### Séminaires de l’année 2019

# Séminaire du LPTMS: Fabio Franchini

## Spontaneous Ergodicity Breaking in Invariant Matrix Models

### Fabio Franchini (Institut Ruđer Bošković, Zagreb, Croatia)

We reconsider the study of the eigenvectors of a random matrix, to better understand the relation between localization and eigenvalue statistics. Traditionally, the requirement of base invariance has lead to the conclusion that invariant models describe only extended (conductive) systems. We show that deviations of the eigenvalue statistics from the Wigner-Dyson universality reflects itself on the eigenvector distribution. In particular, gaps in the eigenvalue density spontaneously break the*U*(

*N*) symmetry to a smaller one, hence rendering the system not anymore ergodic. Models with log-normal weights, recently considered also in string theory models such as ABJM theories, show a critical eigenvalue distribution which would indicate a critical breaking of the

*U*(

*N*) symmetry, supposedly resulting into a multi-fractal eigenvector statistics. These results pave the way to the exploration of localization problems using random matrices via the study of new classes of observables and potentially to novel, interdisciplinary, applications of matrix models.

# Physics-Biology interface seminar: Philippe Marcq

## From cells to tissue: A continuum model of epithelial mechanics

### Philippe Marcq (Institut Curie, Paris)

**SPECIAL LOCATION**

A continuum model of epithelial tissue mechanics is formulated using cellular-level mechanical ingredients and cell morphogenetic processes, including cellular shape changes and cellular rearrangements. This model incorporates stress and deformation tensors, which can be compared with experimental data. Focusing on the interplay between cell shape changes and cell rearrangements, we elucidate dynamical behavior underlying passive relaxation, active contraction-elongation, and tissue shear flow. Extensions of the model allow to incorporate additional ingredients, such as cell division, cell death, and cell motility.

# Séminaire du LPTMS: Gatien Verley

## Non-equilibrium conductance -- Assembly of energy converter

### Gatien Verley (LPT, Bât. 210, Université Paris-Sud)

Stochastic thermodynamic is the appropriated theory to study small non-equilibrium systems modeled by Markov processes. These systems are driven by several heat reservoirs (or work sources) and are crossed by strong energy currents. In this framework and for stationary Markov jump processes, I will introduce the concept of non-equilibrium conductance matrix that connects energy currents and thermodynamic forces, as Onsager matrices do for close-to-equilibrium systems. I will discuss the Thermodynamic Uncertainty Relations (TUR) that generalizes far from equilibrium the fluctuations-dissipation theorem. A central result is that current covariances are bounded from below by the conductance matrix (Loewner partial order). In the close to equilibrium limit, this TUR saturates restoring the fluctuations-dissipation theorem. Non-equilibrium conductance matrices are also convenient to study energy converters and power efficiency trade-off. In the second part, I will describe an energy converter made of many small interacting elementary machines. This energy converter has many original features that I found inspiring for understanding the physics of non-equilibrium systems : - emergent strong coupling in the mean-field treatment (as evidenced by the non-equilibrium conductance matrix), - collective enhancement of efficiency and power, - dynamical phase transition and emergent ergodicity breaking (non-convex large deviation functions for currents) - non-typical efficiency fluctuations# Physics-Biology interface seminar: Mathilde Badoual

## Modeling evolution of brain tumors

### Mathilde Badoual (IMNC, U. Paris-Sud)

Diffuse low-grade gliomas are slowly-growing tumors. After tens of years, they transform inexorably into more aggressive forms, jeopardizing the patient’s life. Mathematical modeling could help clinicians to have a better understanding of the natural history of these tumors and their response to treatments. We present here different models of these tumors: the first one is discrete and describes the appearance of the first glioma cells and the genesis of a tumor. The second model is continuous and consists in a partial differential equation that describes the evolution of the cell density. This model can describe the natural evolution of gliomas and their response to treatments such as radiotherapy. The discrete and the continuous models are designed to be close to the biological reality. The results are quantitatively compared with either biological data or clinical data, at the cellular level (histological samples) and at the organ level (clinical imaging, such as MRI scans).

# Soutenance de thèse: Giulio Bertoli

Soutenance de thèse:*Many-body localization of two-dimensional disordered bosons*

par

## Giulio Bertoli

**Jury**:

- Rapporteur :
**Anna Minguzzi**(LPMMC, Grenoble) - Rapporteur :
**Vladimir Yudson**(Russian Quantum Center, Moscow) - Examinateur :
**Vladimir Kravtsov**(ICTP, Trieste) - Examinateur :
**Nicolas Pavloff**(LPTMS, Orsay) - Directeur de thèse :
**Georgy Shlyapnikov**(LPTMS, Orsay)

**Résumé**:

The study of the interplay between localization and interactions in disordered quantum systems led to the discovery of the interesting physics of many-body localization (MBL). This remarkable phenomenon provides a generic mechanism for the breaking of ergodicity in quantum isolated systems, and has stimulated several questions such as the possibility of a finite-temperature fluid-insulator transition. At the same time, the domain of ultracold interacting atoms is a rapidly growing field in the physics of disordered quantum systems.

In this Thesis, we study many-body localization in the context of two-dimensional disordered ultracold bosons. After reviewing some importance concepts, we present a study of the phase diagram of a two-dimensional weakly interacting Bose gas in a random potential at finite temperatures. The system undergoes two finite-temperature transitions: the MBL transition from normal fluid to insulator and the Berezinskii-Kosterlitz-Thouless transition from algebraic superfluid to normal fluid. At T=0, we show the existence of a tricritical point where the three phases coexist. We also discuss the influence of the truncation of the energy distribution function at the trap barrier, a generic phenomenon for ultracold atoms. The truncation limits the growth of the localization length with energy and, in contrast to the thermodynamic limit, the insulator phase is present at any temperature. Finally, we conclude by discussing the stability of the insulating phase with respect to highly energetic particles in systems defined on a continuum.

# Séminaire du LPTMS: Marie Farge

**Comment faire évoluer le système de publication académique en faisant passer l'intérêt des chercheurs devant celui des maisons d'édition?**

### Marie Farge (CNRS-INSMI et ENS Paris)

La publication des résultats de la recherche se fait encore aujourd'hui selon un modèle hérité de l'imprimerie, où les revues académiques appartiennent aux maisons d'édition et où les chercheurs, ou leur institution, doivent payer pour lire les articles, voire même pour publier. Ce modèle est inadapté à l'ère de l'édition électronique et du Web et le préserver tel quel est contre-productif pour l'avancement de la recherche. Il est important que les chercheurs proposent de nouveaux modèles mieux adaptés à leurs besoins. Nous donnerons différents exemples d'outils développés par les chercheurs pour les chercheurs, dont les plateformes http://dissem.in et https://www.centre-mersenne.org/.

http://openscience.ens.fr/MARIE_FARGE https://hal.archives-ouvertes.fr/hal-01503303 https://hal.archives-ouvertes.fr/hal-01503304 https://hal.archives-ouvertes.fr/hal-01503306 https://hal.archives-ouvertes.fr/hal-01503287 https://zenodo.org/record/1227093 ________________________________________________________________________________Marie Farge est Directrice de Recherche au CNRS. Elle est rattachée à l'Institut National des Mathématiques et de leurs interactions (INSMI) et affectée au Laboratoire de Météorologie Dynamique (LMD) de l'ENS-Paris. Ses domaines de recherche sont la dynamique non linéaire, la turbulence et l'analyse harmonique, en particulier la transformée en ondelettes. Elle a rédigé en 2011 l'avis du COMETS (Comité d'Ethique du CNRS) sur les relations entre les chercheurs et les maisons d'édition et a participé en 2012 au lancement du mouvement The cost of knowledge. En 2015 elle a co-fondé le Comité pour l'Accessibilité aux Publications en Sciences and Humanités (CAPSH) qui développe la plate-forme http://dissem.in pour aider les chercheurs à mettre leurs articles en accès ouvert. Elle est membre du conseil de l'Academia Europaea depuis 2013 et du conseil scientifique du Centre Mersenne créé en 2018, https://www.centre-mersenne.org/.

http://openscience.ens.fr/MARIE_FARGE http://wavelets.ens.fr dans 'Publications'# Physics-Biology interface seminar: Andela Saric

