Triple hourglass Weyl phonons
Guang Liu, Zhongjia Chen, Peng Wu, Hu Xu
Abstract
Unconventional Weyl phonons with higher topological charges in crystalline solids have attracted increasing attention. By symmetry analysis and a low-energy $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ effective Hamiltonian, we propose the symmetry-enforced triple hourglass Weyl phonons (THWPs) with Chern number $\mathcal{C}=\ifmmode\pm\else\textpm\fi{}3$ protected by ${6}_{3}$ screw rotation symmetry in chiral space groups 173 $(P{6}_{3})$ and 182 $(P{6}_{3}22)$. We take ${\mathrm{LiIO}}_{3}$ 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 ${k}_{x}\text{\ensuremath{-}}{k}_{y}$ 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.