LPTMS Internship and PhD Proposal: Out-of-equilibrium dynamics of integrable open spin chains

Contact: Veronique Terras (veronique.terras@universite-paris-saclay.fr)

Video: Out-of-equilibrium dynamics of integrable open spin chains

Quantum spin chains constitute a rare example of quantum many body interacting systems where several important physical quantities can be exactly computed. This property (quantum integrability) together with numerous theoretical and experimental applications makes these systems a perfect testing ground for non-perturbative approaches in quantum field theory. One of the most intriguing (experimentally observable) features of quantum integrable systems including spin chains is their out-of-equilibrium behaviour. We propose to study, using integrability techniques, the out-of-equilibrium properties of open spin chains where a rapid change of a boundary magnetic field can produce a macroscopic change of the system. The goal of the internship is to learn the main methods of quantum integrability (algebraic Bethe Ansatz, separation of variables) and how these methods can be applied for the computation of scalar products and form factors in open spin chains. This first step will be necessary for the further study of the relaxation dynamics after a sudden change (quench) of a boundary magnetic field. This internship will ideally be followed by a PhD, in co-direction between V. TERRAS (LPTMS, Université Paris-Saclay) and N. KITANINE (professor, Université de Bourgogne). The location of the internship and PhD will be in LPTMS.

LPTMS Internship and PhD proposal: Rayonnement de Hawking dans les systèmes analogues

De nombreux systèmes au comportement fortement quantique hébergent des phénomènes non-linéaires. On a ainsi pu observer des solitons, des vortex, des ondes de choc et des murs de domaine dans des condensats de vapeurs atomiques ultra-froides, dans des systèmes de polaritons condensés en micro-cavités, dans l'hélium superfluide, dans des supra-conducteurs... Cette double caractéristique, quantique et non-linéaire, fait de ces systèmes des modèles permettant de mettre en évidence des eff ets tels le rayonnement de Hawking et/ou l'e ffet Casimir dynamique. Dans de telles études, les fluctuations quantiques de l'état fondamental du système (le "vide" de la théorie) peuvent être non triviales grâce à la possible non homogénéite du vide, qui est elle même sous-tendue par la nonlinéarité de l'onde décrivant le système.
La description théorique de certains systèmes permettant d'implémenter un analogue de trou noir a beaucoup progressé ces dernières années, au point qu'on peut maintenant obtenir une comparaison détaillée avec les résultats d'expériences récentes.
Un des aspects du travail de thèse consistera, apres s'être familiarisé avec le bagage théorique, à étudier les  fluctuations de la frontière séparant une région d'écoulement subsonique et une région d'écoulement supersonique dans un condensat de Bose-Einstein atomique. Ces  fluctuations, intrinsèquement nonlinéaires, résultent de l'émission quantique spontanée d'un rayonnement acoustique. Ce rayonnement est l'analogue sonique du rayonnement de Hawking induit par les fluctuations quantiques du vide au voisinage de l'horizon d'un trou noir gravitationnel.
Un deuxième axe de recherche consistera a proposer de nouvelles plateformes permettant de réaliser des analogues de trous noirs dans un contexte di erent de celui des vapeurs ultra-froides (optique nonlinéaire, systèmes magnétiques, ...) en ayant toujours en vue de possibles implémentations expérimentales.
Il s'agit donc d'une thèse théorique durant laquelle on s'attachera à décrire des systèmes susceptibles d'être étudiés expérimentalement. La thèse aura une composante numérique dont la proportion pourra varier en fonction des dispositions de l'étudiant et de la pertinence d'une description précise (au vu des possibles réalisations expérimentales). Le cadre théorique général est celui des équations aux dérivées partielles non-linéaires (décrivant la dynamique quasi-classique du système) et de la théorie quantique des champs (pour l'étude des fluctuations quantiques). Il est prévu que la thèse débute par un stage dont les modalités dépendent du parcours suivi par l'étudiant.

Contact: Nicolas Pavloff nicolas.pavloff@universite-paris-saclay.fr

LPTMS / PMMH ESPCI Internship and PhD Proposal: Stress reversal by a strong nonlinearity: an elastic sheet toy model

Living cells move thanks to nanometer-size molecular motors whose forces are transmitted up to the scale of the cell by a fiber network known as the cytoskeleton. On much larger length scales, individual cells generate forces that are similarly transmitted to the tissue level through the fibrous extracellular matrix. While the biology of these processes is rather well characterized, the simple problem of force transmission through these highly nonlinear elastic media is far from trivial, and leads to a conversion of local extensile forces to contractile stresses, with crucial biological implications.
To better understand this surprising physical behavior, we will set up a model force transmission experiment where the role of the nonlinear elastic medium will be played by a thin plastic sheet floating on water. By locally exerting extensile forces at the center of the sheet by inflating a balloon, we will directly observe how the forces are rectified through the wrinkling of the sheet. The goal is to help explain why the cytoskeleton is always contractile despite containing a significant number of extensile motors, and to inspire the design of counter-intuitive materials that contract when they should extend.

Expected skills: The student will have a taste for experimental physics. He/She will set up and run a model experiment, and participate in the theoretical analysis of the measurements.

