Flat and Tunable Moiré Phonons in Twisted Transition-Metal Dichalcogenides

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanotechnology

Background:

• Twisted van der Waals multilayers utilize displacement fields to engineer flat electronic band dispersions.
• Noncentrosymmetric materials in moiré superlattices offer potential for novel electronic and vibrational properties.

Purpose of the Study:

• To investigate the effect of electric fields on phonon dispersion in twisted transition metal dichalcogenide homobilayers.
• To understand how electric fields modify moiré superlattice properties, particularly phonon spectra and vibrational density of states.

Main Methods:

• Theoretical analysis of phonon dispersion in moiré superlattices under electric fields.
• Focus on twisted transition metal dichalcogenide homobilayers with noncentrosymmetric properties.
• Examination of the interplay between local stacking, polar domains, and phonon behavior.

Main Results:

• Electric fields universally tune moiré phonon spectra for small twist angles.
• A substantially softened longitudinal acoustic phason mode and a flat optical phonon mode are observed.
• Phasons exhibit chiral character with a nonzero angular momentum spread.

Conclusions:

• Electric field is a viable tuning knob for moiré phonon spectra, complementing displacement fields.
• Tunable moiré phonon spectra can significantly enhance vibrational density of states.
• The observed tunability may influence electronic properties, including van der Waals ferroelectricity.