Abstract
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 the classical nucleation theory that the forward and backward switching dynamics are asymmetric:going 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.