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  • Open Access

Nonreciprocity as a Generic Mechanism for Demixing in Flocking Mixtures

Charlotte Myin* and Benoît Mahault†,‡

  • *Contact author: charlotte.myin@ds.mpg.de
  • †Contact author: benoit.mahault@umontpellier.fr
  • ‡Present address: Laboratoire Charles Coulomb (L2C), Univ. Montpellier, CNRS, Montpellier, France.
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Phys. Rev. Lett. 137, 018301 – Published 29 June, 2026

DOI: https://doi.org/10.1103/v435-qnhl

Abstract

We show that even weak nonreciprocal alignment leads to large-scale structure formation in flocking mixtures. By combining numerical simulations of a binary Vicsek model and the analysis of coarse-grained continuum equations, we demonstrate that nonreciprocity destabilizes the ordered phase formed by mutually aligning or antialigning species in a large part of the phase diagram. For aligning populations, this instability results in one species condensing in a single band that travels within a homogeneous liquid of the other species. When interactions are antialigning, both species self-assemble into polar clusters with large-scale chaotic dynamics. In both cases, the emergence of structures is accompanied by the demixing of the two species, despite the absence of repulsive interactions. Our theoretical analysis allows us to elucidate the origin of the instability, and show that it is generic to nonreciprocal flocks.

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Phys. Rev. Lett. 137, 018301– Published 29 June, 2026
  • Received 16 February 2026
  • Accepted 2 June 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

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