Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the density of states that can be tuned by changing the twisting angle θ. A θ-defined tBLG has been produced and characterized with optical reflectivity and resonance Raman scattering. The θ-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.
@article{arxiv.1406.7344,
title = {Optical-phonon resonances with saddle-point excitons in twisted-bilayer graphene},
author = {Ado Jorio and Mark Kasperczyk and Nick Clark and Elke Neu and Patrick Maletinsky and Aravind Vijayaraghavan and Lukas Novotny},
journal= {arXiv preprint arXiv:1406.7344},
year = {2015}
}