▎ 摘　　要

We theoretically investigate the effects of Coulomb interaction, at the level of the unscreened Hartree-Fock approximation in an equation of motion framework, on third harmonic generation from undoped graphene. The unperturbed electronic states are described by a widely used two-band tight-binding model and the Coulomb interaction is described by the Ohno potential. The ground state is renormalized by taking into account the Hartree-Fock term and the optical conductivities are obtained by numerically solving the equations of motion. The absolute values of conductivity for third-harmonic generation depend on the photon frequency Omega as Omega(-n) for h Omega < 1, and then show a peak as 3h Omega approaches the renormalized energy of the M point. Taking into account the Coulomb interaction, n is found to be 5.5, which is significantly greater than the value of 4 found with the neglect of the Coulomb interaction. Therefore, the Coulomb interaction enhances third-harmonic generation at low photon energies-for our parameters h Omega < 0.8 eV-and then reduces it until the photon energy reaches about 2.1 eV. The effect of the background dielectric constant is also considered.