## Sensing and creating mechanical forces by protein assembly

### Andela Saric (University College London, UK)

Responding to mechanical cues and producing mechanical forces is central to survival and adaptation of all organisms. To do so, cells dynamically organise a large number of membrane proteins into functional nanoscale structures. Due to their multiscale nature, such processes are challenging to resolve with current experimental techniques and are too complex for detailed molecular simulations. Coarse-grained computer simulations, rooted in statistical mechanics and soft-matter physics, can be of great value in determining how functional protein nanostructures operate. Here I will discuss cooperative gating of mechanosensitive membrane channels, and the production of mechanical forces by elastic filaments adsorbed on membranes, using minimal coarse-grained models. I will provide quantitative predictions for cooperative action of bacterial mechanosensitive channels, from molecular to cellular scales. I will propose how geometrical transitions of ESCRT-III filaments can remodel and sever membranes of various shapes and topologies, creating membrane cones, tubes, buds, and dividing tubular and spherical membranes. Beyond their biological context, our findings can also guide the design of artificial structures that mechanically sculpt cells and sense mechanical forces at the nanoscale.# Séminaire du LPTMS: Isabelle Bouchoule

## Generalised hydrodynamics on an atom chip

### Isabelle Bouchoule (Institut d'Optique)

Describing the out-of-equilibrium dynamics of many-body quantum systems is a priori a tremendously difficult task. However, a recent theoretical development provides an ab-initio description of the long wavelength dynamics of 1D integrable quantum systems, the so-called Generalised Hydrodynamics (GHD). In contrast to conventional hydrodynamics, GHD does not assume that gas is locally described by the Gibbs ensemble but it keeps track of all conserved quantities of the integrable system. In cold atom experiments, 1D bosonic gases are realised, which are well described by the famous integrable Lieb-Liniger model. Cold atom experiments thus offer an ideal platform to test GHD. We use the atom-chip experiment of LCF, where 1D gases of^{87}Rb are realized, to test experimentally GHD. Starting from a cold atomic cloud at thermal equilibrium, dynamics is generated by a sudden quench of the longitudinal potential. The measured time evolution of the density profiles are in excellent agreement with predictions from GHD. We also compare our data with predictions from the conventional hydrodynamics method, which assumes locally a thermal equilibrium described by a Gibbs ensemble. Except for the special case of harmonic potentials, we find that conventional hydrodynamics completely fails to reproduce our data. Hydrodynamics even predicts the development of sharp structures leading to a chock phenomena, such a phenomena being absent in the data and in the GHD description.

# Physics-Biology interface seminar: Fernando Luis Barroso Da Silva

## Developing and applying fast constant pH methods in biological systems: From biomaterials to virus

### Fernando Luis Barroso Da Silva (University of São Paulo, Brazil)

**SPECIAL TIME & DATE**

pH is a key parameter for biological and technological processes. Different numerical schemes were developed during the last years for such simulations ranging from Poisson-Boltzmann approaches to explicit solvent based methods. Ideally, the proton equilibria should correctly describe the experimental system without hampering the calculation time. A fast proton titration scheme (FPTS), rooted in the Kirkwood model of impenetrable spheres, where salt is treated at the Debye-Huckel level, was specially developed for proteins and nucleic acids. This method has now been coupled with OPEP5 force field for constant pH molecular dynamics simulations. A benchmark study will be presented. Despite our approximations, both the robustness and its ability to proper describe the system physics by these numerical methods can be confirmed. FPTS was also applied to quantify protein stability and biomolecular interactions. In this talk, I will present results for some protein systems with importance in different applied fields from biomaterials to public health.

# Séminaire du LPTMS: Patrick Azaria

## Topological States in Massless One-dimensional Fermionic Systems

### Patrick Azaria (LPTMC-Sorbonne Universités)

Topological states are characterized by the unique exponentially localized excitations they host at their ends in an open geometry. The topological protections of these systems relies on the bulk of these systems being gapped. This is the case in one-dimensional systems for superconductors proximity coupled to a bulk superconductor. However in a purely one-dimensional system, where superconductivity arises from intrinsic charge conserving attractive interactions, the bulk of the system remains gapless due to large quantum fluctuations of the phase. One may then naturally question the existence of such topological phases in the presence of massless bulk excitations. Indeed, systems of spinless fermions with either attractive or repulsive interactions are described by a Luttinger liquid and are topologically trivial: they display quasi-long range dominant superconducting or 2*k*charge density wave instabilities without exponentially localized modes at their ends. In this talk I shall show that in coupled chains, or in interacting multi-flavor fermionic systems, topological states with non trivial end modes may exist provided there is enough symmetry protection. This includes systems with tendency toward superconducting order as well as the more seemingly innocuous situations of coupled density waves with repulsive interactions. In the particular case with

_{F}*N*=3 flavors, I shall argue that a non trivial topological superconducting phase, corresponding to the gapless analogue of the Haldane phase in spin-1 chains, as well as non trivial chiral gapless spin density waves phases hosting Z

_{3}parafermionic edge modes may be stabilized.

# séminaire du LPTMS: Serge Dmitrieff

## Simple elasticity problems inspired by biology

### Serge Dmitrieff (Institut Jacques Monod)

Biological cells are physical elements under internal and external mechanical constraints. For instance, cell need to oppose exterior stress to maintain their shapes, and overcome large internal drag forces to transport cargo. I will discuss about the role of semi-flexible polymers called*microtubules*in these processes. We will see that we can use simple physical notions from elasticity to address these problems : the maintenance the shape of blood cells can be understood as the buckling of a visco-elastic bundle in confinement, while some examples of intracellular transport to the cell center appears akin to Newton's first theorem, albeit with elastic forces. Even minimal formulations of these problems are deceivingly simple, and yield interesting new physics, such as the metastability of disc-like cells, and unexpected scaling laws for centering forces.

# Physics-Biology interface seminar: Karim Benzerara

## Formation of intracellular amorphous carbonates by bacteria

### Karim Benzerara (Sorbonne Universités, Paris)

Living cells can sustain out-of-equilibrium states in a given environment by consuming free energy. The formation of some intracellular mineral phases provides some examples of this. Here I will review some of the work we have performed in the last years to describe and understand how some cyanobacteria, which are abundant photosynthesizing bacteria appeared several billions years ago at the surface of the Earth, manage to form intracellular amorphous carbonates. I will show the environmental conditions under which they catalyze this process, detail the methodologies (including cryo-TEM and spectroscopies) we used to characterize these phases and finally address the involved (bio)molecular mechanisms. The question of the selective advantage(s) provided by this process, if any, will be asked. The implications for basic and applied research will also be addressed. Overall, this talk should convince you that a highly interdisciplinary work is crucially needed to globally understand this intracellular biomineralization process.

# Séminaire exceptionnel du LPTMS: Manas Kulkarni

## Duality in a model integrable even in a box-like confinement: Multi-soliton solutions and field theory

### Manas Kulkarni (International Centre for Theoretical Sciences, Bengaluru)

Models that remain integrable even in confining potentials are extremely rare and almost non-existent. Here, we consider the Hyperbolic Calogero (HC) model which remains integrable even in confining potentials (which has box-like shapes). We present a first-order formulation of the HC model in external confining potential. Using the rich property of duality, we find multi-soliton solutions of this confined integrable model. We demonstrate the dynamics of these multi-soliton solutions via brute-force numerical simulations. We study the physics of soliton collisions and quenching using numerical simulations. We have examined the motion of dual variables and found an analytic expression for the time period in a certain limit. We give the field description of this model and find analytical solutions for background (absence of solitons) in the large-N limit (which has the form of a table-top). Analytical expressions of soliton solutions have been obtained in the absence of confining potential. Our work is of importance to understand general features of trapped interacting particles that remain integrable and can be of relevance to the collective behaviour of trapped cold atomic gases.# Séminaire du LPTMS: Sergej Moroz

## Low-energy physics of a superfluid vortex lattice

### Dr. Sergej Moroz (TU Munich)

Superfluidity is one of the most intriguing quantum phenomena that emerges at temperatures close to absolute zero. Vortices with quantized circulation are the hallmark of superfluidity and are created under external rotation. If the number of vortices is much smaller than the number of particles, vortices arrange themselves in a triangular lattice that exhibits intriguing collective dynamics. Quantum vortex crystals in superfluids and superconductors have been one of the exciting avenues of research for many decades. In this talk I will introduce an effective field theory built from low-energy degrees of freedom which captures universal properties of vortex lattices in bosonic superfluids. I will present our results for current and stress tensor Hall responses and will provide some arguments that the Hall viscosity of a quantum vortex crystal vanishes at zero frequency and momentum. In the end of the talk, I will describe new directions that we are currently exploring using this effective field theory.# Séminaire exceptionnel du LPTMS : Hugo Vanneuville

