Drude-Interband Coupling, Screening, and the Optical Conductivity of Doped Bilayer Graphene

We present a theory of the influence of band renormalization and excitonic electron-electron interaction effects on the optical conductivity $\sigma(\omega)$ of doped bilayer graphene. Using the Keldysh formalism, we derive a kinetic equation from which we extract numerical and approximate analytic results for $\sigma(\omega)$. Our calculations reveal a previously unrecognized mechanism which couples the Drude and interband response and renormalizes the plasmon frequency, and suggest that screening must play an essential role in explaining the weakly renormalized conductivity seen in recent experiments.