Single $\pi$-flux hosting topological defect modes in bilayer acoustic metamaterials

TL;DR

This paper demonstrates how to create and observe topological defect modes in a bilayer acoustic metamaterial by introducing a $ ext{pi}$ gauge flux, revealing robust localized modes linked to topological invariants.

Contribution

It introduces a method to realize topological defect modes in acoustic systems using gauge flux and verifies their robustness experimentally.

Findings

Topological defect modes are successfully realized in a bilayer acoustic network.

The defect modes are highly robust to symmetry-preserving disorder.

The study confirms the bulk-defect correspondence via topological invariants.

Abstract

The bulk-boundary correspondence, which relates topological properties of a material in the bulk to the presence of robust modes localized on the edge, is at the core of the now mature field of topological wave physics. More recently, it was realized that in crystalline structures, certain types of defects can host localized modes, in which case the bulk-boundary correspondence has to be replaced by a bulk-defect correspondence. These defect-localized modes are expected to have robust properties owing to their topological origin. In this work, we show how to obtain topological defect modes in a lattice possessing both mirror and chiral symmetry. The defect is obtained by endowing a plaquette with a non-trivial gauge flux. We show that the bulk-defect correspondence is satisfied by introducing appropriate topological invariants. Moreover, the topological defect modes are shown to be highly robust to the introduction of symmetry-preserving disorder. The model is then realized in an acoustic system made of a bilayer network of tubes, and the presence of topological defect modes is experimentally clearly demonstrated.