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

TL;DR

This study investigates surface collective modes in topological insulators Bi$_2$Se$_3$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3-x}$Se$_{x}$ using inelastic electron scattering, revealing bulk-derived surface plasmons dominate over the predicted spin plasmon.

Contribution

The paper provides experimental evidence that bulk-derived surface plasmons dominate over the predicted spin plasmon in these topological insulators, challenging previous theoretical expectations.

Findings

Surface plasmons from bulk carriers dominate the surface spectral weight.

The observed mode likely causes quasiparticle dispersion kinks in photoemission.

The spin plasmon may interact with the bulk-derived surface mode.

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.