Polymer nanocomposites, systems of polymers containing nanoparticles (NPs), are fascinating systems that have many applications in material science, biology and medicine, but also pose challenges to theoretical physics. One of the fundamental problems in the physics of nanocomposites is to understand how the structure and dynamics of the system depends on key parameters, such as NP size and volume fraction and the typical size of the polymeric mesh. In this thesis we use molecular dynamics simulations to study the structural and dynamic properties of NPs embedded in liquid and solid polymer nanocomposites. We observe that when weakly attractive, well dispersed NPs are added to a dense polymer solution, both the polymers and the NPs experience a dynamical slowing down. We find that, in qualitative agreement with experiments, this dynamical slowing down is captured by a confinement parameter in the form h/λ, where h is the average distance between the surfaces of neighboring NPs (interparticle distance). We are able to show that for the NPs, λ can be interpreted as the hydrodynamic radius of the NP, whereas for the polymers it behaves as a cooperativity length scale. Simulating disordered, polydisperse polymer networks containing purely repulsive NPs, we find that small NPs can freely diffuse through the entanglement mesh, while large NPs are transiently trapped and can only move through a sequence of “jumps” (hopping motion). We find that the parameter controlling NP localization is the ratio between the NP diameter and the localization length of the crosslinks. Finally, we propose a new method to characterize the geometrical mesh size in polymer liquids, a quantity that is important to describe the diffusion of NP in a disordered medium.

The increased flexibility in how commercial microscopes can be modified to add external features has opened the possibility of numerous new
types of experiments. In this talk I will present work on two such projects.



In a first study, the growth of condensed colloidal states from a fluid phase is explored in the presence of an external drive of the system by a controlled thermal
field and thermophoresis. A thermal gradient is formed in a fluid bath of colloids by adding and trapping with optical tweezers a high refractive index particle.
By absorbing a portion of the laser light from the tweezers, the particle acts as a point-like heat source and causes a temperature gradient in the mixture that is
highly localized in both space and time. This leads to the clustering and phase transition by thermophoresis of colloids from the bath around the trapped particle.
I will show that the steady-state size of the aggregates can be determined by the shape of the external field and the bulk volume fraction of the system and that condensed
structures with various order can be generated (crystalline, amorphous and icosahedral order). This novel technique to trigger phase transitions opens the way to probe the dynamics
of growth of phase transitions from highly out-of-equilibrium conditions and to observe the behaviour of interfaces in potentials that are inhomogeneous at the scale of the fluctuations
of the interface.



In the second project, I explore the strength of soft materials against puncture and penetration by sharp objects. These are properties of condensed matter relevant to various
situations spanning the design of strong gloves, to understanding animals bite mechanics or the invasion of filament-shaped yeasts breaking through human skin. Puncture and penetration
are described by an interplay between several mechanisms: the material elastic response, fracture, crack propagation, friction and adhesion. This variety of mechanisms raises the question
of which are mostly relevant to any particular situation, and my focus will be here on two types of micron-scale objects indenting soft polydimethyl siloxane (PDMS). First, nanoindentation
is used to measure the force acting on micropipettes as they are pushed into a PDMS block. I will show that, here, the critical force to puncture is highly dependent on the speed of the indenter
and that there is an optimal polymer crosslinker concentration to resist against puncture. Second, deformations of PDMS are measured as an invasive yeast, Candida albicans, grows in this
substrate in the shape of a filament with a tip continuously moving forward by cell wall creation. Using a dedicated "compression force microscopy" technique, the 3d the displacement of
fluorescent particles in the PDMS is monitored and highlights that it sustains deformations that are surprisingly smaller compared to the nanoindentation tests as well as other detailed
features of the growth.

We will encode the quantum entropy function into modular structures such as Jacobi Forms and Siegel modular forms to uncover interesting structures pointing to an underlying Calogero model as a putative CFT that is the holographically dual model to quantum gravity in ADS_2.

Observing and controlling macroscopic quantum systems has long been a driving force in research on quantum physics. In this endeavor, coupling individual quantum systems to mechanical oscillators is of great interest. While both read-out of mechanical motion using two-level spin systems and spin read-out using oscillators have been demonstrated, temperature control of the motion of a macroscopic object using electronic spins is still a daunting task. We will present our observations of spin-dependent torque and dynamical back-action from a micro-diamond levitating in a Paul Trap [2]. Using a combination of microwave and laser excitation enables the spin of nitrogen-vacancy centers to act on the diamond orientation and to observe cooling of the diamond libration. Further, driving the system in the non-linear regime, we demonstrate bistability and self-sustained lasing of the librational mode. The final temperature is currently limited by microwave excitation on the blue side of the spin-resonance since the trapping frequency is lower than the spin decoherence rate. We discuss ways to achieve spin-cooling to the ground state and present first steps towards a platform that uses levitating magnets and CVD grown diamond samples in order to reach the sideband resolved regime [3].

