Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Electron-phonon interactions are critical for electronic device performance.
• Understanding these interactions in twisted bilayer graphene (TBG) near the magic angle is currently limited.

Purpose of the Study:

• To investigate electron-phonon coupling and hot-electron cooling dynamics in TBG at the magic angle.
• To elucidate the mechanisms behind ultrafast energy relaxation in this material system.

Main Methods:

• Utilized time- and frequency-resolved photovoltage measurements.
• Employed experimental and theoretical analysis to probe phonon-mediated hot-electron cooling.

Main Results:

• Observed a significant speedup in hot-electron cooling in magic-angle TBG, with cooling times of a few picoseconds from room temperature to 5 Kelvin.
• Demonstrated that cooling in pristine bilayer graphene is considerably slower at lower temperatures.
• Identified superlattice formation, low-energy moiré phonons, compressed Wannier orbitals, and a reduced superlattice Brillouin zone as key factors for efficient cooling.

Conclusions:

• The ultrafast cooling in magic-angle TBG is attributed to efficient electron-phonon Umklapp scattering, overcoming momentum mismatch.
• Twist angle emerges as a powerful parameter for controlling energy relaxation and electronic heat flow in graphene-based systems.