Flat and tunable moire phonons in twisted transition-metal dichalcogenides

TL;DR

This paper demonstrates that electric fields can effectively tune the phonon dispersion in twisted transition-metal dichalcogenide bilayers, leading to flat and chiral moiré phonon modes that influence electronic properties.

Contribution

It introduces electric field tuning of moiré phonons in twisted TMDs, revealing universal spectra with softened modes and chiral characteristics, a novel control mechanism for phononic properties.

Findings

Electric fields tune moiré phonon spectra in twisted TMDs.

Identification of softened longitudinal acoustic and flat optical phonon modes.

Emergence of chiral phonon modes with angular momentum.

Abstract

Displacement fields are one of the main tuning knobs employed to engineer flat electronic band dispersions in twisted van der Waals multilayers. Here, we show that electric fields can also be used to tune the phonon dispersion of moir\'e superlattices formed by non-centrosymmetric materials, focusing on twisted transition metal dichalcogenide homobilayers. This effect arises from the intertwining between the local stacking configuration and the formation of polar domains within the moir\'e supercell. For small twist angles, increasing the electric field leads to a universal moir\'e phonon spectrum characterized by a substantially softened longitudinal acoustic phason mode and a flat optical phonon mode, both of which cause a significant enhancement in the vibrational density of states. The phasons also acquire a prominent chiral character, displaying a nonzero angular momentum spread across the Brillouin zone. We discuss how the tunability of the moir\'e phonon spectra may affect electronic properties, focusing on the recently discovered phenomenon of van der Waals ferroelectricity.