## Level set percolation for Gaussian fields

### Hugo Vanneuville (University of Lyon)

In this talk, we consider a random smooth Gaussian function from the plane to R and, given a level u, we colour the points where the function is larger than u in black and the points where the function is less than u in white. By relying on the recent work by V. Beffara and D. Gayet, we study the percolation properties of this random colouring and we try to compare it with Bernoulli percolation (in particular by proposing a mathematical study of the Harris criterion). Our work is motivated by the work by E. Bogomolny and C. Schmit in the case of random planar waves. Joint w. S. Muirhead and A. Rivera.# Séminaire du LPTMS: Naomi Oppenheimer

## Membrane Hydrodynamics and Their Role on Protein Interactions

### Prof. Naomi Oppenheimer (Flatiron Institute, USA)

Since the 1970s vast research has been devoted to studying the equilibrium properties of membranes as elastic surfaces. Much less attention has been given to their in-plane dynamics, which is crucial for protein function. In this talk, I will describe the basics of membrane hydrodynamics and give a few implications of the results. From a coarse-grained perspective, the membrane could be considered as a quasi-two-dimensional (Q2D) colloidal suspension, where the lipids play the role of the molecular fluid, and the proteins play the role of particle inclusions. We will see a couple of implications of membrane hydrodynamics: (1) Hindrance of chemical reactions taking place in a membrane, (2) Fluid mediated self-assembly of ATP synthase proteins in a membrane, or of artificial rotating colloids in a soap film.# Physics-Biology interface seminar: Daria Bonazzi

## Bacterial aggregates of Neisseria meningitidis flow inside blood vessels like a honey-like viscous liquid

### Daria Bonazzi (Institut Pasteur, Paris)

During human meningococcal infections, bacteria accumulate and finally fill up blood vessels, causing severe diseases such as septicaemia and meningitis. However, the mechanisms governing formation of bacterial aggregates and their impact on infection remained elusive. In this multidisciplinary work we unravel that aggregates of Neisseria meningitidis behave as a viscous liquid similar to honey. This is due to type-IV pili, long adhesive filaments which are constantly extending and retracting at the bacterial surface. By these means, bacteria can find each other and transiently come into contact. Aggregation is therefore based on an intermittent process of attraction between bacteria, and gives rise to a new type of active material. Importantly, the unique physical properties of meningococcal aggregates allow them to gradually adapt to the complex geometry of the vascular network. As a consequence, these physical properties are essential for the progression of infection.

# Séminaire du LPTMS: Kris Van Houcke

## Summation of diagrammatic series for a strongly correlated fermionic theory with zero convergence radius

### Kris Van Houcke (École Normale Supérieure, Paris)

In this talk I will mainly focus on the unitary Fermi gas (spin 1/2 fermions with contact interactions in 3D, which describes cold atomic gases at a Feshbach resonance) in the normal phase. Thanks to a diagrammatic Monte Carlo algorithm, we accurately sample all skeleton diagrams (built on dressed single-particle and pair propagators) up to order nine [1]. The diagrammatic series is divergent and there is no small parameter so that a resummation method is needed. We compute the large-order asymptotics of the diagrammatic series, based on a functional integral representation of the skeleton series and the saddle-point method. We show that the radius of convergence is actually zero, but the series is still resummable, by a generalised conformal-Borel transformation that incorporates the large-order asymptotics [2]. This yields new high-precision data, not only for the equation of state, but also for Tan's contact coefficient and for the momentum distribution [3]. I will also highlight some recent developments in (determinant) diagrammatic Monte Carlo and present new high-precision data for the Fermi polaron, which is a single impurity atom immersed in a Fermi sea. References: [1] K. Van Houcke, F. Werner, T. Ohgoe, N. V. Prokof'ev, and B. V. Svistunov, "Diagrammatic Monte Carlo algorithm for the resonant Fermi gas", Phys. Rev. B 99, 035140 (2019). [2] R. Rossi, T. Ohgoe, K. Van Houcke, F. Werner, "Resummation of diagrammatic series with zero convergence radius for strongly correlated fermions", Phys. Rev. Lett. 121, 130405 (2018) [3] R. Rossi, T. Ohgoe, E. Kozik, N. Prokof'ev, B. Svistunov, K. Van Houcke, F. Werner, "Contact and momentum distribution of the unitary Fermi gas", Phys. Rev. Lett. 121, 130406 (2018)# séminaire du LPTMS: Mathilde Badoual and Christophe Deroulers

## How modelling and statistical physics can contribute to understand brain tumors?

### Mathilde Badoual and Christophe Deroulers (IMNC)

Diffuse low-grade gliomas are slowly-growing tumors. After tens of years, they transform inexorably into more aggressive forms, jeopardizing the patient’s life. In our team, we combine mathematical modelling, statistical physics and image analysis, in order to have a better understanding of the natural history of these tumors and their response to treatments. We present here two projects: We will start with the effect of radiotherapy on gliomas. We developed a mathematical model that describes the evolution of cell density in gliomas and their response to treatments such as radiotherapy. We will show how biological data and clinical data were used to design and validate the model, and we will discuss the results of the model. The second part of the talk will be about the collective behaviour of precursor cells that are suspected to be at the origin of gliomas. In their "normal state", these cells proliferate and die in order to maintain a constant cell density all over the adult brain (homeostasis). We modelled these phenomena with a cellular automaton and we will show that in the cases of almost uniform conditions, we observe oscillations, but in the case of non-uniform conditions, we observe intriguing phenomena such as propagating waves, spiral waves, large transient oscillations, and even population extinction. We will also show how the appearance of an abnormal cell can lead to the appearance of a glioma.# Physics-Biology interface seminar: Stefan Karpitschka

## SEMINAR CANCELLED

### Stefan Karpitschka (MPI Göttingen)

# Journal Club: Denis Ullmo

**Semiclassical expectation value for an out of equilibrium system**

### Denis Ullmo (LPTMS/Institut Pascal)

# Séminaire du LPTMS: Gregory Astrakharchik

## Andreev-Bashkin Effect in quantum gases

### Gregory Astrakharchik (UPC, Barcelona)

We study a mixture of two superfluids with density–density and current–current (Andreev–Bashkin) interspecies interactions. The Andreev–Bashkin coupling gives rise to a dissipationless drag (or entrainment) between the two superfluids. Within the quantum hydrodynamics approximation, we study the relations between speeds of sound, susceptibilities and static structure factors, in a generic model in which the density and spin dynamics decouple. Due to translational invariance, the density channel does not feel the drag. The spin channel, instead, does not satisfy the usual Bijl–Feynman relation, since the f-sum rule is not exhausted by the spin phonons. The very same effect on one dimensional Bose mixtures and their Luttinger liquid description is analysed within perturbation theory. Using diffusion quantum Monte Carlo simulations of a system of dipolar gases in a double layer configuration, we confirm the general results. Given the recent advances in measuring the counterflow instability, we also study the effect of the entrainment on the dynamical stability of a superfluid mixture with non-zero relative velocity. We quantify the non-dissipative drag effect, resulting from the Andreev-Bashkin current-current interaction between the two components of the gas, and we show that in the regime of strong coupling it causes a significant suppression of the spin-wave velocity. [1] L. Parisi, G. E. Astrakharchik, and S. Giorgini, Spin Dynamics and Andreev-Bashkin Effect in Mixtures of One-Dimensional Bose Gases, Phys. Rev. Lett. 121, 025302 (2018) [2] Jacopo Nespolo, Grigori E. Astrakharchik, Alessio Recati, Andreev-Bashkin effect in superfluid cold gases mixture, New J. Phys. 19, 125005 (2017)# Séminaire du LPTMS: Jérémie Bouttier

## Last passage percolation, Schur processes and free fermions

### Jérémie Bouttier (IPhT-Saclay and ENS Lyon)

We consider the model of (directed 2D) last passage percolation (LPP), defined as follows. To every site of Z^2 we assign a nonnegative random number called weight. To every directed (north-east) path in Z^2, we assign a weight equal to the sum of of the weights of the sites that it visits. If x,y are two sites such that y is north-east of x, we define the LPP time L(x,y) as the maximal weight of a directed path from x to y. In his seminal paper arXiv:math/9903134 [math.CO], Johansson considered the situation where the weights are all independent and drawn according to the same geometric or exponential distribution. This case is intimately connected with the Totally Asymmetric Simple Exclusion Process (TASEP) and the so-called Corner Growth Model. He proved that the fluctuations of L(x,y), when |y-x| gets large, are asymptotically governed by the Tracy-Widom GUE distribution. Several variants have been considered. In particular, Baik and Rains considered the situation where the weight array has symmetries, for instance a reflection symmetry along the main diagonal i-j=0. This is equivalent to considering LPP in the half-plane i-j≥0. They have shown that the asymptotic fluctuations of L(x,y), for x and y close to the main diagonal, are now governed by the Tracy-Widom GOE or GSE distribution. What allows to obtain such precise results is that, in these models, there exists an exact formula for the distribution of L(x,y) in the form of a Fredholm determinant or pfaffian. Such property holds for the more general class of Schur processes, which may be studied using free fermions. After reviewing these results, I will discuss Schur processes with periodic and free boundary conditions. In the LPP picture, this corresponds to imposing some periodicity/symmetry properties on the weight array. We will see that, in suitable asymptotic regimes, the fluctuations of the LPP time are governed by nontrivial deformations of the Tracy-Widom distributions. This talk is based on joint work with Dan Betea, Peter Nejjar and Mirjana Vuletić.# Séminaire du LPTMS: Thibault Congy

