# Physics-Biology interface seminar: Leïla Perié

## Deciphering the family of immune cells at the single cell level

### Leïla Perié (Institut Curie, Paris)

How heterogeneous systems of cells constituting multicellular organisms establish, organize and achieve coordination persists as a central question in natural sciences. Whereas stochastic gene or protein expressions have clearly demonstrated their role in cellular heterogeneity and are widely studied (Wang and Bodovitz, 2010), the role of cell heterogeneity in the organization of multicellular organisms has been less interrogated. Addressing this question requires adequate tools that quantitatively study ensembles of cells individually rather than group of cells.

My research aims at addressing cell heterogeneity in dynamical and complex systems of cells using the hematopoietic system as a model of study. Strikingly hematopoietic cells (immune cells, platelets and red blood cells) compose over 90% of total human cells and correspond to approximately ten trillions of cells (Sender R, 2016). More importantly they all originate from the same cells, the hematopoietic stem cells (HSC), through a process called hematopoiesis. In addition, as immune and blood cells have a short life span (from hours to months) and can response to perturbations like infections, this process is highly dynamical. It is therefore an interesting and challenging model to study differentiation in a complex system at the single cell level.

To achieve this, Leïla Perié’s lab combines different experimental and mathematical/computational approaches of single cell tracing to study hematopoiesis in vivo. For example cellular barcoding is one of the lineage tracing approaches used by the Perié’s lab. It simultaneously traces the in vivo differentiation of individual cells, allowing to reconstitute the relationship between cell lineages with single cell resolution. In this seminar, we will discuss some of our recent results using cellular barcoding in hematopoiesis.

# Séminaire du LPTMS: Simon Pigeon

## Controlling superfluid vortices in polariton fluids

### Simon Pigeon (LKB, Université Pierre et Marie Curie, Paris)

Exciton-polaritons, microcavity half-matter half-light quasi-particles,  when resonantly driven exhibit a superfluid regime. Accordingly,  topological excitations similar to those predicted in equilibrium  superfluids may spontaneously appear. However, the non-equilibrium  nature of polaritons requires the system to be continuously pumped in  order to compensate for losses. This driving plays a key role in the  formation and dynamics of such topological excitations. Excited through  a resonant pumping, the coherent field driving the system imposes a  phase on the polariton fluid which inhibits the formation of vortices or  solitons. This unique feature of coherently driven polariton superfluids  has been used to trap vortices. To that purpose an engineered pumping  profile, alternating between spatial driven and non driven regions was  proposed and successfully implemented. Based on this same idea of spatially engineered pumping profils, many  works done lately focused spontaneous formation of vortices in polariton  superfluids. For practical experimental reasons, such as high speeds,  small propagation length and a short lifetimes, the hydrodynamics of  these vortices propagating were largely ignored. I will present a recent investigation of an optical method allowing us  to overcome these difficulties, together with giving the unique  opportunity to directly control and manipulate the properties of the  vortices. The general idea is to sustain the superfluid polariton flow,  something which generally does not last long outside the excitation  region, with a secondary supporting excitation of low intensity. This  way the flow may propagate over a much longer distance. The vortices  formed in the superfluid are still permitted due to the weakness of the  support pump. This allows their propagation along large distances and  the observation of their hydrodynamic behaviour. However, the secondary  coherent driving acting as a support, allows for some other rich and  unique features. At the same time as increasing the propagation length,  the same support driving gives direct control over the vortex-antivortex  pair properties, offering a unique opportunity to manipulate vortex  locally and accurately. The new approach proposed allows for the observation of hydrodynamic  behaviour of superfluid vortices, and initiates a new step regarding the  control of such fundamental entities, thus paving the way toward their  practical use. References:

# Séminaire du LPTMS: Eric Vernier *** séminaire exceptionnel ***

## Non equilibrium dynamics of quantum systems : the Loschmidt echo

### Eric Vernier (SISSA, Trieste)

The non-equilibrium dynamics of quantum many-body systems has attracted a large interest over  the last decade, prompted by formidable advances in cold-atomic experiments.  While much progress has been done in understanding the relaxation mechanisms of physical observables and the characterization of the stationary state following, for instance, a quantum quench (where an isolated system is let evolve after one or several parameters have been suddenly changed), very few analytical results exist about the full time dynamics despite the existence of prototypical integrable models. Indeed, the time dynamics involves contributions for arbitrarily excited eigenstates of the Hamiltonian, making calculations prohibitively difficult.
In this talk I will present some progress made recently in this direction (based on the preprint arXiv:1611.06126), namely an analytical computation of the Loschmidt echo, which measures the overlap between the state of the system at a given time and its initial state, for various types of quenches in the Heisenberg XXZ spin chain. The latter has attracted a renewed interest recently in the context of dynamical phase transitions, which it signals through its non-analyticities as a function of the time. Using a reformulation of the problem in terms of an auxiliary boundary quantum transfer matrix and using an infinite set of functional relations, we write the Loschmidt echo as the solution of an infinite set of Non Linear Integral Equations, which allows for its exact determination at arbitrarily large time. This method overcomes the time limitations experienced by numerical approaches, and may serve as a basis for the computation of other physical observables.

# Séminaire du LPTMS: Laura Foini

## Spatio-temporal patterns in ultra-slow domain wall creep dynamics

### Laura Foini (LPT-ENS, Paris)

In presence of impurities, ferromagnetic and ferroelectric domain walls slide only above a finite external field. Close to this depinning threshold, they proceed by large and abrupt jumps, called avalanches, while, at much smaller field, these interfaces creep by thermal activation.
In this talk I will present our results for the creep dynamics at vey low forces, obtained by a novel numerical technique that captures this ultra-slow regime over huge time scales. We point out the existence of activated events that involve collective reorganizations similar to avalanches, but, at variance with them, display correlated spatio-temporal patterns that resemble the complex sequence of aftershocks observed after a large earthquake. Remarkably, we show that events assembly in independent clusters that display at large scales the same statistics as critical depinning avalanches. We foresee this correlated dynamics being experimentally accessible by magneto-optical imaging of ferromagnetic films.

# Physics-Biology interface seminar: Antoine Jégou

## Building and disassembling actin filaments with proteins and forces

### Antoine Jégou (Institut Jacques Monod, France)

The actin cytoskeleton comprises several networks essential for the cell to perform many key functions (motility, cell division, tissue cohesion, …). Their assembly and disassembly is tightly regulated, in space and time, by a myriad of actin binding proteins but also by the mechanical stress applied to those networks. We take advantage of a simple setup based on microfluidics and fluorescence microscopy, to manipulate actin filaments in vitro and assay the regulation of actin assembly. Focusing first on the assembly of filaments by formins, which are able to track filament barbed ends and accelerate their elongation from profilin-actin, we will show how tracking and rapid elongation are modulated by filament tension and regulatory proteins. We will then focus on ADF/cofilin isoforms, which play a central in filament disassembly. We will show how ADF/cofilin fragments and depolymerizes filaments through different mechanisms, targeting both the side and the ends of the filaments.

# Séminaire du LPTMS: Oleksandr Gamayun *** séminaire exceptionnel ***

## Quantum dynamics of a mobile impurity in a one-dimensional Fermi gas

### Oleksandr Gamayun (Leiden Unversity, The Netherlands)

A single impurity particle in an ultracold atomic gas is a prospective model for probing relaxation dynamics of an interacting out-of-equilibrium quantum system. I will focus on a case of an impurity that propagates in a one-dimensional gas of free fermions and interacts with them through the contact interaction. It is the simplest and yet fundamental model capturing a peculiar physics and mathematics of the non-equilibrium processes. A particular feature of this model is integrable when masses of the impurity and gas particles are equal. In this case, I will present a full nonpertubative solution and express physical quantities in terms of the Fredholm determinants of integrable integral operators. After a detailed analysis of the mathematical structure of the Fredholm determinants, I will discuss several striking physical phenomena such as incomplete relaxation, Bloch oscillations, and momentum-dependent impurity statistics. To describe the non-integrable case, I will develop a kinetic theory approach. I will then present its modification that allows description of the integrable case as well. Finally, I will give some arguments about the persistence of the discussed phenomena at a finite temperature.

# Séminaire du LPTMS: John Martin

## Cold atom superradiance

### John Martin (Université de Liège, Belgique)

In my talk, I will report on the derivation and resolution of a novel master equation governing the cooperative spontaneous emission from an ensemble of cold atoms [1]. Our master equation for the atomic internal degrees of freedom incorporates the effects of the quantization of the atomic center-of-mass motion. It provides a unifying picture of the consequences of recoil and indistinguishability of atoms beyond the Lamb-Dicke regime, both on their dissipative and conservative internal dynamics. I will discuss general expressions for the decay rates and dipole-dipole shifts entering our master equation. As an application of our formalism, I will then investigate the consequences of indistinguishability of cold atoms on two hallmarks of cooperative emission processes: Dicke super- and subradiance [2].
Références :
1. [1] F. Damanet, D. Braun, and J. Martin, Master equation for collective spontaneous emission with quantized atomic motion, Phys. Rev. A 93, 022124 (2016) ; Cooperative spontaneous emission from indistinguishable atoms in arbitrary motional quantum states, ibid. 94, 033838 (2016).
2. [2] R. H. Dicke, Coherence in Spontaneous Radiation Processes, Phys. Rev. 93, 99 (1954).

# Séminaire du LPTMS: Pascal Naidon

### Pascal Naidon (RIKEN, Japan)

The properties of two particles immersed in a Bose-Einstein condensate are investigated using a variational approach. These properties can be interpreted in terms of a boson-mediated interaction. For a weak particle-boson attractive interaction, the two particles form two free attractive polarons that interact through a weak boson-mediated Yukawa attraction. For suffciently strong interactions, the boson-mediated interaction turns into an Efimov attraction leading to the binding of the two particles and one boson into Efimov trimers. It is found that these in-medium Efimov trimers exist for weaker interactions than in vacuum.

Reference: P. Naidon, Two impurities in a Bose-Einstein condensate: from Yukawa to Efimov attracted polarons, preprint cond-mat arXiv:1607.04507

# Séminaire du LPTMS: duo Xiangyu Cao & Kirill Plekhanov

### Liouville field theory and log-correlated Random Energy Models'' by Xiangyu Cao (LPTMS, Université Paris-Sud)

An exact mapping is established between the c ≥ 25 Liouville field theory (LFT) and the Gibbs measure statistics of a thermal particle in a 2D Gaussian Free Field plus a logarithmic confining potential. The probability distribution of the position of the minimum of the energy landscape is obtained exactly by combining the conformal bootstrap and one-step replica sym- metry breaking methods. Operator product expansions in LFT allow to unveil novel universal behaviours of the “log-correlated Random Energy Models” class. Applications will include multi- fractality (inverse participation ratios and their corrections) and the overlap distribution in the directed polymer on a Cayley tree model. Ref.: Xiangyu Cao, Pierre Le Doussal, Alberto Rosso and Raoul Santachiara, preprint arXiv:1611.02193.

