Anomalous topological active matter

Systems with topologically nontrivial band structures, which are at the center of the modern condensed matter physics, exhibit unidirectional modes that propagate along the edge of a sample and are immune to disorder. Here, we show that the topological edge modes can ubiquitously emerge in active matter, the collection of self-propelled particles, under influences of spatially periodic structures [1]. Topological edge modes have been found in some active systems by constructing the counterparts of quantum Hall effect. However, to realize the effective external magnetic field, they introduce intricate structures, which can be the bottleneck for an experimental realization. We conclude that, without those intricate structures, it is hard to construct an active-matter counterpart of quantum Hall effect. Instead, by constructing the analogy with quantum anomalous Hall effect, our theoretical proposal eliminates the bottleneck for realizing topological active matter and broadens its applicability. We also discuss its relevance to ongoing experiments in biological systems.
[1] K. Sone and Y. Ashida, arXiv:1905.08999 (to be published in Phys. Rev. Lett.).