## Wave-mean flow interactions in dispersive hydrodynamics

### Thibault Congy (Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne, UK)

The interaction of waves with a mean flow is a fundamental and longstanding problem in fluid mechanics. The key to the study of such an interaction is the scale separation, whereby the length and time scales of the waves are much shorter than those of the mean flow. The wave-mean flow interaction has been extensively studied for the cases when the mean flow is prescribed externally—as a stationary or time-dependent current (a "potential barrier"). In this talk, I will describe a new type of the wave-mean flow interactions whereby a short-scale linear wave is incident on a evolving large-scale nonlinear dispersive hydrodynamic state: a rarefaction wave or a dispersive shock wave (DSW). Modulation equations are derived for the coupling between the wavepacket and the mean flow in the nonlinear dispersive hydrodynamic state. These equations admit particular classes of solutions that describe the transmission or trapping of the linear wave by an unsteady hydrodynamic state. Two adiabatic invariants of motion are identified in both cases that determine the transmission, trapping conditions and show that wavepackets incident upon smooth expansion waves or compressive, rapidly oscillating DSWs exhibit so-called hydrodynamic reciprocity. The developed theory is general and can be applied to a broad class of nonlinear dispersive wave equations in various physical contexts including nonlinear optics and cold atom physics. The Korteweg-de Vries equation will be considered as a concrete example. This is joint work with Gennady El and Mark Hoefer, and the talk is based on Ref. [1]. [1] T. Congy, G. A. El and M. A. Hoefer, Interaction of linear modulated waves with unsteady dispersive hydrodynamic states, arXiv:1812.06593.# Séminaire exceptionnel du LPTMS: Maxime Dupont

## Dynamics in quantum magnets

### Maxime Dupont (UC Berkeley)

Theoretically challenging, the understanding of the dynamical response in quantum magnets is of great interest, in particular for both inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments. In this talk, I will address this question for quasi-one-dimensional quantum magnets, e.g., weakly coupled spin chains for which many compounds are available in nature. In this class of systems, the dimensional crossover between a three-dimensional ordered regime at low temperature towards one-dimensional physics at higher temperature is a nontrivial issue, notably difficult concerning dynamical properties. I will present a comprehensive theoretical study based on both analytical calculations and numerical simulations which allows us to describe the full temperature crossover for the NMR relaxation rate 1/T1, from one-dimensional Tomonaga-Luttinger liquid physics to the three-dimensional ordered regime, as a function of interchain couplings.# Séminaire du LPTMS: Cécile Appert-Rolland

## Experiment driven models for pedestrians and crowds

### Cécile Appert-Rolland (LPT - U-Psud Orsay)

The growing interest for crowd modeling has led to the development of various types of models, in part inspired by fluid mechanics. However we are still lacking full knowledge of pedestrian behavior throughout all the scales (from individuals to crowds, from low to high densities, etc). In this seminar I will show how experiments can be used to develop or improve microscopic or macroscopic models. Several series of experiments will be reported. Some, performed in Rennes, deal with interactions at low and moderate densities (PEDIGREE and PEDINTERACT Project) while other ones, performed in Orsay and in Argentina (PERCEFOULE Project), have provided intriguing observations of high density crowds that question current models. In another perspective, models can be useful to scan the mechanisms responsible for, for example, pattern formation occurring at the crossing of perpendicular pedestrian flows.# Physics-Biology interface seminar: Pierre-Henri Puech

## Some aspects of immune cells mechanobiology: activation of T lymphocytes and parasite phagocytosis by macrophages

### Pierre-Henri Puech (Adhesion and Inflammation Lab - Aix-Marseille Universités)

Surface proteins of cells which are implicated in cell recognition and adhesion are also subjected to forces, since the interactions with their partner is often occuring at the interface between two cells. The study of the roles of physical cues, such as forces, substrate mechanics and dynamics has lead to the emergence of mechanobiology. One idea is to dissect how these physical cues "help" the cells to read and interpret, sometimes react to, biochemical messages, a process known as mechanotransduction.

In this seminar, we will present why and how forces and mechanics are thought to be key regulators of immune cell recognition using our work using two biological models:

- the crucial step of T cell activation which leads to an efficient immune response
- the phagocytic response of macrophages to the parasite Toxoplasma Gondii, responsible of toxoplasmosis.

# Séminaire exceptionnel du LPTMS: Anthony Mays

## Determinantal polynomials, vortices and random matrices: an exercise in experimental mathematical physics

### Anthony Mays (University of Melbourne)

Inspired by the interpretation of eigenvalues of random matrices as a gas of interacting particles we develop a toy model of vortex or particle dynamics using determinantal polynomials of random matrices. By introducing quaternionic structures we generate vortex/anti-vortex systems, and from studying the phase surface of the associated wavefunction, we identify topological rules governing the creation and annihilation of the vortices. We also discuss an interpretation of annihilation events in terms of quaternionic states# Séminaire du LPTMS: Zhan Shi

## The Derrida-Retaux conjecture

### Zhan Shi (Labo. de Probabilités, Statistique et Modélisation, Paris VI)

I am going to present elementary discussions on the Derrida-Retaux conjecture concerning the free energy of some max-type recursive systems. Ongoing work with Xinxing Chen (Shanghai), Victor Dagard (ENS), Bernard Derrida (Collège de France), Yueyun Hu (Paris Nord), Mikhail Lifshits (St. Petersburg).# Soutenance de thèse: Bertrand Lacroix-à-chez-Toine

Soutenance de thèse:*Extreme value statistics of strongly correlated systems: fermions, random matrices and random walks*

#### par

## Bertrand Lacroix-à-chez-Toine

Jury: -Djalil Chafaï (CEREMADE, Université Paris-Dauphine) -Andrea Gambassi (Scuola Internazionale Superiore di Studi Avanzati, Italy) -Jean-Marc Luck (IPhT, CEA Saclay) -Satya N. Majumdar (LPTMS, Université Paris-Sud) -Grégory Schehr (LPTMS, Université Paris-Sud), directeur de thèse -Christophe Texier (LPTMS, Université Paris-Sud) -Patrizia Vignolo (INPHYNI, Université de Nice-Sophia Antipolis) -Pierpaolo Vivo (King’s College London, UK) Résumé:Predicting the occurrence of extreme events is a crucial issue in many contexts, ranging from meteorology to finance. For independent and identically distributed (i.i.d.) random variables, three universality classes were identified (Gumbel, Fréchet and Weibull) for the distribution of the maximum. While modelling disordered systems by i.i.d. random variables has been successful with Derrida's random energy model, this hypothesis fail for many physical systems which display strong correlations. In this thesis, we study three physically relevant models of strongly correlated random variables: trapped fermions, random matrices and random walks.

In the first part, we show several exact mappings between the ground state of a trapped Fermi gas and ensembles of random matrix theory. The Fermi gas is inhomogeneous in the trapping potential and in particular there is a finite edge beyond which its density vanishes. Going beyond standard semi-classical techniques (such as local density approximation), we develop a precise description of the spatial statistics close to the edge. This description holds for a large universality class of hard edge potentials. We apply these results to compute the statistics of the position of the fermion the farthest away from the centre of the trap, the number of fermions in a given domain (full counting statistics) and the related bipartite entanglement entropy. Our analysis also provides solutions to open problems of extreme value statistics in random matrix theory. We obtain for instance a complete description of the fluctuations of the largest eigenvalue in the complex Ginibre ensemble.

In the second part of the thesis, we study extreme value questions for random walks. We consider the gap statistics, which requires to take explicitly into account the discreteness of the process. This question cannot be solved using the convergence of the process to its continuous counterpart, the Brownian motion. We obtain explicit analytical results for the gap statistics of the walk with a Laplace distribution of jumps and provide numerical evidence suggesting the universality of these results.