### Chiral spin state of the frustrated XY model on the honeycomb lattice'' by Kirill Plekhanov (LPTMS, Université Paris-Sud)

Abstract: Currently, it was suggested that the frustrated XY model for spin-1/2 on the honeycomb lattice can support a chiral spin liquid (CSL) phase, that is characterized by the spontaneous breaking of the time-reversal and parity symmetries. The ground state of the system in the CSL phase will be characterized by the gapped bulk and gapless chiral modes along the edges, if open boundary conditions are used. In this talk I will present analytical arguments that lead to this conclusion and a numerical verification of analytical results based on the calculations of local order parameters, structure factors and scalar spin chirality using the exact diagonalization technique. An experimental realization of the model in the context of ultra-cold atoms and circuit-QED systems, based on the application of time-dependent periodic (Floquet) modulations, will be discussed.

# Séminaire du LPTMS: Beatriz Seoane Bartolomé

## Matching experiments and theory in spin glasses

### Beatriz Seoane Bartolomé (LPT-ENS, Paris)

The unifying feature of glass formers (such as polymers, supercooled liquids, colloids, granular materials, spin glasses, superconductors, ...) is a sluggish dynamics at low temperatures. Indeed, their dynamics is so slow that thermal equilibrium is never reached in macroscopic samples: in analogy with living beings, glasses are said to age. This fact suppose a difficulty to describe these systems theoretically. The reasons are two fold. One the one hand, most of the calculations are obtained in the experimentally unreachable low-temperature equilibrium, and on the other hand, they concern microscopic observables (obtained mainly with the replica method) that are hard to measure in experiments. In two recent works [1,2], we have shown that it is possible to quantitatively relate both realms, using large-scale simulations on the special-purpose computers Janus and Janus II, two dedicated supercomputers to simulate spin-glasses. In the first work [1], we have performed a very accurate computation of the non-equilibrium fluctuation-dissipation ratio for the three-dimensional Edwards-Anderson Ising spin glass. This ratio (computed for finite times on very large, effectively infinite, systems) is compared with the equilibrium probability distribution of the spin overlap for finite sizes. The resulting quantitative statics-dynamics dictionary, based on observables that can be measured with current experimental methods, could allow the experimental exploration of important features of the spin-glass phase without uncontrollable extrapolations to infinite times or system sizes. On the other hand, in Ref. [2], we have reproduced in simulations a milestone experiment that measures the spin-glass coherence length through the lowering of free-energy barriers induced by the Zeeman effect. Secondly we determine the scaling behavior that allows a quantitative analysis of a new experiment of amorphous Ge:Mn films [3].  The value of the coherence length estimated through the analysis of microscopic correlation functions turns out to be quantitatively consistent with its measurement through macroscopic response functions. Further, non-linear susceptibilities, recently measured in glass-forming liquids, scale as powers of the same microscopic length.
[1] Janus Collaboration (2017), Probing the spin-glass phase with non-equilibrium measurements: statics-dynamics equivalence through the fluctuation-dissipation ratio'', accepted for PNAS, https://arxiv.org/abs/1610.01418
[2] Janus Collaboration (2017), Matching microscopic and macroscopic responses in glasses'', under review in PRL.
[3] Samaresh Guchhait and Raymond L. Orbach (2017), Magnetic Field Dependence of Spin Glass Free Energy Barriers'', under review in PRL.

# Physics-Biology interface seminar: Michele Castellana

## Enzyme clustering can induce metabolic channeling

### Michele Castellana (Institut Curie, Paris)

Direct channeling of intermediates via a physical tunnel between enzyme active sites is an established mechanism to improve the efficiency of metabolic pathways. In this seminar, I will present a theoretical model which demonstrates that coclustering multiple enzymes into proximity can yield the full efficiency benefits of direct channeling. The model predicts the separation and size of coclusters that maximize metabolic efficiency, and this prediction is in agreement with the inter-cluster spacing in yeast and mammalian cells. In addition, the model predicts that enzyme agglomerates can regulate steady-state flux division at metabolic branch points: we experimentally test this prediction for a fundamental branch point in Escherichia coli bacterium, and the results confirm that enzyme colocalization within an agglomerate can accelerate the processing of a shared intermediate by one branch. Our studies establish a quantitative framework to understand coclustering-mediated metabolic channeling, as well as its application to efficiency improvement and metabolic regulation.

# Séminaire du LPTMS: Shlomi Reuveni

## First Passage Under Restart

### Shlomi Reuveni (Department of Systems Biology, Harvard Medical School, Boston)

Stopping a process in its midst—only to start it all over again—may prolong, leave unchanged, or even shorten the time taken for its completion. Among these three possibilities the latter is particularly interesting as it suggests that restart can be used to expedite the completion of complex processes involving strong elements of chance. This turned out to be important in computer science where restart drastically improves performance of randomized algorithms, but is not less relevant to many physical, chemical, and biological processes since restart is inherent to the stochastic Michaelis-Menten reaction scheme and this is ubiquitous throughout the sciences. I will introduce the problem of first passage under restart using the example of simple diffusion, but will then explain why many unknowns compel us to generalize to arbitrary first passage processes and restart mechanisms. This has recently led us to discover universal features of the problem which I will derive and describe, and the framework developed will also serve to make the case for a much required paradigm shift in our understanding of enzymatic catalysis and inhibition at the single-molecule level. References :
1. S. Reuveni, M. Urbakh and J. Klafter, Role of Substrate Unbinding in Michaelis–Menten Enzymatic Reactions, PNAS 111 (12), 4391 (2014).
2. T. Rotbart, S. Reuveni and M. Urbakh, Michaelis-Menten Reaction Scheme as a Unified Approach Towards the Optimal Restart Problem, Phys. Rev. E 92, 060101(R) (2015).
3. S. Reuveni, Optimal Stochastic Restart Renders Fluctuations in First Passage Times Universal, Phys. Rev. Lett. 116, 170601 (2016).
4. A. Pal and S. Reuveni, First Passage Under Restart, Phys. Rev. Lett. 118, 030603 (2017).
5. T. Rotbart, S. Reuveni and M. Urbakh, Single-enzyme approach predicts natural emergence of inhibitor-activator duality, bioRxiv 095562.

# Séminaire du LPTMS: Denis Ullmo

## Non-linear Schrödinger approach to mean field games

### Denis Ullmo (LPTMS, Université Paris-Sud)

Mean field games theory is a rather recent research area, at the frontier between applied mathematics, social sciences and physics. It was initiated a decade ago by Pierre-Louis Lions and Jean-Michel Lasry as a tool to model certain social phenomena involving a significant number of actors - through a game theory approach - while maintaining a reasonable level of simplicity thanks to the concept of mean-field imported from physics. In this presentation, after a general introduction to Mean Field Games, I will show that there is a formal, but deep, link between an important class of these models and the nonlinear Schrödinger equation familiar to physicists. This link makes it possible to develop a deeper understanding of the behavior of Mean Field Games.  A specific example will be treated in details and will demonstrate how  various concepts developed by physicist while studying the non-linear Schrödinger equation can be used with  profit in the context of Mean Field Games. Reference:

# Physics-Biology interface seminar: Daniel Jost

## Physical biology of chromatin: understanding the functional role of 3D chromosome folding using polymer physics

### Daniel Jost (Université Grenoble Alpes)

Cellular differentiation occurs during the development of multicellular organisms and leads to the formation of many different tissues where gene expression is modulated without modification of the genetic information. These modulations are in part encoded by chromatin-associated proteins or biochemical tags that are set down at the chromatin level directly on DNA or on histone tails. These markers are directly or indirectly involved in the local organization and structure of the chromatin fiber, and therefore may modulate the accessibility of DNA to transcription factors or enzymatic complexes, playing a fundamental role in the transcriptional regulation of gene expression. Propagation, maintenance and inheritance of these epigenetic marks are crucial mechanisms in development, phenotype stabilization and disease. Experimental evidence have shown that the pattern of chromatin markers along chromosomes is strongly correlated with the 3D chromatin organization inside the nucleus. This suggests a coupling between epigenomic information and large-scale chromatin structure. Here, I will discuss our recent works using polymer physics and numerical simulations trying to understand the basic principles behind such coupling and and to propose possible functional roles for the 3D organization of chromosomes.

# Séminaire LPTMS: Tommaso Comparin *** séminaire exceptionnel ***

## Thermodynamics of the unitary Bose gas from first principles

### Tommaso Comparin (LPS-ENS, Paris)

Ultracold atomic gases offer access to a broad class of quantum systems. The strength of interatomic interactions, in particular, may be tuned across a wide range, through the Feshbach-resonance technique. In this talk, I consider a gas of bosonic atoms in the unitary limit, where the scattering length is infinite. For three bosons, unitary interactions lead to the Efimov effect, consisting in an infinite sequence of scale-invariant three-body bound states. The Efimov scenario (predicted in nuclear physics in the 1970s, and first observed in 2006 with ultracold atoms) is correctly described through a model with zero-range two-body interactions and a hard-core three-body repulsion. We use this model to describe a many-body system of unitary bosons, which we study through a dedicated path-integral quantum Monte Carlo algorithm. At thermal equilibrium, the phase diagram of the system includes the normal-gas phase at high temperature and two low-temperature phases: The Efimov liquid and the Bose-Einstein condensate. We determine the critical temperature for Bose-Einstein condensation, which is found to be 10% smaller than for non-interacting bosons. We also compute the momentum distribution of the gas, and compare it with available experimental results which are part of the current effort towards the realization of a degenerate unitary Bose gas. Reference: T. Comparin and W. Krauth, Phys. Rev. Lett. 117, 225301 (2016)

# Séminaire du LPTMS: Kay Wiese

## The Field theory of avalanches

### Kay Wiese (LPT-ENS, Paris)

When elastic systems like contact lines on a rough substrate, domain walls in disordered magnets, or tectonic plates are driven slowly, they remain immobile most of the time, before responding with strong intermittent motion, termed avalanche. I will describe the field theory behind these phenomena, explain why its effective action has a cusp, and how such intricate objects as the temporal shape of an avalanche can be obtained.

# Séminaire du LPTMS: Randy Kamien *** séminaire commun avec le LPS ***

## Developments in the Theory of Smectic Liquid Crystals

### Randy Kamien (University of Pennsylvania)

The theory of liquid crystals combines principles of symmetry, geometry, and topology to understand materials that exhibit compelling, complex configurations and textures.  In this talk I will provide an overview of the theory of smectic liquid crystals and will show how it touches on superconductivity, fluid mechanics, and special relativity.

