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

• Published in: Science advances Vol. 10; no. 6; p. eadj1361
• Main Authors: Mehew, Jake Dudley, Merino, Rafael Luque, Ishizuka, Hiroaki, Block, Alexander, Mérida, Jaime Díez, Carlón, Andrés Díez, Watanabe, Kenji, Taniguchi, Takashi, Levitov, Leonid S, Efetov, Dmitri K, Tielrooij, Klaas-Jan
• Format: Journal Article
• Language: English
• Published: United States 09.02.2024
• ISSN: 2375-2548; 2375-2548
• DOI: 10.1126/sciadv.adj1361

Understanding electron-phonon interactions is fundamentally important and has crucial implications for device applications. However, in twisted bilayer graphene near the magic angle, this understanding is currently lacking. Here, we study electron-phonon coupling using time- and frequency-resolved photovoltage measurements as direct and complementary probes of phonon-mediated hot-electron cooling. We find a remarkable speedup in cooling of twisted bilayer graphene near the magic angle: The cooling time is a few picoseconds from room temperature down to 5 kelvin, whereas in pristine bilayer graphene, cooling to phonons becomes much slower for lower temperatures. Our experimental and theoretical analysis indicates that this ultrafast cooling is a combined effect of superlattice formation with low-energy moiré phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone. This enables efficient electron-phonon Umklapp scattering that overcomes electron-phonon momentum mismatch. These results establish twist angle as an effective way to control energy relaxation and electronic heat flow.