Twist Angle-Dependent Phonon Hybridization in WSe₂/WSe₂ Homobilayer

Area of Science:

• Condensed Matter Physics
• Materials Science
• Spectroscopy

Background:

• Moiré superlattices in twisted transition metal dichalcogenides (TMDs) create novel electronic and optical properties.
• Atomic reconstruction in these structures influences phonon behavior, which is challenging to observe with conventional methods.

Purpose of the Study:

• To investigate the evolution of phonon modes in twisted WSe2/WSe2 (t-WSe2) homobilayers using Raman spectroscopy.
• To understand the relationship between atomic reconstruction, twist angle, and phonon renormalization in moiré superlattices.

Main Methods:

• Noninvasive Raman spectroscopy was employed on twisted WSe2/WSe2 homobilayers with varying twist angles (1-7°).
• Detailed theoretical calculations were performed to model phonon hybridization and moiré potential effects.

Main Results:

• A splitting of the A1g/E2g phonon mode into a doublet was observed in twisted samples, with maximum splitting around 2-3° twist angles.
• Theoretical calculations qualitatively reproduced the observed splitting and its twist-angle dependence.
• Anharmonic phonon-phonon interactions were found to be higher in twisted samples compared to natural bilayers, decreasing with larger twist angles.
• Anomalous Raman frequency softening and line-width increase were observed below 50 K, indicating enhanced electron-phonon coupling and cubic anharmonic interactions.

Conclusions:

• Raman spectroscopy is a viable method for observing twist-angle-dependent phonon evolution in moiré superlattices.
• The moiré potential plays a crucial role in phonon hybridization and anharmonic interactions.
• Twisted TMDs exhibit unique low-temperature phonon behavior driven by electron-phonon and anharmonic interactions.