Flocking in weakly nonreciprocal mixtures

It is known that strongly nonreciprocal interactions may give rise to a wealth of collective phenomena, such as traveling and oscillatory states. Here, we show that, in the context of flocking mixtures, even weak nonreciprocity generically leads to large-scale structure formation. Numerical simulations of a binary Vicsek model with mutually aligning or anti-aligning species —albeit with unequal strength— reveal that nonreciprocity destabilizes the ordered phase in a large part of the phase diagram. For mutually aligning interactions, this instability results in a phase where one species condenses in a single dense band traveling within a homogeneous liquid of the other species. When interactions are anti-aligning, both species self-assemble into polar clusters with large-scale chaotic dynamics. Importantly, in both cases, the emergence of structures is accompanied by the demixing of the two species despite the complete absence of repulsive interactions. By coarse-graining the microscopic model into hydrodynamic equations, we further elucidate the origin of the instability and demonstrate that it is a generic feature of nonreciprocal flocks.