Symmetry-Driven Phonon Circular Polarization in <i>α</i> -Graphyne: Unveiling the Interplay between Hybridization and Chirality

Chirality is a fundamental and ubiquitous concept in physical systems and has recently attracted growing interest in phononics. As a two-dimensional carbon allotrope related to graphene, α -graphyne contains both sp- and sp 2 -hybridized carbon atoms, leading to distinct lattice symmetries and rich vibrational characteristics. In this work, we employ first-principles calculations to systematically investigate the effects of time-reversal and spatial inversion symmetries on phonon circular polarization in α -graphyne. We find that, in the presence of an effective magnetic field, extrema of phonon circular polarization occur not only at high-symmetry points but also at generic k points in the Brillouin zone, reflecting the hybridized nature of the lattice. At the Γ point, the effective magnetic field breaks time-reversal symmetry, lifts phonon degeneracies, and polarizes the originally degenerate modes, thereby generating finite phonon angular momentum. Away from high-symmetry points, the field removes accidental degeneracies and induces circular polarization of the corresponding phonon modes. Furthermore, breaking spatial inversion symmetry through site-selective isotope substitution gives rise to intrinsic circularly polarized phonon modes with momentum-dependent characteristics. These results elucidate the symmetry-governed mechanisms underlying phonon circular polarization in α -graphyne and demonstrate the feasibility of tuning vibrational angular momentum in carbon-based materials via magnetic and structural symmetry breaking.