Low-energy moir\'e phonons in twisted bilayer van der Waals heterostructures

TL;DR

This paper introduces a continuum model for low-energy phonons in twisted bilayer van der Waals heterostructures, enabling efficient and accurate analysis of phonon behavior across various twist angles.

Contribution

The authors develop a configuration-space continuum model that leverages DFT data to accurately compute phonon properties in twisted bilayers at any small twist angle.

Findings

Low-energy phonon modes vary with twist angle.

Phonon eigenmodes develop moiré-scale periodicity.

Model successfully applied to graphene and TMD bilayers.

Abstract

We develop a low-energy continuum model for phonons in twisted moir\'e 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 moir\'e length scale. We demonstrate the capabilities of our model by calculating the phonon properties of three specific structures: bilayer graphene, bilayer molybdenum disulfide (MoS$_2$), and molybdenum diselenide-tungsten diselenide (MoSe$_2$-WSe$_2$).