Flow can order: Phases of live XY spins in two dimensions

TL;DR

This paper develops a hydrodynamic theory for active XY spins coupled with flow in 2D, revealing various ordered and disordered phases depending on activity and number conservation, with implications for understanding active matter systems.

Contribution

It introduces a comprehensive hydrodynamic framework for active XY spins with flow, analyzing phase behavior under different conservation and activity conditions, including novel order types.

Findings

Active flow can induce synchronization and various orders in 2D active XY spins.

Strong activity or flow coupling can lead to quasi long-range order or new order types.

Disordering mechanisms resemble those in active XY spins and KPZ surfaces.

Abstract

We present the hydrodynamic theory of active XY spins coupled with flow fields, for systems both having and or lacking number conservation in two dimensions (2D). For the latter, with strong activity or nonequilibrium drive, the system can synchronize, or be phase-ordered with various types of order, e.g., quasi long range order (QLRO) or new kind of order weaker or stronger than QLRO for sufficiently strong active flow-phase couplings. For the number conserving case, the system can show QLRO or order weaker than QLRO, again for sufficiently strong active flow-phase couplings. For other choices of the model parameters, the system necessarily disorders in a manner similar to immobile but active XY spins, or 2D Kardar-Parisi-Zhang surfaces.