Higher Order Topological Insulator: Acoustic Quadrupole Topological Insulator
Our recent experimental work about acoustic quadrupole topological insulators is published in Physical Review Letters.
Recently, tremendous efforts have been devoted to realizing high-order topological insulators (HOTIs). Quadrupole topological insulators (QTIs), featured with nontrivial bulk quadrupole moments, are proposed as the first type of HOTIs. However, the tight-binding model proposed for such emergent topological insulators demands both positive and negative hopping coefficients, which poses a great challenge in practical realizations.
In this Letter, we introduce a simple mechanism to construct positive and negative hoppings in acoustics. We present the first experimental realization of the acoustic QTI that stringently fulfills the tight-binding model proposed for QTI theory. The hierarchy topology of our acoustic QTI has been conclusively identified through detecting the acoustic responses at the bulk, edge and corner. The arbitrary controlling of the real-valued hoppings enables the further investigation of rich physics inherent in Z2-gauge transformation. Our study can also be extended to other HOTIs such as three-dimensional octupole topological insulators and semimetals, and open new application avenues for the robust and highly confined topological in-gap states.
Fig. 1. Acoustic realizations of negative and positive hoppings.
Fig. 2. Realizations of acoustic quadrupole topological insulators and experimental demonstration.