Demonstration of a quantized acoustic octupole topological insulator

Understand

Recently extended from the modern theory of electric polarization, quantized multipole topological insulators (QMTIs) describe higher-order multipole moments, lying in nested Wilson loops, which are inherently quantized by lattice symmetries.

• Overlooked in the past, QMTIs reveal new types of gapped boundaries, which themselves represent lower-dimensional topological phases and host topologically protected zero-dimensional (0D) corner states.
• Inspired by these pioneering theoretical predictions, tremendous efforts have been devoted to the experimental observation of topological quantized quadrupole phase in a variety of two dimensional (2D) metamaterials.
• However, due to stringent requirements of anti-commuting reflection symmetries in crystals, it has been challenging to achieve higher-order quantized multipole moments, such as octupole moments, in a realistic three-dimensional (3D) structure.