[1] Delord T. et al. Phys. Rev. Lett. 121 053602 (2018)
[2] Delord T., et al. arXiv :1905.11509 (2019)
[3] Huillery P. et al. ArXiv 1903.09699 (2019)

Dans ce séminaire seront présentés les développements méthodologiques en IRM et RMN bas champ pour la caractérisation des tissus végétaux. Deux principales questions sont abordées : d’une part l’acquisition des connaissances sur les mécanismes de relaxation dans les tissus végétaux permettant de progresser dans l’interprétation des signaux RMN/IRM en termes de paramètres physico-chimiques et d’autre part le développement des méthodologies d’IRM pour des mesures précises et robustes de paramètres quantitatifs. La démarche globale est de pouvoir in fine développer les approches RMN et IRM comme techniques d’analyse et de phénotypage des matrices végétales, permettant d’accéder à des informations directes sur des caractéristiques tissulaires. Des exemples des travaux sur les tissus foliaires et des fruits viendront illustrer cette démarche.

We will consider solutions of three physically important problems by the renormalization group methods:
i) Perturbative renormalization of su(2) Yang-Mills. The main part of the work is to prove at all loop orders the existence of the vertex
functions and the restoration of the Slavnov-Taylor identities in the
renormalized theory.
ii) Dynamical symmetry breaking in su(2) Yang-Mills. We make progress towards a derivation of a low energy effective theory for su(2) Yang–Mills theory. This low energy action is computed to 1-loop using the renormalization group technique. After that, we perform the Spin-Charge decomposition in a way proposed by L.D. Faddeev and A.J. Niemi. The effective potential is of Mexican hat type.
iii) Lamb shift in heavy atoms. We formulate the Hartree–Fock method in the contemporary language of QED using a functional integral approach. This allows us to set up the corresponding renormalization group equation which can be solved numerically in the large-N approximation.

Rigidity is the hallmark of solids. It manifests itself as a resistance to flow: for small applied strain, solids provide an elastic, restoring, reversible response, as opposed to liquids. However, for large enough strain, irreversible plastic flow occurs at the so-called yielding point. Recently, yielding of crystals has been rationalised within a nucleation framework, suggesting that a rigid solid is always metastable to an infinitesimal deformation [1]. I shall revisit these topics presenting a recent numerical work exploring the metastability of oscillatory sheared crystals in three dimensions [2]. The main finding is that a transition from a reversible to an irreversible driving regime occurs and that it is affected by the specific composition and structure of the crystal. For large enough oscillation frequency, a yielding pattern reminiscent of spinodal decomposition emerges, consistent with the recent nucleation scenarii.

[1] P. Nath et al. PNAS 115, E4322 (2018)
[2] É. Brillaux and F. Turci, Soft Matter 15, 4371 (2019)

Titre : Émetteur(s) optique(s) en cavité

Résumé : Il est bien connu que les propriétés d'un émetteur de lumière ponctuel, tel qu'un atome ou une boîte quantique, peuvent être drastiquement modifiées à l'aide d'une cavité optique. Je passerai en revue les principales stratégies pour y parvenir, en évoquant notamment le cas des nano-antennes métalliques avec lesquelles nous travaillons au laboratoire pour façonner l'émission de LEDs à boîtes quantiques colloïdales. Nous verrons à travers ces différents exemples que l'intuition que possède la communauté sur ces systèmes repose pour l'essentiel sur des modèles et expériences impliquant des émetteurs uniques. Je présenterai ensuite des mesures nouvelles qui montrent que cette intuition n'est en fait pas pertinente dès lors que le système comprend un ensemble d'émetteurs en interaction. Je rationaliserai ces observations et discuterai des potentialités de ce régime d'émission pour l'optique et l'optoélectronique à l'échelle nano.

Perturbations in CMB photons and LSS of the Universe are sourced primarily by the curvature perturbation produced during inflation. I will present a study of a two-field inflationary model in which the inflaton couples to a spectator field. The one-loop interaction between the spectator and the inflaton induces a rapid growth of the momentum of the curvature perturbation and the associated Gaussian van Neumann entropy during inflation such that the state of the curvature perturbation at the end of inflation differs substantially from the standard one, in which the state is highly squeezed and the momentum is very suppressed. Consequently, one ought to reconsider the kinetic equations describing the evolution of the photon, dark matter, and baryonic fluids in radiation and matter eras by modelling the curvature perturbation and its canonical momentum as two a priori independent stochastic fields. If time permits, I will briefly analyze possible imprints on the CMB temperature fluctuations from the more general inflationary scenario which contains light spectator fields coupled to the inflaton by using a simplistic linear evolution in radiation and matter eras.