### Séminaires de l’année 2020

# Séminaire du LPTMS: Fernando Iemini (Universidade Federal Fluminense)

## Boundary time crystals

### Fernando Iemini (Universidade Federal Fluminense)

Spontaneous symmetry breaking is a cornerstone of physics and occurs at all energy scales, in cosmology and high-energy physics as well as in condensed matter. Breaking space translation symmetry for example gives rise to crystals, while superfluids and ferromagnets are manifestations or the spontaneous breaking of gauge or rotational invariance, respectively. Amongst all possible complex situations that were considered and experimentally verified so far breaking the time-translation symmetry, generating a "time-crystal", had not received attention. The possible existence of time crystals, first addressed by Wilczek in 2013, prompted an intense discussion. In this seminar we will discuss about this new phase of matter, and present our recent results predicting a novel form of time-translation symmetry breaking: continuous Boundary Time Crystals (BTCs). In the BTC phase, symmetry breaking appears in a (macroscopic) fraction of the system. Moreover, a BTC breaks the continuous time-translation symmetry, i.e. the system self-organises in a time periodic pattern with a period which only depends on its coupling constants.# Séminaire du LPTMS: Martial Mazars (LPT U-PSUD)

## Topological defects and the melting in two dimensions

### Martial Mazars (LPT U-PSUD)

Topological defects have a preponderant role in the physics of (quasi) two-dimensional systems. One of the first applications is the description of the XY model by Kosterlitz and Thouless in the 1970s; this theoretical description of the XY model made possible in the 1980s to formulate a theory for the fusion in two dimensions, the KTHNY theory (Kosterlitz - Thouless - Halperin - Nelson - Young). In KTHNY, the transition from the solid phase to the liquid phase occurs with an intermediate phase, the hexatic phase, and each of the transitions: solid / hexatic and hexatic / liquid is of the KT type. The microscopic mechanisms in KTHNY leading to phase transitions are dissociations of clusters of topological defects: the dissociation of dislocation pairs into free dislocations (solid / hexatic) and the dissociation of dislocations into free disclinations (hexatic / liquid). This description of the two-dimensional fusion is in competition with a first-order transition between solid and liquid, without hexatic phase. The existence of an intermediate hexatic phase for simple systems in two dimensions remained hypothetical until the mid-2000s when it was observed in experiments on superparamagnetic colloids confined to a water-air interface (Keim, Maret 2007). This observation reinforces the relevance of KTHNY. Then, with computer simulations of hard disks (Krauth - 2011) and in colloidal systems (Thorneywork, Dullens - 2017), the hexatic / liquid transition is found to be first order, in contradiction with KTHNY. In this seminar, we show that the mechanisms of dissociation of the KTHNY topological clusters are responsible for the solid / hexatic and hexatic / liquid transitions, and that, by a statistical analysis of the topological clusters, the KTHNY theory is compatible with a first order transition for the hexatic / liquid. References : - P. Keim, G. Maret and H.H. von Grünberg, 2007, Phys. Rev. E., 75, 031402. - E.P. Bernard, W. Krauth, 2011, Phys. Rev. Lett., 107, 155704. - A. L. Thorneywork, J. L. Abbott, D. G. A. L. Aarts and R. P. A. Dullens, 2017, Phys. Rev. Lett., 118, 158001. The results presented in this seminar have been published in : - M. Mazars, 2015, EPL, 110, 26003. - M. Mazars and R. Salazar, 2019, EPL, 126, 56002.# Physics-Biology interface seminar: Ronald Melki

## Prion-like propagation of alpha-synuclein assemblies in synucleinopathies, similarities with Tau in Tauopathies

### Ronald Melki (Institut François Jacob, CEA & CNRS)

The aggregation of proteins within the central nervous system is deleterious and associated to neurodegenerative disorders. The aggregation of the proteins alpha-synuclein and Tau are associated to synucleinopathies, in particular Parkinson's disease, and tauopathies, among which Alzheimer’s disease, respectively. How alpha-synuclein and Tau aggregates, how those aggregates traffic between cells, amplify by recruiting endogenous monomeric alpha-synuclein or Tau and cause distinct synucleinopathies or tauopathies is unclear. I will explain the molecular events that lead to alpha-synuclein or Tau aggregation. I will show that alpha-synuclein and Tau aggregates bind to neurons cell membranes and explain the cellular consequences of binding. The similarities and differences between alpha-synuclein and Tau will be highlighted. I will explain how alpha-synuclein and Tau aggregates penetrate the cells and get transported. Finally, I will describe how the structure of the fibrillar polymorphs alpha-synuclein and Tau aggregation yield distinct diseases.**References:**

- Brundin P et al. (2010) Nat Rev Mol Cell Biol. 11:301-7.
- Bousset L et al. (2013) Nat Commun. 4:2575.
- Peelaerts W et al. (2015) Nature 522:340-4.
- Shrivastava AN et al. (2015) EMBO J. 34:2408-23.
- Brahic M et al. (2016) Acta Neuropathol. 131:539-48.
- Makky A et al. (2016) Sci Rep. 6:37970.
- Flavin W et al. (2017) Acta Neuropathol. 134:629-653.
- Shrivastava AN et al. (2017) Neuron 95:33-50.
- Melki R (2018) Neurobiol Dis. 109:201-208.
- Gribaudo S et al. (2019) Stem Cell Reports. 12:230-244.
- Shrivastava AN et al. (2019) EMBO J. 38. pii: e99871.
- Fenyi A et al., (2019) Neurobiol Dis 129:38-43.

