Chiral phonons

A rapidly increasing body of work reporting phenomena associated with lattice vibrations carrying angular momentum has led to the emergence of the field of chiral phonons. Some of these properties, such as the phonon magnetic moment, also occur in achiral phonons that are circularly or elliptically polarized, while the presence of chirality has additional implications for the types of interaction allowed between the phonons and light, electrons and other quasiparticles. In this Perspective we introduce a framework for classifying phonons with angular momentum, and provide illustrations of the different types using examples from the recent literature. Specifically, we suggest the term ‘axial phonon’ to encompass all phonons that carry angular momentum, real or pseudo, and reserve the term ‘chiral phonon’ for those phonons that break improper rotational symmetry. We hope that this scheme provides clarification on the matter of phonon chirality and will serve as a guide for future research.