Observing one-step melting pathway in two-dimensional hard circular particle system

The nature of two-dimensional melting remains a matter of debate. Based on Langevin dynamics simulation, we present a surprising observation that the melting pathway of hard circular-particles/disks is relevant to the dynamical properties of particles. Using hard circular-particles/disks, where particle friction is proportional to particle size, results show the melting pathway of small size particles obeys two-step scenario, consistent with previous reports on melting behavior of hard disks. Conversely, in large size particle systems, we observe a one-step melting pathway. Further investigation reveals that the emergent one-step melting pathway is non-equilibrium phase transition that relates to cooperative relaxation of cage dynamics. This originates from the particularly stable/stiff configuration in large size particle system, which is a metastable state arrested by changed dynamics of particles, thus inducing a non-equilibrium melting pathway that distinguishes from the equilibrium melting behavior. Our findings shed light on the debate on melting behavior of hard disks.