SPONTANEOUSLY ORDERED MOTION OF SELF-PROPELLED PARTICLES

We study a biologically inspired, inherently non-equilibrium model con sisting of self-propelled particles. In the model, particles move on a plane with a velocity of constant magnitude; they locally interact wi th 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 nu merical simulations that, unlike in the equilibrium XY model, a long-r ange ordered phase characterized by non-vanishing net dow, phi, emerge s in this system in a phase-space domain bordered by a critical line a long which the fluctuations of the order parameter diverge. The corres ponding phase diagram as a function of two parameters, the amplitude o f noise eta and the average density of the particles rho is calculated and is found to have the form eta(c)(rho) similar to rho(1/2). We als o find that phi scales asa function of the external bias h (field or ' wind') according to a power law phi similar to h(0.9). in the ordered phase the system shows long-range correlated fluctuations and 1/f nois e.