Probing the bright exciton state in twisted bilayer graphene via resonant Raman scattering

The band structure of bilayer graphene is tunable by introducing a relative twist angle between the two layers, unlocking exotic phases, such as superconductors and Mott insulators, and providing a fertile ground for new physics. At intermediate twist angles around 10 degrees, highly degenerate electronic transitions hybridize to form excitonic states, a quite unusual phenomenon in a metallic system. We probe the bright exciton mode using resonant Raman scattering measurements to track the evolution of the intensity of the graphene Raman G peak, corresponding to the E-2g phonon. By cryogenically cooling the sample, we are able to resolve both the incoming and outgoing resonances in the G peak intensity evolution as a function of excitation energy, a prominent manifestation of the bright exciton serving as the intermediate state in the Raman process. For a sample with twist angle 8.6 degrees, we report a weakly temperature dependent resonance broadening gamma approximate to 0.07 eV. In the limit of small inhomogeneous broadening, the observed gamma places a lower bound for the bright exciton scattering lifetime at 10 fs in the presence of charges and excitons excited by the light pulse for the Raman measurement, limited by the rapid exciton-exciton and exciton-charge scattering in graphene.