Surface Phonons in the Topological Insulators Bi2Se3 and Bi2Te3

TL;DR

This study uses temperature- and wavelength-dependent Raman spectroscopy combined with density functional theory to identify surface phonons in Bi2Se3 and Bi2Te3, clarifying their origin and relation to topological properties.

Contribution

It provides the first combined experimental and theoretical evidence for surface phonons in Bi2Se3 and Bi2Te3, showing they are not linked to topological surface states.

Findings

Identification of additional surface phonon modes in Raman spectra

Theoretical confirmation of surface phonons near observed frequencies

Surface phonons are unrelated to spin-orbit coupling and topological properties

Abstract

Raman scattering [K. M. F. Shahil et al., Appl. Phys. Lett. 96, 153103 (2010), V. Gnezdilov et al., Phys. Rev. B 84, 195118 (2011) and H. H. Kung et al., Phys. Rev. B 95, 245406 (2017)], inelastic helium scattering [X. Zhu et al., Phys. Rev. Lett. 107, 186102 (2011)] and photoemission experiments [J. A. Sobota et al., Phys. Rev. Lett. 113, 157401 (2014)] on the topological insulators Bi2Se3 and Bi2Te3 show features in the range ~ 50-160 cm-1, which have been assigned alternatively to Raman-forbidden, bulk infrared modes arising from symmetry breaking at the surface or to surface phonons, which couple to the topologically protected electronic states. Here, we present temperature- and wavelength- dependent Raman studies showing additional modes we ascribe to surface phonons in both Bi2Se3 and Bi2Te3. Our assignment is supported by density functional theory calculations revealing surface phonons at frequencies close to those of the extra peaks in the Raman data. The theoretical results also indicate that these modes are not a consequence of spin-orbit coupling and, thus, that their occurrence is unrelated to the topological properties of these materials.