Signatures of Chiral Phonons in MnPS$_3$ from first principles
Two-dimensional (2D) materials may host circular phonons, considered as chiral if the presence of a substrate breaks mirror symmetry. In 2D transition metal dichalcogenide (TMDC) monolayers lacking inversion symmetry, phonons with a given chirality can be observed in the non-equilibrium state triggered by optical excitations using circularly polarized light. Backed by first-principles calculations, we present the antiferromagnetic semiconductor MnPS$_3$ with a hexagonal crystal structure and bandstructure similar to TMDCs, but a larger unit cell, as a novel candidate material that may allow for excitation of circular phonons. Using DFT+U and the finite displacement method we obtain in-plane chiral phonon modes at the valley points of a monolayer MnPS$_3$. These modes can be classified according to the Mn or S atoms performing circular motions about their equilibrium positions. In each case, the quantized angular momentum of the phonons is calculated. Moreover, we point out ways to populate the chiral phonons selectively via optical excitation with circularly polarized light.