiv:1506.05322

Séminaire du LPTMS : Alexios Polychronakos

Field theory on fuzzy spaces: effective action and phase transitions

Alexios Polychronakos (City College NY)

Tri-séminaire de Physique Statistique : Matthieu Wyart

Marginal stability and flow in granular materials

Matthieu WYART (New York University)

Complex systems are characterized by an abundance of meta-stable states.This is the case for granular materials, that can flow until one jammed configuration (among the exponentially many possible ones) is reached. I will argue that at least in popular simplified models of granular materials, these dynamically-accessible configurations are stable, but barely so. Such marginal stability offers a new perspective on both the solid and the liquid phase. I will discuss dense suspensions and propose to describe flow as a gas of elementary excitations, corresponding to the opening and closing of contact between particles. This approach leads to a detailed scaling description of rheological properties and length scales of dense flows. If time permits I will discuss a classification of athermal glassy systems based on the stability of low-energy excitations.

Séminaire du LPTMS: Eric Akkermans

Topology and fractals : waves and fields in quasicrystals (theory and experiments with polaritons)

Eric Akkermans, Technion - Israel Institute of Technology

I will present recent results obtained both theoretically and experimentally  on fractal spectral properties of a polariton gas in a Fibonacci quasi-periodic potential. The observed spectrum is accurately reproduced from a theoretical model that we shall present. We have observed for the first time log-periodic oscillations and the opening of mini-gaps following the gap labeling theorem. These results illustrate the potential of cavity polaritons as a quantum simulator in complex topological geometries.

Soutenance de thèse : Caterina de Bacco

Decentralized network control, optimization and random walks on networks

In the last years several problems been studied at the interface between statistical physics and computer science. The reason being that often these problems can be reinterpreted in the language of physics of disordered systems, where a big number of variables interacts through local fields dependent on the state of the surrounding neighborhood. Among the numerous applications of combinatorial optimisation the optimal routing on communication networks is the subject of the first part of the thesis. We will exploit the cavity method to formulate efficient algorithms of type message-passing and thus solve several variants of the problem through its numerical implementation. At a second stage, we will describe a model to approximate the dynamic version of the cavity method which allows to decrease the complexity of the problem from exponential to polynomial in time. This will be obtained by using the Matrix Product State formalism of quantum mechanics. Another topic that has attracted much interest in statistical physics of dynamic processes is the random walk on networks. The theory has been developed since many years in the case the underneath topology is a d-dimensional lattice. On the contrary the case of random networks has been tackled only in the past decade, leaving many questions still open for answers. Unravelling several aspects of this topic will be the subject of the second part of the thesis. In particular we will study the average number of distinct sites visited during a random walk and characterize its behaviour as a function of the graph topology. Finally, we will address the rare events statistics associated to random walks on networks by using the large-deviations formalism. Two types of dynamic phase transitions will arise from numerical simulations, unveiling important aspects of this problems. We will conclude outlining the main results of an independent work developed in the context of out-of-equilibrium physics. A solvable system made of two Brownian particles surrounded by a thermal bath will be studied providing details about a bath-mediated interaction arising for the presence of the bath.

Soutenance de thèse : Silvia Grigolon

Modeling and inference for biological systems: from auxin dynamics in plants to protein evolution


All biological systems are made of atoms and molecules interacting in a non-trivial manner. Such non-trivial interactions induce complex behaviours allowing organisms to fulfil their many vital functions. These features can be found in all biological systems at different levels, from molecules and genes up to cells and tissues. In the past few decades, physicists have been paying much attention to these intriguing aspects by framing them in network approaches for which a number of theoretical methods offer many powerful ways to tackle systemic problems. At least two different ways of approaching these challenges may be considered: direct modelling methods and approaches based on inverse methods. In this defence I am going to show how we made use of both approaches to study three different problems occurring on three different biological scales. The first part concerns the very early stages of tissue development in plants; it covers the model we proposed for understanding which features drive the spontaneous collective behaviour in space and time of the transporters which pump the phytohormone auxin out of plant cells. Then, at the cell level, I will go through my study of the intracellular molecular networks that implement auxin signalling in plants, examining how network structures affect network functions. Finally, I will talk about inference problems on structural properties of proteins. I will introduce a method we formulated to understand how conservation of protein function across different organisms constrains the evolution of protein sequences and their diversity.