# Physics-Biology interface seminar: Greg Grason

## Block Copolymer Assemblies Beneath the Surface: Modeling Intra-Domain Textures and Chirality Transfer to Mesodomains

### Greg Grason (U. of Massachussets, Amherst)

This seminar replaces that of Thierry Bizebard, was rescheduled to May 31st. Please note the more soft matter focus. Self-assembled block copolymer (BCP) melts are a chemically-versatile platform for generating a rich spectrum of periodically-ordered nanostructures of various morphologies, from arrays of layers and columns to cubic arrays of spheres and bicontinuous networks. They are also a model system for understanding processes and properties of self-assemblies, more broadly. Decades of study of BCP assembly have uncovered the principles that connect molecular BCP structure to the translational order of the (scalar) composition profiles in the ordered states. In this talk, I will describe recent efforts to understand the generic, yet poorly known, patterns of orientational ordering of constituent chain segments that underlie the otherwise well-known “standard” BCP morphologies1. From generic properties of the random-walk statistics in BCPs, we show that the direction and degree of alignment of segments varies non-trivially from place-to-place in self-organized domains, and from one morphology to another, leading to new opportunities to manipulate and harness the physics sub-domain textures. Specifically, I will discuss how our efforts to model chiral BCPs2 which have been observed to transfer handedness of chain chemistry to the chiral symmetry of mesodomain shapes that are not formed in achiral BCPs. Our generalized orientational self-consistent field (oSCF) theory framework3 shows that the thermodynamic drive for twisted, or cholesteric, packings of segments of chiral blocks stabilizes observed helical cylinder morphologies, and suggests new mechanisms for driving formation as of yet, unobserved mesochiral domain symmetries4. References
1. I. Prasad, Y. Seo, L. Hall and G. M. Grason arxiv.org/1612.07994 (2016)
2. G. M. Grason ACS MacroLetters 4, 526 (2015). Front Cover Story
3. W. Zhao, T. Russell and G. M. Grason, J. Chem. Phys. 137, 104911 (2012).
4. W. Zhao, T. Russell and G. M. Grason, Phys. Rev. Lett. 110, 058301 (2013).

# Soutenance de thèse: Xiangyu Cao

## Disordered statistical physics in low dimensions: extremes, glass transition, and localization.

### Xiangyu Cao

This thesis presents original results in two domains of disordered statistical physics: logarithmic correlated Random Energy Models (logREMs), and localization transitions in long-range random matrices.

In the first part devoted to logREMs, we show how to characterize their common properties and model--specific data. Then we develop their replica symmetry breaking treatment, which leads to the freezing scenario of their free energy distribution and the general description of their minima process, in terms of decorated Poisson point process. We also report a series of new applications of the Jack polynomials in the exact predictions of some observables in the circular model and its variants. Finally, we present the recent progress on the exact connection between logREMs and the Liouville conformal field theory.

The goal of the second part is to introduce and study a new class of banded random matrices, the broadly distributed class, which is characterid an effective sparseness. We will first study a specific model of the class, the Beta Banded random matrices, inspired by an exact mapping to a recently studied statistical model of long--range first--passage percolation/epidemics dynamics. Using analytical arguments based on the mapping and numerics, we show the existence of localization transitions with mobility edges in the stretch--exponential'' parameter--regime of the statistical models. Then, using a block--diagonalization renormalization approach, we argue that such localization transitions occur generically in the broadly distributed class.

The defense presentation will focus on the logREM--Liouville connection and the broadly distributed random matrices.
Directeurs de thèse: Alberto Rosso, Raoul Santachiara Jury: Bertrand Georgeot, Jonathan Keating, Christopher Mudry, Didina Serban; invité: Pierre Le Doussal

# Séminaire du LPTMS: Jean-Baptiste Delfau

## A counter-intuitive case of spontaneous aggregation : crystals of clusters

### Jean-Baptiste Delfau (Universitat de les Illes Balears, Palma de Mallorca, Spain)

Particles interacting via purely repulsive soft-core potentials can spontaneously aggregate, despite repelling each other, and form periodic crystals of particle clusters. We studied this phenomenon - relevant to colloidal or polymer solutions - for passive (Brownian) and active (self-propelling) particles at two levels of description: performing numerical simulations of the discrete particle dynamics, and by linear and nonlinear analysis of the corresponding Dean-Kawasaki equations for the macroscopic particle density. In this seminar, I will explain what are the mechanisms of the instability leading to clustering which turn out to be the interplay between diffusion or self-propulsion, the intracluster forces and the forces between neighboring clusters.
References :
• J.-B. Delfau, H. Ollivier, C. López, B. Blasius & E. Hernández-García, Pattern formation with repulsive soft-core interactions: Discrete particle dynamics and Dean-Kawasaki equation, Phys. Rev. E 94, 042120 (2016)
• J.-B. Delfau, C. López, E. Hernández-García, Active cluster crystals, arXiv:1701.02639

# Séminaire du LPTMS: Deepak Bhat

## Dynamics of a piston pushed by a single particle gas as a microscopic model for Szilard engine

### Deepak Bhat (ICTS, Bangalore, India)

Motivated by the Szilard engine, we have studied the dynamics of a piston of large mass M inside a 1D cylinder, pushed by repeated elastic collisions with a single particle (of smaller mass m) which is in contact with a thermal reservoir of temperature T. Additionally, piston is assumed to be moving under influence of an external force. By expanding the underlying master equation of the piston-particle system in terms of the small parameter (m/M)1/2, we show that, an effective equation of motion of the piston can be deduced by integrating out the small particle. Given finite but large mass of the piston, we show that at times of order √M the Piston’s motion is described by a determinstic dynamics while at larger times, the piston’s motion becomes stochastic and is described by a Langevin equation. Interestingly, we note that the dissipation coefficient γ depends on piston position X, and is also function of time t. In particular, it changes from an initial value (1/X)(8mkBT/π)1/2 and increases as log(t)/X asymptotically. We argue that the logarithmic increase of γ is associated to the interesting and unusual slow relaxation of particle confined within the reservoir and the piston.

# Séminaire du LPTMS: Maurizio Fagotti

## Intermediate-time dynamics in  out-of-equilibrium spin chains

### Maurizio Fagotti (LPT-ENS, Paris)

I discuss some aspects of quench dynamics in quantum many-body systems. In the first part of the talk, I present an unusual mechanism of prethermalization. This is based on the presence of a symmetry of the pre-quench Hamiltonian which is spontaneously broken at zero temperature and is explicitly broken by the post-quench Hamiltonian.  In the second part of the talk, I focus on the non-equilibrium time evolution of piecewise homogeneous states. The inhomogeneity of the initial state gives rise to a peculiar intermediate-time dynamics that is captured by a hydrodynamic description. I present the solution to the dynamics in the XXZ spin-1/2 chain and describe some remarkable properties of the profiles of charges and currents. References:
• V. Alba and M. Fagotti, Prethermalization at low temperature: the scent of a spontaneously broken symmetry, arXiv:1701.05552 (2017).
• B. Bertini, M. Collura, J. De Nardis, and M. Fagotti, Transport in Out-of-Equilibrium XXZ Chains: Exact Profiles of Charges and CurrentsPhys. Rev. Lett. 117, 207201 (2016).

# Physics-Biology interface seminar: Alessandro Barducci

## Molecular chaperones as cellular non-equilibrium machines.

### Alessandro Barducci (Centre de Biochimie Structurale-INSERM, Montpellier)

Molecular chaperones are a vast class of proteins that maintain protein homeostasis in the cell and are thus essential for cell viability. In order to assist protein folding and prevent misfolding, most chaperones proceed through conformational cycles that are regulated by complex interaction networks and fueled by ATP-hydrolysis. A remarkable example are the 70-kDalton heat shock proteins (Hsp70s), which are essential in prokaryotes and eukaryotes and are involved in co-translational folding, refolding of misfolded and aggregated proteins, protein translocation, and protein degradation. While the investigation of Hsp70 cycle has attracted great attention in the last decades, the actual role of ATP-hydrolysis and, thus of energy consumption, in the chaperone function has been long unaddressed. Here we will prove how biochemical data, recent single-molecule fluorescence experiments and molecular simulations can be combined into an appropriate theoretical framework to show that: i) ATP hydrolysis allows Hsp70 chaperones to increase their affinity for the client proteins beyond the bounds imposed by equilibrium thermodynamics ii) This ultra-affinity can be exploited to perform mechanical work on client proteins thus avoiding the formation of misfolded and potentially cytotoxic species.

# Séminaire du LPTMS: Grégoire Misguich

## Entanglement and Shannon entropies in low dimensional magnets

### Grégoire Misguich (IPhT, CEA, Saclay)

The entanglement (von Neumann) entropy is now routinely used by condensed-matter theorists as a tool to probe quantum many-body  systems, since it often gives access to properties that are not easily visible using more conventional observables. I will illustrate this using a few examples taken from the literature on 1D and 2D spin systems. I will then discuss a related but different entropy, the Shannon entropy, which can also be used as a tool to probe many-body  systems. The Shannon entropy of a many-body state |ψ>  is defined by expanding the wave-function in some preferential configuration basis { |i> }. The projections of  |ψ> onto the basis states define a set of normalized probabilities p[i] = |<ψ|i>|2, and these probabilities can used to define an associated Shannon entropy S = - Σi p[i]*log(p[i]). This quantity measures hos 'localized" is |ψ>, in the chosen basis. While the entanglement entropy requires the choice of a specific subsystem, the Shannon entropies is basis-dependent but is defined for the system as a whole. In this talk we discuss a few examples in 1D and 2D where some universal information about the long-distance physics of the system can be obtained by analyzing the scaling of S as a function of the system size. In particular, we show that the spontaneous symmetry breaking of a continuous symmetry (as in Néel-ordered antiferromagnets) leads in 2D to some universal logarithmic terms in the Shannon entropy Reference:
• G. Misguich, V. Pasquier & M. Oshikawa, Shannon-Rényi entropy for Nambu-Goldstone modes in two dimensions, preprint arXiv:1607.02465

# Séminaire du LPTMS: Alexander Hartmann

## Large deviations for equilibrium and non-equilibrium processes

### Alexander Hartmann (Universität Oldenburg, Germany)

Large deviations and rare events play an ever increasing role in science, economy and society. Large deviations play a crucial role for example for the estimation of impacts of storms, the calculation of  probabilities of stock-market crashes or the sampling of transition  paths for conformation change of  proteins. More fundamentally, when  studying any random process, only the full probability distribution,  including the large-deviation tails, gives a complete information about  the underlying system. The basic principal to study large deviations using numerical  simulations is simple: make unlikely events more probable and correct in  the end for the bias. Here, we present a very general black-box method,  based on  sampling vectors of random numbers within an artificial  finite-temperature (Boltzmann). This allows to access rare events and  large deviation for almost arbitrary equilibrium and non-equilibrium  processes. In this way, we obtain probabilities as  small as $10^{-500}$    and smaller, hence rare events and large deviation properties  can be  easily obtained. The method can be applied to equilibrium/static sampling problems, e.g., the distribution of the number and size of connected components of  random graphs. Here, applications from different fields are presented: 1. Distribution of work performed for a  critical (T=2.269) two-dimensional  Ising system of size LxL=128x128 upon rapidly changing  the external magnetic field(also applying  theorems of Jarzynski and  Crooks to obtain the free energy difference  of such a large system) 2. Distribution of perimeters and area of convex hulls of two- and  higher dimensional single and multiple random walks. 3. Distribution of the flow for the Nagel-Schreckenberg traffic model.

