Optical-phonon resonances with saddle-point excitons in twisted-bilayer   graphene
  Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the density of states that can be tuned by changing the twisting angle $\theta$. A $\theta$-defined tBLG has been produced and characterized with optical reflectivity and resonance Raman scattering. The $\theta$-engineered optical response is shown to be consistent with persistent saddle-point excitons. Separate resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the two-dimensional saddle-point excitons, similar to what has been previously observed for one-dimensional carbon nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the two processes are correlated and that they share the same phonon. 
Ado Jorio, Mark Kasperczyk, Nick Clark, Elke Neu, Patrick Maletinsky,   Aravind Vijayaraghavan, Lukas Novotny
cond-mat.mes-hall physics.optics