Plasmon-polaron of the topological metallic surface states

We report a plasmon-polaron mode of a two-dimensional electron gas occupying surface states of a three-dimensional topological crystal. This 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 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 Bi 2 Se 3.