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Short range Coulomb correlations render massive Dirac fermions massless

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.