# Séminaire du LPTMS: Baruch Meerson

## Geometrical optics of constrained Brownian motion: three short stories

### Baruch Meerson (Racah Institute of Physics, Hebrew University of Jerusalem)

The optimal fluctuation method — essentially geometrical optics — gives a deep insight into large deviations of Brownian motion, and it achieves this purpose by simple means. Here we illustrate these points by telling three short stories about Brownian motions, ``pushed" into a large-deviation regime by constraints. Story 1 deals with a long-time survival of a Brownian particle in 1 + 1 dimension against absorption by a wall which advances according to a power law x_{w}(t)~ t

^{γ}, where γ > 1/2. We also calculate the large deviation function (LDF) of the particle position at an earlier time, conditional on the survival by a later time. Story 2 addresses a stretched Brownian motion above an absorbing obstacle in the plane. We compute the short-time LDF of the position of the surviving Brownian particle at an intermediate point. In story 3 we compute the short-time LDF of the winding angle of a Brownian particle wandering around a reflecting disk in the plane. In all three stories we uncover singularities of the LDFs which can be interpreted as dynamical phase transitions and which have a simple geometric origin. We also use the small-deviation limit of the geometrical optics to reconstruct the distribution of typical fluctuations. We argue that, in stories 1 and 2, this is the Ferrari–Spohn distribution. The talk is based on a recent paper by B. Meerson and N. R. Smith, J. Phys. A: Math. Theor. 52, 415001 (2019).

# Séminaire du LPTMS: Giulio Biroli (LPENS)

## Multiple Equilibria, Aging Dynamics and Chaos in Large Interacting Ecosystems

### Giulio Biroli (LPENS)

I will focus on Lotka-Volterra equations, which provide a general model to study large assemblies of strongly interacting degrees of freedom in many different fields: biology, economy and in particular ecology. I will present our analysis of the generalised Lotka-Volterra model of an ecological community formed by a large number of species and focus in particular on two different phases that emerge: when interactions are symmetric we find a regime, akin to the spin-glass phase of random magnets, characterized by an exponential number of multiple equilibria, all poised at the edge of stability. For non-symmetric interactions, this phase is replaced by a chaotic one or a dynamical aging regime depending on the presence of immigration from the mainland. Finally, I will consider spatially coupled communities and show that migration between them plays a key role in enhancing chaos and diversity of the ecosystem.# Physics-Biology interface seminar: Stéphane Vassilopoulos

## Native membrane visualization by meta-replica electron microscopy

### Stéphane Vassilopoulos (Sorbonne Université)

In cell biology, particular Importance is given to developing new methods of sample preparation that will achieve a more natural appearance of samples in the microscope. To accomplish this, scientists have developed the metal-replica electron microscopy (EM) technique directly on unroofed cultured cells. This particular EM technique permits visualization of the inner side of cell membranes and their components with an extremely high resolution using transmission EM. Metal-replicas have profoundly shaped our understanding of the functional morphology of the cell and used to capture several different cellular processes including remodeling of cytoskeletons, membrane trafficking, cell-to-cell contacts and formation of extracellular matrices. During this presentation, I will outline the history of this particular EM technique and describe different methodologies and key applications in cell biology with a particular emphasis on visualization of actin filament networks at the cell cortex.**Selected publications:**Vassilopoulos S*, Gibaud S, Jimenez A, Caillol G, Leterrier C. Ultrastructure of the axonal periodic scaffold reveals a braid-like organization of actin rings. Nat Commun. 2019 Dec 20;10(1):5803. Ferrari R*, Martin G, Tagit O, Guichard A, Cambi A, Voituriez R, Vassilopoulos S*, Chavrier P*. MT1-MMP directs force-producing proteolytic contacts that drive tumor cell invasion. Nat Commun. 2019 Oct 25;10(1):4886. Franck A, Lainé J, Moulay G, Lemerle E, Trichet M, Gentil C, Benkhelifa-Ziyyat S, Lacène E, Bui MT, Brochier G, Guicheney P, Romero N, Bitoun M, Vassilopoulos S*. Clathrin plaques and associated actin anchor intermediate filaments in skeletal muscle. Mol Biol Cell. 2019 Mar 1;30(5):579-590. Randrianarison-Huetz V, Papaefthymiou A, Herledan G, Noviello C, Faradova U, Collard L, Pincini A, Schol E, Decaux JF, Maire P, Vassilopoulos S, Sotiropoulos A. Srf controls satellite cell fusion through the maintenance of actin architecture. J Cell Biol. 2018 Feb 5;217(2):685-700. Elkhatib N, Bresteau E, Baschieri F, Rioja AL, van Niel G, Vassilopoulos S, Montagnac G. Tubular clathrin/AP-2 lattices pinch collagen fibers to support 3D cell migration. Science. 2017 Jun 16;356(6343). Vassilopoulos S*, Gentil C, Lainé J, Buclez PO, Franck A, Ferry A, Précigout G, Roth R, Heuser JE, Brodsky FM, Garcia L, Bonne G, Voit T, Piétri-Rouxel F, Bitoun M. Actin scaffolding by clathrin heavy chain is required for skeletal muscle sarcomere organization. J Cell Biol. 2014 May 12;205(3):377-93.

