Circular Raman responses from angular-momentum inequivalence in CoSi

Abstract

Circularly polarized Raman scattering in solids exhibits distinct phenomena such as Raman optical activity (ROA) and chiral-phonon-induced frequency splitting, whose relationship has remained unclear. Here we show that these seemingly different responses can be understood within a common framework based on the inequivalence of phonon states carrying opposite crystal angular momenta. Using helicity-resolved Raman spectroscopy of the chiral crystal CoSi, we find that ROA and frequency splitting arise from different symmetry channels, namely axial multipolar symmetry and structural chirality, respectively. First-principles calculations reproduce both effects and clarify their symmetry origins. These results establish angular-momentum inequivalence as a unifying principle of circular Raman responses and link helicity-resolved Raman spectroscopy to the angular-momentum structure of chiral phonons in topological materials.