  The feature-rich electronic excitations of monolayer germanene lie in the
significant spin-orbital coupling and the buckled structure. The collective and
single- particle excitations are diversified by the magnitude and direction of
transferred momentum, the Fermi energy and the gate voltage. There are four
kinds of plasmon modes, according to the unique frequency- and
momentum-dependent phase diagrams. They behave as two-dimensional acoustic
modes at long wavelength. However, for the larger momenta, they might change
into another kind of undamped plasmons, become the seriously suppressed modes
in the heavy intraband e-h excitations, keep the same undamped plasmons, or
decline and then vanish in the strong interband e-h excitations. Germanene,
silicene and graphene are quite different from one another in the main features
of the diverse plasmon modes.
