Resonance Raman enhancement by the intralayer and interlayer electron-phonon processes in twisted bilayer graphene

Area of Science:

• Condensed Matter Physics
• Materials Science
• Spectroscopy

Background:

• Twisted bilayer graphene (TBG) exhibits tunable electronic and optical properties based on the twist angle.
• The coupling of Dirac cones in TBG creates van Hove singularities (vHs) influencing electronic states.
• Raman spectroscopy is crucial for studying TBG, with enhanced responses and new peaks appearing due to Moiré superlattice-activated phonons.

Purpose of the Study:

• To investigate how intralayer and interlayer electron-phonon processes enhance Raman peak intensities in TBG.
• To differentiate the contributions of acoustic and optical phonon branches to the Raman response.
• To understand the role of quantum interference and symmetry in the double resonance Raman process.

Main Methods:

• Performed resonance Raman spectroscopy on TBG samples with twist angles from 0° to 2.1°.
• Utilized various laser excitation energies in the NIR and visible ranges (1.39–2.71 eV).
• Conducted theoretical calculations of double-resonance Raman intensity incorporating momentum conservation for electron-phonon processes.

Main Results:

• Demonstrated distinct enhancement of different graphene phonon branches by intralayer and interlayer processes.
• Experimental findings were well-explained by theoretical calculations.
• Observed that quantum interference and symmetry requirements significantly affect Raman response enhancement.

Conclusions:

• Intralayer and interlayer electron-phonon processes uniquely enhance specific phonon modes in TBG.
• Theoretical modeling accurately predicts experimental Raman spectra.
• The study highlights the importance of quantum interference and symmetry in TBG's resonant Raman scattering.