Surface Collective Modes in the Topological Insulators Bi2Se3 and Bi0.5Sb1.5Te3-xSex

Abstract

We used low-energy, momentum-resolved inelastic electron scattering to study surface collective modes of the three-dimensional topological insulators Bi2Se3 and Bi0.5Sb1.5Te3-xSex. Our goal was to identify the "spin plasmon" predicted by Raghu and co-workers [Phys. Rev. Lett. 104, 116401 (2010)]. Instead, we found that the primary collective mode is a surface plasmon arising from the bulk, free carriers in these materials. This excitation dominates the spectral weight in the bosonic function of the surface chi (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.