Short-range Coulomb correlations render massive Dirac fermions massless

¹ Department of Science, Tarbiat Modares University - Tehran, Iran
² Department of Physics, Sharif University of Technology - Tehran 11155-9161, Iran
³ School of Physics, Institute for Research in Fundamental Sciences (IPM) - Tehran 19395-5531, Iran

^a akbar.jafari@gmail.com

Received: 12 January 2012
Accepted: 27 March 2012

Abstract

Tight-binding electrons on a honeycomb lattice are described by an effective Dirac theory at low energies. Lowering symmetry by an alternate ionic potential (Δ) generates a single-particle gap in the spectrum. We employ the dynamical mean-field theory (DMFT) technique to study the effect of on-site electron correlation (U) on massive Dirac fermions. For a fixed mass parameter Δ, we find that beyond a critical value U_c1(Δ) massive Dirac fermions become massless. Further increasing U beyond U_c2(Δ), there will be another phase transition to the Mott insulating state. Therefore, the competition between the single-particle gap parameter, Δ, and the Hubbard U restores the semi-metallic nature of the parent Hamiltonian. The width of the intermediate semi-metallic regime shrinks by increasing the ionic potential. However, at small values of Δ, there is a wide interval of U values for which the system remains semi-metal. The implication of this result for graphene is that in contrast to a single-particle picture, the on-site Coulomb repulsion makes the Dirac cone spectrum robust against small values of the symmetry breaking parameter Δ.