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Résumé |
Understanding the collective motion of self-propelling particles, such as flocking birds, is a problem that is
almost 30 years old but remains topical. Considering minimal flocking models (the Vicsek model and its
variants) I will present several results that tend to show the fragility of the polar ordered phases of collective
motion. First, any amount of spatial anisotropy destroys the long-range correlations and giant density
fluctuations of these states. Then, the ordered state of the "Malthusian" model, with population dynamics, is
unstable to the nucleation of topological defect as well as the ordered state in the Active Ising model that has
discrete symmetry. |
