Nucleation-induced transition to collective motion in active systems

While the existence of polar ordered states in active systems is
well established, the dynamics of the self-assembly processes are
still elusive. We study a lattice gas model of self-propelled
elongated particles interacting through excluded volume and
alignment interactions, which shows a phase transition from an
isotropic to a polar ordered state. By analyzing the ordering
process we find that the transition is driven by the formation of a
critical nucleation cluster and a subsequent coarsening process.
Moreover, the time to establish a polar ordered state shows a
power-law divergence.