Past PhD 2020

Thibault Bonnemain

24 fevrier 2020
LPTM, universite Cergy Pontoise

Soutenance de thèse

Quadratic mean field games with negative coordination

Mean Field Games provide a powerful theoretical framework to deal with stochastic optimization problems involving a large number of coupled subsystems. They can find application in several fields, be it finance, economy, sociology, engineering … However, this theory is rather recent and still poses many challenges. Its constitutive equations, for example, are difficult to analyse and the set of behaviours they highlight are ill-understood. While the large majority of contributions to this discipline come from mathematicians, economists or engineering scientists, physicist have only marginally be involved in it. In this thesis I try and start bridging the gap between Physics and Mean Field Games through the study of a specific class of models dubbed « quadratic ».

 

Directeurs : Denis Ullmo, Thierry Gobron

Jury : Cecile Appert-Rolland, Olivier Gueant, Max-Olivier Hongler, Jean-Pierre Nadal, Filippo Santambrogio


Samuel Cazayus-Claverie

25 fevrier 2020
Petit amphitheatre, batiment Pascal

Soutenance de thèse

Effect of residual stress on the elasticity of fiber networks

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

 

Directeur : Martin Lenz

Jury : Anael Lemaitre, Chaouqi Misbah, Giuseppe Foffi, Cecile Leduc, Raphael Voituriez


Giampaolo Folena

10 mars 2020
Universite de Rome, La Sapienza

Soutenance de thèse

The mixed p-spin models: selecting, following, and losing states

The main driving notion behind my thesis research is to explore the connection between the dynamics and the static in a prototypical model of glass transition, i.e. the mean-field p-spin spherical model. This model was introduced more than 30 years ago with the purpose of offering a simplified model that had the same equilibrium dynamical slowing down, theoretically described a few years earlier by mode-coupling theory. Over the years, the p-spin spherical model has shown to be a very meaningful and promising model, capable of describing many equilibrium and out-of-equilibrium aspects of glasses. Eventually it came to be considered as a prototypical model of glassiness. Having such a simple but rich reference model allows a coherent examination of a subject, in our case the glass behavior, which presents a very intricate phenomenology. Thus, the main purpose is not to have a quantitative prediction of the phenomena, but rather a broader view with a strong analytical basis. In this sense the p-spin model has assumed a role for disordered systems which is comparable to that of the Ising model for understanding ferromagnetism. My research is a natural path to reinforce our knowledge and comprehension of this model.

Directeurs : Silvio Franz, Federico Ricci-Tersenghi

Jury : Luca Leuzzi, Chiara Cammarota, Patrick Charbonneau, Florent Krzakala, Luca Dall'Asta, Pierfrancesco Urbani,