Active Matter Class with Second-Order Transition to Quasi-Long-Range Polar Order

TL;DR

This paper introduces a new class of active matter in two dimensions that transitions directly to polar order without phase separation, exhibiting unique critical behavior distinct from classical universality classes.

Contribution

It identifies a novel active matter class with a second-order transition to quasi-long-range order, challenging existing universality class assumptions.

Findings

Displays quasi-long-range polar order with variable scaling exponents.

Transition does not follow BKT universality, indicating a different critical point.

Interplay between order and density alters defect roles.

Abstract

We introduce and study in two dimensions a new class of dry, aligning, active matter that exhibits a direct transition to orientational order, without the phase-separation phenomenology usually observed in this context. Characterized by self-propelled particles with velocity reversals and ferromagnetic alignment of polarities, systems in this class display quasi-long-range polar order with continuously-varying scaling exponents and yet a numerical study of the transition leads to conclude that it does not belong to the Berezinskii-Kosterlitz-Thouless universality class, but is best described as a standard critical point with algebraic divergence of correlations. We rationalize these findings by showing that the interplay between order and density changes the role of defects.