# Séminaire du LPTMS: Eugenio Valdano (UCLA)

## Predicting epidemic risk from contact and mobility data

### Eugenio Valdano (UCLA)

The vulnerability of a host population to a specific disease measures how likely pathogen introduction will lead to an epidemic outbreak, and how hard it is to contain or eliminate an ongoing one. Predicting vulnerability is thus key to designing risk-reduction strategies that limit disease burden on public health and economic development. To do that, highly-resolved data tracking contacts and mobility of the host population need to integrate into detailed models of disease dynamics. This represents a twofold challenge. Firstly, we need theoretical frameworks that turn data feeds into predictors of epidemic risk, and can identify which of the structural features of the host population drive its vulnerability. Secondly, we need new ways to access, analyze, and share the relevant contact and mobility data: a necessary step to make our predictions realistic and reliable. In my talk, I will address both issues. I will show how to analytically derive the conditions that discriminate between epidemic regime and quick pathogen extinction, by representing diseases spreading on empirically measured contacts as dynamical processes on time-evolving complex networks. The analytical core of this theory leads to a broad range of applications. At the same time, its data-driven nature prompts context-specific predictions that can inform policymaking, as I will show in two case studies: reorganizing nurse scheduling to reduce the risk of spread of healthcare-associated infections; linking the features of livestock trade movements to the spatial spread of cattle diseases. The latter application is also an example of how limited access and incomplete data collection represent a big hurdle to predictive vulnerability analysis. To overcome this, I will present a collaborative platform for analyzing and comparing trade networks coming from several European countries. Using a bring code to the data approach, our platform surmounts the strict regulations preventing data sharing, and builds an algorithm that predicts vulnerability even in situations when limited data on cattle trade are available.# Séminaire exceptionnel du LPTMS: Francesco Sciortino (Uni Roma La Sapienza)

## Patchy DNA-made nanoparticles

### Francesco Sciortino (Uni Roma La Sapienza)

DNA oligomers can nowadays be assembled to produce a large variety of nanometric constructs, via a cascade of self-assembly processes, each one guided by the length of complementary sequences of distinct DNA strands. In the lecture I will show that it is possible to build bulk quantities of DNA-made nanoparticles that closely match idealized colloids, transferring modern in-paper and in-silico intuitions into experimental realizations. I will show how unconventional collective behaviors, recently explored theoretically and numerically, can indeed be reproduced in the lab. Specifically I will discuss:- how to exploit limited valence interactions to suppress phase separation, enhancing the stability of the equilibrium gel phase;
- how to exploit competing interactions to generate a material that is fluid both at high and at low temperatures and a solid-like disordered open network structure in between and
- how to exploit bond-swap dynamics to create an all-DNA vitrimer.

# Soutenance de thèse: Thibault Bonnemain

## Quadratic Mean Field Games with Negative Coordination

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### Thibault Bonnemain

Jury: Cécile APPERT-ROLLAND, Univeristé Paris-Saclay, examinatrice Thierry GOBRON, Université Cergy-Pontoise, directeur de thèse Olivier GUEANT, Université Paris 1 Panthéon-Sorbonne, examinateur Max-Olivier HONGLER, EPFL, rapporteur Jean-Pierre NADAL, École Normale Supérieure, examinateur Filippo SANTAMBROGIO, Université Claude Bernard - Lyon 1, rapporteur Denis ULLMO, Univeristé Paris-Saclay, co-directeur de thèse Resumé: Mean Field Games provide a powerful theoretical framework to deal with stochastic optimization problems involving a large number of coupled subsystems. They can find application in several fields, be it finance, economy, sociology, engineering ... However, this theory is rather recent and still poses many challenges. Its constitutive equations, for example, are difficult to analyse and the set of behaviours they highlight are ill-understood. While the large majority of contributions to this discipline come from mathematicians, economists or engineering scientists, physicist have only marginally be involved in it. In this thesis I try and start bridging the gap between Physics and Mean Field Games through the study of a specific class of models dubbed "quadratic".# Soutenance de thèse: Samuel Cazayus-Claverie

## Effect of residual stress on the elasticity of fiber networks

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### Samuel Cazayus-Claverie

Jury: Anaël Lemaître - rapporteur Chaouqi Misbah - rapporteur Giuseppe Foffi - examinateur Cécile Leduc - examinatrice Martin Lenz - directeur de thèse Raphaël Voituriez - examinateur Resumé: Cells are the basic units of all living organisms. Eukaryotic cells are stuctured on top of a scaffold of fibers ranging from stiff microtubules to semiflexible actin : the cytoskeleton. As such the cytoskeleton is involved into a broad family of processes of translocation and deformation of cells, it is also responsible for cells mechanical stiffness. The actin filaments into cytoskeleton can be cross-linked into bundles built of as much as 30 parallel filaments, but filaments can get bound at a finite angle also. These processes are in competition during network’s self-assembly and result in strong residual stresses. In this thesis, we study the effect of these residual stresses on the elasticity of fiber networks in 2 dimensions of space. We develop an original method to compute stress on the boundaries of a network and its elastic moduli. We find that residual stress induces a stiffening in the infinitesimal response of the network. Residual stress also affects the non linear response of the network : we find that it makes the network unstable under compression, and that they control the onset of non linear response to shear.# Physics-Biology interface seminar: Anne-Virginie Salsac

## SEMINAR CANCELLED

### Anne-Virginie Salsac (Université de Technologie de Compiègne)

# Séminaire du LPTMS: Jacopo De Nardis (Ghent University)

## Universalities of transport in quantum and classical chains: from diffusion to KPZ dynamics

### Jacopo De Nardis (Ghent University)

Finding a theoretical framework to explain how phenomenological transport laws on macroscopic scales emerge from microscopic deterministic dynamics poses one of the most significant challenges of condensed matter and statistical physics. Recently there has been a flood of new numerical and analytical results in the transport theory of quantum and classical many-body1D Hamiltonian systems, both integrable and not. I will provide a general framework to understand the main classes of transport observed: diffusion, super-diffusion and logarithmic corrections to diffusion. I will review how diffusive spreading is generically present in integrable chains and how KPZ dynamics emerges in both integrable and non-integrable rotationally invariant magnets.# Séminaire du LPTMS: Muhittin Mungan (Uni Bonn)

