Demonstration of acoustic high-order Stiefel-Whitney semimetal in bilayer graphene sonic crystals

TL;DR

This paper demonstrates an acoustic high-order Stiefel-Whitney semimetal in bilayer graphene sonic crystals, revealing novel topological phases with protected hinge and surface states at the same frequency.

Contribution

It introduces a new acoustic topological semimetal characterized by both first and second Stiefel-Whitney charges, with unique bulk-edge correspondence protected by a single topological invariant.

Findings

Existence of high-quality surface and hinge states at the same frequency

Experimental validation of topological hinge and surface states

Observation of bound states in the continuum (BICs)

Abstract

Recently, higher-order topological phases have endowed materials many exotic topological phases. For three-dimensional (3D) higher-order topologies, it hosts topologically protected 1D hinge states or 0D corner states, which extend the bulk-boundary correspondence of 3D topological phases. Meanwhile, the enrichment of group symmetries with exploration of projective symmetry algebras redefined the fundamentals of nontrivial topological matter with artificial gauge fields, leading to the discovery of new topological phases in classical wave systems. In this Letter, we construct an acoustic topological semimetal characterized by both the first and the second Stiefel-Whitney (SW) topological charges by utilizing the projective symmetry. Different from conventional high-order topologies with multiple bulk-boundary correspondences protected by different class topological invariants, acoustic high-order Stiefel-Whitney semimetal (HOSWS) has two different bulk-edge correspondences protected by only one class (SW class) topological invariant. Two types of topological hinge and surface states are embedded in bulk bands at the same frequency, featuring similar characteristics of bound states in the continuum (BICs). In experiments, we demonstrate the existence of high-quality surface state and hinge state at the interested frequency window with polarized intensity field distributions.