ABSTRACT Chiral phonons, circularly‐polarized lattice vibrations with intrinsic angular‐momentum, offer novel pathways for controlling heat transport, spin‐phonon interactions, and various quantum phenomena. Broken inversion symmetry is often required for chiral phonon modes with non‐zero angular‐momentum to develop. We demonstrate that in‐plane phonon modes in anisotropic MoO 2 can be chiral despite inversion and time‐reversal symmetries. Based on first‐principles symmetry analysis, we predict that anisotropic MoO 2 may have chiral phonon modes without having mirror symmetry but breaking the joint symmetry operation . We experimentally verified the chirality of the phonons by performing helicity‐resolved Raman (HRR) measurements and computing the degree of circular‐polarization (DOCP). The MoO 2 flakes exhibit both helicity‐reversed (chiral‐ B g ) and helicity‐conserved (achiral‐ A g ) Raman modes. Notably, the sign of DOCP offers a rapid and unambiguous way to differentiate between the A g and B g modes of MoO 2 . A giant frequency shift (∼5‐10 cm −1 ) is also observed with reversed‐helicity mode due to the non‐zero pseudo‐magnetic field in the MoO 2 . Additionally, the degree of anisotropy in MoO 2 is investigated through angle‐resolved, polarized Raman (ARPR) experiments. This thorough investigation of chiral phonons, including in‐plane Raman anisotropy, may facilitate the development of MoO 2 ‐based polarization‐sensitive devices support chiral optics applications in the future.
Kumar et al. (Thu,) studied this question.