Quantifying chirality of phonons

Abstract

Recent years have witnessed growing interest in chiral phonons, lattice vibrations carrying angular momentum and exhibiting handedness, as revealed by helicity-dependent optical phenomena. Despite this progress, a quantitative characterization of phonon chirality as a dynamical property has remained elusive. In this work, we propose a theoretical framework to quantify the dynamical chirality of lattice vibrations. We introduce two quantitative measures: momentum-resolved dynamical chirality, which provides a mode- and wave-vector-resolved picture of phonon chirality, and the bulk dynamical chirality, which characterizes the collective behavior of thermally populated chiral phonons. Using first-principles calculations for both chiral and achiral materials, we demonstrate how these quantities capture the handedness and population imbalance of phonon modes and serve as a means to distinguish the enantiomers of chiral crystals.