Asymmetric Nucleation Processes in Spontaneous Mode Switch of Active Matter

Flocking and vortical are two typical motion modes in active matter. Although it is known that the two modes can spontaneously switch between each other in a finite-size system, the switching dynamics remain elusive. In this work, by computer simulation of a two-dimensional Vicsek-like system with 1000 particles, we find from the perspective of classical nucleation theory that the forward and backward switching dynamics are asymmetric: from flocking to vortical is a one-step nucleation process, while the opposite is a two-step nucleation process with the system staying in a metastable state before reaching the final flocking state.