Physics-Biology interface seminar: Agnese Seminara

Osmotic spreading of Bacillus subtilis biofilms

Agnese Seminara (LPMC, Université de Nice)

Bacterial biofilms are organized communities of cells living in association with surfaces. The hallmark of biofilm formation is a well defined spatio-temporal pattern of gene expression, leading to differentiation and a complex morphology. While this process resembles the development of a multicellular organism, biofilms are only transiently multicellular. More importantly the functions associated to the biofilm phenotype are largely unknown.

A common feature of biofilm formation is the secretion of a polymeric matrix rich in sugars and proteins in the extracellular space. In Bacillus subtilis, secretion of the exopolysaccharide (EPS) component of the extracellular matrix is genetically coupled to the inhibition of flagella-mediated motility. The onset of this switch results in slow expansion of the biofilm on a substrate. Different strains have radically different capabilities in surface colonization: Flagella-null strains spread at the same rate as wild type, while both are dramatically faster than EPS mutants. Multiple functions have been attributed to the EPS, but none of these provides a physical mechanism for generating spreading. We propose that the secretion of EPS drives surface motility by generating osmotic pressure gradients in the extracellular space. A simple mathematical model based on the physics of polymer solutions shows quantitative agreement with experimental measurements of biofilm growth, thickening, and spreading. We discuss the implications of this osmotically driven type of surface motility for nutrient uptake that may elucidate the reduced fitness of the matrix-deficient mutant strains.

Physics-Biology interface seminar: Christophe Le Clainche

Regulation of actin assembly and mechanotransduction in cell-matrix adhesion complexes: a biochemical study of the talin-vinculin complex

Christophe Le Clainche (I2BC, Gif-sur-Yvette)

Cell migration is involved in many physiological and pathological processes. Force is produced by the growth and the contraction of the actin cytoskeleton (1). To produce force in adherent cells, these actin networks must be anchored to the extracellular matrix (ECM) by adhesion complexes (1,2). These structures contain transmembrane integrins that mechanically couple the ECM to the intracellular actin cytoskeleton via actin binding proteins (ABPs) (2). This system acts as a molecular clutch that controls force transmission across adhesion complexes. This molecular clutch is a complex interface made of multiple layers of regulated protein-protein interactions (2). The multiple activities of the ABPs present in these structures play a critical role in the dynamics of this interface. In addition to the control of actin filament binding and polymerization (1-3), these proteins sense and respond to the force applied by the actomyosin cytoskeleton to adjust the anchoring strength (4,5). Our goal is to determine the molecular mechanisms by which these ABPs cooperate to control the mechanical coupling between the actin cytoskeleton and cell-matrix adhesion complexes. To study these ABPs, our laboratory combines the measurement of actin polymerisation kinetics in fluorescence spectroscopy, single actin filament observations in TIRF microscopy and the reconstitution of actin-based mechanosensitive processes on micropatterned surfaces. Our model system is the mechanosensitive complex made of the two ABPs talin and vinculin. Our results showed that vinculin controls actin filament elongation (3). More recent results revealed that talin also regulates actin polymerisation in response to integrin binding (unpublished data). In addition, we have developed a microscopy assay with pure proteins in which the self-assembly of actomyosin cables controls the association of vinculin to a talin-micropatterned surface in a reversible manner (4, 5). This in vitro reconstitution revealed the mechanism by which a key mechanosensitive molecular switch senses and controls the connection between adhesion complexes and the actomyosin cytoskeleton. References
  • (1) Christophe Le Clainche and Marie-France Carlier. Regulation of actin assembly associated with protrusion and adhesion in cell migration. Physiological Reviews (2008) Apr;88(2):489-513.
  • (2) Corina Ciobanasu, Bruno Faivre, Christophe Le Clainche. Integrating actin dynamics, mechanotransduction and integrin activation: The multiple functions of actin binding proteins in focal adhesions. European Journal of Cell Biology (2013) (92) 339-348.
  • (3) Christophe Le Clainche, Satya P Dwivedi, Dominique Didry, Marie-France Carlier. Vinculin is a dually regulated actin filament barbed-end capping and side-binding protein. Journal of Biological Chemistry (2010) Jul 23;285(30):23420-32.
  • (4) Corina Ciobanasu, Bruno Faivre, Christophe Le Clainche. Actomyosin dependent formation of the mechanosensitive talin-vinculin complex reinforces actin anchoring. Nature Communications (2014) 5:3095
  • (5) Corina Ciobanasu, Bruno Faivre, Christophe Le Clainche. Reconstituting actomyosin-dependent mechanosensitive protein complexes in vitro. Nature Protocols (2015) Jan ;10(1):75-89

