Opening of a Gap in Graphene Due to Supercell Potential: Group Theory   Point of View
  We analyze in the framework of the space group theory the change of the dispersion law in grapenein the vicinity of the (former) Dirac points due to application of supercell potential with the $\sqrt{3}\times\sqrt{3}$ space periodicity and the same point symmetry as graphene. 
E. Kogan and M. Kaveh
cond-mat.mes-hall