We investigate the chirality of phonon modes in twisted bilayer ${\mathrm{WSe}}_2$ 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 nondegenerate $K/K^{\prime }$ valley phonons are found for twist angles near ${60}^{\circ }$ whereas no nondegenerate 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 ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ 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 twisted bilayer ${\mathrm{WSe}}_2$. 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.

Close

PrevNext

View figure in article

• Crystal phenomena
• Lattice dynamics
• Phonons
• Quasiparticles & collective excitations
• Structural properties
• 2-dimensional systems
• Honeycomb lattice
• Transition metal dichalcogenides
• Molecular dynamics