The liquid solid transition of monodisperse active particles*

We simulate dense systems of monodisperse self propelled particles in two dimensions. We explore how the character of the liquid solid transition changes as the system is gradually tuned from an equilibrium Brownian system to an active system with increasingly persistent particle motion. We are especially interested in how activity affects the existence and extent of the hexatic phase. We find that activity shifts the transition towards higher packing fractions and widens the interval where hexatic order is observed.
As the activity is increased further we investigate how the liquid-solid transition interacts with the motility induced phase separation. In the crossover region between these two phenomena, at high activity but still outside the phase separated region, we observe that the system can remain dynamically active even at packing fractions far above the liquid-solid transition observed in equilibrium systems. In these dense active systems we observe the formation of short lived highly ordered clusters that constantly forms, melts and flows giving rise to some very interesting dynamics.