Twist Angle-Dependent\nPhonon Hybridization in WSe₂/WSe₂ Homobilayer

Abstract

The emerging moiré superstructure of twisted transition\nmetal dichalcogenides (TMDs) leads to various correlated electronic\nand optical properties compared to those of twisted bilayer graphene.\nIn such a versatile architecture, phonons can also be renormalized\nand evolve due to atomic reconstruction, which, in turn, depends on\nthe twist angle. However, observing this reconstruction and its relationship\nto phonon behavior with conventional, cost-effective imaging methods\nremains challenging. Here, we used noninvasive Raman spectroscopy\non twisted WSe₂/WSe₂ (t-WSe₂) homobilayers\nto examine the evolution of phonon modes due to interlayer coupling\nand atomic reconstruction. Unlike in the natural bilayer (NB), ∼0°\nas well as ∼60° t-WSe₂ samples, the nearly\ndegenerate A_1g/E_2g mode in the twisted samples\n(1–7°) split into a doublet in addition to the nondegenerate\nB_2g mode, and the maximum splitting is observed around\n2–3°. Our detailed theoretical calculations qualitatively\ncapture the splitting and its dependence as a function of the twist\nangle and highlight the role of the moiré potential in phonon\nhybridization. Additionally, we found that around the 2° twist\n angle, the anharmonic phonon–phonon interaction is higher\nthan the natural bilayer and decreases for larger twist angles. Interestingly,\nwe observed anomalous Raman frequency softening and line-width increase\nwith the decreasing temperature below 50 K, pointing to the combined\neffect of enhanced electron–phonon coupling and cubic anharmonic\ninteractions in moiré superlattice.