Soutenance de thèse : Clélia de Mulatier

A random walk approach to stochastic neutron transport

One of the key goals of nuclear reactor physics is to determine the distribution of the neutron population within a reactor core. This population indeed fluctuates due to the stochastic nature of the interactions of the neutrons with the nuclei of the surrounding medium: scattering, emission of neutrons from fission events and capture by nuclear absorption. Due to these physical mechanisms, the stochastic process performed by neutrons is a branching random walk. For most applications, the neutron population considered is very large, and all physical observables related to its behaviour, such as the heat production due to fissions, are well characterised by their average values. Generally, these mean quantities are governed by the classical neutron transport equation, called linear Boltzmann equation.
During my PhD, using tools from branching random walks and anomalous diffusion, I have tackled two aspects of neutron transport that cannot be approached by the linear Boltzmann equation. First, thanks to the Feynman-Kac backward formalism, I have characterised the phenomenon of "neutron clustering" that has been highlighted for low-density configuration of neutrons and results from strong fluctuations in space and time of the neutron population. Then, I focused on several properties of anomalous (non-exponential) transport, that can model neutron transport in strongly heterogeneous and disordered media, such as pebble-bed reactors. One of the novel aspects of this work is that problems are treated in the presence of boundaries. Indeed, even though real systems are finite (confined geometries), most of previously existing results were obtained for infinite systems.

Séminaire du LPTMS: Hajime Yoshino

Hierachical rigidities of glassy systems

Hajime Yoshino, Osaka University

Recent progress of the replica theory for the hardsphere glass in large dimensional limit suggests an intriguing possibility of hierarchical energy landscape reminiscent of spin-glasses [1]. In this talk we argue that response of glasses to external shear would be very similar to the response of spin-glasses to external magnetic field. First we show theoretically that  shear-modulus becomes hierarchical reflecting the hierarchical structure of the glass order parameter [2]. Then we discuss our MD simulations which reveal the anticipated difference between "field cooled" (FC) and "zero field cooled" (ZFC) shear-modulus [3].  Finally we show theoretical and numerical suggestions of negatively diverging non-linear shear-modulus associated with the marginal stability of continuous replica symmetry breaking.
[1]  P. Charbonneau, J. Kurchan, G. Parisi, P. Urbani, F. Zamponi, Nat. Comm. 5, 3725 (2014).
[2]  H. Yoshino and F. Zamponi, Phys. Rev. E 90, 022302 (2014).
[3]  D. Nakayama, H. Yoshino and F. Zamponi, in preparation.

Soutenance de thèse : Ricardo Marino

Number statistics in random matrices and applications to quantum systems

Ricardo Marino

Random matrix theory has found many applications spanning a vast number of fields in physics and mathematics in the last two decades. Most recently, the equivalence between the statistics of eigenvalues of Gaussian Hermitian matrices  and the position of ground-state harmonically confined 1-D fermionic particles has been studied to obtain many interesting and universal results in cold atoms. In my thesis, I explore this connection to solve the problem of determining quantum fluctuations of cold fermions using techniques from random matrix theory, expanding previous results that were restricted only to specific scaling limits of the spectrum to yield a full picture of the behavior of fluctuations of fermionic particles in one dimensional traps.

Séminaire du LPTMS : Viktor Eisler & Zdzislaw Burda

Entanglement in mixed states: the negativity

Dr. Viktor Eisler (TU Graz)

In pure states of many-body systems, entanglement is routinely studied via the Renyi entropies, which give a complete characterization of the bipartite case. The situation becomes more complicated, if the system is composed of more than two parts, and one is interested in the entanglement between two non-complementary pieces. Such a scenario can be studied by introducing a new entanglement measure, the so-called negativity, which has been the focus of recent interest. In this talk I would like to give an overview about the available methods to calculate the negativity, and present some new results on mixed-state entanglement.


