Observation of an acoustic octupole topological insulator

Understand

The Berry phase associated with energy bands in crystals can lead to quantized quantities, such as the quantization of electric dipole polarization in an insulator, known as a one-dimensional (1D) topological insulator (TI) phase.

• Recent theories have generalized such quantization from dipole to higher multipole moments, giving rise to the discovery of multipole TIs, which exhibit a cascade hierarchy of multipole topology at boundaries of boundaries: A quantized octupole moment in the three-dimensional (3D) bulk can induce quantized quadrupole moments on its two-dimensional (2D) surfaces, which then produce quantized dipole moments along 1D hinges.
• The model of 2D quadrupole TI has been realized in various classical structures, exhibiting zero-dimensional (0D) in-gap corner states.
• Here we report on the realization of a quantized octupole TI on the platform of a 3D acoustic metamaterial.