Low-energy moire phonons in twisted bilayer van der Waals heterostructures
JZ Lu and ZY Zhu and M Angeli and DT Larson and E Kaxiras, PHYSICAL REVIEW B, 106, 144305 (2022).
We develop a low-energy continuum model for phonons in twisted moire bilayers, based on a configuration space approach. In this approach, interatomic force constants are obtained from density functional theory (DFT) calculations of untwisted bilayers with various in-plane shifts. This allows for efficient computation of phonon properties for any small twist angle, while maintaining DFT-level accuracy. Based on this framework, we show how the low-energy phonon modes, including interlayer shearing and layer breathing modes, vary with the twist angle. As the twist angle decreases, the frequencies of the low-energy modes are reordered and the atomic displacement fields corresponding to phonon eigenmodes break translational symmetry, developing periodicity on the moire length scale. We demonstrate the capabilities of our model by calculating the phonon properties of three specific structures: Bilayer graphene, bilayer molybdenum disulfide (MoS2), and molybdenum diselenide-tungsten diselenide (MoSe2-WSe2).
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