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Scattering between particles represents a crucial class of microscopic physical processes that govern transport properties and excited-state relaxation phenomena, among others. In crystals, as well as normal scattering, the discrete translation symmetry allows Umklapp scatter- ing events that do not conserve momentum. Phonon-phonon Umklapp scattering is a ubiquitous process that governs the thermal conductiv- ity of semiconductors and insulators, while electron-electron Umklapp scattering is only observed in ultraclean systems, usually at low temper- atures. In contrast, Umklapp scattering between electrons and phonons has not been demonstrated experimentally. Here we reveal the occurrence of electron-phonon Umklapp scattering in twisted bilayer graphene near the magic angle using time- and frequency-resolved photocurrent mea- surements. In magic-angle twisted bilayer graphene, a dramatic speedup of hot carrier cooling occurs: the cooling time is a few picoseconds from room temperature down to 5 K, where in pristine graphene coupling to acoustic phonons takes nanoseconds. Our theoretical calculations show that this ultrafast cooling is the result of the formation of a super- lattice with low-energy moir ́e phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone, enabling Umklapp scattering and thus overcoming electron-phonon momentum mismatch. These results demonstrate the ability to engineer electron- phonon coupling in twistronic systems, and could contribute to the fundamental understanding of their transport properties, while enabling applications in thermal management and ultrafast photodetection.

Rights:

• Creative Commons BY-NC Attribution-NonCommercial 4.0 International
  

Keyword: Electron-phonon interactions, twisted bilayer graphene, hot-electron cooling

Date published: 2024-02-09

Publisher: American Association for the Advancement of Science (AAAS)

Journal:

• Science Advances (ISSN: 23752548) vol. 10 issue. 6 eadj1361

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Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1126/sciadv.adj1361

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Updated at: 2025-02-23 22:46:44 +0900

Published on MDR: 2025-02-23 22:46:44 +0900