Dresden 2026 – scientific programme
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DY: Fachverband Dynamik und Statistische Physik
DY 44: Poster: Active Matter, Soft Matter, and Fluids
DY 44.9: Poster
Wednesday, March 11, 2026, 15:00–18:00, P5
Absence of chiral long-range order in the 2d non-reciprocal Vicsek model — •Chul-Ung Woo1, Heiko Rieger1, and Jae Dong Noh2 — 1Department of Theoretical Physics and Center for Biophysics, Saarland University, Saarbrücken, Germany — 2Department of Physics, University of Seoul, Seoul, Korea
Nonreciprocal interactions in active matter have been predicted to generate homogeneous chiral phases, in which the polarization order parameter rotates at a constant frequency as a result of a nonreciprocal phase transition. Here we revisit the non-reciprocal Vicsek model in two space dimensions with short-range interactons and ask whether the putative chiral phase survives in the thermodynamic limit. Using large-scale simulations, we show that a spatially homogeneous chiral state, while long-lived in small systems, is generically unstable to the spontaneous nucleation of spiral defects that invade the system and drive it into spatio-temporal chaos. The global chiral order parameter, τ exhibits a robust finite-size scaling τ(J−;L)=J− τr(L J−), where J=JAB−JBA is the anti-symmetric part of the interaction matrix, with the asymptotic behavior τr(x)∼ x−1 at large x, implying that global chirality vanishes in the thermodynamic limit. Controlled droplet-seeding experiments and a Boltzmann kinetic description support the droplet nucleation and growth scenario for the breakdown of chiral order. Our results demonstrate that paradigmatic non-reciprocal flocking models display chiral long-range order only in mean-field, whereas it is only metastable in two space dimensions and replaced there by spatio-temporal chaos.
Keywords: Non-reciprocal interaction; Flocking model