Observation of an acoustic octupole topological insulator

Haoran Xue, Yong Ge, Hong-Xiang Sun (), Qiang Wang, Ding Jia, Yi-Jun Guan, Shou-Qi Yuan, Yidong Chong () and Baile Zhang ()
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Haoran Xue: Nanyang Technological University
Yong Ge: Jiangsu University
Hong-Xiang Sun: Jiangsu University
Qiang Wang: Nanyang Technological University
Ding Jia: Jiangsu University
Yi-Jun Guan: Jiangsu University
Shou-Qi Yuan: Jiangsu University
Yidong Chong: Nanyang Technological University
Baile Zhang: Nanyang Technological University

Nature Communications, 2020, vol. 11, issue 1, 1-6

Abstract: Abstract Berry phase associated with energy bands in crystals can lead to quantised observables like quantised dipole polarizations in one-dimensional topological insulators. Recent theories have generalised the concept of quantised dipoles to multipoles, resulting in the discovery of multipole topological insulators which exhibit a hierarchy of multipole topology: a quantised octupole moment in a three-dimensional bulk induces quantised quadrupole moments on its two-dimensional surfaces, which in turn induce quantised dipole moments on one-dimensional hinges. Here, we report on the realisation of an octupole topological insulator in a three-dimensional acoustic metamaterial. We observe zero-dimensional topological corner states, one-dimensional gapped hinge states, two-dimensional gapped surface states, and three-dimensional gapped bulk states, representing the hierarchy of octupole, quadrupole and dipole moments. Conditions for forming a nontrivial octupole moment are demonstrated by comparisons with two different lattice configurations having trivial octupole moments. Our work establishes the multipole topology and its full hierarchy in three-dimensional geometries.

Date: 2020
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DOI: 10.1038/s41467-020-16350-1

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