# Physics-Biology interface seminar: Volker Bormuth

## Whole-brain imaging during vestibular stimulation in zebrafish with a novel rotatable light-sheet microscope

### Volker Bormuth (Laboratoire Jean Perrin, Université Pierre et Marie Curie)

Light-sheet microscopy allows cell resolved whole-brain calcium imaging at several brain scans per second in zebrafish larvae. Currently this technique is not compatible with dynamic stimulation of the vestibular system. We developed an ultra-stable miniaturized light-sheet microscope that can be rotated while performing whole-brain recordings. Rotating the microscope rotates the fish and stimulates the vestibular system while imaging always the same plane in the brain. We demonstrate volumetric whole-brain neuronal activity recordings during vestibular stimulation. We mapped the brain activity with cellular resolution of the vestibule-ocular reflex (VOR) which drives compensatory eyes movements to maintain clear vision during body rotation. Our long-term goal is study with this system multisensory signal processing by the vertebrate brain by combining visual with vestibular stimuli.

# Séminaire du LPTMS: Tridib Sadhu

## Generalized arcsine law in fractional Brownian motion

### Tridib Sadhu (LPT-ENS, Paris)

The three arcsine laws for the standard Brownian motion are a cornerstone of extreme value statistics. For a standard Brownian motion evolving in a time window, one can consider the following three observables: (1) the fraction of time it remained positive, (2) the last time it crossed the origin, (3) and the time when it reached its maximum. All three observables have the same cumulative probability distribution expressed as an arcsine function. I shall discuss how these three laws change for a fractional Brownian motion. The fractional Brownian motion is a non-Markovian Gaussian process indexed by Hurst exponent H which generalizes Brownian motion (H=1/2). I shall show that the three observables have different distributions for general H. I shall present a perturbation expansion scheme using which one can derive these probability distributions

# Séminaire du LPTMS: Satya Majumdar

## KPZ story

### Satya N. Majumdar (LPTMS, Université Paris Sud)

The celebrated KPZ equation (Kardar, Parisi, Zhang, 1986) is an important  milestone in statistical physics, originally introduced to describe the  late time dynamics in various growth models. Over the last 30 years, the  KPZ story has evolved in various interesting directions, making links on the way to different areas of physics and mathematics. This includes in  particular the link to the famous Tracy-Widom distribution in random  matrix theory. The story of KPZ is a very successful one, involving  theoretical physics, mathematics and experiments--a fertile playground for  interdisciplinary science. In this talk, I will review the evolution of  the KPZ story, pointing out the important landmarks as I go along. At the  very end, I will discuss some recent developments establishing a nice link between the KPZ height fluctuations and the edge physics in cold atom  systems.

# Physics-Biology interface seminar: Julien Husson

## Single-cell leukocyte mechanics: force generation, viscoelasticity, and rupture mechanics

### Julien Husson (LadHyX, École polytechnique, France)

Leukocytes are very soft cells that perform many diverse functions: they adhere, crawl, transmigrate, kill, phagocytose or interact with other cells. During their activation, leukocytes both generate mechanical forces and change their viscoelastic properties (i.e. they stiffen/soften, get more or less viscous). We have developed micropipette-based setups to quantify single-leukocyte mechanical properties and monitor them over time while a leukocytes gets activated by a relevant stimulus. We further quantify rupture properties of cell membrane, as these help us to better understand cell structure and dynamics. We use this approach in diverse contexts involving leukocytes: activation of T lymphocytes, phagocytosis of a target by a neutrophil, or transmigration of a lymphoblast across an endothelial monolayer. We perform microrheology experiments with a profile microindentation setup [1,2], measure forces generated by T lymphocytes [3,4], characterize cell-substrate adhesion [5] or establish a rupture criteria for membrane rupture [2,6] (Figure 1). These mechanical measurements shed a new light on how cell mechanical properties evolve over a short period of time (seconds), how they adapt to the stiffness of their environment, and how intracellular signaling is involved.

T-lymphocytes in the human body routinely undergo large deformations, both passively when going through narrow capillaries and actively when transmigrating across endothelial cells or squeezing through tissue. In this artistic rendering, a T-lymphocyte is aspirated in a micropipette to mimic passive deformations that occur when squeezing through narrow capillaries. The fluorescent signal is due to the entry of propidium iodide into the cell and indicates membrane rupture (Image: Julien Husson, LadHyX, Ecole polytechnique, cellmechanics.jimdo.com/galleries)

1. Guillou, L., Babataheri, A., Puech, P.-H., Barakat, A.I. & Husson, J. Dynamic monitoring of cell mechanical properties using profile microindentation. Scientific Reports 6:21529 (2016). 2. Guillou, L., Babataheri, A., Saitakis, M., Bohineust, A., Dogniaux, S., Hivroz, C., Barakat, A.I. & Husson, J. T lymphocyte passive deformation is controlled by unfolding of membrane surface reservoirs. Molecular Biology of the Cell 27(22): 3574-3582. (2016, journal cover). 3. Husson, J., Chemin, K., Bohineust, A., Hivroz, C. & Henry, N. Force Generation upon T Cell Receptor Engagement. PLoS One 6(5):e19680 (2011). 4. Basu, R.*, Whitlock, B.M.*, Husson, J.*, Le Floc’h, A., Jin, W., Dotiwala, F., Giannone, G., Hivroz, C., Lieberman, J., Kam, L.C. & Huse, M. Cytotoxic T Cells Use Mechanical Force to Potentiate Target Cell Killing. Cell 165(1):100–110 (2016). 5. Hogan, B., Babataheri, A., Hwang, Y., Barakat, A.I. & Husson, J. Characterizing Cell Adhesion by Using Micropipette Aspiration. Biophysical Journal 109(2):209-19 (2015). 6. Gonzalez-Rodriguez, D., L. Guillou, F. Cornat, J. Lafaurie-Janvore, A. Babataheri, E. de Langre, A.I. Barakat, and J. Husson. Mechanical Criterion for the Rupture of a Cell Membrane under Compression. Biophys. J. 111: 2711–2721 (2016).

# Séminaire du LPTMS: Inti Sodemann *** séminaire exceptionnel ***

## Quantum Hall ferroelectrics and the hidden link between composite fermions and the exciton condensate

### Inti Sodemann (MIT, Cambridge, USA)

In this talk, I will describe two recent developments in our understanding of phases in the quantum Hall regime. In the first part, I will describe our recent discovery of a remarkable connection between the composite fermion metal state realized in half-filled landau levels and the exciton superfluid realized in quantum Hall bilayers. Specifically, I will show that the exciton condensate is identical in all its universal properties to an interlayer paired state of composite fermions, and the two states can be connected in an analogous fashion to the BEC-BCS crossover. In the second part, I will describe our recent proposal to realize a “quantum Hall ferroelectric”: an integer quantum Hall state that spontaneously breaks inversion symmetry. I will argue for the ubiquity of these and nematic states in multi-valley systems whose low energy dispersions are anisotropic, and discuss material platforms for these systems.

# Séminaire du LPTMS: Ken Sekimoto

## Coarse-Graining of Momentum Flow

### Ken Sekimoto (MSC, Université Denis Diderot & Gulliver, ESPCI)

Like the energy and mass, the momentum is also a conserved quantity.  Usually, however, we pay less attention to the role played by the last  one as compared with the former ones. In the present talk, I will  discuss the three examples in which the concept of momentum flow helps  and simplifies our understanding of phenomena. First we discuss the adiabatic-piston'' for which the physical origin  of the non-equilibrium force has long been unclear. The consideration of  the momentum flux of both the gases and piston allows to reach the  central results in a few lines of elementary calculation, elucidating  how the non-equilibrium force emerges. Secondly we present a method of coarse-graining the overdamped dynamics  of dense cellular packing of matters.'' We show that the coarse graining  of the momentum and angular momentum fluxes leads naturally to the  Irving-Kirkwood type formula for the stress tensor irressective of the underlying dynamics. Lastly we discuss the Newton's cradle with modified repulsive laws. This  classical 1D system can show the similarity to the quantum Mössbauer  absorption and emission. By considering the momentum flow through the  cluster, we reach the coarse-grained dynamics for the center of mass coupled with an evanescent'' wave.

# Séminaire du LPTMS: Robin Guichardaz

## Robin Guichardaz (E.N.S Lyon)

Les attracteurs étranges apparaissant dans des systèmes dynamiques chaotiques présentent une structure fractale, caractérisée par une dimension fractionnaire et positive. Une des définitions possibles de la dimension fractale, la dimension de corrélation, est liée à la densité de points dans l'espace des phases autour d'une trajectoire donnée. Cette densité évolue généralement en une loi de puissance en fonction de la séparation des trajectoires, de sorte que la dimension de corrélation est reliée simplement à l'exposant de cette loi de puissance. Nous avons étendu cette notion au cas de systèmes dynamiques stochastiques stables, pour lesquels tous les exposants de Lyapunov sont strictement négatifs, ce qui signifie que toutes les trajectoires convergent in fine vers un attracteur ponctuel. Cependant, si nous ajoutons du bruit dans les équations du mouvement, contrairement à un étalement gaussien autour de l'attracteur, nous retrouvons une distribution en loi de puissance pour la séparation des trajectoires, indépendante du bruit additif. Nous interprétons l'exposant correspondant comme une dimension fractale, qui s'avère prendre des valeurs négatives. Plus spécifiquement, nous avons étudié un modèle unidimensionnel décrivant la dynamique de particules inertielles dans un flot aléatoire. Nous exprimons la dynamique de séparation de trajectoires proches comme un processus de sédimentation dans un espace adapté. Nous présenterons des résultats issus de simulations numériques, ainsi que des outils analytiques permettant de déterminer la dimension de corrélation. Indépendamment du modèle, nous proposons une approche basée sur les grandes déviations et pouvant être utilisée dans l'étude de systèmes intermittents. ----- Negative fractal dimensions in random dynamical systems : In chaotic dynamical systems, strange attractors have fractal structures, characterized by a (positive) dimension. In particular, the correlation dimension is linked to the density of points in the phase space around a given trajectory. This density usually scales as a power-law in terms of the separation of trajectories, so that the correlation dimension is linked to the exponent of the power-law. We extended this notion to the case of stable stochastic dynamical systems for which all the Lyapunov exponents are negative, meaning that all trajectories converge to a point-like attractor with probability unity. Nevertheless, in the presence of an additive noise, instead of a gaussian distribution linked to the additive noise we retrieve a power-law distribution for the separation of trajectories, independent of the additive noise. We interpret the corresponding exponent as a fractal dimension, which turns out to be negative. Specifically, we illustrate these notions with a simple one-dimensional model of colloidal particles in a randomly generated flow. We reformulate the dynamics of close trajectories in terms of a sedimentation process in a suitable space. Both numerical determinations and analytic tools to compute the correlation dimension will be presented. We propose a large deviation approach, without reference to any specific model, which can be applied to the study of intermittent systems.