## Driving with maps or: Finding your way in the glassy landscape of an amorphous solid

### Muhittin Mungan, University of Bonn

The energy landscape of sheared amorphous solids, even when probed under athermal and quasistatic (AQS) conditions, reveals rich features, such as a transition from brittle to ductile response and yielding. In this talk I will argue that a natural (and rigorous) way of representing this AQS response is via "maps", i.e. state transition graphs. This opens the way for understanding dynamical features in terms of the topological structure of the underlying transition graph. Moreover, such graphs can be readily extracted from simulations of sheared amorphous solids.# Physics-Biology interface seminar: Davy Martin

## Prion replication and structural diversification mechanisms

### Davy Martin (INRA Jouy-en-Josas)

Prions are proteinaceous infectious agents responsible for a range of neurodegenerative diseases in animals and humans. Prion particles are assemblies formed from a misfolded, β-sheet rich, aggregation-prone isoform (PrPSc) of the host-encoded cellular prion protein (PrPC). Prions replicate by recruiting and converting PrPC into PrPSc. PrPSc is a pleiomorphic protein as different conformations can dictate different disease phenotypes in the same host species. This is the basis of the strain phenomenon in prion diseases. Moreover, recent experimental evidence suggests further structural heterogeneity in PrPSc assemblies within specific prion populations and strains. Still, this diversity is rather seen as a size continuum of assemblies with the same core structure, while analysis of the available experimental data points to the existence of structurally distinct arrangements. The atomic structure of PrPSc has not been elucidated so far, making the underlying mechanistical processes of emergence and coevolution of structurally distinct assemblies difficult to understood. Here, we will present our latest experimental results and replication model providing mechanistic insights into prion structural diversification, a key determinant for prion adaptation and toxicity.# CANCELLED -- Séminaire du LPTMS: Alexandre Solon (LPTMC)

## Pressure and forces for active matter

### Alexandre Solon (LPTMC)

Active matter, composed of self-propelling entities, is found across scales in nature, from cellular tissues to animal groups. Such systems, as well as engineered active materials, exhibit many types of collective behaviors and unusual mechanical properties. In this talk, I will focus on different aspects of the interactions between active fluids and boundaries or passive objects, and show that they lead to intriguing effects, specific to active systems. In particular, I will discuss the absence of equation of state for the pressure of active fluids, the instability of a filament in an active bath, long-range interactions mediated by an active fluid and the localization of active particles in a random potential.# CANCELLED -- Séminaire exceptionnel du LPTMS: Joachim Krug (Uni Cologne)

## Fisher's geometric model and spin glasses

### Joachim Krug (Uni Cologne)

Fisher's geometric model describes biological fitness landscapes by combining a linear map from the discrete space of genotype sequences of length L to an n-dimensional Euclidean phenotype space with a nonlinear, single-peaked phenotype-fitness map. Recent work has shown that the interplay between the genotypic and phenotypic levels gives rise to a range of different landscape topographies that can be characterised by the number of local fitness maxima. I will present new results for the distribution of the number of maxima. The typical scale of the number of maxima is derived for general n, and the full scaled probability density and two point correlation function of maxima are determined for the one-dimensional case. The model is closely related to the anti-ferromagnetic Hopfield model with n random continuous pattern vectors, and most results carry over to this setting. The talk is based on joint work with Sungmin Hwang and Su-Chan Park.# Séminaire du LPTMS: Julien Tailleur (MSC - Diderot)

## Collective behaviors of active particles in and out of equilibrium

### Julien Tailleur (MSC - Université Paris-Diderot)

Active matter describes systems in which individual units dissipate energy to exert forces on their environment. Dissipation and injection of energy are then disconnected at the microscopic scale, driving the system strongly out of equilibrium. This leads to phenomenology markedly different from that of equilibrium physics, such as the emergence of dense phases in the absence of cohesive attractive forces. Nevertheless, for a large class of systems, non-trivial mappings exist between active and passive dynamics. In this talk, I will review the existing work on motility-induced phase separation and discuss its relationship with equilibrium physics. I will then show how a large class of tactic active matter models admit an exact mapping onto equilibrium dynamics at the fluctuating hydrodynamic level.# Physics-Biology interface seminar: Marion Jasnin

## SEMINAR CANCELLED

### Marion Jasnin (MPI for Biochemistry, Martinsried, Germany)

# Séminaire du LPTMS: Charlie Duclut (MPI PKS Dresden)

## Quorum effects in assemblies of chemotactic cells

### Charlie Duclut (MPI PKS Dresden)

ZOOM MEETING ID: 908 161 299 - PASSWORD: 026475 -- To self-organise into complex structures such as tissues and organs, individual cells need to interact. A generic mechanism for this interaction is a chemical signalling. The ability of an individual cell to follow a gradient of chemicals is called chemotaxis. Starting from a microscopic description of chemotactic cells, I will present a coarse-graining procedure to describe an assembly of such particles. This system exhibits a phase transition that I will then study using a dynamical renormalisation group approach. This analysis indicates the crucial role of a polarity-based chemotactic interaction, usually overlooked in phenomenological approaches. Finally, I will discuss an emergent symmetry of the model that allows us to compute scaling exponents exactly. In particular, we deduce a superdiffusive behavior of the particles in all physical dimensions.# Séminaire du LPTMS: Cécile Sykes (Institut Curie)

