Mathias Casiulis (LPTMC)
Collective motion in an ideal spin fluid
Collective motion, the macroscopic alignment of velocities in a system of particles, is a key feature of active matter systems. From simple Vicsek-like models to real-life experiments, many different systems seem to feature collective motion, often accompanied by exotic correlations and phase separation properties. These phenomena are however generally observed in non-equilibrium dynamics only, and many models are built up in an ad hoc manner to reproduce experimental data.
In the last few years, some works tried to link the exotic features of active systems to well-known equilibrium dynamics, by studying the low Peclet number limit of active systems for instance , thereby finding links between the usual 2d melting transition and the acclaimed MIPS (Motility-Induced Phase Separation), for instance.
The aim of my work is to study, in a toy-model that was designed to be as simple as possible , whether collective motion can exist in a conservative framework, when introducing a spin-velocity coupling in a spin fluid. In this talk, I will discuss the model itself, its numerical phenomenology and its links to various other systems and results of statistical mechanics.
 L. Cugliandolo, P. Digregorio, G. Gonella, and A. Suma, Phase Coexistence in Two-Dimensional Passive and Active Dumbbell Systems, PRL 119, 268002 (2017)
 S. Bore, M. Schindler, K. Lam, E. Bertin, and O. Dauchot, Coupling Spin to velocity: collective motion of Hamiltonian polar particles, J. Stat. Mech. 033305 (2016)
Alexis Poncet (LPTMC)
Tagged particles in single-file systems
Single-file systems, in which particles confined to a channel cannot overtake each other, exhibit a well-known subdiffusive scaling.
This anomalous behavior is a direct consequence of strong spatial correlations induced by the geometrical constraints. Even if this fact has been recognized qualitatively for a long time, up to now there has been no full quantitative determination of these correlations.
In this talk, we present several theoretical approaches that enable us to derive expressions for multiple-tag observables in the Symmetric Exclusion Process (SEP). We will cover the cases of both high-density and arbitrary-density systems, and of both unbiased and biased tagged particles.