Quaternionic R transform and non-hermitian random matrices

Dr. Zdzislaw Burda (AGH University of Science and Technology)

Using the Cayley-Dickson construction we rephrase and review the non-hermitian diagrammatic formalism [R. A. Janik, M. A. Nowak, G. Papp and I. Zahed, Nucl.Phys. B 501, 603 (1997)], that generalizes the free probability calculus to asymptotically large non-hermitian random matrices. The main object in this generalization is a quaternionic extension of the R transform which is a generating function for planar (non-crossing) cumulants. We demonstrate that the quaternionic R transform generates all connected averages of all distinct powers of X and its hermitian conjugate X:  <<(1/N) Tr [Xa Xb Xc …] >>  for N → ∞. We show that the R transform for Gaussian elliptic laws is given by a simple linear quaternionic map R(z+wj) = x + σ² (μ e^{2iφ} z + w j) where (z,w) is the Cayley-Dickson pair of complex numbers forming a quaternion q=(z,w)≡ z+ wj. This map has five real parameters Re[x], Im[x], φ, σ and μ. We use the R transform to calculate the limiting eigenvalue densities of several products of Gaussian random matrices.

Tri séminaire de Physique Statistique : Martin Evans

Explosive condensation in nonequilibrium systems

Martin EVANS, School of Physics and Astronomy, Univ. of Edinburgh

In this talk I will discuss condensation in the zero range process (ZRP) and related models. The ZRP is a simple model of interacting classical particles hopping between sites of a one-dimensional lattice, but with rates that do not obey detailed balance and thus generate a nonequilibrium state. Real-space condensation is the phenomenon whereby a finite fraction of the particles are typically found at a single site in the limit of a large system size. I'll show how and why condensation arises and how it is related to large deviations of sums of random variables. I'll also discuss a variation of the model exhibiting `explosive condensation' - the condensate moves rapidly through the system and the relaxation time to the stationary state decreases to zero for large system size. This provides a first example of instantaneous gelation in a spatially extended system. If time permits I will also discuss how condensation generally arises when there are two constraints - say the sum and the variance - on a collection of random variables.


Soutenance de thèse : Andrii Gudyma

Non-equilibrium dynamics of a trapped one-dimensional Bose gas

Andrii Gudyma

A study of breathing oscillations of a one-dimensional trapped interacting Bose gas is presented. Oscillations are initiated by an instantaneous change of the trapping frequency. In the thesis a 1D quantum Bose gas in a parabolic trap at zero temperature is considered, and it is explained, analytically and numerically, how the oscillation frequency depends on the number of particles, their repulsive interaction, and the trap parameters. We have focused on the many-body spectral description, using the sum rules approximation.  The oscillation frequency is identified as the energy difference between the ground state and a particular excited state.

The existence of three regimes is demonstrated, namely the Tonks regime, the Thomas-Fermi regime and the Gaussian regime. The transition from the Tonks to the Thomas-Fermi regime is described in the terms of the local density approximation (LDA). For the description of the transition from the Thomas-Fermi to the Gaussian regime the Hartree approximation is used. In both cases the parameters where the transitions happen are found. The extensive diffusion Monte Carlo simulations for a gas containing up to N = 25 particles is performed. As the number of particles increases, predictions from the simulations converge to the ones from the Hartree and LDA in the corresponding regimes. This makes the results for the breathing mode frequency applicable for arbitrary values of the particle number and interaction. The analysis is completed with the finite N perturbative results in the limiting cases. The theory predicts the reentrant behavior of  the breathing mode frequency when moving from the Tonks to the Gaussian regime and fully explains the recent experiment of the Innsbruck group.

