Emergence of synchronised rotations in dense active matter with disorder

We consider the rich variety of collective motion patterns emerging when aligning active particles move in the presence of randomly distributed obstacles - representing quenched noise in two dimensions. In order to get insight into the involved complex flows and the transitions between them we use a simple model allowing the observation and analysis of behaviors that are less straightforwardly accessible by experiments or analytic calculations. We find a series of symmetry breaking states in spite of the applied disorder being isotropic. In particular, as the level of perturbations is increased, the system of self-propelled particles changes its collective motion patterns from i) directed flow ii) through a mixed state of locally directed or locally rotating flow to iii) a novel, globally synchronized rotating state thereby the system violating overall chiral symmetry. Finally, this phase crosses over to a state in which iv) clusters of locally synchronized rotations are observed. The way of change from polar flow to overall synchronization can be interpreted as indicating a non-reciprocal phase transition. Our simulations suggest that when both present, quenched rather than shot noise dominates the behaviors.