Origin of van Hove singularities in twisted bilayer graphene

In this work, we present a resonance Raman spectroscopy study of more than 100 samples of twisted bilayer graphene (TBG) with a continuous distribution of twisting angles from 0° to 30°, using three different laser excitation energies (1.96, 2.33 and 2.54 eV). From the Raman images of all investigated samples we could observe giant enhancements of the G band for samples with twisting angles between 9°and 17°, in agreement with previous Raman studies of TBGs. However, although we investigated a large number of samples with low (<9°) or high (>17°) twisting angles, we could not observe the G-band enhancement that was predicted for these samples from the zone-folding of the single layer graphene electronic structure. Our results allow us to conclude that the van Hove singularities in the density of states are associated with the Moiré pattern, which does not necessarily exhibit a translational symmetry. Moreover, we could observe that the Raman intensity ratio I_TBG/I_SLG in non-resonance cases is always smaller than two, and this value depends on the laser energy. This result is discussed in terms of the relaxation of photo-excited carriers in TBG, which is expected to increase with increasing laser excitation energy.