Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle  523 du LPTMC - Tour 12-13 


André Thiaville (LPS Orsay)

Magnétisme et topologie [titre provisoire]

Christina Kurzthaler (Univ. Innsbruck)

Spatiotemporal dynamics of active agents

Various challenges are faced when microorganisms or artificially synthesized self-propelled particles move autonomously in aqueous media at low Reynolds number. These active agents are intrinsically out of equilibrium and exhibit peculiar dynamical behavior due to the complex interplay of stochastic fluctuations and directed swimming motion. In particular, these particles display fascinating physics ranging from the run-and-tumble motion of bacteria to the noisy circular trajectories of biological or artificial microswimmers due to hydrodynamic couplings in the vicinity of interfaces or chiral body shapes. Here, we provide a theoretical analysis of the spatiotemporal dynamics of different types of active particles in terms of the experimentally accessible intermediate scattering function. Our analytical predictions characterize the spatiotemporal dynamics of catalytic Janus particles, a paradigmatic class of synthetic active agents, from the smallest length scales where translational Brownian motion dominates, up to the largest ones, which probe the randomization of the swimming direction due to rotational diffusion. We also show that our theoretical framework finds application in different areas such as polymer physics.

André Estevez-Torres (Laboratoire Jean Perrin, CNRS et Sorbonne Université, Paris)
Writing down reaction-diffusion equations with DNA 
The question that motivates my research is: to what extent non-equilibrium molecular systems can create order at the macroscopic scale, in particular spatial order.
A major mechanism of spatial self-organization is reaction-diffusion. For decades reaction-diffusion has been experimentally investigated using the Belousov-Zhabotinsky reaction and its cousins. The problem is that the dynamics of this reaction is difficult to change, limiting the number of questions that one can address using this system.
As a possible solution to this drawback I will introduce an approach to engineer chemical dynamical systems using DNA and three enzymes. I will then describe the experimental implementation of various reaction-diffusion patterns with this system: monostable and bistable traveling fronts, predator-prey waves and spirals, and immobile fronts. Finally, I will suggest open theoretical questions that we have encountered while investigating these systems.
Gilles Montambaux (LPS Orsay)

Généralisation de la loi de Stefan-Boltzmann et quelques remarques sur la naissance de la mécanique quantique

La loi de Stefan-Boltzmann, « la puissance rayonnée par un corps varie comme la puissance quatrième de sa température », est une des plus importantes de notre quotidien puisqu’elle détermine  la température de la terre et notre existence même. Elle est souvent enseignée comme une conséquence de la loi de Planck du corps noir (1900), conséquence de la quantification des échanges d’énergie, acte de naissance de la mécanique quantique. En fait, cette loi expérimentale de Stefan fut démontrée par Boltzmann 16 ans avant la loi de Planck, par des arguments purement classiques. En revenant sur cette démonstration, je montrerai comment on peut la généraliser, sans utiliser la mécanique quantique, à d’autres gaz de particules massifs ou sans masse, de fermions ou de bosons, retrouver classiquement  la thermodynamique d’un gaz de Bose par exemple.On montrera ainsi comment unifier  la thermodynamique de systèmes jusque-là considérés comme très différents.
L’exposé sera agrémenté de plusieurs remarques historiques sur la naissance de la mécanique quantique.

 Leonardo Mazza (ENS Paris)

Majorana fermions in particle-conserving settings

The paradigmatic condensed-matter models where zero-energy localized Majorana fermions have been studied so far have the distinguishing feature of not conserving the number of fermions. Moreover, the accepted definition of "Majorana fermion" naturally belongs to this scenario. Is it possible to discuss Majorana fermions in "canonical" particle-conserving settings?

In this seminar I will present several exact and numerical results on Majorana fermions in particle-conserving scenarios. I will start from the discussion of a model for bosons and fermions where, in a proper limit, the physics of the celebrated Kitaev's chain appears. I will continue by presenting exact results on Majorana fermions in ladder models where the two legs of the system can only exchange pair of particles. Finally, I will comment on the possibility of making experiments with Majorana fermions in particle-conserving settings.


Iemini, LM, Rossini, Fazio and Diehl, PRL *115*, 156402 (2015)

Iemini, LM, Fallani, Zoller, Fazio, Dalmonte, PRL *118*, 200404 (2017)