# Physics-Biology interface seminar: Thierry Bizebard

## Ribosome assembly studied by single-molecule force measurements

### Thierry Bizebard (IBPC, Paris)

Ribosomes belong to the most complicated structures in biology. Their assembly is a question of fundamental interest, but is still poorly understood. In vitro reconstitution studies have shown that the ribosome assembly process is highly cooperative and starts with the binding of a few ribosomal (r-) proteins to rRNA, but how these early binders act is unknown. Our work focuses on the initial phase of the assembly of the large subunit (50S) of the E. coli ribosome, which involves 23S rRNA, five r-proteins and a selection of assembly “helper” proteins. Our force measurements on single RNA molecules have allowed us to pinpoint several important properties of the early-binding r-proteins we have studied:

- These proteins bind with high cooperativity to the rRNA (as would be expected to obtain a high yield of fully assembled and active ribosomes).
- The r-proteins act as molecular clamps, stabilising the RNA 3D structure.
- As such, they afford a strong mechanical and energetical stabilisation of the ribonucleoprotein structure (which is also probably necessary for optimum activity).

In the near future, we intend to further improve the potential of our single-molecule measurements by implementing combined force/fluorescence manipulations, and apply this methodology to our study of the early phase of E. coli large ribosomal subunit assembly.

# Séminaire du LPTMS: Ada Altieri et Inés Rodríguez-Arias

## The jamming paradigm: from computer science to field theory and beyond

### Ada Altieri  (LPTMS, Université Paris-Sud)

A recent challenging problem concerns the study of glassy systems at low temperature. We present a parallel derivation of an effective thermodynamic potential in two high-dimensional models: the negative  perceptron and its generalization to sphere systems. They both define continuous constrained satisfaction problems with a critical jamming transition characterized by the same exponents. The disclosed method enables us to study the vibrational spectrum of soft modes and to deepen the diverging behavior of the stiffness coefficients. A pivotal feature emerging close to jamming is that the effective thermodynamic potential has a subleading logarithmic contribution, which turns out to be dominant in a suitable scaling limit. A detailed analysis of higher-order corrections to the potential might also help in accounting for finite-dimensional systems and interpolating between different regimes. Another interesting outcome concerns the extension of the jamming transition topic to generic Von Neumann problems, especially focusing on ecosystems in high dimension.

Réf: Ada Altieri, Silvio Franz & Giorgio Parisi, The jamming transition in high dimension: an analytical study of the TAP equations and the effective thermodynamic potentialJ. Stat. Mech. (2016) 093301.

## A story of disorder, interactions and a quantum fridge inside a microwave oven

### Inés Rodríguez-Arias (LPTMS, Université Paris-Sud)

In my presentation I will show two models for dynamic nuclear polarization (DNP), an extremely promising technique that enhances the nuclear spin polarization in order to improve the nuclear magnetic resonance (NMR) signal-to-noise ratio. We have found this technique to represent a good scenario to check the ergodicity properties of the eigenstates of the system in a standard DNP protocol. The structure of the eigenstates is indeed modified upon changing the strength of the dipolar interactions between electrons keeping a constant disorder. Firstly, I will present a spin model that shows a many-body localization (MBL) transition and that reproduces the so far unexplained results from the experiments. Secondly, I will compare this to an exactely solvable Anderson model, which undergoes an Anderson localization (AL) transition.
Réfs: * Inés Rodríguez-Arias, Markus Müller, Alberto Rosso and Andrea De Luca, An exactly solvable model for Dynamic Nuclear polarization, preprint cond-mat arXiv:1703.05416. * Andrea De Luca, Inés Rodríguez-Arias, Markus Müller and Alberto Rosso, Thermalization and many-body localization in systems under dynamic nuclear polarization, Phys. Rev. B 94, 014203 (2016)​.

# Séminaire du LPTMS: Duncan O'Dell *** séminaire exceptionnel ***

## Quantum catastrophes

### Duncan O’Dell (McMaster University)

Catastrophe theory provides a unified description of a broad range of singularities and defects in fields and has been applied extensively in optics. A key idea is that of scale: at large scales the catastrophe appears singular but at smaller scales it is smoothed, e.g. by wave interference. In 2004 Michael Berry and Mark Dennis suggested that waves might themselves display singularities which are only smoothed by the fundamental discreteness of quantum field excitations (e.g. photons). In this talk I will give examples of such “quantum catastrophes” appearing in the dynamics of simple quantum systems such as a Josephson junction made from two BECs following a quench.  I will emphasize that, owing to the structural stability of catastrophes and their scaling properties, quantum catastrophes represent a universal aspect of quantum dynamics.

# Séminaire FAST-LPTMS: Mark Hoefer

## The hydraulic analogy to superfluid-like dispersive hydrodynamics

### Mark Hoefer (Department of Applied Mathematics, University of Colorado,  Boulder, USA)

The classic "hydraulic analogy" between supercritical shallow water flows and gas dynamics was leveraged in the early to mid 20th century as a means to model the flow patterns around wings with a relatively simple water table. But this analogy breaks down when water wave dispersion is strong relative to dissipative effects as in oscillatory, undular bores. In this regime, the more appropriate analogy is to dispersive hydrodynamic media such as superfluids and nonlinear optical diffractive or dispersive patterns, where dispersive shock waves (DSWs), also called quantum and optical shock waves, have been observed. In this talk, the problem of steady pattern formation in supercritical, shallow water flows will be examined theoretically and experimentally. Two new, non-classical types of oscillatory DSWs will be identified within the context of the fifth order Kawahara equation resulting from a balance between capillarity and gravity at water depths on the order of 0.5 cm. Experiments with a shallow water table demonstrate their physical existence and properties. The results presented have implications for dispersive hydrodynamic media with non-convex linear dispersion curvature such as ultracold atomic superfluids and nonlinear fiber optics. Voir l'annonce du FAST: https://semmeca.limsi.fr/affiche/res17_15.html

************************* ATTENTION **************************** l'exposé est donné au FAST : accès, cf. http://www.fast.u-psud.fr/fr/plan ******************************************************************

# Séminaire du LPTMS: Salvatore Manmana

## Finite-temperature dynamics in low-dimensional quantum magnets

### Salvatore Manmana (University of Göttingen, Germany)

Quantum magnetic materials are a fascinating research topic, which  explores novel behavior in quantum systems caused by strong  correlations. In my talk, I will review basic properties of such strongly correlated quantum magnets. A particular focus is given to  dynamical properties, which reveal the elementary excitations and their  properties. An interesting aspect here is the effect of temperature.  This is discussed for one-dimensional systems. In particular, the finite-temperature dynamics in the antiferromagnetic spin-one chain is discussed, which considerably is one of the most fundamental quantum many-body systems, with symmetry protected topological order in the ground state. Increasing the temperature leads to additional structures  in the dynamical response functions, which we propose to measure in experiments. Reference :
• Jonas Becker, Thomas Köhler, Alexander C. Tiegel, Salvatore R. Manmana, Stefan Wessel and Andreas Honecker, Finite-temperature dynamics and thermal intra-band magnon scattering in Haldane spin-one chains,  preprint cond-mat arXiv:1703.04652

# Physics-Biology interface seminar: Ivo Sbalzarini

## Investigating embryogenesis using numerical simulations of biophysics

### Ivo Sbalzarini (Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany)

#### Abstract:

Development and morphogenesis of tissues, organs, and embryos emerges from the collective self-organization of cells that communicate though chemical and mechanical signals. Decisions about growth, division, and migration are taken locally by each cell based on the collective information. In this sense, a developing tissue is akin to a massively parallel computer system, where each cell or processor computes robust local decisions, integrating communication with other cells/processors. Mechanistically understanding and reprogramming this system is a grand challenge. Our vision is to develop a virtual computer model of a developing embryo, incorporating the known biochemistry and biophysics into a computational model in 3D-space and time, in order to understand the information-processing aspects of development on an algorithmic basis. While the “hardware” (proteins, lipids, etc.) and the “source code” (genome) are increasingly known, we known virtually nothing about the algorithms that this code implements on this hardware. Using examples from our work, I outline our roadmap toward a virtual embryo, and highlight challenges along the way. These range from globally optimal approaches to image analysis, to novel languages for parallel high-performance computing, to virtual reality and real-time graphics for 3D microscopy and numerical simulations of biochemical and biomechanical models. This cooperative interdisciplinary effort contributes to all involved disciplines.

#### Bio:

Ivo Sbalzarini is the Chair of Scientific Computing for Systems Biology on the faculty of computer science of TU Dresden, and director of the TUD-Department in the Center for Systems Biology Dresden. He also is a permanent Senior Research Group Leader with the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden. He graduated in Mechanical Engineering from ETH Zurich in 2002 (Willi Studer Award). He completed his doctorate in computer science in 2006 at ETH Zurich (Chorafas Award, Weizmann Institute of Science), where he formed a close collaboration between biology and computer science. In 2006, he was named Assistant Professor for Computational Science in the Department of Computer Science of ETH Zurich. In 2012, Ivo and his group moved to Dresden, where he became one of the founding members of the new Max Planck Center for Systems Biology and the TU-Dresden Chair of Scientific Computing for Systems Biology. He also serves as a co-leader of the biological systems path of the Center for Advancing Electronics Dresden, Dean of the International Max Planck Research School in Cell, Developmental, and Systems Biology, and Vice-Dean of the Faculty of Computer Science.

