Spontaneously ordered motion of self-propelled particles

We study a biologically inspired, inherently non-equilibrium model consisting of self-propelled particles. In the model, particles move on a plane with a velocity of constant magnitude; they locally interact with their neighbours by choosing at each timestep a velocity direction equal to the average direction of their neighbours. Thus, in the limit of vanishing velocities the model becomes analogous to a Monte Carlo realization of the classical XY ferromagnet. We show by large-scale numerical simulations that, unlike in the equilibrium XY model, a long-range ordered phase characterized by non-vanishing net flow, φ, emerges in this system in a phase-space domain bordered by a critical line along which the fluctuations of the order parameter diverge. The corresponding phase diagram as a function of two parameters, the amplitude of noise η and the average density of the particles % is calculated and is found to have the form ηc(%) ∼ %1/2. We also find that φ scales as a function of the external bias h (field or ‘wind’) according to a power law φ ∼ h0.9. In the ordered phase the system shows long-range correlated fluctuations and 1/f noise.