Revisiting the flocking transition using active spins

TL;DR

This paper studies an active Ising model with biased diffusion, revealing a first-order flocking transition in 2D predicted by coarse-grained theory, while fluctuations prevent symmetry breaking in 1D.

Contribution

It introduces a coarse-grained framework predicting a first-order flocking transition in active spins, validated by simulations in 2D and analyzed in 1D.

Findings

First-order flocking transition in 2D confirmed by simulations.

Fluctuations prevent symmetry breaking in 1D.

Theoretical predictions match simulation results in 2D.

Abstract

We consider an active Ising model in which spins both diffuse and align on lattice in one and two dimensions. The diffusion is biased so that plus or minus spins hop preferably to the left or to the right, which generates a flocking transition at low temperature/high density. We construct a coarse-grained description of the model that predicts this transition to be a first-order liquid-gas transition in the temperature-density ensemble, with a critical density sent to infinity. In this first-order phase transition, the magnetization is proportional to the liquid fraction and thus varies continuously. Using microscopic simulations, we show that this theoretical prediction holds in 2d whereas the fluctuations alter the transition in 1d, preventing for instance any spontaneous symmetry breaking.