Plasmon modes of spatially separated double-layer graphene

We derive the plasmon dispersion in doped double-layer graphene (DLG), made of two parallel graphene monolayers with carrier densities n(1) and n(2), respectively, and an interlayer separation of d. The linear chiral gapless single-particle energy dispersion of graphene leads to DLG plasmon properties with several unexpected experimentally observable characteristic features such as a nontrivial influence of an undoped (n(2)=0) layer on the DLG plasmon dispersion and a strange influence of the second layer even in the weak-coupling d ->infinity limit. At long wavelengths (q -> 0), the density dependence of the plasma frequencies is different from the usual two-dimensional (2D) electron system with quadratic energy dispersion. Our predicted DLG plasmon properties clearly distinguish graphene from the extensively studied usual parabolic 2D electron systems.