Reentrant phase behavior in binary topological flocks with nonreciprocal alignment

Abstract

We study a binary metric-free Vicsek model involving two species of self-propelled particles aligning with their Voronoi neighbors, focusing on a weakly nonreciprocal regime, where species $A$ aligns with both $A$ and $B$, but species $B$ does not align with either. Using agent-based simulations, we find that even with a small fraction of $B$ particles, the phase behavior of the system can be changed qualitatively, which becomes reentrant as a function of noise strength: traveling bands arise not only near the flocking transition, but also in the low-noise regime, separated in the phase diagram by a homogeneous polar liquid regime. We find that the ordered bands in the low-noise regime travel through an ordered background, in contrast to their metric counterparts. We develop a coarse-grained field theory, which can account for the reentrant phase behavior qualitatively, provided the higher-order angular modes are taken into consideration.