Chiral phonons in honeycomb sublattice of layered CoS-like compounds

Hexagonal and kagome lattices exhibit extraordinary electronic properties. It is a natural consequence of additional discrete degree of freedom associated with a valley or the occurence of electronic flat-bands. Combination of both types of lattices, observed in CoSn-like compounds, leads not only to the topological electronic behavior, but also to the emergence of chiral phonon modes. Here, we study CoSn-like compounds in the context of realization of chiral phonons. Previous theoretical studies demonstrated that the chiral phonons can be found in ideal two-dimensional hexagonal or kagome lattices. However, it turns out that in the case of CoSn-like systems, the kagome lattice formed by d-block element is decorated by the additional p-block atom. This results in a two dimensional triangular lattice of atoms with non-equal masses and the absence of chiral phonons in the kagome plane. Contrary to this, the interlayer hexagonal lattice of p-block atoms is preserved and allows for the realization of chiral phonons. We discuss properties of these chiral phonons in seven CoSn-like compounds and demonstrate that they do not depend on atomic mass ratio or the presence of intrinsic magnetic order. The chiral phonons of d-block atoms can be restored by removing the inversion symmetry.