Topological node lines in mechanical metacrystals

Abstract

Topological elastic waves have recently emerged as an exciting interdisciplinary field which is still mainly focused on low-dimensional systems. Here we propose phononic topological node lines emerging from three-dimensional mechanical metacrystals. The resultant ribbon- or drumhead-like phononic edge states exhibit unusual frequency stability against perturbations, which can be exploited for robust elastic wave trapping for various applications. Our design leads to symmetry-guaranteed phononic topological node lines which are protected by both nonsymmorphic crystalline symmetry and fundamental conservation laws. The band topology, characterized by the nontrivial Zak phase, is also revealed through the reflection phase of the band gap.