The 1-loop vacuum polarization for a graphene-like medium in an external magnetic field; corrections to the Coulomb potential
I calculate the 1-loop vacuum polarization $\Pi_{\mu\nu}(k,B,a)$ for a photon of momentum $k=(\hat k,k_3)$ interacting with the electrons of a thin medium of thickness $2a$ simulating graphene, in the presence of a constant and uniform external magnetic field $B$ orthogonal to it (parallel to $k_3$). Calculations are done with the techniques of Schwinger, adapted to the geometry and Hamiltonian under scrutiny. The situation gets more involved than for the electron self-energy because the photon is now allowed to also propagate outside the medium. This makes $\Pi_{\mu\nu}$ factorize into a quantum, "reduced" $T_{\mu\nu}(\hat k,B)$ and a transmittance function $V(k,a)$, in which the geometry of the sample and the resulting confinement of the $\gamma\,e^+\,e^-$ vertices play major roles. This drags the results away from reduced QED$_{3+1}$ on a 2-brane. The finiteness of $V$ at $k^2=0$ is an essential ingredient to fulfill suitable renormalization condition for $\Pi_{\mu\nu}$ and to fix the corresponding counterterms. Their connection with the transversality of $\Pi_{\mu\nu}$ is investigated. The corrections to the Coulomb potential and their dependence on $B$ strongly differ from QED$_{3+1}$.