Active diffusing crystals in a 2D non-equilibrium system

TL;DR

This study explores a 2D non-equilibrium system of monodisperse disks with purely repulsive interactions, revealing complex phase transitions including static, active crystal, and liquid states driven by the magnitude of random kicks.

Contribution

It introduces a novel 2D dynamical model showing rich phase behavior driven solely by repulsive displacements, highlighting unconventional order-disorder transitions.

Findings

Disordered static absorbing state exists with no overlaps.

Second-order transition to an active hexagonal crystal with collective diffusion.

First-order transition from crystal to an active isotropic liquid.

Abstract

We investigate a 2D 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 $\epsilon$, the maximum size of the random repulsive kicks. Small $\epsilon$ facilitates self-organization into an ordered state, but large $\epsilon$ 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.