Determination of scattering lifetime of the bright excitonic state in twisted bilayer graphene <i>via</i> 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.