Microscopic chiral phonon junction identification in vdW ferroelastic WO ₂ I ₂

Abstract Chiral phonons (CPs)—vibrational modes carrying angular momentum—offer a new degree of freedom for phononic quantum information and topological energy transport. However, their microscopic identification and control within van der Waals materials remain limited. Here, we report the direct observation of domain-dependent CP junctions in layered ferroelastic WO₂I₂, whose intrinsic micro-strain twinning provides a natural platform for CP localization. Using helicity-resolved and angle-resolved Raman spectroscopy, we reveal distinct circular polarization responses in specific domains, where A g modes at 115 cm −1 and 753 cm −1 exhibit strong helicity contrast, while the B g mode remains helicity-independent. These chiral responses are confined to ferroelastic domains with higher local strain. Density functional theory (DFT + U ) calculations accurately reproduce the experimental Raman features and show that strain induces degeneracy lifting in the A g modes, consistent with domain-resolved observations. Our results demonstrate that ferroelastic strain engineering can be used to generate and manipulate microscopic CP junctions, establishing WO₂I₂ as a promising candidate for strain-tunable quantum phononic devices.