Discovering and Engineering two-dimensional Weyl phonons from First principles

Topological properties in phononic systems have attracted great attention in the past decade, as phonons differ fundamentally from electrons in many aspects. Phonon Weyl points (WPs) and Weyl nodal lines(WNLs), as typical topological features in 3D systems, have been widely predicted and studied in different compounds. In 2D systems, although dirac nodes in phonon dispersions are well-studied in hexagonal lattices (for example graphene), Weyl phonons did not receive equal attention. The lack of the third free variable k_z imposes symmetry conditions on the existence of WPs in 2D, and materials realizations of electronic Weyls points in systems such as monolayer FeB₂ and Cr₂C are predicted only recently. In this study we extend the concept of 2D Weyl electrons to 2D Weyl phonons. We use DFT to calculate phonon band structures to explore type-I/type-II WPs and WNLs for a series of 2D hexagonal materials with inversion symmetry breaking. By tuning crystal structures, we engineer the phonon band structures, thus transform WPs and WNLs.