Surface dynamics and atom-surface interaction of topological insulators from helium atom scattering

Materials with peculiar electronic surface effects such as the novel group of topological insulators are particularly interesting for truly surface sensitive measurement methods such as helium atom scattering (HAS). Using HAS we are able to obtain an accurate He-Bi2Te3(111) atom-surface interaction profile. Diffraction experiments from in-situ cleaved single crystals of Bi2Te3(111) show a multitude of distinct resonance features which are used to identify the bound state energies of the interaction potential. A full three-dimensional interaction profile can be fitted to a an azimuthal scan using an extended close-coupling (CC) calculation. Modelling the scattering intensities via an elastic CC calculation provides a quantum mechanically accurate way of testing a provided interaction profile against measured data [1,2]. Moreover, using temperature dependent measurements of the elastically scattered intensities we are able to extract the electron-phonon (e-ph) coupling based on the new approach by Manson et al. [3]. The e-ph coupling constant characterises the interaction strength between electrons and phonons, but conflicting values had been reported for topological insulator surfaces [4-6]. Even though some individual phonons may provide a large e-ph interaction [5], we determine a rather modest value for the global e-ph coupling on Bi2Te3(111). These results are further confirmed by our findings about the surface phonon modes, where theoretical studies showed that the e-ph interaction cannot account for the absence of the Rayleigh phonon mode [6]: Previous experimental studies claimed that the Rayleigh mode on Bi2Te3(111) is absent[5] whereas we observe an intense Rayleigh wave in the surface phonon dispersion.

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[3] J. R. Manson et al., Phys. Chem. Lett. 7, 1016 (2016).
[4] X. Zhu et al., Phys. Rev. Lett. 108, 185501(2012).
[5] C. Howard et al., Phys. Rev. B 88, 035402 (2013).
[6] V. Parente et al., Phys. Rev. B 88, 075432 (2013).