# THz Surface Modes and Electron-Phonon Coupling in Bi$_2$Se$_3$(111)

## TL;DR

This study combines experimental helium atom scattering and theoretical calculations to analyze surface vibrational modes and electron-phonon interactions in Bi$_2$Se$_3$, revealing new surface excitations and precise coupling parameters.

## Contribution

It provides the first detailed phonon dispersion of Bi$_2$Se$_3$ surface modes below 10 meV, including the observation of additional surface excitations linked to electron density structures.

## Key findings

- Dominance of a strong Rayleigh surface phonon mode.
- Absence of Kohn-anomaly in the phonon dispersion.
- Electron-phonon coupling parameter $$ = 0.23 consistent with other spectroscopies.

## Abstract

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator (TI) Bi$_2$Se$_3$ with particular emphasis on the low-energy region below 10 meV that has been difficult to resolve experimentally. By applying inelastic helium atom scattering (HAS), the entire phonon dispersion was determined and compared with density functional perturbation theory (DFPT) calculations. The intensity of the phonon modes is dominated by a strong Rayleigh mode, in contrast to previous experimental works. Moreover, also at variance with recent reports, no Kohn-anomaly is observed. These observations are in excellent agreement with DFPT calculations. Besides these results, the experimental data reveal$-$via bound-state resonance enhancement$-$two additional dispersion curves in the gap below the Rayleigh mode. They are possibly associated with an excitation of a surface electron density superstructure that we observe in HAS diffraction patterns. The electron-phonon coupling paramenter $\lambda$ = 0.23 derived from our temperature dependent Debye-Waller measurements compares well with values determined by angular resolved photoemission or Landau level spectroscopy. Our work opens up a new perspective for THz measurements on 2D materials as well as the investigation of subtle details (band bending, the presence of quantum well states) with respect to the electron-phonon coupling.