# Realization of an acoustic third-order topological insulator

## TL;DR

This paper reports the first experimental realization of a third-order topological insulator in a 3D acoustic metamaterial, demonstrating topological corner states and expanding the understanding of higher-order topological phases.

## Contribution

The work introduces a 3D acoustic metamaterial that exhibits third-order topological insulator behavior, extending topological corner states from 2D to 3D systems.

## Key findings

- Observation of corner states in a 3D acoustic structure
- Confirmation of nontrivial topology via Wannier centers
- Extension of topological insulator concepts to third-order in 3D

## Abstract

The recent discovery of higher-order topological insulators (TIs) has opened new possibilities in the search for novel topological materials and metamaterials. Second-order TIs have been implemented in two-dimensional (2D) systems exhibiting topological 'corner states', as well as three-dimensional (3D) systems having one-dimensional (1D) topological 'hinge states'. Third-order TIs, which have topological states three dimensions lower than the bulk (which must thus be 3D or higher), have not yet been reported. Here, we describe the realization of a third-order TI in an anisotropic diamond-lattice acoustic metamaterial. The bulk acoustic bandstructure has nontrivial topology characterized by quantized Wannier centers. By direct acoustic measurement, we observe corner states at two corners of a rhombohedron-like structure, as predicted by the quantized Wannier centers. This work extends topological corner states from 2D to 3D, and may find applications in novel acoustic devices.

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Source: https://tomesphere.com/paper/1903.01194