Non-reciprocal forces and exceptional phase transitions in metric and topological flocks

TL;DR

This paper investigates how microscopic non-reciprocal interactions influence phase transitions and collective behavior in flocking models, revealing new phases and restoring flocking order through hydrodynamic descriptions.

Contribution

It demonstrates the impact of microscopic non-reciprocal forces on phase transitions and introduces modified hydrodynamic models for metric and topological flocking.

Findings

Non-reciprocal interactions induce exceptional phase transitions.

Hydrodynamic models predict clustered and restored flocking phases.

Large-scale simulations confirm theoretical predictions.

Abstract

Many models of flocking involve alignment rules based on the mean orientation of neighboring particles, which we show introduces microscopic non-reciprocal interactions. In the absence of this microscopic non-reciprocity an exceptional phase transition is predicted at low noise strength within the Toner-Tu framework of polar aligning matter; we demonstrate this transition via large-scale numerical simulations. By coarse-graining the microscopic non-reciprocal forces found in more common models of flocking, we identify additional terms in a hydrodynamic description which lead to a highly ordered clustered phase in metric models and restore the homogeneous flocking phase in topological models.