Chiral phonons in two-dimensional materials

There has been growing interest in investigating chiral phonons since they are theoretically found and experimentally verified recently. In a magnetic system with time inversion symmetry breaking, phonon can have nonzero angular momentum, which makes a correction to the gyromagnetic ratio measured in the Einstein-de Haas effect. Though total phonon angular momentum is zero in a nonmagnetic two-dimensional (2D) hexagonal system with space inversion symmetry breaking, phonons at high symmetry points of the Brillouin zone can have nonzero phonon angular momentum, which means they are chiral phonons. Chiral phonons decide selection rules in the electronic intervalley scattering, which has been experimentally verified in tungsten-diselenide monolayers very recently (Zhu et al 2018 Science 359 579). In this review, after a brief introduction of related background and some basic concepts, we mainly report recent progress of phonon angular momentum in magnetic systems and chiral phonon in nonmagnetic systems. We also review known experiments in verifications of the phonon chirality and finally conclude with an outlook of future developments.