Interaction and excitonic insulating transition in graphene

The strong long-range Coulomb interaction between massless Dirac fermions in graphene can drive a semimetal-insulator transition. We show that this transition is strongly suppressed when the Coulomb interaction is screened by such effects as thermal fluctuation, doping, disorder, and finite volume. It is completely suppressed once the screening factor mu is beyond a threshold mu(c) even for infinitely strong coupling. However, such transition is still possible if there is an additional strong contact four-fermion interaction. The differences between screened and contact interactions are also discussed.