## Active deformation of the cell membrane through actin assembly, and how we may infer nucleus deformation

### Cécile Sykes (Institut Curie)

ZOOM MEETING ID: 919 2003 3856 - PASSWORD: 041219 -- In all cell functions, a common observation is that cytoskeleton assembly correlates with membrane deformation based on active forces. The exact role, in particular, of the actin cytoskeleton in cell membrane deformation, with pushing or pulling forces, is what we address both experimentally and theoretically. We conceive stripped-down experimental systems that reproduce cellular behaviours in simplified conditions: cytoskeleton dynamics are reproduced on liposome membranes. Actin polymerization through the growth of a branched actin network is able to initiate membrane tubules and spikes by pushing or pulling, and mimics cellular deformations. By changing experimentally membrane tension and the structural details of the cytoskeleton architecture, we displace the system within a phase diagram where inward or outward deformations are favoured [1]. Moreover, shells of branched actin networks grown around liposomes display buckling and wrinkling under osmotic deflation, thereby confirming their elastic properties. The time during which we let the network grow around liposomes allows us to vary the shell thickness, and to specify the length scale of buckling versus wrinkling [2]. Our results illustrate the generic mechanism of buckling and wrinkling found in various systems spanning from pollen grains to the development of the gut or the brain. Inspired by actin forces exerted on membranes and organelles, we address now how the nucleus, which is the most rigid cell organelle, is deformed by the actin cytoskeleton during cell translocation. When cells move through narrow spaces that are smaller than their nuclei, we find that proteins of the nuclear membrane, such as nesprins, accumulate at the nucleus front and pull the nucleus forward [3]. We want to address in the future how we could characterize this active process and infer its molecular details. References: [1] Simon et al., Nature Physics (2019) [2] Kusters et al., Soft Matter (2019) [3] Davidson et al., in revision# CANCELLED -- Séminaire du LPTMS: Matthieu Mangeat

TBA# CANCELLED Séminaire du LPTMS: Marco PICCO (LPTHE) CANCELLED

TBA# Séminaire du LPTMS : Lenka Zdeborova POSTPONED

## POSTPONED

### Lenka Zdeborova (IPT, Gif-sur-Yvette)

# Séminaire du LPTMS: Claudio Benzoni (TU Muenchen)

## Rayleigh edge waves in two-dimensional chiral crystals

### Claudio Benzoni (Technische Universitaet Muenchen)

Meeting ID: 940 9301 2897 -- Password: 464951 We investigate, within the framework of linear elasticity theory, edge Rayleigh waves of a two-dimensional elastic solid which breaks time-reversal and parity symmetries due to the Coriolis-Lorentz force. We find that the direction of propagation of the Rayleigh modes is determined not only by the sign of the magnetic field but also by the Poisson ratio of the elastic system. We discover three qualitatively different regions distinguished by the chirality of the low-frequency edge waves and study their universal properties. To illustrate the Rayleigh edge-waves in real time we have carried out finite-difference simulations of the model. https://arxiv.org/abs/2004.09517# POSTPONED: Séminaire du LPTMS: Sandro Stringari (Uni Trento)

TBA# Séminaire du LPTMS : Ricard Alert Zenon

## Topological Defects, Turbulence, and Phase Separation

### Ricard Alert Zenon (Princeton)

ZOOM Meeting ID: 929 5958 3836 Password: 813501 Active fluids display striking collective phenomena not possible in passive fluids. Examples include spontaneous flows powered by microscopic constituents, and phase separation in the absence of attractive interactions. In this talk, I will discuss three aspects of the physics of active fluids, connected with experiments in both biological and artificial soft matter systems. First, I will show that dense colonies of the rod-shaped motile bacterium Myxococcus xanthus form active liquid crystals. I will show that topological defects of the cell alignment field induce flows that lead to the formation of new cell layers, which triggers the development of multicellular structures called fruiting bodies. Second, I will discuss chaotic flows in active liquid crystals. I will show that, as in classic turbulence, the statistical properties of these active flows at low Reynolds number are described by universal scaling laws. Unlike classic turbulence, however, this type of active turbulence requires no energy transfer across scales. Finally, I will present a new mechanism of liquid-gas phase separation in active fluids. I will show that, in addition to repulsive forces that oppose phase separation, artificial self-propelled Janus colloids experience torques that reorient particle motion toward high-density regions, providing a novel route to phase separation.# Soutenance de thèse: Mathieu Isoard

*Theoretical study of quantum correlations and nonlinear fluctuations in quantum gases*

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### Mathieu Isoard

Jury: Matteo Conforti, Université Lille, rapporteur Patrik Öhberg, Heriot-Watt University, rapporteur Élisabeth Giacobino, CNRS, examinatrice Sandro Stringari, University of Trento, examinateur Chris Westbrook, Institut d’Optique, examinateur Nicolas Pavloff, Université Paris-Saclay/CNRS , directeur de thèse Resumé This thesis is dedicated to the study of nonlinear-driven phenomena in two quantum gases which bear important similarities: Bose-Einstein condensates of ultracold atomic vapors and paraxial nonlinear laser beam. These systems can be described within a hydrodynamic framework which make it possible to investigate their short and long time propagation by means of mathematical methods developed by Riemann and Whitham. In particular, we study the formation and the propagation of dispersive shock waves which arise after a wave breaking event. We obtain a kind of a weak shock theory, from which we can extract a quantitative description of experimentally relevant parameters, such as the velocity of the solitonic edge of the shock or the contrast of its fringes.# Séminaire du LPTMS : Ilya Galanov

## On Self-Assembly of Aperiodic Tilings

### Ilya Galanov (Laboratoire d’Informatique de Paris Nord - Université Paris 13)

Seminar taking place at Batiment Pascal with ZOOM streaming. ZOOM Meeting ID: 973 9827 8038 Password: 742352 Self-assembly is the process in which the components of a system, whether molecules, polymers, or macroscopic particles, are organized into ordered structures as a result of local interactions between the components themselves, without exterior guidance. This talk is devoted to the self-assembly of aperiodic tilings. Aperiodic tilings serve as a mathematical model for quasicrystals - crystals that do not have any translational symmetry. Because of the specific atomic arrangement of these crystals, the question of how they grow still remains open. Our aim is to develop a growth algorithm for a particular class of aperiodic tilings - octagonal tilings of finite type. In order to mimic the growth of real-world quasicrystals, we demand the algorithm be local: the tiles must be added one by one, using only the local information, and no data must be stored between the steps. Simulations strongly support the evidence that this algorithm grows aperiodic tilings, up to an unavoidable but neglectable proportion of missing tiles.# Séminaire du LPTMS : Lenart Zadnik

