  In this paper, we predict the existence of low-frequency nonlocal plasmon
excitations at the vacuum-surface interface of a superlattice of $N$ graphene
layers interacting with a thick conducting substrate. This is different from
graphite which allows inter-layer hopping. A dispersion function is derived
which incorporates the polarization function of the graphene monolayers (MLGs)
and the dispersion function of a semi-infinite electron liquid at whose surface
the electrons scatter specularly. We find that this surface plasmon-polariton
is not damped by the particle-hole excitations (PHE's) or the bulk modes and
separates below the continuum mini-band of bulk plasmon modes. For a conducting
substrate with surface plasmon frequency $\omega_s=\omega_p/\sqrt{2}$, the
surface plasmon frequency of the hybrid structure always lies below $\omega_s$.
The intensity of this mode depends on the distance of the graphene layers from
the surface of the conductor, the energy band gap between the valence and
conduction bands of MLG and, most importantly, on the number of two-dimensional
(2D) layers. Furthermore, the hybrid structure has no surface plasmon for a
sufficiently large number ($N\stackrel{>}{\sim} 7$) of layers. The existence of
two plasmons with different dispersion relations indicates that quasiparticles
with different group velocity may coexist for various ranges of wavelength
which is determined by the number of layers in the superlattice.
