Topological chiral interface phonons beyond mirror symmetry in h-GaN/h-BAs lateral heterostructures

Topological chiral phonons have so far been realized primarily in mirror-symmetric interface structures, raising the fundamental question of whether mirror symmetry is required for their existence. Here, using first-principles calculations, we demonstrate propagating topological chiral interface phonons in intrinsically asymmetric zigzag h-GaN/h-BAs heterostructure nanoribbons. The heterointerface naturally breaks mirror symmetry because of the distinct atomic species, masses, and bonding environments on the two sides, yet it supports interface-localized phonon modes with pronounced valley-contrasting chirality. We show that the opposite sublattice mass distributions of the constituent monolayers give rise to opposite phonon circular polarizations accompanied by complete valley-topology inversion, characterized by opposite valley Chern numbers for the corresponding in-plane phonon bands, thereby providing the topological contrast required for interface-state formation. Furthermore, the large overlapping phononic bandgaps of h-GaN and h-BAs create a broad and clean frequency window that effectively isolates the interface modes from bulk phonons. Unlike conventional valley chiral phonons in pristine two-dimensional materials, whose group velocities vanish at the valley extrema, the interface modes maintain finite group velocities throughout the Brillouin zone, enabling propagating topological phonon transport along the heterointerface. These results identify intrinsically asymmetric heterointerfaces as a viable route for realizing topological chiral interface phonons beyond mirror symmetry and offer new opportunities for low-dissipation phononic information processing and interface-engineered phononic devices.