He atom scattering has been demonstrated to be a sensitive probe of the electron−phonon interaction parameter λ at metal and metal-overlayer surfaces. Here it is shown that the theory linking λ to the thermal attenuation of atom scattering spectra (the Debye−Waller factor) can be applied to topological semimetal surfaces, such as the quasi-onedimensional charge-density-wave system Bi(114) and the layered pnictogen chalcogenides. The electron−phonon coupling, as determined for several topological insulators belonging to the class of bismuth chalcogenides, suggests a dominant contribution of the surface quantum well states over the Dirac electrons in terms of λ.

Gas-surface scattering experiments yield high accuracy interaction potentials in the van der Waals regime. A perspective overview for topological semimetals & insulators is provided and the importance for benchmarking ab initio calculations outlined.

Scanning tunnelling microscopy and low energy electron diffraction show a dimerization-like reconstruction in the one-dimensional atomic chains on Bi(114) at low temperatures. While onedimensional systems are generally unstable against such a distortion, its observation is not expected for this particular surface, since there are several factors that should prevent it: One is the particular spin texture of the Fermi surface, which resembles a one-dimensional topological state, and spin protection should hence prevent the formation of the reconstruction. The second is the very short nesting vector 2kF , which is inconsistent with the observed lattice distortion. A nesting-driven mechanism of the reconstruction is indeed excluded by the absence of any changes in the electronic structure near the Fermi surface, as observed by angle-resolved photoemission spectroscopy. However, distinct changes in the electronic structure at higher binding energies are found to accompany the structural phase transition. This, as well as the observed short correlation length of the pairing distortion, suggest that the transition is of the strong coupling type and driven by phonon entropy rather than electronic entropy.

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Helium-atom scattering (HAS) spectroscopy from conducting surfaces has been shown to provide direct information on the electron–phonon interaction, more specifically the mass-enhancement factor λ from the temperature dependence of the Debye–Waller exponent, and the mode-selected electron–phonon coupling constants λQν from the inelastic HAS intensities from individual surface phonons. The recent applications of the method to superconducting ultra-thin films, quasi-1D high-index surfaces, and layered transition-metal and topological pnictogen chalcogenides is briefly reviewed.