Ab-initio calculation of surface phonons at the Sb2Te3(111) surface

Density functional perturbation theory is used to study the phonon dispersion relations at the (111) surface of the topological insulator Sb2Te3. Inelastic Helium atom scattering experiments at the (111) surface of the similar Bi2Se3 topological insulator suggested the occurrence of a deep Kohn anomaly related to the presence of the topological surface metallic state. A slab made of three Sb2Te3 quintuple layers is used here to model the (111) surface. This geometry is shown to be sufficient to generate metallic Dirac surface states in good agreement with experimental data from Angle Resolved Photoemission Spectroscopy. The inclusion of spin-orbit interaction in the calculations of surface phonons at wavevectors corresponding to Fermi surface nesting inside the Dirac cone allow us to address the possible occurrence of a Kohn anomaly in Sb2Te3(111) as well. No evidence is found in our calculations for any Kohn anomaly in Sb2Te3 due to electronic transitions across the Dirac cone.