Physics-Biology interface seminar: Romain Koszul

Insights on the regulatory principles of genome organization in unicellular microorganisms

Romain Koszul (Institut Pasteur)

Chromosomes of a broad range of kingdoms, from bacteria to mammals, are structured by large topological domains, whose precise functional roles and regulatory mechanisms remain elusive. Using chromosome conformation capture technology, we unraveled the higher-order organization of the Bacillus subtilis, Escherichia coli and Vibrio cholerae genomes, in a variety of growth and mutant conditions. Different types of topological domains were found to structure these chromosomes, ranging from a few dozens to a thousand kb. We show that the matP/matS and parB/parS systems generate specific types of topological structures, regulated by replication and cell cycle progression. We have also functionally characterized some of the global organizational principles of these domains, in link with replication/segregation during the cell cycle. Overall, the comparative analysis of these different species provide striking insights on the diversity of the regulatory mechanisms of genome structure of the bacterial world. In addition, I will also present and discuss recent data obtained during the cell cycle of the eukaryotic species Saccharomyces cerevisiae.

Séminaire du LPTMS: Marie Piraud (ATTENTION horaire inhabituel)

Interacting bosons on ladder in the presence of gauge fields: Swimming against the tide !

Marie Piraud, Ludwig-Maximilians-Universität München

We present the phase diagrams of the interacting Bose-Hubbard model defined on two- and three-leg ladder geometries in the presence of a homogeneous flux. Our work is motivated by recent experiments using laser assisted-tunneling in optical lattices [1] and lattices in synthetic dimensions [2], which studied the regime of weak interactions. Based on extensive density matrix renormalization group simulations and a bosonization analysis for the two-leg ladder, we explore the parameter space and calculate experimentally accessible observables.
The phase diagrams comprise gapless and gapped Meissner and vortex phases, with the gapped states emerging in Mott-insulating regimes [3,4]. We show that vortex lattices also form at certain commensurate vortex densities [4,5]. On the two-leg ladder, we also find the so-called 'biased leg phase', which shows density-imbalance between the two legs [6].
Very interestingly, an enlarged unit cell forms in the vortex lattice phases, which can lead to the reversal of the current circulation-direction. We demonstrate this effect for arbitrarily weak interactions and at sufficiently low temperature, and show that it is significant for intermediate interactions [5].
[1] Atala etal., Nature Phys. 10, 588 (2014)
[2] Mancini etal., Science 349, 1510-1513 (2015)
[3] Piraud etal., PRB 91, 140406(R) (2015)
[4] Kolley etal., NJP 17, 092001 (2015)
[5] Greschner etal., to appear in PRL (2015)
[6] Wei and Mueller, PRA 89, 063617 (2014)

Séminaire du LPTMS: Jerome K. Percus

Random walks that generate their own environment

J.K. Percus, Courant Inst and Physics Dept, New York University

The term "random walk in a random environment" covers many phenomena in applied science. Most frequently, it refers to two independent random processes. We will look at the opposite extreme, in which the previous state of a - typically next neighbor walk on a lattice -changes the parameters of the current site. A primitive example that can be interpreted in this way is that of the statistics of the maximum of a walk on the half line, where we would expect a substantially increased mean and a substantially decreased variance over that of the underlying walk. How extreme are these? A more structured example, of relevance to synaptic strengthening, is that of a reinforced random walk, in which the first visit to a given site strengthens the "attraction" of that site for future visits, and this is quite simple in the Brownian motion limit. Still more nuanced is the situation in which the "attraction" depends upon the number of previous visits. (in collaboration with O.E. Percus)

Physics-Biology interface seminar: Jens Elgeti

Simulating Growing Tissues

Jens Elgeti (Forschungszentrum Jülich)

Growth of solid tumors or metastasis requires, besides massive biomedical changes, also a spatial remodelling of the tissue. This remodelling, often including displacements of healthy tissue around, requires mechanical work to be done. These mechanics of growth has attracted a lot of attention in recent years, but still remains poorly understood.

We use particle based simulations to study mechanical properties and effects in growing and motile tissues. These simulations have been helpful in understanding, interpreting and designing experiments. I will present an overview of the simulation technique, and how it contributed to recent developments in three dimensional tissue growth and collective cell migration. In a recent series of simulations and close experimental collaborations we found important interfacial and surface effects that lead to novel phenomena. For example, the tissue divides favorably at a free surface, even without any nutrient effects. This leads to the possibility and stability of a negative homeostatic pressure. In turn, a negative homeostatic pressure leads to naturally to finite steady states and tensile states.