# Physics-Biology interface seminar: Matthew Turner

## Intelligence, a recipe

### Matthew Turner (Warwick University, UK)

We study information-processing (artificial), or “intelligent” (living), agents. These agents seek maximal control of their environment via future state maximisation (FSM), a principle that we argue may relate to intelligent behaviour more generally. Here we study moving, re-oreintable agents that seek to maximise their space of accessible (visual) environments, out to some time horizon. The action of each agent is (re)established by exhaustive enumeration of its future decision tree at each time step - each agent chooses the branch of its tree leading from the present to the richest future state space. Remarkably, cohesive swarm-like motion emerges that is similar to that observed in animal systems, such as bird flocks. We develop heuristics that mimic computationally intensive FSM but that could also operate in real time under animal cognition. Finally, we show that iterative application of these heuristics as the model for the behaviour of others, when determining the dynamics of self under full FSM, can lead to a form of closure for the problem. I will argue that this offers a philosophically attractive, bottom-up mechanism for the emergence of swarming.

# Séminaire exceptionnelle du LPTMS: Leo Radzihovsky

## Fracton-elasticity duality

### Leo Radzihovsky (University of Colorado, Boulder)

I will discuss a recent discovery that elasticity theory of a two-dimensional crystal is dual to a fracton tensor gauge theory, providing a concrete realization of the fracton phenomenon. The disclinations and dislocations respectively map onto charges and dipoles of the tensor gauge theory. The fractionalized mobility of fractons matches the constrained dynamics of lattice defects. This duality leads to predictions for fractonic phases and phase transitions, such as the counterparts to the commensurate crystal, supersolid, hexatic, and isotropic fluid phases of the elasticity theory. Extensions of this duality to generalized elasticity theories provide a route to discovery of new fracton models.# Séminaire du LPTMS: Lenart Zadnik

## Inhomogeneous matrix product ansatz and exact solutions for strongly boundary driven spin chains

### Lenart Zadnik (University of Ljubljana)

I will present a novel inhomogeneous Lax structure and demonstrate the exact solvability of a dissipatively boundary driven XYZ spin-1/2 chain in the limit of strong dissipation. I will describe the explicit inhomogeneous matrix product ansatz for the nonequilibrium steady state of the problem. The constituent matrices of this ansatz satisfy a simple set of linear recurrence relations. Although they can be embedded into an infinite-dimensional auxiliary space, they cannot be simultaneously put into a tridiagonal form, not even in the case of axially symmetric (XXZ) bulk interactions and general nonlongitudinal boundary dissipation. The results are expected to have further fundamental applications for the construction of nonlocal integrals of motion for the open XYZ model with arbitrary boundary fields.# Physics-Biology interface seminar: Stéphanie Mangenot & Aurélie Bertin

## Membrane reshaping induced by Curvature sensitive Septin filaments: A story of paired filaments told by a pair of Orsay alumni

### Stéphanie Mangenot & Aurélie Bertin (Institut Curie, Paris)

Septins are cytoskeletal proteins that assemble into a variety of supramolecular organizations from paired filaments to bundles, ring like structures or gauzes of orthogonal filaments [1‐3]. Septins are bound to the inner plasma membrane through specific interactions with phosphoinositides [1,4]. Septins are essential for cell division, participate in the formation of diffusion barrier and might be involved in membrane deformation and rigidity. Throughout cell division, septins undertake major rearrangements. Septin filaments are first aligned toward the mother‐daughter cell axis and then rotate to be circumferential around the constriction site. We have shown that septins arrange differently on positive or negative curvatures using Scanning Electron Microscopy on micro‐patterned PDMS periodic undulated substrates. Besides, this curvature preference is closely related to the ability of septins to reshape and deform membranes. Indeed, bound to Giant unilamellar Vesicles (GUVs), septins induce striking deformations with regular spikes and hollow micrometric deformations at the surface of liposomes, as visualized by fluorescence microscopy. Smaller vesicles (LUVs of 100‐300 nm in diameter), highly positively curved, are flattened by Septin filaments into “pancake”like objects as shown in 3D by cryo‐electron tomography. With the resolution of cryo‐EM and sub‐tomogram averaging we visualize both the septin filaments and the deformed vesicles. We propose a simple model where the filamentous properties of septins control their curvature sensitivity and thus impose their orientation in situ [5].- [1] A. Bertin, et al. (2010), Phosphatydinositol 4,5 biphosphate promotes budding yeast septin filament assembly and organization, J. Mol. Biol., 404(4), 711‐31.
- [2] G. Garcia et al. (2011), The regulatory budding yeast septin Shs1 promotes ring and gauze formation in a phosphorylation dependent manner., J. Cell. Biol., 195(6), 993‐1004.
- [3] A.Bertin, et al. (2008), Saccharomyces Cerevisiae septins: supramolecular organization of heterooligomers and the mechanism of filament assembly, Proc.Natl. Acac. Sci USA, 105, 8274‐8279
- [4] Beber A et al.,Septin‐based readout of PI(4,5)P2 incorporation into membranes of giant unilamellar vesicles. (2018) Cytoskeleton. doi: 10.1002/cm.21480
- [5] A. Beber et al. (2019), Membrane reshaping by micrometric curvature sensitive septin filaments, Nat. Commun., 10, 420.

# Modern trends in closed and open quantum systems

### A welcome workshop for Davide Rossini (Università di Pisa and LPTMS, Orsay)

Monday, July 1 2019 Université Paris Diderot - Bâtiment Condorcet, 4th floor, Salle Luc Valentin (454A)*Free participation*

**Program:**

**13h15**- Welcome coffee break

**13h45**- Davide Rossini (Univ. Pisa & LPTMS)

*Discrete time crystal in a finite chain of Rydberg atoms without disorder*

**14h30**- Cristiano Ciuti (MPQ)

*Quantum cavities: from vacuum manipulation to photon simulation of quantum materials*

**15h00**- Guillaume Roux (LPTMS)

*Trapped interacting Rydberg atoms : cat states, thermometry and quantum transitions*

**15h30**- Coffee break

**16h00**- Marco Schirò (IPHT & Collège de France)

*Fluctuations Effects in Driven-Dissipative Quantum Systems*

**16h30**- Fabrice Gerbier (LKB & Collège de France)

*Anomalous momentum diffusion of strongly interacting bosons in optical lattices*

**17h00**- Christophe Mora (LPENS)

*Topology and strong correlation in twisted bilayer graphene*

**17h30**- Maurizio Fagotti (LPTMS)

*The spectrum of the time averaged state in a quantum spin lattice system*Organizers: Leonardo Mazza (LPTMS) and Alberto Biella (MPQ)

# Séminaire du LPTMS: Yacine Ikhlef

## Three-point functions in the fully packed loop model on the honeycomb lattice

### Dr. Yacine Ikhlef (LPTHE)

The Fully-Packed Loop (FPL) model on the honeycomb lattice is a critical model of non-intersecting polygons covering the full lattice, and was introduced by Reshetikhin in 1991. Using the two-component Coulomb-Gas approach of Kondev, de Gier and Nienhuis (1996), we argue that the scaling limit consists of two degrees of freedom: a field governed by the imaginary Liouville action, and a free boson. We introduce a family of three-point correlation functions which probe the imaginary Liouville component, and we use transfer-matrix numerical diagonalisation to compute finite-size estimates. We obtain good agreement with our analytical predictions for the universal amplitudes and spatial dependence of these correlation functions. Finally we conjecture that this relation between non-intersecting loop models and the imaginary Liouville theory is in fact quite generic. We give numerical evidence that this relation indeed holds for various loop models.# Physics-Biology interface seminar: Wolfgang Keil

## Using microfluidics for quantitative studies of post-embryonic development in C. elegans

### Wolfgang Keil (Institut Curie, Paris)

**Special location and time; seminar hosted by Carsten Janke**

The development of most metazoans can be divided in an early phase of embryogenesis and a subsequent phase of post-embryonic development. Developmental dynamics during the post-embryonic phase are generally much slower and often controlled by very different molecular mechanisms that, e.g., ensure tissue synchrony and integrate metabolic queues. However, obtaining long-term in-vivo quantitative imaging data post-embryonically with good statistical and cellular resolution has been highly challenging because animals must be allowed to grow, feed, and move in order to properly develop after embryogenesis. In this talk, I will discuss our recent progress in overcoming these challenges in the model organism C. elegans, using microfluidics technology. I will then outline two of our recent studies, in which quantitative in-vivo imaging data of post-embryonic development allows novel insights into mechanisms of cell-fate acquisition and the regulation of oscillatory gene expression in C. elegans.

