Chiral phonons in honeycomb sublattice of layered CoSn-like compounds

TL;DR

This paper investigates the presence of chiral phonons in layered CoSn-like compounds, revealing their dependence on interlayer hexagonal lattices and symmetry, and demonstrating how to restore chiral phonons by symmetry reduction.

Contribution

It provides a detailed analysis of chiral phonons in CoSn-like compounds, showing their realization in interlayer lattices and how symmetry breaking can restore them.

Findings

Chiral phonons are present in interlayer hexagonal lattices of CoSn-like compounds.

Chiral phonons do not depend on atomic mass ratio or magnetic order.

Reducing symmetry in certain compounds can restore chiral phonons.

Abstract

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 with the $P6/mmm$ symmetry, 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. The latter is possible in the crystal structure of CoGe and RhPb with the reduced symmetry ($P\bar{6}2m$) and distorted-kagome-like lattice.