Observation of degenerate zero-energy topological states at disclinations in an acoustic lattice

TL;DR

This paper demonstrates the experimental realization of topological states bound to disclinations in acoustic lattices, which are protected by chiral symmetry and exhibit maximal confinement without fractional density of states.

Contribution

The study introduces a new paradigm for topological states at disclinations that preserve chiral symmetry, ensuring protected mid-gap states with integer density of states in acoustic systems.

Findings

Topological states are bound to disclinations with preserved chiral symmetry.

States are pinned at mid-gap, ensuring protection and maximal confinement.

Two-fold degeneracy due to integer density of states and symmetry constraints.

Abstract

Building upon the bulk-boundary correspondence in topological phases of matter, disclinations have recently been harnessed to trap fractionally quantized density of states (DoS) in classical wave systems. While these fractional DoS have associated states localized to the disclination's core, such states are not protected from deconfinement due to the breaking of chiral symmetry, generally leading to resonances which, even in principle, have finite lifetimes and suboptimal confinement. Here, we devise and experimentally validate in acoustic lattices a paradigm by which topological states bind to disclinations without a fractional DoS but which preserve chiral symmetry. The preservation of chiral symmetry pins the states at the mid-gap, resulting in their protected maximal confinement. The integer DoS at the defect results in two-fold degenerate states that, due to symmetry constraints, do not gap out. Our study provides a fresh perspective on the interplay between symmetry-protection in topological phases and topological defects, with possible applications in classical and quantum systems alike.