References: [1] M.Basan et al, PNAS 110:2452 (2013) [2] F. Montel et al, N. J. Phys. 14:055008 (2012) [3] F. Montel et al, Phys. Rev. Lett. 107:188102, (2011) [4] M. Basan et al, Phys. Biol. 8:026014, (2011) [5] J. Ranft et al, PNAS 107:20863, (2010)

Séminaire du LPTMS: Pierre-Elie Larré

Superfluid and quantum features in the hydrodynamic flow of a fluid of light

Pierre-Élie Larré, INO-CNR BEC Center, Dipartimento di Fisica, Università di Trento, Via Sommarive 14, 38123 Povo, Italia

In the presence of a significant Kerr optical nonlinearity, a many-photon light beam can behave as a quantum fluid of interacting bosons. This has opened the way to active experimental and theoretical investigations of many-body hydrodynamic and quantum features in photon-based systems, the research field of the so-called quantum fluids of light. A promising platform to study photon-fluid physics consists in the paraxial propagation of a quasimonochromatic light wave through a nonabsorbing cavityless nonlinear optical medium of Kerr type. In contrast to semiconductor-planar-microcavity architectures where driving and dissipation play a major role in the evolution of the fluid of light, the photon field in a cavityless, propagating, geometry obeys a fully conservative Gross–Pitaevskii-type quantum dynamics. The statistical properties of the photon beam entering the dielectric fix the initial conditions of the problem and the ones of the light exiting the medium determine the final state of the photon field. The first part of my talk will be dedicated to a review of a very general quantum theory of light propagation in such a configuration. As a first application of the formalism, we will see in a second part that the occurrence of a frictionless flow of superfluid light past a solid dielectric immersed into a nonlinear optical liquid may be revealed from the dramatic suppression of the optomechanical deformation of the object, demonstrating that, in the optical case also, superfluidity is associated with a drop in the force exerted by the fluid on obstacles stymying its flow. In a third part, I will show that the paraxial-propagation geometry constitutes a very simple platform to investigate quantum-quench physics in closed systems of many interacting bosons, including, e.g., the acoustic analog of the dynamical Casimir effect, the light-cone-like spreading of the two-body correlations following a quantum quench, or the emergence of prethermalization features in one-dimensional configurations. Before concluding, I will present ongoing experiments aiming at measuring the Bogoliubov dispersion relation in a one-dimensional nonlinear optical waveguide (in Trento) and at detecting superfluid features in the flow of a photon fluid past a localized optical defect (in Nice and Edinburgh). Finally, I  will briefly expose in-progress works carried out in Trento and Trieste, on the study of the strong-interaction, Tonks–Girardeau, regime in one-dimensional cavityless geometries and on the investigation of the relaxation of a quantum-quenched photon gas towards the Bose–Einstein statistics.

Séminaire du LPTMS: Jacopo de Nardis

Time evolution of out-of-equilibrium integrable models

Jacopo de Nardis, LPS ENS

Non-equilibrium quantum mechanics has received a considerable attention in the last years and in particular the dynamics observed after a quantum quench, when a global parameter in the Hamiltonian of the system is rapidly changed. We focus here on the post-quench time evolution of the simplest interacting integrable models as the Lieb-Liniger delta-Bose gas and the spin-1/2 XXZ spin chain. We address the problem of determining the steady state after a quench in the interaction strength and the time evolution towards it. While the steady state is determined by sets of recently introduced quasi-local conserved quantities, the time evolution is related to the thermodynamic limit of the matrix elements and of the overlap coefficients between the eigenstates of the model and the initial state.

Physics-Biology interface seminar: Christoph Schmidt


Séminaire du LPTMS: Oleg Lychkovskiy

Perpetual motion and driven dynamics of a mobile impurity in a quantum fluid.

Oleg Lychkovskiy, Russian Quantum Center, Moscow

We study the dynamics of a mobile impurity in a quantum fluid at zero temperature. Two related settings are considered. In the first setting, the impurity is injected in the fluid with some initial velocity v_0 (which is below the critical velocity), and we enquire about its velocity at infinite time, v_inf. We derive a rigorous upper bound on |v_0 − v_inf|. In the limit of vanishing impurity-fluid coupling, this bound amounts to v_inf = v_0, which is anticipated from the Landau criterion of superfluidity. In the case of a finite coupling, the velocity of the impurity can decrease, but not to zero; the bound quantifies the maximal possible decrease.