## Large-anisotropy limit of Heisenberg antiferromagnet: Towards description of pre-relaxation phenomena

### Lenart Zadnik (LPTMS)

Seminar taking place at Batiment Pascal with ZOOM streaming. ZOOM Meeting ID: 959 1349 7684 Password: 878617 The recent decade has witnessed several breakthroughs in the description of relaxation of local observables in quantum many-body systems prepared far from equilibrium. In contrast, the time evolution on intermediate time scales that precede relaxation towards the stationary state has remained much less explored. Strong coupling expansions offer a good starting point for investigation of such problems, since they provide a natural time scale, on which the pre-relaxation can occur. We will discuss the effective Hamiltonian that generates time evolution of states on intermediate time scales in the large-anisotropy limit of Heisenberg spin-1/2 chain. Such an effective Hamiltonian describes a simple Bethe-Ansatz-solvable model with local interactions. Starting from a microscopic picture of the dynamics, I will explain the ballistic-scale hydrodynamic theory describing the time evolution after a quench, in which two halves of the system, thermalised at different temperatures, are abruptly joined.# Soutenance de thèse: Sebastian GRIJALVA

*Boundary effects in Quantum Spin Chains and Finite-Size Effects in the Toroidal Correlated Percolation model*

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### Sebastian Grijalva

Jury: Christian Hagendorf, Université catholique de Louvain, examinateur Jacopo De Nardis, ENS, invité Nikolai Kitanine, Institut de Mathématiques de Bourgogne, examinateur Karol Kozlowski, ENS de Lyon, rapporteur Vincent Pasquier, IPhT, examinateur Pierre Pujol, Université Paul Sabatier, rapporteur Raoul Santachiara, LPTMS, Université Paris Saclay, co-directeur de thèse Véronique Terras, LPTMS, Université Paris Saclay, directrice de thèse Résumé:**ZOOM Meeting ID: 962 2604 2868 Password: 708393**

# Soutenance de thèse: Nina Javerzat

*New conformal bootstrap solutions and percolation models on the torus*

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### Nina Javerzat

Jury: Bertrand Duplantier, IPhT, examinateur Malte Henkel, LPCT, Université de Lorraine, rapporteur Eveliina Peltola, University of Bonn, examinatrice Marco Picco, LPTHE, examinateur Silvain Ribaud, IPhT, examinateur Raoul Santachiara, LPTMS, Université Paris Saclay, directeur de thèse Erik Tonni, SISSA, rapporteur Jacopo Viti, Istituto Nazionale di Fisica Nucleare, examinateur Résumé: The geometric properties of critical phenomena have generated an increasing interest in theoretical physics and mathematics over the last thirty years. Percolation-type systems are a paradigm of such geometric phenomena, their phase transition being characterised by the behaviour of non-local degrees of freedom: the percolation clusters. At criticality, these clusters are examples of random objects with a conformally invariant measure. Even in the simplest model, uncorrelated percolation, important universal properties remain out of reach, in particular the connectivity properties −the probabilities that points are connected by clusters. In two dimensions, the presence of conformal symmetry has especially important implications, which can be exploited to obtain a complete determination of the universality classes. We will present results on the universal properties of two families of long-range correlated percolation models, obtained using a conformal bootstrap approach and the study of the clusters connectivities on a torus topology. The first family is the random cluster Q− state Potts model, whose conformal field theory is not yet −albeit almost, fully solved today. We test conjectures on this conformal field theory by computing the finite size effects induced on the connectivities by the torus topology, and comparing with numerical measurements. The second family is obtained from the excursion sets of fractional Gaussian surfaces. We use conformal field theory predictions on the torus cluster connectivities, together with numerical simulations, to establish the conformal invariance of these systems, and obtain the first features of this new conformal field theory.**Zoom meeting**

**ID de réunion : 968 8791 8326**

**Code secret : 666675**

# Séminaire du LPTMS : Nicolas Levernier (IUSTI)

## Nicolas Levernier (IUSTI Aix-Marseille Université)

### First-passage time of non markovian random walks

Seminar in presence @LPTMS with ZOOM streaming -- ZOOM Meeting ID: 973 9548 5969 -- Password: 396890 -- In this talk, I will present part of the results I got during my PhD, and part of those obtained during my first postdoc I) The computation of the encounter time of particles is a key question in many contexts, as this time quantifies the reactivity rate for diffusion-limited processes. In the case of markovian random walks, such as brownian motion, some analytic results can be obtained. But in the case of non-markovian processes, much fewer results do exist, although "non-markov is the rule and markov is the exception" (Van Kampen). In this talk I will present a formalism we have developed to deal with non-markovian gaussian random walks and show its application to Fractional Brownian Motion, a paradigmatic example of highly-correlated process. If time allows, I will briefly present how aging of the dynamics can deeply modify the encounter time statistics. II) In the second part of my talk, I will briefly present results I got during my first postdoc. I will show how chaotic motion can arise in an extended active gel layer, typically describing cortical cytoskeleton, where polymerization and contraction due to molecular motors are combined. This result questions the usual description of the cortex as a thin layer, as such a description cannot describe this instability. I will also briefly present recent experimental evidences of this predicted phenomenon.# Séminaire du LPTMS : Jérôme Beugnon (LKB)