# Séminaire exceptionnel du LPTMS: Daniel Cabra

## Daniel Cabra (Universidad Nacional de La Plata, Argentina)

### Multiferroics: magneto-electric coupling through phonons

We propose a microscopic magneto-electric model in which the coupling between spins and electric dipoles is mediated by lattice distortions. The magnetic sector is described by a spin S=1/2 Heisenberg model coupled directly to the lattice via a standard spin-Peierls term and indirectly to the electric dipole variables via the distortion of the surrounding electronic clouds. Electric dipoles are described by Ising variables for simplicity. We show that the effective magneto-electric coupling which arises due to the interconnecting lattice deformations is quite efficient in one-dimensional arrays. More precisely, we show using bosonization and extensive DMRG numerical simulations that increasing the magnetic field above the spin Peierls gap, a massive polarization switch-off occurs due to the proliferation of soliton pairs. We also analyze the effect of an external electric field E when the magnetic system is in a gapped (plateau) phase and show that the magnetization can be electrically switched between clearly distinct values. More general quasi-one-dimensional models and two-dimensional systems are also discussed.# Journal club: Serguei Brazovskii

## When the intuition betrays or saves: messages from the 1d world

**Abstract :**

*Series of two lectures with four excursions*

Theory of (quasi) one-dimensional electronic systems attracts us thanks to its special facilities but also because of some specific curiosities. In these excursions we shall meet some examples of not quite expected relations, difficulties and even mistakes which one can pickup from the half-a-century history of this field.

First excursion will descend to the contradictory story of Luttinger liquid with its curiosities in historical , personal and linguistic aspects. It will also introduce the once unattended role of so called chiral or Schwinger anomalies.

Second excursion will meet the chiral anomalies as they appear already in simplest, MF or BCS types, models particularly at finite temperatures when normal excitations are present. The resulting effective Ginzburg-Landau theory will prove to be quite different from what is commonly expected.

Third excursion will undermine the common belief of the spin-charge separation which is seemingly endorsed by the bosonization and exact solutions. Actually, spin excitations must carry the electric current as it takes place for free fermions. The resolution comes from correct definition of current carrying states and current operators taking into account the band curvature.

Forth excursion will lead to the early days when 1D models were studies to understand the phase diagram of real quasi-1D systems. We will see that, unlike the common beliefs, the 1D (g-ological) phase diagram based upon diverging power-law susceptibilities, does not want at all to reproduce itself when electrons acquire a bandwidth in interchain directions. The system falls to the Fermi-liquid regime unless the “imaginary gaps” appear from external symmetry lowering due to the crystal field of the magnetic field.

# Séminaire du LPTMS: Zorana Zeravcic (ESPCI)

## Towards Living Matter with Colloidal Particles

### Zorana Zeravcic (ESPCI)

Biological systems inspire a new paradigm for material synthesis, aiming to design materials that emulate living systems. Colloidal particles endowed with specific interactions provide a particularly promising approach for realizing this vision. In this talk I will focus on a set of colloidal spheres with specific, time-dependent interactions, and demonstrate that the interactions can be programmed so that three fundamental properties of living materials are realized: The ability to spontaneously assemble complex structures; the ability to self-replicate; and the ability to perform complex and coordinated reactions that enable transformations impossible to realize if a single structure acted alone.

# Séminaire exceptionnel du LPTMS: Elia Macaluso

## Charge and statistics of lattice quasiholes from density measurements

### Elia Macaluso (Università di Trento, Italy)

In the last decades, the lattice counterparts of the fractional quantum Hall effect have attracted strong attention. These systems, generally known as fractional Chern insulators (FCIs), support elementary bulk excitations carrying fractional charge and anyonic statistics. In this context, I will first introduce the FCI state at filling ν = 1/2 described by the bosonic Harper-Hofstadter model with hard-core interactions. Then, I will show how the aformentioned bulk excitations can be studied by means of a Tree Tensor Network (TTN) technique and, in particular, how the TTN ansatz for the many-body state allows us to study relatively large samples (up to N=18 particles on a 16x16 lattice), with the experimentally relevant open boundary conditions. Finally, I will explain how, not only the fractional charge, but also the anyonic statistics of the FCI excitations can be obtained through simple density measurements.# Journal club: Serguei Brazovskii

## When the intuition betrays or saves: messages from the 1d world

**Abstract :**

*Series of two lectures with four excursions*

Theory of (quasi) one-dimensional electronic systems attracts us thanks to its special facilities but also because of some specific curiosities. In these excursions we shall meet some examples of not quite expected relations, difficulties and even mistakes which one can pickup from the half-a-century history of this field.

First excursion will descend to the contradictory story of Luttinger liquid with its curiosities in historical , personal and linguistic aspects. It will also introduce the once unattended role of so called chiral or Schwinger anomalies.

Second excursion will meet the chiral anomalies as they appear already in simplest, MF or BCS types, models particularly at finite temperatures when normal excitations are present. The resulting effective Ginzburg-Landau theory will prove to be quite different from what is commonly expected.

Third excursion will undermine the common belief of the spin-charge separation which is seemingly endorsed by the bosonization and exact solutions. Actually, spin excitations must carry the electric current as it takes place for free fermions. The resolution comes from correct definition of current carrying states and current operators taking into account the band curvature.

Forth excursion will lead to the early days when 1D models were studies to understand the phase diagram of real quasi-1D systems. We will see that, unlike the common beliefs, the 1D (g-ological) phase diagram based upon diverging power-law susceptibilities, does not want at all to reproduce itself when electrons acquire a bandwidth in interchain directions. The system falls to the Fermi-liquid regime unless the “imaginary gaps” appear from external symmetry lowering due to the crystal field of the magnetic field.

# Séminaire du LPTMS: Marco Schirò

## Defying Quantum Thermalization through Interactions and Disorder: From Many Body Localization to Quantum Glassiness

### Marco Schirò (IPHT and Collège de France)

Generic interacting quantum many body systems are expected to reach thermal equilibrium when isolated from their environment and let evolve under their own quantum dynamics. Exceptions to this paradigm can emerge in presence of quenched random disorder, due to many body localization (MBL) or quantum glassiness. Those two robust scenarios for ergodicity breaking have recently attracted considerable interest both from a purely theoretical viewpoint and for their implications on the robustness of future quantum technologies. In this talk I will study the quantum dynamics of two prototype models for MBL and quantum glassy systems. I will first introduce a theoretical framework based on flow equations to study the properties MBL systems, in particular the emergence of localised integral of motions, and their dynamics. Then I will consider a mean field model of a quantum glass and I will study its isolated dynamics after a quantum quench. I will show that, contrary to the conventional wisdom based on thermodynamics, quantum fluctuations and non equilibrium effects result in an enhanced glassiness and ageing behavior.# Physics-Biology interface seminar: Carine Douarche

## Collective motion in bacterial suspensions

### Carine Douarche (Université Paris-Sud)

**Warning: special seminar time.**

Suspending highly motile Escherichia coli in a liquid lowers the viscosity of the solution at low shear rate. At higher cell concentrations, a regime of near zero viscosity can be reached. In this work, we investigate the system-size dependence of the rheological response of an E. coli suspension as a function of shear rate and bacteria concentration using a low-shear Couette rheometer. Additionally, we image the suspensions in a cone-plate rheo-imaging setup allowing direct visualisation of the collective organisation under shear. We find the flow becomes banded and viscosity decreases to near zero at a bacterial concentration close to where collective motion appears in absence of flow.

Indeed, dense suspensions of swimming bacteria display remarkable collective motion, i.e. local bacterial ordering associated with a characteristic correlation length, reminiscent of turbulent flow behaviour. Using video microscopy over large fields of view (up to 3 mm x 4 mm), and particle image velocimetry, we calculate the spatial correlation of the velocity vectors and extract a characteristic length scale. At sufficiently high bacterial concentrations, we find this length to be proportional to the smallest system size. However, the absence of saturation towards large system-sizes suggests there is no intrinsic length-scale in these dense populations of such ‘pusher-like’ swimmers.

