Kohn anomalies in topological insulator thin films: first-principles study

Kohn anomaly is a non-smooth phonon softening induced by electron-phonon coupling in low-dimensional metals. Some measurements claimed that Kohn anomalies are present in topological materials due to the Dirac fermions in the bulk or in the surface. However, first-principles calculations have not reproduced the Kohn anomalies, especially, on the surface of topological insulators. It is still unclear about the origin of the controversy for the existence of the Kohn anomaly whether it is a numerical shortcoming or misinterpretation in measurement. In this study, we investigate the surface Kohn anomaly in two topological insulators Bi₂Se₃and SnSe using the state-of-the-art Wannier interpolation schemes. We find that Bi₂Se₃exhibits the Kohn anomaly but only in the bulk-like phonon modes by structural confinement along thec-axis. Interestingly, SnSe exhibits the surface Kohn anomaly in support of the experimental report on Pb_0.7Sn_0.3Se. We show that double Dirac cones in SnSe surface states are responsible for the Kohn anomaly, which is even enhanced if the subsurface states are partially occupied.