Anomalous behavior in the phonon dispersion of the (001) surface of Bi$_2$Te$_3$ determined from helium atom-surface scattering measurements

TL;DR

This study uses helium atom-surface scattering to measure phonon dispersion on Bi$_2$Te$_3$ surface, revealing mode softening due to electron-phonon interactions and quantifying the coupling strength.

Contribution

It provides the first direct measurement of surface phonon dispersion and electron-phonon coupling in Bi$_2$Te$_3$ using helium atom scattering.

Findings

Observation of phonon mode softening due to electron-phonon interaction.

Quantification of the electron-phonon coupling constant as 1.44.

Determination of phonon lifetimes from the imaginary part of the self-energy.

Abstract

We employ inelastic helium atom-surface scattering to measure the low energy phonon dispersion along high-symmetry directions on the surface of the topological insulator Bi$_2$Te$_3$. Results indicate that one particular low-frequency branch experiences noticeable mode softening attributable to the interaction between Dirac fermion quasiparticles and phonons on the surface. This mode softening constitutes a renormalization of the real part of the phonon self-energy. We obtain the imaginary part, and hence lifetime information, via a Hilbert transform. In doing so we are able to calculate an average branch specific electron-phonon coupling constant $<\lambda_\nu> = 1.44$.