Competing ferromagnetic and nematic alignment in self-propelled polar particles

TL;DR

This paper investigates a Vicsek-style model with competing ferromagnetic and nematic alignments, revealing phase diagram similarities to equilibrium models and distinct transition types in metric versus metric-free interactions.

Contribution

It demonstrates that the phase behavior of a nonequilibrium XY model with competing alignments mirrors that of equilibrium models, with robustness to interaction biases.

Findings

Phase diagram similar to equilibrium XY models

Discontinuous polar-nematic transitions in metric case

Continuous transitions in metric-free case, in Ising class

Abstract

We study a Vicsek-style model of self-propelled particles where ferromagnetic and nematic alignment compete in both the usual "metric" version and in the "metric-free" case where a particle interacts with its Voronoi neighbors. We show that the phase diagram of this out-of-equilibrium XY model is similar to that of its equilibrium counterpart: the properties of the fully-nematic model, studied before in [F. Ginelli, F. Peruani, M. Baer, and H. Chat\'e, Phys. Rev. Lett. 104, 184502 (2010)], are thus robust to the introduction of a modest bias of interactions towards ferromagnetic alignment. The direct transitions between polar and nematic ordered phases are shown to be discontinuous in the metric case, and continuous, belonging to the Ising universality class, in the metric-free version.