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Breakdown of Emergent Chiral Order and Defect Chaos in Nonreciprocal Flocks

Charlotte Myin1,*, Suropriya Saha1,†, and Benoît Mahault2,1,‡

  • *Contact author: charlotte.myin@ds.mpg.de
  • †Contact author: suropriya.saha@ds.mpg.de
  • ‡Contact author: benoit.mahault@umontpellier.fr
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Phys. Rev. Lett. 137, 118302 – Published 8 September, 2026

DOI: https://doi.org/10.1103/c6dy-38z1

Abstract

We show that chiral order in two-dimensional nonreciprocal flocking mixtures is generically unstable. Combining large-scale agent-based simulations with a coarse-grained continuum description, we demonstrate that rotating chiral states emerging from antisymmetric couplings are destroyed by the proliferation of topological defects. The resulting dynamics is spatiotemporally chaotic and characterized by a finite correlation length that diverges as nonreciprocity vanishes. On length scales below this cutoff, density and orientational order fluctuations remain scale-free, but the associated scaling exhibits nonuniversal exponents. We attribute this atypical behavior to the coupling between density and order, which causes topological defects to act as persistent sources of nonlinear fluctuations.

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Physics Subject Headings (PhySH)

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Nonreciprocity Sends Flocks into Chaos

Published 8 September, 2026

Two intermingled species of active matter can exhibit coherent rotation or disorderly scrambling depending on their mutual interactions.

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Extensive Spatiotemporal Chaos in Nonreciprocal Flocking

Chul-Ung Woo, Jae Dong Noh, and Heiko Rieger
Phys. Rev. Lett. 137, 118301 (2026)

Article Text

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Phys. Rev. Lett. 137, 118302– Published 8 September, 2026
  • Received 4 May 2026
  • Accepted 16 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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