Seminário de Matéria Condensada - 3/09/2014, 11:00, sala 201

Twisted bilayer graphene (tBLG) is a two-graphene layers system with a mismatch angle θ between the two hexagonal structures. The interference between the two rotated layers generates a supperlattice with a θ-dependent wavevector that gives rise to van Hove singularities in the electronic density of states and activates phonons in the interior of the Brillouin zone. We use Raman spectroscopy to study tBLG, exploring the -dependent effects, corroborated by independent microscopy analysis. The phonon frequencies give a Raman signature of the specific tBLG. Rich resonance effects, including single and multiple-resonances, intra- and intervalley scattering events make it possible to accurately measure the energy of superlattice-induced van Hove singularities in the electronic joint density of states, as well as the phonon dispersion relation in tBLG, including the layer breathing vibrational modes. The resonance effects are explained based on the θ dependence of the tBLG electronic structure, as calculated by ab initio methodologies. The twist angle also defines the observation of a “D-like” band which obeys the double-resonance process, but relies on the superlattice along with long-range defects to fulfill momentum conservation.