Origin of universal optical conductivity and optical stacking sequence identification in multilayer graphene

Recently, experiments have demonstrated that the conductivity per layer in multilayer graphene has the universal value $\sigma_{uni}=(\pi/2) \, e^2/h$ in the optical frequency range. We show that the origin of the universal optical conductivity in normal $N$-layer graphene multilayers is an emergent chiral symmetry which guarantees that $\sigma(\omega)=N\sigma_{uni}$ in both low and high frequency limits. In the intermediate frequency regime, the optical conductivity shows qualitatively different trends depending on the stacking sequence; thus, the optical conductivity measurement can provide a convenient qualitative characterization of multilayer graphene stacks. \\[4pt] Ref) Hongki Min and A. H. MacDonald, Phys. Rev. Lett. {\bf 103}, 067402 (2009).