Nanoscale Surface Dynamics of Bi$_2$Te$_3$(111): Observation of a Prominent Surface Acoustic Wave and the Role of van der Waals Interactions

TL;DR

This study combines experimental helium-3 spin-echo spectroscopy and density functional theory to reveal the surface vibrational modes, including the Rayleigh wave, of Bi$_2$Te$_3$(111), emphasizing the importance of van der Waals interactions.

Contribution

It provides the first experimental observation of the Rayleigh surface acoustic wave on Bi$_2$Te$_3$(111) and highlights the critical role of van der Waals corrections in theoretical models.

Findings

Observation of the Rayleigh mode on Bi$_2$Te$_3$(111)

Excellent agreement between experiments and van der Waals corrected calculations

Van der Waals interactions significantly influence lattice dynamics and surface acoustic waves

Abstract

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi$_2$Te$_3$. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi$_2$Te$_3$(111). The low energy region of the lattice vibrations is mainly dominated by the Rayleigh mode which has been claimed to be absent in previous experimental studies. The appearance of the Rayleigh mode is consistent with previous bulk lattice dynamics studies as well as theoretical predictions of the surface phonon modes. Density functional perturbation theory calculations including van der Waals corrections are in excellent agreement with the experimental data. Comparison of the experimental results with theoretically obtained values for films with a thickness of several layers further demonstrate, that for an accurate theoretical description of three-dimensional topological insulators with their layered structure the inclusion of van der Waals corrections is essential. The presence of a prominent surface acoustic wave and the contribution of van der Waals bonding to the lattice dynamics may hold important implications for the thermoelectric properties of thin-film and nanoscale devices.