Mixed higher-order topology: Boundary non-Hermitian skin effect induced by a Floquet bulk

We show that anomalous Floquet topological insulators generate intrinsic, non-Hermitian topology on their boundary. As a consequence, removing boundary hopping from the time-evolution operator stops propagation of chiral edge modes, leading to skin effect. This does not occur in Chern insulators, however, which modes continue propagating. The effect is consequence nontrivial bulk operator, we by designing real-space invariant. Our work introduces form `mixed' higher-order topology, providing bridge between research periodically-driven systems and study non-Hermiticity. It suggests periodic driving, has already been demonstrated wide range experiments, may be used effects.