Hourglass charge-three Weyl phonons

The unconventional Weyl point with nonlinear dispersion features higher topological charge |C|>1 and multiple topologically protected Fermi arc states at its boundary. As a novel topological state, it has been attracting widespread attention. However, the unconventional Weyl point with |C|=3 has not yet been reported in realistic materials, even though it has been theoretically proposed for more than a decade. In this work, based on first-principles calculations and theoretical analysis, we predict the existing material α-LiIO3 as the first realistic example with this unconventional Weyl point. Specifically, in the phonon spectra of α-LiIO3, two Weyl points with C=-3, connected by time-reversal symmetry, appear at the neck crossing point of an hourglass-type band, leading to two hourglass charge-3 Weyl phonons. The symmetry protection and the associated novel triple- and sextuple-helicoid surface arc states of the hourglass charge-3 Weyl phonons are revealed. Our results uncover a hidden topological character of α-LiIO3 and also show that the phonon spectra are a great platform for exploring unconventional topological states.