Chiral phonons in kagome lattices

Chiral phonons have been theoretically proposed [Zhang and Niu, Phys. Rev. Lett. 115, 115502 (2015)] and experimentally verified [Zhu et al., Science 359, 579 (2018)] in materials with hexagonal lattice. Here, we theoretically predict and investigate the chiral phonons in the kagome lattice. We show that with broken inversion symmetry, chiral phonons with large circular polarization can emerge at the $K$ and $K^{\prime }$ high symmetry points. Different from the hexagonal lattice, these chiral phonons have all three sublattices vibrating with the same chirality, and the vibration orbit can take an elliptical shape. They possess valley-contrasting Berry curvatures sharply peaked at the $K$ and $K^{\prime }$ valleys, which can be used to separate the chiral phonons according to their valley label via the valley phonon Hall effect. The pseudoangular momentum for these chiral phonons has also been studied, which determines the selection rules of electron-phonon interaction in the intervalley scattering process. Furthermore, we show that by tuning the relative bond strength, there can be a change in phonon chirality associated with a phonon branch switching. Our work enriches the studies on chiral phonons and offers a probe for many fascinating quantum materials which possess the kagome type lattice.