## Scale-invariant dynamics of a planar Bose gas : dynamical symmetry, solitons and breathers.

### Jérôme Beugnon (LKB)

Online seminar --- ZOOM Meeting ID: 913 5556 5450 -- Password: 966156 -- Low temperature quasi-2D Bose gases feature an approximate scale invariance. I will discuss several consequences of this scale-invariance on the equilibrium and dynamics of the gas and illustrate them through experiments with box-trapped ultracold atoms. I will first present experimental signatures of the SO(2,1) dynamical symmetry. In a two-component gas, I will report the observation of two-dimensional scale-invariant solitons, the so-called Townes solitons. Finally, I will show the surprising existence of breathers that might be related to the scale-invariance of the gas.# Séminaire du LPTMS : David Zwicker (MPI DS Goettingen)

## Controlling phase separation in biological cells

### David Zwicker (Max Planck Institute for Dynamics and Self-Organization, Göttingen)

Online seminar --- Zoom Meeting ID: 944 4358 5604 --- Passcode: 322346 --- Phase separation has emerged as an essential concept for the spatial organization inside cells. In particular, phase separation explains how droplets can form spontaneously to create subcellular compartments. However, traditional theories of phase separation cannot explain how cells could control these droplets. To unveil part of this mystery, I will present two different mechanisms used by cells in this talk. I will first discuss chemical reactions that influence the physical properties of droplet components. Driving such reactions by exploiting the non-equilibrium environment of biological cells can stabilize multiple droplets and control their size. I will then focus on the elastic properties surrounding droplets, e.g., provided by the cytoskeleton. I will show that stiffness gradients, which are present in heterogeneous environments like cells, can influence the positioning of droplets and thus determine their overall arrangement. These two examples demonstrate that heterogeneous, living cells can regulate the size, number, and position of their droplets. Moreover, similar mechanisms may allow controlling droplets in technical applications in the future.# Séminaire du LPTMS : Thierry Lahaye (LCF)

## Tunable arrays of single Rydberg atoms for quantum simulation of spin models

### Thierry Lahaye (Institut d'Optique - LCF)

Online seminar --- Zoom Meeting ID: 927 0748 7986 -- Passcode: 865632 -- Over the last years, a novel platform for quantum simulation of spin Hamiltonians, using arrays of single atoms held in optical tweezers and excited to Rydberg levels to make them interact, has emerged. In this seminar, I'll explain how we can now create arrays of up to 200 individually-controlled atoms with almost arbitrary geometries in 1,2 and even 3 dimensions, and illustrate the quantum simulation of Ising and XY spin Hamiltonians on a variety of recent experiments performed in our lab.# Séminaire du LPTMS : Mikhail Zvonarev (LPTMS)

## Mobile impurity in a quantum fluid

### Mikhail Zvonarev (LPTMS)

Online seminar --- ZOOM Meeting ID: 994 9374 5810 -- Password: 467383 -- I am planning to discuss several recent findings (theoretical and experimental) in the dynamics of an impurity particle injected into a quantum liquid: (i) The momentum distribution of the impurity in one dimension subject to a constant external force exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. As a consequence, the impurity exhibits periodic dynamics that is interpreted as Bloch oscillations, which arise even though the quantum liquid is translationally invariant. (ii) The impurity injected into a one-dimensional liquid with some initial momentum sheds only a part of it to the background gas, and forms a correlated state that no longer decays in time; furthermore, if the initial momentum is large enough, the impurity undergoes long-lived oscillations. The value of the impurity's velocity at infinite time lies between zero and the speed of sound in the gas, and is determined by the injection protocol. This way, the impurity's frictionless motion is a dynamically emergent phenomenon whose description goes beyond accounting for the kinematic constraints of Landau's approach to superfluidity. (iii) The impurity traveling through a weakly interacting three-dimensional Bose-Einstein condensate (BEC) of ultra-cold atoms shows a phase transition in the ground state and far-from-equilibrium properties of the system as a function of the impurity's velocity. Above a critical initial velocity, the impurity's final velocity reaches a fundamental kinematic boundary, the BEC's speed of sound. For weak and intermediate impurity-BEC interactions, the critical line is determined by the effective mass of the polaron. This quantum transition can be interpreted as a dynamical Cherenkov effect; the impurity-BEC interaction causes the dynamics of the system to appear as if the bare impurity is propagating through a BEC whose speed of sound is altered by an effective refractive index.# Séminaire du LPTMS : Thibaud Maimbourg (LPTMS)

## Bath-induced Zeno localization in driven many-body quantum systems

### Thibaud Maimbourg (LPTMS)

Online seminar --- ZOOM Meeting ID: 979 6114 1696 --- Password: 105367 --- In this talk I will first discuss the main conditions for a many-body quantum system to reach a thermal stationary state. I will then focus on the mechanism by which, relaxing one of these conditions, the system in contact with a bath of spatially-localized vibrational modes can fail to reach such a state. As a realistic illustration, I shall consider a quantum interacting spin system subject to an external drive, serving as a model for dynamic nuclear polarization (DNP) protocols. Those protocols, besides their concrete applications, represent an experimental platform to investigate ergodicity of the stationary state. Even when the isolated system is ergodic, a sufficiently strong coupling to the bath may break ergodicity, due to many-body quantum Zeno effect. In the DNP context, this result provides an explanation of the breakdown of the so-called "thermal mixing" regime (effective equilibrium), experimentally observed above 4 – 5 Kelvin. Ref.: arXiv:2009.11784. Joint work with D. M. Basko, M. Holzmann (LPMMC Grenoble), A. Rosso (LPTMS).# Séminaire du LPTMS : Gwendal Fève (LPENS)

