Universal collisionless transport of graphene

The impact of the electron-electron Coulomb interaction on the optical conductivity of graphene has led to a controversy that calls into question the universality of collisionless transport in this and other Dirac materials. Using a lattice calculation that avoids divergences present in previous nodal Dirac approaches, our work settles this controversy and obtains results in quantitative agreement with experiment over a wide frequency range. We also demonstrate that dimensional regularization methods agree, if the regularization of the theory in modified dimensions is correctly implemented. Tight-binding lattice and nodal Dirac theory calculations are shown to coincide at low energies even when the nonzero size of the atomic orbital wave function is included, conclusively demonstrating the universality of the optical conductivity of graphene.