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Authors: Christopher Bock, Christoph Weber, Erwin Frey.
To uncover the principles underlying the emergence of swarms a plethora of minimal models have been designed throughout the last two decades. A famous minimal model is the one published by Vicsek et al. [1], where point particles move off-lattice at constant speed, adjusting their direction instantaneously to the average velocity of their neighbors, slightly disturbed by some noise term. Here we study the Vicsek model in the presence of a weak repulsive interaction and find new states distinguishable by their kinetic behavior. We observe mostly non-stationary states with various kinetic properties - amongst them are intermittent states, which switch continuously between flow and disorder. In addition we find fast "shock"-waves, and show that their occurrence is correlated with the fluid's ordering capability. The shockwaves' transport properties, such as propagation speed and relaxation time, are analyzed. Moreover, we identify the density to be an essential control parameter for the emergence of a polar flow state. Upon increasing the density, a strong increase of the characteristic time for a bulk polar state to develop is found. For large densities, particles' movement is found to be either ballistic, diffusive or caged - indicated by long time periods of sub-diffusive motion. In this regime hexatic order occurs independent of polarity. [1] T. Vicsek, A. Czirok, E. Ben-Jacob, I. Cohen, O. Shochet. Novel Type of Phase Transition in a system of self-propelled particles. PRL, Vol 75, No 6. |
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