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

Area of Science:

• Condensed Matter Physics
• Materials Science
• Surface Science

Background:

• Kohn anomalies are phonon softening phenomena in metals driven by electron-phonon coupling.
• Previous studies reported Kohn anomalies in topological materials, but first-principles calculations often failed to reproduce them, particularly on surfaces.
• This discrepancy created a debate regarding experimental interpretations versus computational limitations.

Purpose of the Study:

• To investigate the existence and origin of surface Kohn anomalies in topological insulators Bi2Se3 and SnSe.
• To resolve the controversy surrounding the observation of Kohn anomalies in topological materials.

Main Methods:

• Utilized state-of-the-art Wannier interpolation schemes for first-principles calculations.
• Analyzed phonon spectra and electronic band structures of Bi2Se3 and SnSe.
• Investigated the role of surface states and Dirac cones in phonon anomalies.

Main Results:

• Bi2Se3 showed Kohn anomalies only in bulk-like phonon modes under structural confinement along the c-axis.
• SnSe exhibited a clear surface Kohn anomaly, consistent with experimental findings in related materials like Pb0.7Sn0.3Se.
• Identified double Dirac cones in SnSe surface states as the cause of the surface Kohn anomaly, with enhancement upon partial occupation of subsurface states.

Conclusions:

• The study confirms the presence of surface Kohn anomalies in specific topological insulators like SnSe.
• Double Dirac cones in the surface states of SnSe are the key mechanism driving the observed surface Kohn anomaly.
• This work reconciles theoretical predictions with experimental observations, clarifying the role of electronic structure in phonon anomalies within topological materials.