Combinatorics

Physics

Topology (electrical circuits)

Mathematics

Topological insulator

Band gap

Acoustic metamaterials

Insulator (electricity)

Condensed matter physics

Quadrupole

Quantum mechanics

Optoelectronics

Physical Review Letters

A quadrupole topological insulator, being one higher-order topological insulator with nontrivial quadrupole quantization, has been intensely investigated very recently. However, the tight-binding model proposed for such emergent topological insulators demands both positive and negative hopping coefficients, which imposes an obstacle in practical realizations. Here we introduce a feasible approach to design the sign of hopping in acoustics, and construct the first acoustic quadrupole topological insulator that stringently emulates the tight-binding model. The inherent hierarchy quadrupole topology has been experimentally confirmed by detecting the acoustic responses at the bulk, edge and corner of the sample. Potential applications can be anticipated for the topologically robust in-gap states, such as acoustic sensing and energy trapping.

Acoustic Realization of Quadrupole Topological Insulators