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Critical behaviour of reduced QED$_{4,3}$ and dynamical fermion gap generation in graphene

The dynamical generation of a fermion gap in graphene is studied at the infra-red Lorentz-invariant fixed point where the system is described by an effective relativistic-like field theory: reduced QED$_{4,3}$ with $N$ four component fermions ($N=2$ for graphene), where photons are $(3+1)$-dimensional and mediate a fully retarded interaction among $(2+1)$-dimensional fermions. A correspondence between reduced QED$_{4,3}$ and QED$_3$ allows us to derive an exact gap equation for QED$_{4,3}$ up to next-to-leading order. Our results show that a dynamical gap is generated for $α> α_c$ where $1.03 < α_c < 1.08$ in the case $N=2$ or for $N < N_c$ where $N_c$ is such that $α_c \to \infty$ and takes the values $3.24 < N_c < 3.36$. The striking feature of these results is that they are in good agreement with values found in models with instantaneous Coulomb interaction. At the fixed point: $α= 1/137 \ll α_c$, and the system is therefore in the semi-metallic regime in accordance with experiments.