Spatial Distribution of Topological Surface State Electrons in   Bi$_{2}$Se$_{3}$ Probed by Na$^{+}$ Low Energy Ion Scattering
  Bi$_{2}$Se$_{3}$ is a topological insulator whose unique properties result from topological surface states (TSS) in the band gap. Low energy ion scattering can determine the properties of the outermost few atomic layers of a solid. The deposition of Cs helps to reveal that the neutralization of Na$^{+}$ is larger when scattered from surface Se than from Bi. This is caused by the spatial redistribution of the conductive charges in the TSS, which are primarily positioned between the first and second atomic layers. This provides direct experimental evidence of the spatial distribution of the TSS electrons. 
Weimin Zhou, Haoshan Zhu and Jory A. Yarmoff
cond-mat.mes-hall