Is there any simple justification about graphene having no band gap? How bout its linear E-K? Why bilayer graphene has a quadratic E-K and electric field can open a band gap there? I do not completely understand the broken symmetry argument? Also Why MoS2 which has similar structure, do not have similar properties? I know that we can create the Hamiltonian, plot the band-structure numerically and answer these questions. But I want to know if we can use some simple quantum theory arguments (like perturbation theory, or Fermi golden rule) or symmetry facts to get a qualitative answer for all of these questions.