Determination of scattering lifetime of the bright excitonic state in twisted bilayer graphene via resonant Raman spectroscopy

The band structure of bilayer graphene has been shown to be tunable by introducing a relative twist angle between the two layers, unlocking exotic phases, such as superconductor and Mott insulator, and providing a fertile ground for new physics. At intermediate twist angles around 10°, highly degenerate electronic transitions hybridize to form excitonic states, resulting in a resonant condition that can be observed by Raman spectroscopy. We use a reflection contrast technique to quickly identify this resonance, then follow up with Raman scattering measurements to track the evolution of the intensity of the graphene Raman G peak, corresponding to the E_2G phonon. The intensity profile features broadening due to the finite lifetime of the scattered carriers. For a sample with twist angle 8.6°, we report a temperature dependent broadening ≈ 0.07 eV, putting the lifetime of the bright exciton on the order of 10 fs.