Diverse degeneracy types in topological phonons: A perspective

The study of novel topological quantum states is gaining significant attention. Recently, the concept of topology has expanded from fermions to bosons, leading to the new era of topological phonons. In crystals, distinctive symmetry constraints lead to diverse excitations among quasiparticles. This makes different patterns of phonon degeneracy in the spectrum. In this perspective, we review recent advances in various types of degeneracies found in topological phonons, including nodal points, nodal lines or rings, and unconventional nodal points with higher Chern numbers. We conclude by looking ahead at exciting possibilities in topological phonons.