Université Paris Diderot, Sorbonne Paris Cite, MSC, UMR 7057 CNRS, F75205 Paris, France.
Phys Rev Lett. 2013 Aug 16;111(7):078101. doi: 10.1103/PhysRevLett.111.078101. Epub 2013 Aug 13.
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 and 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 throughout the phase diagram. 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.
我们考虑一个在一维和二维晶格中扩散和对齐的主动伊辛模型。扩散是有偏向的，使得正或负自旋更倾向于向左或向右跳跃，这在低温和高密度下产生了群集转变。我们构建了该模型的粗粒度描述，预测该转变在温度-密度系综中是一级液-气转变，临界密度趋于无穷大。在这个一级相变中，磁化强度与液相分数成正比，因此在整个相图中连续变化。通过微观模拟，我们表明这个理论预测在 2D 中成立，而在 1D 中，涨落改变了转变，例如，阻止了任何自发对称破缺。
