Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle 523 du LPTMC - Tour 12-13
Quantum Spin Liquids in Dipolar-Octupolar Pyrochlores
Owen Benton (MPIPKS Dresden)
Over many years, there has been a concerted research effort to identify systems realizing Quantum Spin Liquid (QSL) ground states. Realization of QSL states is of great interest due to their association with large-scale quantum entanglement, fractional excitations and emergent gauge fields. A particularly interesting subset of QSLs is those that realise emergent electromagnetism, with gapless photons and gapped, fractional charges as excitations.
In this regard the “dipolar-octupolar” pyrochlore oxides R2M2O7 (R=Ce, Sm, Nd) represent an important opportunity. The effective S=1/2 exchange Hamiltonian which governs their low energy physics has an alluringly simple “XYZ” form and is known to be conducive to forming a U(1) QSL ground state, at least in certain limits. Meanwhile, recent experiments on these materials strongly suggest QSL physics.
Motivated by this, we present here a complete analysis of the ground state phase diagram of dipolar-octupolar pyrochlores. Combining perturbation theory, variational arguments and exact diagonalization we discover multiple U(1) QSL phases which together occupy a large fraction of the parameter space. By comparing numerical calculations to published thermodynamic data we can also locate the materials Ce2Zr2O7 and Ce2Sn2O7 on the phase diagram, finding strong support for a QSL ground state.
Statistical properties of energy barriers and activated dynamics in mean-field models of glasses
Valentina Ros (IPhT CEA Saclay)
Understanding the geometrical properties of high-dimensional, random energy landscapes is an important problem in the physics of glassy systems, with plenty of interdisciplinary applications. Among these properties, an important role is played by the statistics of stationary points, which is relevant in determining the evolution of local dynamics within the landscape. In this talk I will focus on the energy landscape of a simple model for glasses (the so-called spherical *p*-spin model) and I will present a framework to compute the statistical properties of the saddle points surrounding local minima of the landscape. I will discuss how this computation allows to extract information on the distribution of energy barriers surrounding the minimum, as well as on its connectivity in configuration space. I will comment on the dynamical implications on these results, especially for the activated regime of the dynamics, relevant when the dimension of configuration space is large but finite.
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 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. 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. These invariants correctly describe chiral modes as we verified numerically on interfaces between different gyrotropic materials.  S. Raghu and F. D. M. Haldane, Phys. Rev. A78, 033834 (2008).  M. G. Silveirinha, Phys. Rev. B92, 125153 (2015).  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).