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

TL;DR

This paper demonstrates ultrafast electron cooling in magic-angle twisted bilayer graphene due to Umklapp scattering facilitated by moiré superlattice effects, revealing new pathways for engineering electron-phonon interactions.

Contribution

It provides the first experimental evidence of Umklapp-assisted electron-phonon cooling in moiré materials, highlighting the role of superlattice phonons and reduced Brillouin zone.

Findings

Cooling time is a few picoseconds near the magic angle.

Ultrafast cooling is due to Umklapp scattering enabled by moiré phonons.

Enhanced electron-phonon coupling in twisted bilayer graphene.

Abstract

Carrier relaxation measurements in moir\'e materials offer a unique probe of the microscopic interactions, in particular the ones that are not easily measured by transport. Umklapp scattering between phonons is a ubiquitous momentum-nonconserving process that governs the thermal conductivity of semiconductors and insulators. In contrast, Umklapp scattering between electrons and phonons has not been demonstrated experimentally. Here, we study the cooling of hot electrons in moir\'e graphene using time- and frequency-resolved photovoltage measurements as a direct probe of its complex energy pathways including electron-phonon coupling. We report on a dramatic speedup in hot carrier cooling of twisted bilayer graphene near the magic angle: the cooling time is a few picoseconds from room temperature down to 5 K, whereas in pristine graphene coupling to acoustic phonons takes nanoseconds. Our analysis indicates that this ultrafast cooling is a combined effect of the formation of a superlattice with low-energy moir\'e phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone, enabling Umklapp scattering that overcomes electron-phonon momentum mismatch. These results demonstrate a way to engineer electron-phonon coupling in twistronic systems, an approach that could contribute to the fundamental understanding of their transport properties and enable applications in thermal management and ultrafast photodetection.