Exotic plasmon modes of double layer graphene

We derive the plasmon dispersion in doped double layer graphene (DLG), made of two parallel graphene mono-layers with carrier densities $n_1$ and $n_2$ respectively, and an inter-layer separation of $d$. The linear chiral gapless single particle energy dispersion of graphene leads to exotic 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 $d\to 0$ limit. Our predicted exotic DLG plasmon properties clearly distinguish graphene from the extensively studied usual parabolic 2D electron systems.