  We report a plasmon-polaron mode of a 2D electron gas occupying surface
states of a 3D topological crystal. This new, low-frequency, acoustic plasmon
mode splits-off from the conventional spin-plasmon mode as a result of strong
interactions of the surface electrons with bulk phonons. We show that, like in
the case of the conventional spin-plasmon, the scattering of this mode is
strongly suppressed in some regions of the phase space. This signature of the
topological protection leads to an Umklapp-free mode dispersion at the
Brillouin zone boundary. Such a plasmon polaron mode has indeed been recently
observed in the topological metal Be2Se3.
