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

TL;DR

This study uses resonant Raman scattering to investigate bright exciton states in twisted bilayer graphene at intermediate angles, revealing their role in the material's optical response and exciton dynamics.

Contribution

It provides the first direct experimental evidence of bright excitons in twisted bilayer graphene using resonant Raman scattering at cryogenic temperatures.

Findings

Resonant Raman G peak intensity reveals bright exciton states.

Bright exciton scattering lifetime is estimated at approximately 10 fs.

Resonance broadening is weakly temperature dependent.

Abstract

The band structure of bilayer graphene is 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{\deg}, 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 E2g phonon. By cryogenically cooling the sample, we are able to resolve both the incoming and outgoing resonance 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{\deg}, we report a weakly temperature dependent resonance broadening ${\gamma}$ ${\approx}$ 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 Raman measurement, limited by the rapid exciton-exciton and exciton-charge scattering in graphene.