# Séminaire du LPTMS: Werner Krauth (LPENS)

## Fast irreversible Markov chains in statistical physics

### Werner Krauth (LPENS)

The Monte Carlo method is an outstanding computational tool in science. Since its origins, it has relied on the detailed-balance condition (that is, the absence of flows in equilibrium) to map general computational problems onto equilibrium-statistical-physics systems. Irreversible Markov chains violate the detailed-balance condition. They realize the equilibrium Boltzmann distribution as a steady state with non-vanishing flows. For one-dimensional particle models we have proven rigorously that local algorithms reach equilibrium on much faster time scales than the reversible algorithms that satisfy detailed balance. The event-chain Monte Carlo algorithm (ECMC) generalizes these irreversible Markov chains to higher dimensions. It relies on a factorized Metropolis filter which is based on a consensus rule rather than on an energy criterion. As applications I will discuss the solution of the famous two-dimensional melting problem for hard disks and related systems, where we showed, using ECMC, that hard disks melt neither following the Kosterlitz-Thouless-Halperin-Nelson-Young prescription nor the alternative first-order liquid-solid scenario. I will also present the use of ECMC for the general classical all-atom N-body problem. Here, the Boltzmann distribution exp(-beta E) is sampled (without any discretization or truncation error) but the potential energy E remains unknown. This is of great interest in the Coulomb problem, where E or its derivatives, the forces, are hard to compute. Our recent JeLLyFysh open-source Python application implements ECMC for models from hard spheres to three-dimensional water systems. I will finish by discussing its features, that closely mirror the mathematical formulation of ECMC, and by presenting its future challenges.# Séminaire du LPTMS: Sylvain Nascimbène (LKB)

## Synthetic quantum Hall system with ultracold Dysprosium atoms

### Sylvain Nascimbène (LKB)

A quantum Hall system is characterized by the quantization of its Hall conductance, and its robustness with respect to material imperfections. In solid states devices disorder induces strong inhomogeneities in the Hall current distribution, making the connection with simple disorder-free models challenging. In this talk I will present the realization of a synthetic quantum Hall system using ultracold atoms of Dysprosium, in which a synthetic dimension is encoded in the electronic spin J=8. Dynamics in this dimension is induced by laser-induced spin couplings, and the Doppler effect occuring in these processes leads to a spin-orbit coupling, interpreted as an artificial magnetic field. We show that our system reproduces several characteristic features of Landau levels. We observe a clear distinction between bulk states -- with inhibited motion due to limited energy dispersion -- and edge modes, free to move in one direction only. We also probe the system excitations, via the measurement of cyclotron and skipping orbits. We finally probe the Hall response of the system, and make the connection to topological properties of the lowest energy band.# Physics-Biology interface seminar: Aurore Loisy

## Active fluids under confinement: spontaneous flow and negative viscosity

### Aurore Loisy (School of Mathematics, University of Bristol)

**Seminar hosted by Harold Auradou**

It has been known for a long time that adding motile bacteria to a liquid lowers the viscosity of the mixture. But only recently came the experimental evidence that this phenomenon can continue until a zero viscosity is reached (Lopez, Gachelin, Douarche, Auradou, & Clément, Phys. Rev. Lett. 115, 028301 (2015)). An even more intriguing result in this experiment was the (single) data point located below the zero-viscosity threshold. So, how is it possible to observe a steady state with a negative viscosity? In this talk I will use a simple model of an active liquid and propose an explanation to this peculiar phenomenon.

Time permitting, I will then consider a drop of active liquid (a minimal model for the actomyosin cytoskeleton) and show how the same physics allows this drop to propel itself on a substrate without the need to exert any traction locally on the surface. This mode of self-propulsion, which does not rely on adhesion, may be relevant to cell motility in crowded environments.

# Séminaire du LPTMS: Jérôme Martin (IAP)

## Cosmic Inflation and Quantum Mechanics

### Jérôme Martin (IAP)

One of the most important insight of modern Cosmology is that galaxies are of quantum-mechanical origin. According to the theory of inflation, they are indeed nothing but quantum fluctuations amplified by gravitational instability and stretched to cosmological distances by cosmic expansion. In this talk, I describe this mechanism and discuss the implications for primordial cosmology but also for foundational issues in Quantum Mechanics.# Séminaire exceptionnel du LPTMS: Martin Evans (Univ. of Edinburgh)

## Active Interface Equations

### Martin Evans (Univ. of Edinburgh)

In this work we consider the role of active inclusions in a growing interface, for example membrane binding proteins which catalyse growth in the plasma membrane of eukaryotic cells. The interface is thus rendered active and is described by two coupled fields: the height field of the interface and the density of the inclusions. The equations generalise to active interface growth the Kardar Parisi Zhang equation, which describes nonequilibrium growth and also represents many other systems driven out of out of equilibrium. In our model inclusions gravitate towards minima of the height field and then catalyse growth which generates interface waves. This leads to complex kinematic waves and pattern formation and the proteins are able to surf the waves they create. The interface width displays a novel superposition of scaling and sustained oscillations distinct from KPZ physics. Reference: F. Cagnetta, M. R. Evans and D Marenduzzo, Phys. Rev. Lett. 120, 258001 (2018)# Séminaire du LPTMS: Denis Boyer (UNAM, Mexico)

## Random walks on networks with stochastic resetting

### Denis Boyer (UNAM, Mexico)

Resetting a stochastic process from time to time to the initial state can represent an efficient search strategy for locating a hidden target. This fact finds applications in statistical physics, computer science, enzymatic reactions or foraging ecology. Random walks on general networks and subject to resetting have not been studied, despite their relevance to epidemic spreading, searching on the web or human mobility, among others. We develop an extension to an arbitrary network topology of the diffusion problem with stochastic resetting. Our formalism applies to finite undirected networks and is implemented from the spectral properties of the random walk without resetting. We show that resetting can allow a faster exploration of many network architectures such as rings, Cayley trees, random networks and complex networks, highlighting the importance of the choice of the starting/resetting node. The small-world effect and the presence of communities, two properties of many real world etworks, strongly affects exploration strategies based on resetting.# Séminaire du LPTMS: Igor Ferrier-Barbut

## Light scattering by cold atomic arrays, towards quantum simulation of dissipative many-body physics

### Igor Ferrier-Barbut (LCF, Institut d’Optique)

Single atom trapping using tightly focused optical tweezers has emerged as a new versatile experimental platform. Optical holographic techniques allow to engineer 1, 2 or 3D ensembles of atoms in vacuum, that have an almost arbitrary geometry and are nearly free of defects. Typically, these techniques are combined with an excitation of the atoms to Rydberg states, that allow to engineer an interaction between the individual atoms, realizing quantum simulators for quantum many-body physics. In this presentation I will focus on our progress to study dissipative quantum many-body systems in such platforms. For this we use light-induced resonant dipole-dipole interactions between two-level atoms, which contain both a conservative and dissipative part. We will present first experiments where we were able to increase the effect of such interactions, and prospects to reach the strongly interacting regime.# Physics-Biology interface seminar: Heiko Rieger

## Biophysics of Killing – Theory and Experiment

### Heiko Rieger (Saarland University)

Cytotoxic T lymphocytes and natural killer cells are the main cytotoxic killer cells of the human body to eliminate pathogen-infected or tumorigenic cells. Various processes are involved in a successful killing event: activation of the killer cell, migration and search for the target, formation of a synapse and polarization upon contact with the target, transport of cytotoxic agents towards the synapse, and finally elimination of the target via necrosis or apoptosis. In this talk I will review various biophysical aspects of killing that were studied in collaboration with experimental groups from biology and medicine. Topics include the analysis of search strategies of migrating killer cells; the mechanistic understanding of the molecular motor driven cytoskeleton rotation towards the synapse during polarization; the efficiency of the spatial organization of the cytoskeleton for search problems occurring in intra-cellular cargo transport; and the stochastic analysis of different killing strategies via inducing necrosis or apoptosis.

