Interaction of Phonons and Dirac Fermions on the Surface of Bi2Se3: A Strong Kohn Anomaly

TL;DR

This study measures surface phonon dispersion on Bi2Se3, revealing a strong Kohn anomaly caused by interactions with Dirac fermions, supported by theoretical analysis.

Contribution

First experimental observation of phonon dispersion on Bi2Se3 surface showing Dirac fermion influence and Kohn anomaly, with theoretical modeling of interactions.

Findings

Observation of a surface phonon branch with a maximum at 1.8 THz

Identification of a V-shaped minimum at 2kF indicating a Kohn anomaly

Theoretical analysis linking phonon renormalization to Dirac fermion interactions

Abstract

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3. The surface phonon measurements were carried out with the aid of coherent helium beam surface scattering techniques. The results reveal a prominent signature of the exotic metallic Dirac fermion quasi-particles, including a strong Kohn anomaly. The signature is manifest in a low energy isotropic convex dispersive surface phonon branch with a frequency maximum of 1.8 THz, and having a V-shaped minimum at approximately 2kF that defines the Kohn anomaly. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model.