Phonon Renormalization in Reconstructed MoS$_2$ Moir\'e Superlattices

TL;DR

This paper reveals that phonon spectra in twisted bilayer MoS2 are significantly renormalized due to atomic reconstructions and mode coupling, with a new continuum model successfully explaining experimental observations and linking structural and optical properties.

Contribution

The study introduces a novel low-energy continuum model for phonons in moiré superlattices, capturing complex phonon behavior and enabling analysis of large supercells.

Findings

Phonon spectra are strongly renormalized in twisted MoS2 bilayers.

A new continuum model accurately predicts phonon behavior and experimental results.

Optical spectroscopy can infer strain and lattice distortions in moiré structures.

Abstract

In moir\'e crystals formed by stacking van der Waals (vdW) materials, surprisingly diverse correlated electronic phases and optical properties can be realized by a subtle change in the twist angle. Here, we discover that phonon spectra are also renormalized in MoS$_2$ twisted bilayers, adding a new perspective to moir\'e physics. Over a range of small twist angles, the phonon spectra evolve rapidly due to ultra-strong coupling between different phonon modes and atomic reconstructions of the moir\'e pattern. We develop a new low-energy continuum model for phonons that overcomes the outstanding challenge of calculating properties of large moir\'e supercells and successfully captures essential experimental observations. Remarkably, simple optical spectroscopy experiments can provide information on strain and lattice distortions in moir\'e crystals with nanometer-size supercells. The newly developed theory promotes a comprehensive and unified understanding of structural, optical, and electronic properties of moir\'e superlattices.