Dense topological defects in weak active nematics

So far, the emergence of half-integer topological defects has been considered a hallmark of dense, spatially uniform active nematics. Dilute or weakly interacting systems, on the other hand, exhibit qualitatively quite different ordering behavior and dynamics. One observes micro-phase separation into nematic bands with intriguing complex dynamics, but so far without the emergence of half-integer topological defects. Here we describe a new type of defects in weakly interacting active nematics that exhibit particle mass condensation and active particle currents near their center. They coexist with nematic lanes and are formed when three curved nematic lanes meet and condense into a topological defect with a high-density core region. We analyze this system combining two approaches: a previously developed agent-based model for weakly interacting self-propelled polymers and a phenomenological hydrodynamic theory of weak active nematics. We reveal the mechanisms underlying the new defects formation and identify the corresponding control parameters. In particular, we show that condensation of density around the defects cores is determined by the interplay between isotropic. Finally, we discuss the potential role of dense topological defects in the formation of active foams.