  The screening properties and collective excitations (plasmons) in AA-stacked
bilayer graphene are studied within the random phase approximation (RPA).
Whereas long lived plasmons in single layer graphene and in AB-stacked bilayer
graphene can exist only in doped samples, we find that coherent plasmons can
disperse in AA-stacked bilayer graphene {\it even in the absence of doping}.
Moreover, we show that the characteristic low energy dispersion relation is
unaffected by changes in the number of carriers, unless the chemical potential
of the doped sample exceeds the inter-layer hopping energy. We further consider
the effect of an external electric field applied perpendicular to the layers,
and show how the dispersion of the modes can be tuned by the application of a
gate voltage.
