Ubiquitous topological states of phonons in solids: Silicon as a model material

TL;DR

This paper identifies silicon as a model material hosting diverse topological phonon states, including nodal lines, nexus phonons, and surface states, expanding the understanding of topological phonons in solids.

Contribution

It reveals silicon's rich topological phonon states, introduces a new type of nexus phonon, and generalizes symmetry analysis to find numerous candidate materials.

Findings

Silicon hosts topological nodal lines with quantized Berry phase.

Discovery of a novel nexus phonon with unique surface states.

Numerous candidate materials for topological phonons are identified.

Abstract

Research on topological physics of phonons has attracted enormous interest but demands appropriate model materials. Our {\it ab initio} calculations identify silicon as an ideal candidate material containing extraordinarily rich topological phonon states. In silicon, we identify various topological nodal lines protected by glide mirror or mirror symmetries and characterized by quantized Berry phase $\pi$, which gives drumhead surface states observable from any surface orientations. Remarkably, a novel type of topological nexus phonon is discovered, which is featured by double Fermi-arc-like surface states and distinguished from Weyl phonons by requiring neither inversion nor time-reversal symmetry breaking. Versatile topological states can be created from the nexus phonons, such as Hopf nodal link by strain. Furthermore, we generalize the symmetry analysis to other centrosymmetric systems and find numerous candidate materials, demonstrating the ubiquitous existence of topological phonons in solids. These findings open up new opportunities for studying topological phonons in realistic materials and their influence on surface physics.