Optical topological chiral modes flowing between non-topological materials

Marco Marciani (Labo de Physique de l'ENS Lyon)

The most remarkable feature of the so-called "topological crystals" is the presence of states flowing at their edge that are robust against disorder. A beautiful mathematical theory allows to predict the properties of such states directly from the topological invariants (e.g. the Chern numbers) of the bulk bands. Given the great success of this theory in terms of theoretical impact and technological advance, in recent years much effort has been put to make the extension from the field electronics to other fields[1] and from crystals to various non-crystaline systems such as quasi-crystals and amorphous materials. In this talk I will show how deal with continuous systems governed by linear Maxwell's equations[2]. Even though bands Chern numbers cannot be defined and optical materials are non-topological, we discover that interface Chern numbers can always be defined by means of the theory of spectral flows[3]. These invariants correctly describe chiral modes as we verified numerically on interfaces between different gyrotropic materials. [1] S. Raghu and F. D. M. Haldane, Phys. Rev. A78, 033834 (2008). [2] M. G. Silveirinha, Phys. Rev. B92, 125153 (2015). [3] M. Marciani and P. Delplace, arXiv:1906.09057 (2019).

Confotronics of biofilaments

Hervé Mohrbach (LPCT Nancy-Metz)

Biofilaments like those of the cytoskeleton show anomalous behaviours in various experiments that can be explained by the existence of conformational excitations. For example, the 2-d confinement of helices form peculiar squeezed conformations often resembling looped waves, spirals or circles. These shapes as well as the unusual statistical mechanics can be understand in terms of moving and interacting localized conformational quasiparticles. A condensate of these quasiparticles emerge in recent experiments realized on intermediate filaments. As a second example, I will consider tubular lattices like microtubules. Despite significant effort, understanding the unusual mechanics of microtubules remains elusive. There are strong evidences for the existence of conformational internal degrees of freedom in the microtubule lattice that lead to filaments with unique characteristics in the world of macromolecules. I will discuss those characteristics, like the mechanical hysteresis and a peculiar rotational zero mode, which not only explain various experimental results but could also inspire novel smart materials.

Interactions of antibodies and bacteria in the digestive tract

Claude Loverdo (LJP Jussieu)

Inside the organism, the immune system can fight generically against any bacteria. However, the lumen of the gut is home to a very important microbiota, so the host has to find alternative ways to fight dangerous bacteria while sparing beneficial ones. While many studies have focused on the complex molecular and cellular pathways that trigger an immune response, little is known about how the produced antibodies act once secreted into the intestinal lumen. Our modeling work is along 3 axes. First, using stochastic models of bacterial population dynamics and branching processes, we infer relevant biological parameters of the dynamics of the bacterial population in the in vivo experiments of our collaborator, Emma Slack (ETH Zurich). We contributed to show that the main physical effect of these antibodies is to cross-link bacteria into clusters as they divide, preventing them from interacting with epithelial cells, thus protecting the host. We then developed a simple ordinary differential equations model of these bacterial clusters, and studied how the interplay of the time scales of bacterial growth and of link breaking could enable the immune system to target the most problematic bacteria. Last, we studied how such immune-mediated bacterial clustering could impact the evolution of drug resistance by using a hybrid cross-scale model (with deterministic within-host bacterial growth, and stochastic transmission).

Séminaire commun avec le LPTHE. Lieu: BIBLIOTHEQUE DU LPTHE (13-14, 4ème étage)

Convergence of Non-Perturbative Approximations to the Renormalization Group

Nicolás Wschebor (Universidad de la República, Montevideo, Uruguay)

We provide analytical arguments showing that a non-perturbative approximation scheme known as the derivative expansion is controlled by a small parameter for very generic model at thermodynanical equilibrium. This approximation must be implemented within the Non-Perturbative Renormalisation Group (a modern version of Wilson's renormalisation group) with a regulator profile properly chosen. We employ the Ising model in three dimensions as a testing ground of the general analysis. In this case the derivative expansion has been recently pushed at order fourth order. We find fast convergence of critical exponents to their exact values, in full agreement with our general arguments. We also analyze preliminary results by employing the same techniques for O(N) models.