  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.
