Full phase diagram of active Brownian disks: from melting to motility-induced phase separation

TL;DR

This paper maps the complete phase diagram of active Brownian disks, revealing how activity influences melting, order transitions, and motility-induced phase separation in two-dimensional systems.

Contribution

It provides the first comprehensive phase diagram of active Brownian disks, detailing the effects of activity on phase transitions and phase separation.

Findings

Equilibrium melting scenario persists at low activity.

Active systems exhibit shifted phase transition densities with increased activity.

Motility-induced phase separation occurs at high activity, leading to coexistence of disordered and ordered phases.

Abstract

We establish the complete phase diagram of self-propelled hard disks in two spatial dimensions from the analysis of the equation of state and the statistics of local order parameters. The equilibrium melting scenario is maintained at small activities, with coexistence between active liquid and hexatic order, followed by a proper hexatic phase and a further transition to an active solid. As activity increases, the emergence of hexatic and solid order is shifted towards higher densities. Above a critical activity and for a certain range of packing fractions, the system undergoes MIPS and demixes into low and high density phases; the latter can be either disordered (liquid) or ordered (hexatic or solid) depending on activity.