In the second setting, a small constant force is exerted upon the impurity. We argue that two distinct dynamical regimes exist—backscattering oscillations of the impurity velocity and saturation of the velocity without oscillations. Which regime is realized depends on the mass of the impurity. A nonequilibrium quantum phase transition occurs at some critical mass.

Séminaire du LPTMS: Hugo Touchette

Markov processes conditioned on large deviations

Hugo Touchette, National Institute for Theoretical Physics (NITheP),Stellenbosch, South Africa

I will discuss in this talk recent works with Raphael Chetrite (Université de Nice) on Markov processes conditioned on rare events - for example, Brownian motion conditioned on having a given asymptotic velocity or on staying positive for a very long time. I will show that a process conditioned on a large deviation can be represented in the long-time limit by a conditioning-free Markov process, called the driven or the equivalent process, which has the same typical states as the conditioned process. The driven process arises in the study of nonequilibrium processes and the simulation of large deviations. Other applications related to reaction paths, stochastic optimal control, and Brownian bridges and meanders will also be mentioned.

Physics-Biology interface seminar: Étienne Fodor

Tracking nonequilibrium physics in living systems

Étienne Fodor (Université Paris-Diderot)

Living systems operate far from equilibrium due to the continuous injection of energy provided by ATP supply. The dynamics of the intracellular components is driven by both thermal equilibrium fluctuations and active stochastic forces generated by the molecular motors. Tracer particles are injected in living cells to study these fluctuations. Alternatively, vesicles which are already present in the cytoplasm serve as probes of the intracellular dynamics.

To sort out genuine nonequilibrium fluctuations from purely thermal effects, we combine passive and active microrheology methods. They consist in measuring the spontaneous tracer fluctuations and extracting the response from an external oscillatory perturbation. By testing the fluctuation-dissipation theorem, we quantify the deviation from equilibrium appearing at low frequency. Removing the thermal contribution in the tracer fluctuations, we estimate the spectrum of the active forces. Eventually, we report non-Gaussian tails in the tracer displacement distribution as a result of directed motion events.

We recapitulate theoretically the observed fluctuations by modeling the dynamics with a confining harmonic potential which experiences random bursts as a result of motor activity [1]. This minimal model allows us to quantify the time and length scales of the active forces, along with the energy scale injected by the ensuing fluctuations [2, 3]. Finally, we estimate the energy dissipated by the tracers in the surrounding environment, leading us to define an efficiency of the energy conversion driving the tracer dynamics [4].

  • [1] É. Fodor et al., Phys. Rev. E 90, 042724 (2014)
  • [2] É. Fodor et al., EPL 110, 48005 (2015)
  • [3] W. W. Ahmed et al., arXiv:1510.08299
  • [4] É. Fodor et al., arXiv:1511.00921

Séminaire du LPTMS: Andrea Gabrielli

Estimating topological properties of weighted networks from limited information

Andrea Gabrielli, Dipartimento di Fisica, Università di Roma La Sapienza

A problem typically encountered when studying complex systems is the limitedness of the information available on their topology, which hinders our understanding of their structure and of the dynamical processes taking place on them. A paramount example is provided by financial networks, whose data are privacy protected: Banks publicly disclose only their aggregate exposure towards other banks, keeping individual exposures towards each single bank secret. Yet, the estimation of systemic risk strongly depends on the detailed structure of the interbank network. The resulting challenge is that of using aggregate information to statistically reconstruct a network and correctly predict its higher-order properties. Standard approaches either generate unrealistically dense networks, or fail to reproduce the observed topology by assigning homogeneous link weights. Here [1], we develop a reconstruction method, based on statistical mechanics concepts, that makes use of the empirical link density in a highly nontrivial way. Technically, our approach consists in the preliminary estimation of node degrees from empirical node strengths and link density, followed by a maximum-entropy inference based on a combination of empirical strengths and estimated degrees. Our method is successfully tested on the international trade network and the interbank money market, and represents a valuable tool for gaining insights on privacy-protected or partially accessible systems. [1] G. Cimini, T. Squartini, A. Gabrielli, and D. Garlaschelli, Phys. Rev. E 92, 040802(R), 2015