Exact RG computation of the optical conductivity of graphene
  The optical conductivity of a system of electrons on the honeycomb lattice interacting through an electromagnetic field is computed by truncated exact Renormalization Group (RG) methods. We find that the conductivity has the universal value pi/2 times the conductivity quantum up to negligible corrections vanishing as a power law in the limit of low frequencies. 
Alessandro Giuliani and Vieri Mastropietro
cond-mat.str-el