Contact: Martin Lenz, Etienne Reyssat, José Bico, Benoît Roman / @: prenom.nom@espci.fr

Internship location: barre Cassan A, campus Jussieu, 75005 Paris


LPTMS Internship and PhD Proposal: Frustrated self-assembly with multiple particle types

Self-organization is key to the function of living cells – but sometimes goes wrong! In Alzheimer’s and many other diseases, normally soluble proteins thus clump up into pathological fiber-like aggregates. While biologists typically explain this on the grounds of detailed molecular interactions, we have started proving that such fibers are actually expected from very general physical principles. We thus show that geometrical frustration builds up when mismatched objects self-assemble, and leads to non-trivial aggregate morphologies, including fibers.
While we have shown that collections of identical particles form aggregates of various dimensionalities, realistic biological examples often involve multiple proteins. We will thus investigate how collections of several types of different particles typically interact and interfere. Our study will first consist in developing multi-geometries variants of the lattice-based numerical model presented in the illustration. We will then ask whether species with different geometries tend to phase separate, or conversely whether the mutiplicity of interactions they offer eases geometrical frustration and favors co-assembly. We will also wonder how this combinatorics affects the dimensionality of the aggregates, and whether we can identify generic features of the particles that distinguish between the two scenarios. We will then conduct off-lattice simulations to assess the robustness of these scenarios. Finally, we will attempt to construct a mean-field theory describing the co-assembly of a large variety of particles (> 10 or so) thus revealing the interplay between frustration and combinatorial freedom in self-assembly.
Beyond protein aggregation, this project opens investigations into a new class of “disordered” systems where the disorder is carried by each identical particle, as opposed to sprinkled throughout the system. This will help define the much-debated notion of frustration in dilute systems. This project will be conducted in collaboration with Pierre Ronceray (Turing Center for Living Systems, Marseille), who will co-direct a possible PhD project.

Expected skills:
A taste for statistical mechanics and numerical simulations connected to analytical aspects.
PMMH at ESPCI & Sorbonne U. and/or LPTMS at U. Paris-Saclay (Orsay)
martin.lenz@espci.fr or martin.lenz@u-psud.fr


LPTMS Internship and PhD Proposal: Self-assembly in space and time

Video: Self-assembly in space and time - Martin Lenz - LPTMS

Recent experimental developments have made assembling machines at the nanometer scales that mimic or even attempt to surpass the functions of biological objects an increasingly reasonable goal (as recognized in 2016). Despite remarkable progress in manufacturing individual nanometer-sized objects with controlled shapes however (see an example in the illustration), assembling many of them into larger structures remains an open challenge and an active field of research.
In this project we will undertake an additional challenge, namely to self-assemble such objects not only in space, but also in time. Specifically, we will explore the design principles for DNA origami particles produced by our collaborator Seth Fraden (Brandeis University, USA) to assemble over a given sequence over time, which will allow for an actin-like treadmilling (coordinated polymerization from one end, depolymerization from the other) of a polymer-like structure under e.g., temperature cycling. Suchmechanisms could be key in controlling the motor action of prospective molecular machines.
In a second stage (e.g., during a PhD), the intern may develop simulations tools to optimize particle shapesfor self-assembly of printed particles produced at PMMH in collaboration with Julien Heuvingh and Olivia du Roure.

Expected skills:  A taste for statistical mechanics, numerical simulations and working with experimentalists.

PMMH at ESPCI & Sorbonne U. and/or LPTMS at U. Paris-Saclay (Orsay)
martin.lenz@espci.fr or martin.lenz@u-psud.fr


LPTMS Intership and PhD Proposal: Mean Field Game description of Pedestrian Dynamics

Responsable: Denis ULLMO + 33 (0)1 69 15 74 76

Video: Mean field games

The situations where large crowds are gathered, and for which one would clearly benefit from a better ability to predict their dynamics, are numerous, and range from daily life optimization in train stations or sport events, to more dramatic circumstances such as the stampedes that have grieved the hadj in 1990 and 2015.

Predictive models for such crowd dynamics have followed essentially two strategies. The first one based on a kind of “cellular automate" approach, where one tries to identify local interaction rules between individuals. These approaches have shown some success for some animal groups such as bird flock, fish school or insect swarm, but have not provided a robust description of human crowd motion. The second strategy is to follow one of the favorite routes of condensed matter physicist and to develop a hydrodynamic description, or in terms of models analog to the ones developed for granular materials.

In some circumstances, these hydrodynamic, or granular material-like, models, fail drastically, even at the qualitative level, and they do so because, as the cellular-automate based model, they lack the ability to include the anticipation and optimization performed by the agents.

To include anticipation and optimization requires a Game Theoretical approach to the problem, which leads one to consider “many-body Game Theory” as many agents in interaction are involved.

The goal of the internship will be to address this problem of crowd dynamics in terms of a mean field approximation to the this many-body game theory that has been developed in the last decade under the name of Mean Field Game.

[For an introduction do Mean Field Games, see : “Quadratic Mean Field Games

Denis Ullmo, Igor Swiecicki, and Thierry Gobron, Physics Report 799, 1-35, (2019)]