# Séminaire du LPTMS: Mark Hoefer (lecture n°1)

## Lectures on Whitham Modulation Theory and Dispersive Hydrodynamics (1)

### Mark Hoefer (Department of Applied Mathematics, University of Colorado,  Boulder, USA)

Nonlinear wave modulation theory, developed by G. B. Whitham over 50  years ago, is a powerful mathematical tool to investigate dispersive  hydrodynamics.  Dispersive hydrodynamics encompass fluid and fluid-like  media in which nonlinear, hydrodynamic phenomena (e.g., shock formation  and expansion waves) are influenced more prominently by wave dispersion  than by irreversible, dissipative processes. Examples include  superfluids, intense light propagation through a nonlinear medium, and  the interface between two classical fluids.  A familiar feature of such  media includes the solitary wave or soliton, whose width represents the  characteristic coherence length of the medium, e.g., the so-called  healing length of a Bose-Einstein condensate.  Whitham theory is used to study modulations of nonlinear waves on a scale much larger than the medium's coherence length.  It has been successfully used to describe  the most fundamental object in dispersive hydrodynamics, a dispersive  shock wave. These lectures will introduce the listener to the basic theory of  Whitham with applications to several modern examples.  Mathematically,  the Whitham modulation equations are a system of first order,  quasi-linear partial differential equations.  Properties of these  hydrodynamic type systems such as (strict) hyperbolicity, ellipticity,  genuine nonlinearity, and simple wave solutions will be elucidated with  a view toward understanding the physical implications of these abstract  concepts.  Fundamental problems in dispersive hydrodynamics such as the  Riemann problem and the piston problem will be described. The theory  will be sufficiently developed to describe a new type of hydrodynamic  interaction where a soliton coherently interacts with a hydrodynamic  flow, termed hydrodynamic soliton tunneling.

# Séminaire du LPTMS: Alexios Polychronakos

## Duality, Solitons and Hydrodynamics in Generalized Calogero Models

### Alexios Polychronakos (The City College and Graduate Center of the CUNY, New York)

I will present a first-order formulation of the Calogero model in external potentials in terms of a generating function, which simplifies the derivation of its dual form. Solitons naturally appear in this formulation as particles of negative mass. Using this method I derive the dual form of Calogero particles in external quartic, trigonometric and hyperbolic potentials, which were known to be  integrable but had no known dual formulation. I will also derive the fluid mechanical formulation of these models and outline the corresponding fluid mechanical soliton solutions.

# Séminaire du LPTMS: Mark Hoefer (lecture n°2)

## Lectures on Whitham Modulation Theory and Dispersive Hydrodynamics (2)

### Mark Hoefer (Department of Applied Mathematics, University of Colorado,  Boulder, USA)

Nonlinear wave modulation theory, developed by G. B. Whitham over 50  years ago, is a powerful mathematical tool to investigate dispersive  hydrodynamics.  Dispersive hydrodynamics encompass fluid and fluid-like  media in which nonlinear, hydrodynamic phenomena (e.g., shock formation  and expansion waves) are influenced more prominently by wave dispersion  than by irreversible, dissipative processes. Examples include  superfluids, intense light propagation through a nonlinear medium, and  the interface between two classical fluids.  A familiar feature of such  media includes the solitary wave or soliton, whose width represents the  characteristic coherence length of the medium, e.g., the so-called  healing length of a Bose-Einstein condensate.  Whitham theory is used to study modulations of nonlinear waves on a scale much larger than the medium's coherence length.  It has been successfully used to describe  the most fundamental object in dispersive hydrodynamics, a dispersive  shock wave. These lectures will introduce the listener to the basic theory of  Whitham with applications to several modern examples.  Mathematically,  the Whitham modulation equations are a system of first order,  quasi-linear partial differential equations.  Properties of these  hydrodynamic type systems such as (strict) hyperbolicity, ellipticity,  genuine nonlinearity, and simple wave solutions will be elucidated with  a view toward understanding the physical implications of these abstract  concepts.  Fundamental problems in dispersive hydrodynamics such as the  Riemann problem and the piston problem will be described. The theory  will be sufficiently developed to describe a new type of hydrodynamic  interaction where a soliton coherently interacts with a hydrodynamic  flow, termed hydrodynamic soliton tunneling.

# Séminaire du LPTMS: Alexander Ossipov

## Scattering approach to Anderson localisation

### Alexander Ossipov (School of Mathematical Sciences, University of Nottingham, UK)

I will introduce a new approach to Anderson localisation problem, which is based on the scattering formalism. The first part of the talk will  review a derivation of some known results for the statistics of Wigner delay times and wavefunction amplitudes in the one-dimensional Anderson  model. Then I will explain how this method can be generalised to higher  dimensions. In particular, the density of the Wigner delay times will be  derived for the two-dimensional Anderson model.

# Soutenance de thèse : Aleksey Fedorov

## Non-conventional many-body phases in ultracold dipolar systems

### Aleksey Fedorov

The problem of revealing and describing novel macroscopic quantum states characterized by exotic and non-conventional properties has the fundamental importance for modern physics. Such states offer fascinating prospects for potential applications in quantum information processing, quantum simulation, and material research. In the present Thesis we develop a theory for describing non-conventional phases of ultracold dipolar gases. The related systems of large-spin atoms, polar molecules, and dipolar excitons in semiconductors are actively studied in experiments. We put the main emphasis on revealing the role of the long-range character of the dipole-dipole interaction. We consider the effect of rotonization for a 2D weakly interacting gas of tilted dipolar bosons in a homogeneous layer, and demonstrate that in contrast to the case of perpendicular dipoles, in a wide range of tilting angles the condensate depletion remains small even when the roton minimum is extremely close to zero. We predict the effect of rotonization for a weakly correlated Bose gas of dipolar excitons in a semiconductor layer and calculate the stability diagram. According to our estimates, the threshold of the roton instability for a bose-condensed exciton gas with the roton-maxon spectrum is achievable experimentally in semiconductor layers. We then consider p-wave superfluids of identical fermions in 2D lattices. The optical lattice potential manifests itself in an interplay between an increase in the density of states on the Fermi surface and the modification of the fermion-fermion interaction (scat- tering) amplitude. The density of states is enhanced due to an increase of the effective mass of atoms. For short-range interacting atoms in deep lattices the scattering amplitude is strongly reduced compared to free space due to a small overlap of wavefunctions of fermions sitting in the neighboring lattice sites, which suppresses the p-wave superfluidity. However, we show that for a moderate lattice depth there is still a possibility to create p-wave superfluids with sizable transition temperatures. For fermionic polar molecules, due to a long-range character of the dipole-dipole interaction the effect of the suppression of the scattering amplitude is absent. It is shown that for microwave-dressed polar molecules a stable topological p+ip superfluid may emerge in the 2D lattice at realistic temperatures. Finally, we discuss another interesting novel superfluid of fermionic polar molecules. It is expected in a bilayer system, where dipoles are oriented perpendicularly to the layers and in opposite directions in different layers. We demonstrate the emergence of interlayer superfluid pairing. In contrast to the already known s-wave interlayer superfluid, when all dipoles are parallel to each other, in our case the s-wave pairing is suppressed and there can be p-wave or higher partial wave superfluids.

Directeurs de thèse: Georgy Shlyapnikov

Jury: Christophe Texier, Mikhail Baranov, Paolo Pedri, Andrey Varlamov

# Séminaire du LPTMS: Thomas Barthel

## Entanglement and computational complexity for 1D quantum systems at finite T

### Thomas Barthel (Duke University, USA)

Using the replica trick in 1+1 dimensional quantum field theory, one can show that the cost for the classical simulation of one-dimensional quantum many-body systems at finite temperatures grows only polynomially with the inverse temperature and is system-size independent - even for gapless systems. In particular, I will show that the thermofield double state (TDS), a widely used purification of the equilibrium density operator, has a faithful matrix product state (MPS) representation. The argument is based on the scaling behavior of Rényi entanglement entropies in the TDS. At finite temperatures, they obey the area law. For gapless conformally invariant systems, the Rényi entropies are found to grow only logarithmically with inverse temperature. The field-theoretical results are complemented by exact and quasi-exact numerical computations for integrable as well as non-integrable spin chains, and the Bose-Hubbard model. This allows us to compare actual matrix product truncation dimensions with upper bounds derived from Rényi entropies. Finally, I will highlight some applications

# Séminaire du LPTMS: Vladimir Gritsev *** séminaire exceptionnel ***

## Integrability and supersymmetry in quantum optics

### Vladimir Gritsev (University of Amsterdam)

First I will give an overview of quantum integrable models used in quantum optics. Then I will discuss some new methods recently developed by us for solving quantum dynamics in various nonlinear models of quantum optics. Finally I am going to talk about supersymmetry in quantum optics and spin-orbit coupled systems.

# Séminaire du LPTMS: Raoul Santachiara

## Riemann-Hilbert problem in 2D Conformal Field Theory

### Raoul Santachiara (LPTMS, Université Paris-Sud)

The Riemann-Hilbert problem asks whether the global properties (i.e. their the analytic continuation) of a space of holomorphic functions allows to determine uniquely the  linear ordinary complex differential equation (of Fuchsian type) of which they are solutions.  The answer is positive for a class of  equations that are called rigid systems. We have recently observed that the rigid systems describe a class of fundamental equations that lie in the heart of the 2D conformal field theories. Moreover we used for the first time a procedure developed in the last decades for the solutions of rigid systems to provide new results in CFTs.  In this talk, I will try to introduce the (beautiful) mathematics behind the Fuchsian systems, and I will try to give you an idea of the reason why these equations are related to the conformal symmetry.

