Charge-two Weyl phonons with type-III dispersion

In recent years, charge-two Weyl point phonons (WPPs) have attracted increasing attention. Charge-two WPPs not only provide a new platform for realizing phonon-based unconventional Weyl points (WPs) but also help in realizing specific phonon-based transport behaviors. Herein, based on the first-principles calculations and symmetry analysis, we propose a realistic material with the P3121 space group, BaZnO2, which has a charge-two WPP and two charge-one WPPs at high-symmetry points. These three unpaired WPPs form a unique triangular Weyl complex. Unlike previously reported material candidates with type-I or type-II charge-two WPPs, the proposed BaZnO2 has a type-III charge-two WPP, which has a constant frequency surface that contains two electronlike or holelike states connected at the charge-two WP. BaZnO2 can support double-helicoid phonon surface states that cover the entire (001) surface Brillouin zone. The clean type-III WPP, unique triangular Weyl complex, and clear and long surface states in BaZnO2 suggest that it is an excellent platform for further research into the physics and applications of type-III charge-two WPPs. Furthermore, we pointed out that charge-two WPPs with type-III band dispersion may appear at high-symmetry points in space groups 75-80, 89-98, 143-146, 149-155, 168-173, 177-182, 196, 207-210. Besides trigonal BaZnO2, some material candidates, including tetragonal MgTiO4, trigonal Li2GeF6, hexagonal CaSO4, and cubic Li10B14Cl2O25, are also shown to be the hosts of type-III charge-two WPPs.