Chiral phonon in the cubic system based on the Laves phase of $A$Bi$_{2}$ ($A=$K, Rb, Cs)

TL;DR

This study investigates chiral phonons in cubic $A$Bi$_{2}$ compounds, showing doping can induce a new phase with non-zero PAM and proposing new materials for phonon engineering and potential phonon Hall effect applications.

Contribution

It demonstrates how doping induces a symmetry change leading to chiral phonons with non-zero PAM and predicts two new stable materials with these properties.

Findings

Doping causes symmetry change to F$ar{4}3$m.

Chiral phonons occur at the Brillouin zone edge.

New stable materials KRbBi$_{4}$ and RbCsBi$_{4}$ are predicted.

Abstract

$A$B$_{2}$ ($A=$K, Rb, Cs) compounds crystallize in the cubic Laves phase (symmetry Fd$\bar{3}$m). The geometry of the crystal structure allows the realization of chiral phonons, which are associated with the circulation of atoms around their equilibrium positions. Due to the inversion symmetry and time reversal symmetry, total pseudo-angular momentum (PAM) of the system vanishes. We show that the doping of these system can lead to a new phase with symmetry F$\bar{4}3$m. New systems (KRbBi$_{4}$ and RbCsBi$_{4}$) do not exhibit soft modes (are stable dynamically). Due to the inversion symmetry breaking, realized chiral phonon modes posses a non-zero total PAM. In both type of systems the chiral phonons are realized for the wavectors at the edge of the Brillouine zone. This study explores the possibility of chiral phonon engineering via doping, and predicts two new materials. Discussing the problem opens a new way to study the phonon Hall effect.