Hybrid-order topological insulators in a phononic crystal

TL;DR

This paper demonstrates a novel hybrid-order topological insulator in a bilayer phononic crystal that hosts both first-order and second-order topological states, including edge and corner states, with potential applications in acoustic devices.

Contribution

It introduces the concept of hybrid-order topological insulators in phononic crystals, combining multiple topological phases in a single system for the first time.

Findings

Coexistence of 1D helical edge states and 0D corner states.

Realization of hybrid-order topological phase in phononic crystal.

Potential for novel topological acoustic device applications.

Abstract

Topological phases, including the conventional first-order and higher-order topological insulators and semimetals, have emerged as a thriving topic in the fields of condensed-matter physics and material science. Usually, a topological insulator is characterized by a fixed order topological invariant and exhibits associated bulk-boundary correspondence. Here, we realize a new type of topological insulator in a bilayer phononic crystal, which hosts simultaneously the first-order and second-order topologies, referred here as the hybrid-order topological insulator. The one-dimensional gapless helical edge states, and zero-dimensional corner states coexist in the same system. The new hybrid-order topological phase may produce novel applications in topological acoustic devices.