Acoustic Polaron on the Surface of Topological Insulators

We report a plasmon-polaron mode of a 2D electron gas occupying the surface states of a 3D
topological crystal. This new, low-frequency, acoustic plasmon mode splits-off from the
conventional spin-plasmon mode [1], as a result of the strong interactions between surface
electrons and 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 Be 2 Se 3 [2].
[1] S. Raghu, S. B. Chung, X. L. Qi, and S. C. Zhang, “Collective Modes of a Helical Liquid”,
Phys. Rev. Lett. 104, 116401 (2010)
[2] X. Jia, S. Zhang, R. Sankar, F-C Chou, W. Wang, K. Kempa, E. W. Plummer, J. Zhang, X.
Zhu, and J. Guo, “Anomalous Acoustic Plasmon Mode from Topologically Protected States”,
Phys. Rev. Lett. 119, 136805 (2017)