Only the Ambidextrous Can Flock: Two-dimensional Chiral Malthusian Flocks, Time Cholesterics, and the KPZ Equation

TL;DR

This paper explores two-dimensional chiral active matter, revealing that fluctuations governed by the KPZ equation lead to short-range order and predicting observable velocity and density correlations.

Contribution

It introduces a theoretical framework for chiral dry Malthusian flocks and connects their fluctuations to the KPZ equation, predicting short-range order in such systems.

Findings

Fluctuations are described by the (2+1)-KPZ equation.

Chiral flocks exhibit short-ranged orientational order.

Predictions are testable in simulations and experiments.

Abstract

We study two-dimensional chiral dry Malthusian flocks; that is, chiral polar-ordered active matter with neither number nor momentum conservation. In the absence of fluctuations, these form a ``time cholesteric", in which the velocity rotates uniformly in time at a fixed frequency. Fluctuations are described by the (2+1)-Kardar-Parisi-Zhang (KPZ) equation, which implies short-ranged orientational order. For weak chirality, the system is in the linear regime of the KPZ equation for a wide range of length scales, over which it exhibits quasi-long-ranged orientational order. Our predictions for velocity and density correlations are testable in both simulations and experiments.