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

Minjae Ghim¹, Seung-Hoon Jhi¹

¹Department of Physics, Pohang University of Science and Technology, Pohang 37673, Republic of Korea.

Journal of Physics. Condensed Matter : an Institute of Physics Journal

|April 11, 2022

Summary

The Kohn anomaly, a phonon softening effect, is investigated in topological insulators. SnSe exhibits a surface Kohn anomaly due to its double Dirac cones, resolving a scientific controversy.

Keywords:

Kohn anomaly phonon superconductivity topological

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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.