  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.
