Defect-Mediated Phase Transitions in Active Soft Matter
  How do topological defects affect the degree of order in active matter? To answer this question we investigate an agent-based model of self-propelled particles, which accounts for polar alignment and short-ranged repulsive interactions. For strong alignment forces we find collectively moving polycrystalline states with fluctuating networks of grain boundaries. In the regime where repulsive forces dominate, the fluctuations generated by the active system give rise to quasi-long-range transitional order, but---unlike thermal system---without creating topological defects. 
Christoph A. Weber, Christopher Bock and Erwin Frey
cond-mat.soft