Coulomb screening and collective excitations in a graphene bilayer

We have investigated the Coulomb screening properties and collective excitations in a graphene bilayer. The static screening effect is anisotropic and is much stronger in the undoped graphene bilayer than in a monolayer graphene. The dynamic screening shows the properties of a Dirac gas at low frequency and that of a Fermi gas at high frequency. The transition from the Dirac to the Fermi gas is also observed in the plasmon spectrum. Finally, we find that an electron gas in a doped graphene bilayer has quite similar properties as those of a Fermi gas in materials containing two energy valleys.