Moiré collective vibrations in atomically thin van der Waals superlattices
Collective vibration is pivotal for materials’ thermal, electrical, phase transition and topological properties. Lately, the rising of moiré superlattices, characterized by overarching periodicity of moiré pattern, generates highly tunable interfacial structures that manipulate collective excitations in material at the atomic scale. Here, we experimentally demonstrate moiré collective vibrations, the mechanical counterparts of moiré excitons, at heterointerfaces of twisted tungsten diselenide/tungsten disulfide heterobilayers. Using helicity-resolved inelastic Raman scattering, we find chiral interfacial phonons carrying angular momentum analogous to that of chiral bulk phonons in quartz, enabling unprecedented spectral resolution of rich vibrational modes at heterointerface in a few atomic layers. Upon mutual torsion of heterobilayers, we observe terahertz interlayer vibrations proportional to moiré periodicity as a periodic function of rotation angles, demonstrating moiré-tuned interlayer modes which couple to Coulomb-bound electron-hole pairs in interlayer moiré excitons. In low-angle strong coupling regime, interlayer dynamics exhibit a distinct long-lived breathing mode with zero angular momentum and pronounced high energy, highlighting phonon-hybridization character wherein intralayer breathing vibrations are folded into moiré mini-Brillouin zone by spatial periodicity and hybridize with interlayer vibrations. Our findings establish moiré collective vibrations as candidates for exploitation in energy-efficient thermal management, strongly correlated electrical engineering, and new emergent topological phononics.
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