  Plasmons, the collective excitations of electrons in the bulk or at the
surface, play an important role in the properties of materials, and have
generated the field of Plasmonics. We report the observation of a highly
unusual acoustic plasmon mode on the surface of a three-dimensional topological
insulator (TI), Bi2Se3, using momentum resolved inelastic electron scattering.
In sharp contrast to ordinary plasmon modes, this mode exhibits almost linear
dispersion into the second Brillouin zone and remains prominent with remarkably
weak damping not seen in any other systems. This behavior must be associated
with the inherent robustness of the electrons in the TI surface state, so that
not only the surface Dirac states but also their collective excitations are
topologically protected. On the other hand, this mode has much smaller energy
dispersion than expected from a continuous media excitation picture, which can
be attributed to the strong coupling with surface phonons.
