  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.
