Today's excellent local seminar, by Uli Zuelicke, discussed the relativistic nature of delocalised electron systems in graphene. There excitations are essentially massless at certain critical points, for which the lack of a gap between conduction and valence bands creates a light cone like surface. We consider a graphene lattice diagram with two hexagonal site colours, much as for benzene molecules.
The optical properties of graphene exhibit universal behaviour, in terms of the fine structure constant $\alpha$. Single atom layers are rather transparent, but some absorption defines an opacity which is a factor of $4 \pi$ different from the universal optical conductivity (at high energies). The stepwise shift in opacity has been measured for a number of atomic layers.
6 years ago