# Séminaire du LPTMS: Erdal Oguz (Tel Aviv University)

## Hyperuniformity of quasicrystals and related structures

### Erdal Oguz (Tel Aviv University)

Density fluctuations in many-body systems are of fundamental importance throughout various scientific disciplines. Hyperuniform systems, which include crystals and quasicrystals, have density fluctuations that are anomalously suppressed at long wavelengths compared to the fluctuations in typical disordered point distributions such as in ideal gases and liquids. In this talk, I will provide the first rigorous hyperuniformity analysis of quasicrystals and related points sets, and its consequences. Most importantly, we reveal that one-dimensional quasicrystals obtained by the cut-and-project method fall into two distinct classes with respect to their large-scale density fluctuations. This distinction provides a new classification scheme of quasicrystalline systems. Furthermore, our results suggest that these two classes exhibit distinct physical properties; for instance, by employing a tight-binding model, we discover that wave localization in each class displays a qualitatively different behavior.# Soutenance de thèse: Ivan Palaia

*Charged systems in, out of, and driven to equilibrium: from nanocapacitors to cement*

Par

### Ivan Palaia

Jury: Rudolf Podgornik, Chinese Academy of Sciences, rapporteur René van Roij, Utrecht University, rapporteur Emanuela del Gado, Georgetown University, examinatrice Patrick Guenoun, CEA Saclay / CNRS, examinateur Manoel Manghi, Université de Toulouse / CNRS, examinateur Benjamin Rotenberg, Sorbonne Université / CNRS, examinateur Emmanuel Trizac, Université Paris-Sud / CNRS, directeur de thèse Résumé : Most systems in soft matter are immersed in solutions with charged species: some can be described by mean-field theories, others require more sophisticated techniques. In this thesis defense we focus on two such systems. Firstly, we analyze the relaxation dynamics of a nanocapacitor within the mean-field approach. We study relaxation times in the linear and nonlinear regime and characterize the behavior of the system as a function of salt density and applied voltage. The problem of designing a smart applied potential, to drive the system from an initial to a chosen final equilibrium state, is also tackled. We then discuss the physics of ionic correlations in charged systems, with particular focus on the phenomenon of like-charge attraction. We show the relevance of the strong coupling theory, that we apply to the nanoscopic constituents of cement. We demonstrate that the strong cohesion force of cement may be explained by a “dry water” picture, in excellent agreement with molecular dynamics simulations, thereby filling a gap in our understanding of the most used synthetic material in the world.# Séminaire du LPTMS: Christine Proust

## Mathématiques en Mésopotamie, un panorama général et une attention particulière portée sur l'arithmétique des nombres flottants

### Christine Proust (CNRS - Université Paris-Diderot - SPHERE)

Ce séminaire s’appuie sur une collection de tablettes d’argile écrites il y a 4000 ans dans les écoles de scribes de Mésopotamie. À première vue, ce sont des exercices très simples, de contenu mathématique élémentaire, et de ce fait, ces modestes écrits n’ont guère attiré l’attention des historiens des mathématiques. Pourtant, à y regarder de plus près, cette simplicité se révèle trompeuse. Le sentiment de familiarité résulte essentiellement d’une projection sur le passé de conceptions profondément ancrées lors de nos propres apprentissages précoces. Une observation attentive des tablettes scolaires révèle des différences subtiles entre les conceptions anciennes et nos idées modernes sur ce que nous appelons les nombres, les quantités, les unités, la mesure, les grandeurs, l’ordre, la divisibilité, etc. Les efforts pédagogiques des maı̂tres scribes laissent percevoir un univers numérique tout à fait original, doté de règles qui lui sont propres, et permettant le développement d’algorithmes de calcul puissants.# Physics-Biology interface seminar: Tuomas Knowles

## Probing proteins in small volumes

### Tuomas Knowles (Cambridge University, UK)

**Warning: special time**

This talk outlines our efforts on exploring experimental strategies to provide a new window into protein self-assembly that are enabled by operation in small volumes. We have shown that microconfinement achieved through droplet microfluidics allows the isolation of single nucleation events in protein aggregation and thus to study a rare event as single molecule resolution. Using this strategy we have also been able to develop an understanding of how aberrant misfolded protein states are transmitted from one molecule to another through time and space. More recently we have exploited measurements of mass transport through fluid streams under laminar flow conditions to generate a platform for probing protein-protein interactions under fully native conditions.

# Soutenance de thèse: Luca Barberi

Soutenance de thèse:*Inferring forces from geometry in biology*

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### Luca Barberi

Jury: Rudolf Podgornik, University of Ljubljana (Slovenia) and Chinese Academy of Sciences (Beijing, China) Anđela Šarić, University College London (United Kingdom) Clément Campillo, Université d'Évry Val d'Essonne (France) Antonio De Simone, Scuola Internazionale Superiore di Studi Avanzati (Trieste, Italy) Aurélien Roux, Université de Genève (Suisse) Pierre Sens, Institut Curie (Paris, France) Martin Lenz, CNRS and Université Paris-Sud (France) Résumé : Inter-molecular forces on which we have poor prior knowledge are often essential for the stability and evolution of biological assemblies. In this thesis, we focus on two such forces that are critically involved in the deformation of either biopolymers or membranes. We infer these forces by reconciling the geometry of such deformation with simple mechanical models. In the first part of the thesis, we consider the attractive force between DNA molecules mediated by multivalent cations. This attraction is required to compensate DNA bending rigidity when packaging large quantities of DNA in comparatively small environments, such as the nuclei of sperm cells. In vitro, multivalent cations drive DNA condensation into dense toroidal bundles. Geometrical data on DNA toroidal bundles give access to the competition between inter-helical attraction and DNA bending rigidity. From these data, we infer inter-helical forces and argue that the toroid curvature weakens the adhesion between DNA molecules. In the second part of the thesis, we turn to the binding force of a membrane remodeling protein complex, ESCRT-III, to cellular membranes. ESCRT-III proteins assemble into membrane-remodeling polymers during many cellular processes, ranging from HIV budding to cytokinesis. The mechanism by which ESCRT-III polymers deform membranes is still unclear. In vitro, ESCRT-III polymers can reshape spherical membrane vesicles into helical tubes. We argue that helical tubes result from the peculiar positioning of membrane-binding sites on the surface of ESCRT-III polymers. Furthermore, we infer the binding force between ESCRT-III and membrane from the geometry of helical tubes.# Séminaire du LPTMS: Alexandra Sheremet

## Light-matter interface based on collective and cooperative effects

### Alexandra Sheremet (Institut Langevin, ESPCI)

Understanding and control of light-matter interactions have been central to the development of modern physics. A promising approach for study of such interactions is based on ensemble of neutral atoms. Manipulations with density and spatial organization of atoms in a system can bring fascinating results, which are interesting for quantum information applications. For example, reducing average distance between atoms up to resonant wavelength induces dipole-dipole interaction and results in cooperative effects. These effects in spatially dense atomic ensembles can modify optical properties of the system. Moreover, spatial organization of atoms in ordered arrays and optical lattices causes a manifestation of collective effects. In such systems, long-range spatial order brings dramatic consequences for the light propagation. Collective and cooperative effects manifistable in an atomic ensemble could provide essential enhancement of the coupling strength between the light and atomic systems. Moreover, recent experimental advances in the trapping technique have made it possible to create 1D, 2D or 3D spatially ordered atomic configurations, where the collective effects play a very important role. In addition, the interaction between light and atoms can be enhanced by trapping atoms in the vicinity of a nanoscale waveguide due to strong confinement of the light. In this talk, I will discuss light propagation in an atomic ensemble where average distance between atoms is comparable with the resonant wavelength. I will consider the light propagation in both free space and trapped near a nanofiber surface atomic ensembles. The light scattering in such dense atomic configuration is described in terms of microscopic approach based on the standard scattering matrix and resolvent operator formalism. We show theoretically and experimentally that spatially dense atomic ensembles allow obtaining effective light-matter interface and reliable light storage with essentially fewer atoms than it can be achieved in dilute gases. Furthermore, we show that the presence of an optical nanofiber modifies the character of atomic interaction and results in long-range dipole-dipole coupling between atoms not only via the free space, but also through the waveguide mode.# Physics-Biology interface seminar: Matthias Merkel

## A new approach unifies mechanical rigidity in cell-based tissue models and biopolymer networks

### Matthias Merkel (Turing Center for Living Systems, Marseille)

Understanding how mechanical properties of biological tissues arise from collective cellular behavior is vital for understanding the mechanisms that guide embryonic development, cancer growth, and wound healing. With my group, I am studying several questions of collective effects and self-organization in biological tissue. Recently, a new type of rigidity transition was discovered in a family of cell-based models for 2D and 3D tissues. Here I discuss these transitions and show that they are an instance of a much more general class of transitions, which appear when introducing geometric incompatibility into so-called under-constrained systems. This kind of transition also provides an important limiting case to understand stiffening in fiber network models, which are used to describe biopolymer networks like collagen. We show that all of these models exhibit generic elastic behavior close to the transition, which is largely independent of the microscopic structure and the disorder in the system. We obtain analytic expressions for the relevant elastic properties and numerically verify our findings by simulations of under-constrained spring networks as well as 2D and 3D vertex models for dense biological tissues. Several of our predictions are parameter-free, and we thus expect them to be general hallmarks for geometric-incompatibility-induced stiffening in under-constrained materials. Hence, they provide quantitative experimental tests for whether stiffening in a given material is due to this effect or not. Finally, I will briefly discuss another project of current interest in my group, where we will explore conditions for robust self-organized oriented deformation of biological tissue.