Chiral valley phonons and flat phonon bands in moiré materials

Abstract

We investigate the chirality of phonon modes in twisted bilayer WSe2 and demonstrate distinct chiral behavior of the $K/K^{\prime }$ valley phonons for twist angles close to $0^{\circ }$ and close to $60^{\circ }$. In particular, multiple chiral non-degenerate $K/K^{\prime }$ valley phonons are found for twist angles near $60^{\circ }$ whereas no non-degenerate chiral modes are found for twist angles close to $0^{\circ }$. Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moiré scale and find similar modes in moiré patterns of strain-engineered bilayers WSe2 and MoSe2/WSe2 heterostructures. At the energy gap between acoustic and optical modes, the formation of flat phonon bands for a broad range of twist angles is observed in the twisted bilayer WSe2. Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moiré materials and also for the design of phononic analogues of flat band electrons.

arXiv commentAccepted as a Letter in Phys. Rev. B

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• Truncated atomic plane wave method for the subband structure calculations of Moiré systems
• Low-energy moiré phonons in twisted bilayer van der Waals heterostructures
• Moiré phonons in graphene/hexagonal boron nitride moiré superlattice
• Moiré phonons and impact of electronic symmetry breaking in twisted trilayer graphene
• Chiral phonons in binary compounds $A$Bi ($A$ = K, Rb, Cs) with P2${}_1$/c structure
• Physics of Phonons in Systems with Approximate Screw Symmetry
• Anomalous circular phonon dichroism in transition metal dichalcogenides
• Chiral phonon in the cubic system based on the Laves phase of $A$Bi${}_2$ ($A=$K, Rb, Cs)