# Séminaire du LPTMS: Sergio Ciliberto

## The Maxwell demon and Landuer's principle: from gedanken to real experiments

### Sergio Ciliberto (Laboratoire de Physique, ENS de Lyon)

During this talk we will recall the connections between information and thermodynamics. We will then discuss a specific example of the measure of the Landauer's bound. Rolf Landauer argued that the erasure of information is a dissipative process . A minimal quantity of heat, proportional to the thermal energy, is necessarily produced when a classical bit of information is deleted. A direct consequence of this logically irreversible transformation is that the entropy of the environment increases unavoidably by a finite amount. We experimentally show the existence of the Landauer bound in a generic model of a one-bit memory. Using a system of a single colloidal particle trapped in a modulated double-well potential, we establish that the mean dissipated heat saturates at the Landauer bound in the limit of long erasure cycles. This result demonstrates the intimate link between information theory and thermodynamics. For a memory erasure procedure, which is a logically irreversible operation, a detailed Jarzynski Equality is verified, retrieving the Landauer limit independently of the work done on the system. References :
• Experimental verification of Landauer’s principle linking information and thermodynamics, A. Bérut, A. Arakelyan, A. Petrosyan, S. Ciliberto, R. Dillenschneider, E. Lutz, Nature 483, 187–189 (08 March 2012)
• Detailed Jarzynski Equality applied to a Logically Irreversible Procedure, A. Bérut, A. Petrosyan, S. Ciliberto, EPL 103, 60002 (2013)
• Information: From Maxwell’s demon to Landauer’s eraser, Eric Lutz, Sergio Ciliberto, Physics Today 68 (9), 30 (2015)

# Séminaire du LPTMS: Antonin Coutant

## Mimicking black holes in fluids

### Antonin Coutant (University of Nottingham, UK)

I will discuss the possibility to mimic black hole physics in fluid flows. The starting point is an analogy discovered by Unruh between the propagation of sound in a flowing fluid and waves around a black hole. I will discuss the analogue of the Hawking effect through which a black hole loses its mass, and its recent experimental verifications. I will also present a recent water wave experiment, where we have observed the analogue of black hole superradiance, that is, the amplification of waves by extraction of angular momentum from a rotating flow

# Soutenance de thèse: Stefano Spigler

## Avalanches in glassy systems

### Stefano Spigler

Beaucoup de systèmes qui ont un certain degré de désordre ont des similarités dans leur structure : le paysage énergétique est aléatoire et il a plusieurs minima locaux de l’énergie. Quand on ajoute une petite perturbation externe au système à basse température, il est raisonnable d’attendre que la dynamique conduira le système d’un minimum à l’autre, et ça donne lieu à une réponse aléatoire et saccadée. Les sautes discontinus que l’on observe sont appelés avalanches, et l’intérêt de ce travail est le calcul de leur distribution. Un des résultats est en effet le développement d’un cadre pour calculer cette distribution dans des systèmes en dimension infinie qui peuvent être décrits avec le replica symmetry breaking. Nous appliquons les résultats à l’un des modèles les plus simples des verres structuraux, c’est à dire les empilements denses de sphères molles avec répulsion harmonique, avec une déformation (shear strain) du volume comme perturbation. Nous soutenons que, quand la déformation est suffisamment petite, une portion de la distribution des avalanches devient une loi de puissance, dont l’exposant peut être directement lié au paramètre d’ordre de la brisure de symétrie de replica. Cet exposant est également lié à la distribution des forces de contact (au moins entre certaines sphères), dont le comportement asymptotique on sait que ne dépend pas fortement de la dimension spatiale ; pour cette raison nous comparons les prédictions de champ moyen en dimension infinie avec des simulations du même système en dimension trois et, remarquablement, on trouve un bon accord. Dans le reste de la thèse nous discutons aussi les similarités avec des travaux précédents et quelques conséquences que la distribution des avalanches donne sur les propriétés élastiques de la matière granulaire dense.

Directeur de thèse: Silvio Franz

Jury: Jean-Louis BARRAT, Giulio BIROLI, Chiara CAMMAROTA, Giuseppe FOFFI, Tommaso RIZZO, Matthieu WYART

# Soutenance de thèse: Thibault Congy

## Fluctuations non-linéaires dans les gaz quantiques à deux composantes

### Thibault Congy

Cette thèse est dédiée à l'étude des fluctuations non-linéaires dans les condensats de Bose-Einstein à deux composantes. Dans un régime de champ moyen, la dynamique des deux espèces du condensat est régie par deux équations de Gross-Pitaevskii couplées et on montre que le spectre des excitations se sépare en deux modes de propagation distincts : un mode dit de densité correspondant au mouvement global des atomes à l'intérieur du condensat et un mode dit de polarisation correspondant au mouvement relatif entre les deux espèces constituant le condensat.

Cette séparation en une dynamique globale et une dynamique relative persiste en présence d'un couplage cohérent entre les deux espèces, et on présentera dans une première partie les équations gouvernant la propagation des excitations non-linéaires du mode de polarisation en présence d'un couplage de Rabi. En particulier on analysera la stabilité modulationnelle de ce mode à l'aide de la méthode des échelles multiples et on montrera que les excitations de polarisation, au contraire des excitations de densité, souffrent d'une instabilité de Benjamin-Feir.

Dans une deuxième partie, on présentera la dynamique de polarisation du condensat proche de la limite dite de Manakov. Plus particulièrement dans le cas où les coefficients d'interaction inter-et-intra espèces sont proches, la dynamique de polarisation se révèle être régie par une équation de Landau-Lifshitz sans dissipation. Les équations de Landau-Lifshitz appartiennent à une hiérarchie d'équations intégrables (hiérarchie Ablowitz-Kaup-Newell-Segur) et on peut déterminer les solutions à une phase à l'aide de la méthode d'intégration "finite-gap". Profitant de l'intégrabilité du système, on résoudra notamment le problème de Riemann à l'aide de la théorie de modulation de Whitham et on montrera que les condensats à deux composantes peuvent propager des ondes de raréfaction ainsi que des ondes de choc dispersives.

Directeur de thèse: N. Pavloff Jury: F. Chevy, B. Gallet, S. Gavrilyuk, C. Josserand, C. Nore, F. Lagoutière

# Séminaire du LPTMS: Davide Squizzato

## KPZ Physics in Exciton Polaritons Systems

### Davide Squizzato (LPMMC, Grenoble)

The experimental demonstration of Bose-Einstein condensation of exciton polaritons gases in driven-dissipative conditions [1] has given rise to several questions on the nature and the characterization of out-of-equilibrium bosonic systems under pump and dissipation. Such systems can be theoretically described by a generalised Gross-Pitaevskii equation (gGPE) in which complex coefficients and noise enrich the equilibrium picture. An analytical mapping between gGPE and the Kardar-Parisi-Zhang (KPZ) equation has been demonstrated at long wavelength if fluctuations of the amplitude of the condensate are negligible with respect to the ones of the phase field. Hence, one expects the long-distance properties of driven-dissipative condensates to pertain to the KPZ universality class. A numerical verification of KPZ scaling was given in (1+1)dimensions [2]; however, the experimental accessibility of the KPZ mapping is still under debate. In this work we use both Keldysh field-theoretical approach and numerical simulations to investigate the effects of inhomogeneities typical of experimental set-ups and study their effects on the universal properties of driven-dissipative polaritons condensates; we furthermore look at the real-time statistics of the phase in both homogeneous and in-homogeneous case in order to better understand the nature of KPZ behaviour, i.e. the geometrical sub-class to which it belongs to. References:
1. [1] J. Kasprzak et al., “Bose-Einstein condensation of exciton polaritons”, Nature 443, pp. 409–414, Sep 2006.
2. [2] L. He, L. M. Sieberer, E. Altman, and S. Diehl, “Scaling properties of one-dimensional driven-dissipative condensates”, Phys. Rev. B 92, 155307, Oct 2015.

# Soutenance de thèse: Maxime Sevelev

## Phase diagram, jamming and glass transitions in the non-convex perceptron

### Maxime Sevelev

This thesis treats the «spherical perceptron model», a simple exactly solvable model for glassy behavior and jamming suitably generalized to negative values of scalar product parameter κ. The classical machine-learning problem of random pattern classification by the perceptron is a convex constraint satisfaction problem (CSP). Even when the «stability parameter» κ of the model becomes negative, the problem still makes sense and can be interpreted as the problem of particles on an N-dimensional sphere trying to avoid randomly placed obstacles. In this case, the corresponding CSP is non-convex. This thesis studies the problem in detail in the non-convex domain. Systematic study is made possible by assigning to a constraint satisfaction problem its corresponding optimization version endowed with a Hamiltonian function (cost function) quantifying the violations of the constraints, as a function of the system's configuration. The connection between random CSP and glassy phenomenology in physics is well known and has been explored in detail for models with discrete variables. The presence of continuous variables in the (spherical) perceptron model enables us to unveil, in random CSP, the characteristic SAT/UNSAT transition where the system transits from the satisfiable regime (where the ground state has zero energy) to the unsatisfiable one (where the ground state energy is positive). This phase transition can also be interpreted as a jamming transition similar to the one that exhibit models with frictionless spheres. The simplicity of the considered model allows the exact determination of the zero temperature phase diagram as a function of the control parameters: the density of obstacles and their size. In the present thesis, the jamming transition thus identified is completely characterized and several glass phases of stable and marginal character are studied in detail.

Keywords: spin glass, phase transition, disordered system, jamming Directeur de thèse: S. Franz Jury: R. Mulet, F. Ricci.tersenghi, V. Lecomte, M. Tarzia, V. Terras, C. Texier.

# Séminaire du LPTMS: Vladimir Gritsev

## Integrable Floquet Dynamics

### Vladimir Gritsev (University of Amsterdam)

I will discuss several classes of integrable Floquet systems, i.e. systems which do not exhibit chaotic behavior even under a time dependent perturbation. The first class is associated with finite-dimensional Lie groups and infinite-dimensional generalization thereof. The second class is related to the row transfer matrices of the 2D statistical mechanics models. The third class of models, called the "boost models", is constructed as a periodic interchange of two Hamiltonians - one is the integrable lattice model Hamiltonian, while the second is the boost operator. The latter for known cases coincides with the entanglement Hamiltonian and is closely related to the corner transfer matrix of the corresponding 2D statistical models. As an interesting application of the boost models I discuss a possibility of generating periodically oscillating states with the period different from that of the driving field. In particular, one can realize an oscillating state by performing a static quench to a boost operator. We term this state a "Quantum Boost Clock". All analyzed setups can be readily realized experimentally, for example in cod atoms.

# Séminaire du LPTMS: Thierry Jolicoeur

## Quantum coherence in bilayer graphene in the quantum Hall regime

### Thierry Jolicoeur (LPTMS, Université Paris-Sud)

Bilayer graphene under a magnetic field has a central octet of quasi-degenerate Landau levels due to spin, valley, and orbital degeneracies. This set of zero-energy Landau level is resolved into several incompressible states whose detailed nature is still elusive. A Hartree-Fock treatment of a realistic tight-binding four-band model can be used to understand the quantum ferromagnetism phenomena expected for integer fillings of the octet levels.  I will discuss various phase diagrams as a function of applied bias and magnetic field. In some corners of these diagrams are interesting phases with quantum coherence involving valley or orbital degrees of freedom. Reference:

# Physics-Biology interface seminar: Paul François

## Untangling the biological hairball of immune recognition networks

### Paul François (McGill University, Canada)

Complex mathematical models of interaction networks are routinely used for prediction in systems biology. However, it is difficult to reconcile network complexities with a formal understanding of their behavior. I will introduce several models of immune recognition by T cells and will show how a simple procedure can be used to reduce them to functional submodules, using statistical mechanics of complex systems combined with a fitness-based approach inspired by in silico evolution. Our procedure works by putting parameters or combination of parameters to some asymptotic limit, while keeping (or slightly improving) the model performance, and requires parameter symmetry breaking for more complex models. An intractable model of immune recognition with close to a hundred individual transition rates is reduced to a simple two-parameter model, and connected to the adaptive sorting" principle that we previously identified and experimentally validated. Our procedure extracts three different mechanisms for early immune recognition, and automatically discovers similar functional modules in different models of the same process allowing for model classification and comparison.

