Nanoscale surface dynamics of Bi₂Te₃(111): observation of a prominent surface acoustic wave and the role of van der Waals interactions

Abstract

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi2Te3. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi2Te3(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.

Statics and dynamics of multivalley charge density waves in Sb(111)

“…Due to the e-ph interaction, large modulations of the surface charge density are induced, 10 which is also important in observing comparatively large inelastic intensities from surface optical modes. 48 The property is common to other layered structures, as recently shown theoretically 49 and experimentally 50,51 for Bi 2 Te 3 (111) and Bi 2 Se 3 (111). In general the overall agreement of the measurements with the calculations is very good.…”

Nanoscopic diffusion of water on a topological insulator

“…Understanding the energy dissipation channels during diffusion on TI surfaces is interesting due to their insulating interior, so that the only contribution to electronic friction arises from the metallic surface state. There is no simple way to disentangle the electronic from the phononic contribution to the friction 54 but it is noteworthy that Bi 2 Te 3 has a low Debye temperature with phonons having correspondingly low frequencies 69,70 . The energy mismatch between the acoustic phonons and the internal modes of a water molecule, together with a mismatch between the mass of the water molecule and the heavy substrate atoms, suggests that phononic friction will occur predominantly through multiphonon processes (see also Supplementary Discussion and Supplementary Table 3).…”

“…Due to the topologically non-trivial nature of Bi 2 Te 3 the inclusion of SOC leads to the formation of the TSS, i.e. the Dirac cones in the surface electronic structure 69 . The Bi 2 Te 3 substrate was modelled with a single quintuple layer in a ð1 1Þ supercell, and an additional 25 Å vacuum layer for separating the periodically repeated supercells in the z-direction.…”

Contrasting the Surface Phonon Dispersion of Pb0.7Sn0.3Se in Its Topologically Trivial and Nontrivial Phases

“…The projected bulk modes as well as surface modes of SH polarization have been removed to avoid clutter. In principle, surface SH modes are inaccessible to HAS in the sagittal plane, however, the finite nature of the angleresolving aperture may relax this constraint [38]. We do not find any overlap of our measured data with calcu- FIG.…”