Abstract
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 are briefly reviewed.
Highlights
Helium-atom scattering (HAS) from a conducting surface can exchange energy and momentum with the lattice vibrations of the surface exclusively via the surface charge-density oscillations produced by the atomic motion, i.e., via electron–phonon (e–ph) interaction
This mechanism has been suggested since the early days of HAS spectroscopy [1], as a consequence of the discovery by HAS of the ubiquitous anomalous longitudinal surface resonance at metal surfaces [2,3,4,5], only more recent theoretical studies based on density functional perturbation theory (DFPT) [6,7] proved that the inelastic HAS intensities from surface phonons are directly proportional to their specific e–ph coupling constants λQν, inaugurating what has been termed as mode-lambda spectroscopy
This permits the direct derivation of λ from the temperature dependence of HAS reflectivity for any conducting surface, as demonstrated in a recent series of papers devoted to metal surfaces [10], ultra-thin metal films [11], layered transition-metal chalcogenides [12,13], topological semimetals [14,15] and one-dimensional metals [16], multidimensional materials [17] and graphene [18]
Summary
Helium-atom scattering (HAS) from a conducting surface can exchange energy and momentum with the lattice vibrations of the surface exclusively via the surface charge-density oscillations produced by the atomic motion, i.e., via electron–phonon (e–ph) interaction. A natural consequence of this mechanism is that the thermal mean-square distortion of the surface charge-density profile, providing the Debye–Waller attenuation of the specular intensity with increasing temperature, is proportional to the mean-square phonon displacement via the total e–ph coupling, represented by the mass-enhancement factor λ This permits the direct derivation of λ from the temperature dependence of HAS reflectivity for any conducting surface, as demonstrated in a recent series of papers devoted to metal surfaces [10], ultra-thin metal films [11], layered transition-metal chalcogenides [12,13], topological semimetals [14,15] and one-dimensional metals [16], multidimensional materials [17] and graphene [18]. After a theoretical summary (Section 2), with the relevant equations whose detailed derivation is found in refs. [7,10,11,15,17], a few examples for different classes of 2D superconducting materials are discussed
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