Experimental characterization of fragile topology in an acoustic metamaterial

TL;DR

This study experimentally demonstrates the fragile topological nature of low-lying Bloch bands in an acoustic metamaterial, highlighting how crystalline symmetries influence topological classifications and spectral responses.

Contribution

The paper provides the first experimental characterization of fragile topology in an acoustic metamaterial, including symmetry analysis and spectral signatures under boundary condition twists.

Findings

Fragile topology confirmed in acoustic metamaterials.

Spectral flow observed under twisted boundary conditions.

Symmetry properties crucial for topological classification.

Abstract

Symmetries crucially underlie the classification of topological phases of matter. Most materials, both natural as well as architectured, possess crystalline symmetries. Recent theoretical works unveiled that these crystalline symmetries can stabilize fragile Bloch bands that challenge our very notion of topology: while answering to the most basic definition of topology, one can trivialize these bands through the addition of trivial Bloch bands. Here, we fully characterize the symmetry properties of the response of an acoustic metamaterial to establish the fragile nature of the low-lying Bloch bands. Additionally, we present a spectral signature in the form of spectral flow under twisted boundary conditions.