# Séminaire du LPTMS: Cécile Repellin

!!!! jour et horaire inhabituels !!!!

## Numerical investigation of gapped edge states in fractional quantum Hall-superconductor heterostructures

### Cécile Repellin (MPI, Dresden, Allemagne)

The possibility of realizing anyons -- quasiparticles with fractional  exchange statistics -- is an exciting prospect in the field of  interacting topological phases. Non-abelian anyons, whose exchange is  characterized by a matrix rather than a simple phase, are of the most  exotic kind. They are highly sought after as they could be used as  qubits for quantum computation intrinsically immune to decoherence. While non-abelian anyons are expected to appear in the fractional  quantum Hall effect, engineering systems that purposefully favor their  emergence might be a better strategy to probe their properties. Progress  in creating quantum devices with Majorana bound states represents an  important achievement in this direction. Realizing the even more elusive  parafermions, which unlike Majoranas could support universal quantum  computation, would be an exciting next step. Parafermion states have been envisioned to occur in heterostructures  using fractional quantum Hall (FQH) states and superconductors. Existing  theoretical studies of these systems have been entirely based on  effective theories of fractional quantum Hall edge states which cannot  offer any quantitative analysis of competing energy scales, correlation  lengths etc. To fill this gap, we propose and implement a numerical  setup for studying edge states of FQH droplets with a superconducting  instability. Using exact diagonalization, we report the first  observation in a fully microscopic model of gapped FQH edge modes with  topological degrees of freedom. The topological nature of these modes is  probed through fractional Josephson effect and flux insertion, and  evidence the cooper pairing of fractionalized quasiparticles.

# Séminaire du LPTMS: Mauricio Pato

## Pseudo-Hermitian random matrix ensembles

Mauricio Pato (Universidade de São Paulo, Brazil) In my talk, I will revise results obtained in an effort to construct ensemble of random matrices which are connected with their adjoint by a similarity transformation. Matrices satisfying this condition have eigenvalues real or complex conjugate and are called pseudo-Hermitian. This condition arose in studies of systems whose Hamiltonians are PT-symmetric invariant, that is, Hamiltonians which though not invariant under the time-reversl and the parity transformations performed isolatedly become invariant when these transformations are combined.

# Séminaire du LPTMS: Martin Evans

## Model of Antibiotic Action on Bacterial Growth

### Martin Evans (University of Edinburgh, UK)

In this talk I will describe a simple  model for the growth of a  bacterial population under the challenge of ribosome-targetting  antibiotics. I shall endeavour to motivate why statistical physics lends itself to the study of bacterial dynamics. The model I will   discuss is statistical physics-like in that it makes a coarse-grained description of the growth process, reduced to three variables within the bacterial cell - the antibiotic concentration, the concentration of ribosomes bound to antibiotics and the concentration of unbound ribosomes. Remarkably the model can explain several observations concerning   antibiotic action and bacterial growth rate. In particular the growth-dependent bacterial susceptibility is controlled by a single, "universal" parameter and the extreme behaviours correspond to the phenomenological classification into bactericidal and bacteriostatic antibiotics. The predictions of the model are backed up by experimental studies.

# Séminaire du LPTMS: Danny Baillie

## Dipolar droplets

### Danny Baillie (University of Otago, New Zealand)

Experiments with Bose-Einstein condensates of dysprosium [1] and erbium [2] atoms have observed the formation of dilute quantum gas droplets that can preserve their form, even in the absence of any external confinement [3]. These droplets are stabilized by Lee-Huang-Yang (LHY) quantum fluctuation corrections to meanfield theory [4]. We discuss the regimes in which such self-bound droplet states are stable [5], and examine the properties of their collective excitations. Notably we observe that the elongated filament-shaped droplets act as a quasi-one-dimensional waveguide along which low angular momentum phonons propagate [6].

References [1] H. Kadau, M. Schmitt, M. Wenzel, C. Wink, T. Maier, I. Ferrier-Barbut, and T. Pfau, Nature 530, 194 (2016). [2] L. Chomaz, S. Baier, D. Petter, M. J. Mark, F. Wächtler, L. Santos, and F. Ferlaino, Phys. Rev. X 6, 041039 (2016). [3] M. Schmitt, M. Wenzel, F. Böttcher, I. Ferrier-Barbut, and T. Pfau, Nature 539, 259 (2016). [4] T. D. Lee, K. Huang, and C. N. Yang, Phys. Rev. 106, 1135 (1957). [5] D. Baillie, R. M. Wilson, R. N. Bisset, and P. B. Blakie, Phys. Rev. A 94, 021602(R) (2016). [6] D. Baillie, R. M. Wilson, and P. B. Blakie, arXiv:1703.07927.

# Séminaire du LPTMS: Giacomo Mazza

## Strong light-matter coupling: transient superconductivity in optical stimulated systems and electronic properties inside optical cavities

### Giacomo Mazza (CPhT, Ecole Polytechnique, Université Paris-Saclay)

The interaction between light and matter has been intensively used for probing the properties of condensed matter systems. In the last years, this paradigm evolved and the light-matter interaction has become a tool for strongly modify the systems properties and accessing states of matter that are not present (or hidden) in equilibrium conditions. In this talk I will discuss two different examples.
In the first case I will consider the optical stimulation of solids by means of strong light pulses. A series of recent experiments reported the observations of light-induced modification of the electronic properties suggestive of the formation of transient superconducting states. Remarkably these transient states extend far above the equilibrium critical temperatures and appear in different compounds, as cuprates and organics superconductors (fullerides) [1,2]. I will discuss from the theoretical point of view possible realisations of such transient superconductivity in simplified models for correlated superconductors. In particular, I will start from the general case of superconductivity induced by a strong attractive interaction [3] and then move the more specific case of fullerides [4].
As a second example I will consider the electronic properties of materials placed inside high-finesse optical cavities, where the vacuum electromagnetic field strongly interact with a given electronic transition. I will discuss how the electronic properties are modified by the light-matter coupling and how the latter can be used to tune the electronic transport properties.

References:
• [1] D. Fausti et al., Light-induced superconductivity in a stripe-ordered cuprate, Science 331, 189 (2011)
• [2] M. Mitrano et al., Possible light-induced superconductivity in K3C60 at high temperature, Nature 530, 461 (2016)
• [3] G. Mazza, From sudden quench to adiabatic dynamics in the attractive Hubbard model, arXiv:1708.01096
• [4] G. Mazza and A. Georges, Nonequilibrium superconductivity in driven alkali-doped fullerides, Phys.Rev.B 96, 064515 (2017).

# Physics-Biology interface seminar: Felix Rico

## Molecular to cellular mechanics probed by high-speed atomic force microscopy

### Felix Rico (Aix-Marseille Université)

The mechanical properties of individual proteins, filaments, and supramolecular assemblies provide structural stability and mechanical flexibility to the living cell. Thus, molecular understanding of the mechanics from the single molecule to the whole cell is relevant to understand biological function. High-speed atomic force microscopy (HS-AFM) is a unique technology that combines nanometric-imaging capabilities at video rate. In this talk, I will present our recent applications of HS-AFM to probe protein and cellular mechanics. In the first part, I will introduce the development of high-speed force spectroscopy (HS-FS) to probe protein unfolding at the timescales of molecular dynamics simulations (1). This provides a unique approach to acquire atomistic understanding of biomolecular processes based on experimental results. In the second part, I will present our recent work on the adaptation of HS-AFM to probe the microrheology of living cells at high frequencies (up to 100 kHz), revealing cytoskeletal dynamics (2). We show that the mechanical response at high frequencies depends on the actin filament tension and pathological state of the cell. Microrheology over a wide dynamic range—up to the frequency characterizing the molecular components—provides a mechanistic understanding of cell mechanics.

1. F. Rico, L. Gonzalez, I. Casuso, M. Puig-Vidal, S. Scheuring, High-Speed Force Spectroscopy Unfolds Titin at the Velocity of Molecular Dynamics Simulations. Science 342, 741 (2013).

2. A. Rigato, A. Miyagi, S. Scheuring, F. Rico, High-frequency microrheology reveals cytoskeleton dynamics in living cells. Nat Phys 13, 771 (2017).

# Séminaire exceptionnel du LPTMS: Natalia Menezes Silva Da Costa

## Quantum field theory in low-dimensional condensed-matter systems

### Natália Menezes Silva Da Costa (Utrecht University)

In this talk I will present two examples of how quantum field theories may be applied to describe the long wavelength regime of condensed-matter systems. The first example [1] concerns the study of electronic interactions on the boundary of a two-dimensional time-reversal-invariant topological insulator. While the bulk of this two-dimensional material is insulating, the boundary exhibits propagating modes that may be described in terms of a one-dimensional Dirac theory.  By assuming that there is an underlying electromagnetic theory mediating the e-e interaction on the edges, and by employing a dimensional reduction procedure, I will show that the effective one-dimensional theory is a non-Fermi liquid, known as the helical Luttinger liquid (HLL).  This HLL resembles a theory of free bosons, however, with a parameter in its kinematics that indicates the strength of the e-e interactions. Within the quantum-field theoretical formalism, I will show that such parameter can be written in terms of the fine structure constant, which allows one not only to predict its value but also to manipulate the nature attractive/repulsive of the interaction. The second example [2] concerns the topological response of a fermionic model defined on the Lieb lattice in presence of an electromagnetic field. The tight-binding model is built in terms of three species of spinless fermions and supports a topological Varma phase due to the spontaneous breaking of time-reversal symmetry. In the low-energy regime, the emergent effective Hamiltonian coincides with the so-called Duffin-Kemmer-Petiau (DKP) Hamiltonian, which describes relativistic pseudospin-0 quasiparticles and goes beyond the commonly studied spin-1/2 Dirac/Weyl paradigm. By considering a minimal coupling between the DKP quasiparticles and an external Abelian gauge field, I will present both the Landau-level spectrum and the emergent Chern-Simons theory. The corresponding Hall conductivity reveals an atypical quantum Hall effect, which can be simulated in an artificial Lieb lattice. [1] N. Menezes, G. Palumbo and C. Morais Smith, Sci. Rep. 7, 14175 (2017). [2] N. Menezes, C. Morais Smith and G. Palumbo, arXiv:1710.07916 (2017).

# Séminaire du LPTMS: Alexios Polychronakos

## The 2017 Physics Nobel prize: the observation of gravitational waves

### Alexios Polychronakos (The City College and Graduate Center of the CUNY, New York, USA)

A sweeping review of the history, drama and excitement behind the recent observation of gravitational waves will be presented, including its human aspects, theoretical and technical challenges, physics and engineering, at a level appropriate for non-experts.