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Seminários de Matéria Condensada

o seminário de matéria condensada ocorrerá essa semana,
excepcionalmente, na quarta-feira, 3/09. Horário e local continuam
sendo 11h, sala 201. O seminário será apresentado pelo Luiz Gustavo
Cançado, da UFMG. O Título é Raman spectroscopy of twisted bilayer
graphene.

Resumo: 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.