Triple hourglass Weyl phonons

Abstract

Unconventional Weyl phonons with higher topological charges in crystalline solids have attracted increasing attention. By symmetry analysis and low-energy $k.p$ effective Hamiltonian, we propose the symmetry enforced triple hourglass Weyl phonons (THWPs) with Chern number $\mathcal C$ = $\pm$ 3 protected by $6_3$ screw rotation symmetry in chiral space groups 173 ($P6_3$) and 182 ($P6_322$). We take LiIO$_3$ with space group 173 as a candidate and confirm that it possesses THWP with linear and quadratic dispersions along the $k_z$ direction and in the $k_x$-$k_y$ plane, respectively. Due to the constraints of crystal symmetry and topological charge conservation, six equivalent single Weyl phonons (SWPs) emerge and lie in the $k_z$ = 0 plane. Therefore, the unique phonon surface arcs connect the projections of two THWPs and six SWPs, leading to nontrivial sextuple-helicoid surface arcs on the (001) surface Brillouin zone. Our work proposes a class of topological phonons and realizes it in realistic materials, providing a perfect platform for experimental observation of THWPs. We expect our work to provide a new idea for detection of unconventional quasiparticles.