Diffusion in nonequilibrium two-dimensional crystals

We investigate a two-dimensional dynamical absorbing state model of monodisperse disks, in which rich phase behavior arises from interactions consisting solely of repulsive displacements between overlapping particles. The phase diagram reveals several unconventional features, including a disordered and static absorbing configuration, where no particles overlap, separated by a second-order phase transition to a continuously evolving active hexagonal crystal with collective ring diffusion, which in turn undergoes a first-order phase transition to an active isotropic liquid. The only driving parameter is ε, the maximum size of the random repulsive kicks. Small ε facilitates self-organization into an ordered state, but large ε prevents this organization from occurring. This is very different from typical order-disorder transitions, where there are two competing influences, energy and entropy, that drive the transition.