## Fractional statistics of anyons in a mesoscopic collider

### Gwendal Fève (LPENS)

Online seminar --- ZOOM Meeting ID: 929 5714 7926 -- Password: 084622 -- In three-dimensional space, elementary particles are divided between fermions and bosons according to the properties of symmetry of the wave function describing the state of the system when two particles are exchanged. When exchanging two fermions, the wave function acquires a phase pi. On the other hand, in the case of bosons, this phase is zero. This difference leads to deeply distinct collective behaviors between fermions, which tend to exclude themselves, and bosons which tend to bunch together. The situation is different in two-dimensional systems which can host exotic quasiparticles, called anyons, which obey intermediate quantum statistics characterized by a phase varying between 0 and pi [1,2]. For example in the fractional quantum Hall regime, obtained by applying a strong magnetic field perpendicular to a two-dimensional electron gas, elementary excitations carry a fractional charge [3,4] and have been predicted to obey fractional statistics [1,2] with an exchange phase pi/m (where m is an odd integer). I will present how fractional statistics of anyons can be demonstrated in this system by implementing and studying anyon collisions at a beam-splitter [5]. The collisions are first studied in the low magnetic field regime, where the elementary excitations are electrons which obey the usual fermionic statistics. It leads to the observation of an antibunching effect in an electron collision: electrons systematically exit in two different arms of the beam-splitter. The observed result is completely different in the fractional quantum Hall regime. Fractional statistics lead to a suppression of the antibunching effect and quasiparticles tend to bunch together in larger packets of charge in a single output of the splitter. This effect leads to the observation of negative correlations of the current fluctuations [5] in perfect agreement with recent theoretical predictions [6]. [1] B. I. Halperin, Phys. Rev. Lett. 52, 1583–1586 (1984). [2] D. Arovas, J. R. Schrieffer, F. Wilczek, Phys. Rev. Lett. 53, 722–723 (1984). [3] R. de Picciotto et al., Nature 389, 162–164 (1997). [4] L. Saminadayar, D. C. Glattli, Y. Jin, B. Etienne, Phys. Rev. Lett. 79, 2526–2529 (1997) [5] H. Bartolomei, M. Kumar et al. Science 368, 173-177 (2020). [6] B. Rosenow, I. P. Levkivskyi, B. I. Halperin, Phys. Rev. Lett. 116, 156802 (2016).# Soutenance de thèse: Hao Pei

*Separation of Variables and Correlation Functions of Quantum Integrable Systems*

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### Hao Pei

##### Online defense - Zoom Meeting ID: 971 0660 6090 - Password: 379764

Jury: Christian Hagendorf, Université catholique de Louvain, examinateur Nikolai Kitanine, Université de Bourgogne, examinateur Karol Kozlowski, CNRS, ENS de Lyon, rapporteur Vincent Pasquier, CEA Saclay, Université Paris Saclay, examinateur Eric Ragoucy, CNRS, Université Savoie Mont Blanc, rapporteur Véronique Terras, Université Paris-Saclay, directrice de thèse Resumé:The aim of this thesis is to develop an approach for computing correlation functions of quantum integrable lattice models within the quantum version of the Separation of Variables (SoV) method. SoV is a powerful method which applies to a wide range of quantum integrable models with various boundary conditions. Yet, the problem of computing correlation functions within this framework is still widely open. Here, we more precisely consider two simple models solvable by SoV: the XXX and XXZ Heisenberg chains of spins 1/2, with anti-periodic boundary conditions, or more generally quasi-periodic boundary conditions with a non-diagonal twist. We first review their solution by SoV, which present some similarities but also crucial differences. Then we study the scalar products of separate states, a class of states that notably contains all the eigenstates of the model. We explain how to obtain convenient determinant representations for these scalar products. We also explain how to generalize these determinant representations in the case of form factors, i.e. of matrix elements of the local operators in the basis of eigenstates. These form factors are of particular interest for the computation of correlation functions since all correlation functions can be obtained as a sum over form factors. Finally, we consider more general elementary building blocks for the correlation functions, and explain how to recover, in the thermodynamic limit of the model, the multiple integral representations that were previously obtained from the consideration of the periodic models by algebraic Bethe Ansatz.

# Séminaire du LPTMS : Ilya Galanov

## On Self-Assembly of Aperiodic Tilings

### Ilya Galanov (Laboratoire d’Informatique de Paris Nord - Université Paris 13)

Online seminar --- ZOOM Meeting ID: 956 4238 1538 --- Password: Ask L. Mazza and D. Petrov Self-assembly is the process in which the components of a system, whether molecules, polymers, or macroscopic particles, are organized into ordered structures as a result of local interactions between the components themselves, without exterior guidance. This talk is devoted to the self-assembly of aperiodic tilings. Aperiodic tilings serve as a mathematical model for quasicrystals - crystals that do not have any translational symmetry. Because of the specific atomic arrangement of these crystals, the question of how they grow still remains open. Our aim is to develop a growth algorithm for a particular class of aperiodic tilings - octagonal tilings of finite type. In order to mimic the growth of real-world quasicrystals, we demand the algorithm be local: the tiles must be added one by one, using only the local information, and no data must be stored between the steps. Simulations strongly support the evidence that this algorithm grows aperiodic tilings, up to an unavoidable but neglectable proportion of missing tiles.# Soutenance de thèse: Antonio Sclocchi

**A new critical phase in jammed models: jamming is even cooler than before**

**A new critical phase in jammed models: jamming is even cooler than before**

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### Antonio Sclocchi

##### Online defense – Zoom Meeting ID: 940 6511 8835 – Password: 520321

Jury:

Jean-Louis Barrat, Université Grenoble-Alpes, examinateur

Giuseppe Foffi, Université Paris-Saclay, examinateur

Silvio Franz, LPTMS, directeur de thèse

Luca Leuzzi, Consiglio Nazionale delle Ricerch, rapporteur

Andrea Liu, University of Pennsylvania, examinatrice

Markus Müller, Paul Scherrer Institut, rapporteur

Pierfrancesco Urbani, CNRS, CEA Saclay, examinateur

Matthieu Wyart, École Polytechnique Fédérale de Lausanne, examinateur

Resumé: