Surface collective modes in the topological insulators Bi$_2$Se$_3$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3-x}$Se$_{x}$

A. Kogar; S. Vig; A. Thaler; M. H. Wong; Y. Xiao; D. Reig-i-Plessis; G. Y. Cho; T. Valla; Z. Pan; J. Schneeloch; R. Zhong; G. Gu; T. L. Hughes; G. J. MacDougall; T. -C. Chiang; P. Abbamonte

doi:10.1103/physrevlett.115.257402

arXiv · 1505.03784

Surface collective modes in the topological insulators Bi$_2$Se$_3$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3-x}$Se$_{x}$

A. Kogar·S. Vig·A. Thaler·M. H. Wong·Y. Xiao·D. Reig-i-Plessis·G. Y. Cho·T. Valla·Z. Pan·J. Schneeloch·R. Zhong·G. Gu·T. L. Hughes·G. J. MacDougall·T. -C. Chiang·P. Abbamonte

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Abstract

We used low-energy, momentum-resolved inelastic electron scattering to study surface collective modes of the three-dimensional topological insulators Bi$_2$Se$_3$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3-x}$Se$_{x}$. Our goal was to identify the "spin plasmon" predicted by Raghu and co-workers [S. Raghu, et al., Phys. Rev. Lett. 104, 116401 (2010)]. Instead, we found that the primary collective mode is a surface plasmon arising from the bulk, free carrers in these materials. This excitation dominates the spectral weight in the bosonic function of the surface, $\chi "(\textbf{q},\omega)$, at THz energy scales, and is the most likely origin of a quasiparticle dispersion kink observed in previous photoemission experiments. Our study suggests that the spin plasmon may mix with this other surface mode, calling for a more nuanced understanding of optical experiments in which the spin plasmon is reported to play a role.

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A. Kogar, S. Vig, A. Thaler, M. H. Wong, Y. Xiao, D. Reig-i-Plessis, G. Y. Cho, T. Valla, Z. Pan, J. Schneeloch, R. Zhong, G. Gu, T. L. Hughes, G. J. MacDougall, T. -C. Chiang, P. Abbamonte. 2015-05-14. Surface collective modes in the topological insulators Bi$_2$Se$_3$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3-x}$Se$_{x}$. https://doi.org/10.1103/physrevlett.115.257402

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