Phononic real Chern insulator with protected corner modes in graphynes

TL;DR

This paper demonstrates that certain 2D graphyne materials host phononic second-order topological insulator states, featuring protected corner modes due to real Chern topology, expanding higher-order topological concepts to phonons in real materials.

Contribution

It reveals phononic second-order topological insulator phases in graphyne materials, characterized by real Chern numbers and protected corner modes, using first-principle calculations and symmetry analysis.

Findings

Identification of nontrivial phononic band gaps with real Chern numbers

Verification of protected phonon corner modes in finite nanodisks

Extension of higher-order topology to phonons in real materials

Abstract

Higher-order topological insulators have attracted great research interest recently. Different from conventional topological insulators, higher-order topological insulators do not necessarily require spin-orbit coupling, which makes it possible to realize them in spinless systems. Here, we study phonons in 2D graphyne family materials. By using first-principle calculations and topology/symmetry analysis, we find that phonons in both graphdiyne and $\gamma$-graphyne exhibit a second-order topology, which belongs to the specific case known as real Chern insulator. We identify the nontrivial phononic band gaps, which are characterized by nontrivial real Chern numbers enabled by the spacetime inversion symmetry. The protected phonon corner modes are verified by the calculation on a finite-size nanodisk. Our study extends the scope of higher-order topology to phonons in real materials. The spatially localized phonon modes could be useful for novel phononic applications.