Phys.org February 12, 2024
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. An international team of researchers (Spain, Japan, USA – MIT, Germany) studied electron-phonon coupling using time- and frequency-resolved photovoltage measurements as direct and complementary probes of phonon-mediated hot-electron cooling. They found a remarkable speedup in cooling of twisted bilayer graphene near the magic angle: the cooling time was a few picoseconds from room temperature down to 5 kelvin, whereas in pristine bilayer graphene, cooling to phonons becomes much slower for lower temperatures. Their experimental and theoretical analysis indicated 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 enabled efficient electron-phonon Umklapp scattering that overcame electron-phonon momentum mismatch. According to the researchers their results establish twist angle as an effective way to control energy relaxation and electronic heat flow… read more. Open Access TECHNICAL ARTICLE
Excited carrier relaxation in MATBG. Credit: SCIENCE ADVANCES, 9 Feb 2024, Vol 10, Issue 6