Measuring the Electron–Phonon Interaction in Two-Dimensional Superconductors with He-Atom Scattering

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 is briefly reviewed.

Material properties particularly suited to be measured with helium scattering: selected examples from 2D materials, van der Waals heterostructures, glassy materials, catalytic substrates, topological insulators and superconducting radio frequency materials

“…One problem has been that, while there are several ways to measure l for bulk materials as discussed above, up to very recently there was no straightforward method for measuring l directly for the low energy phonon regime. 203 Up until now the experimental method usually applied to measure l in 2D materials has been angular resolved photoelectron spectroscopy (ARPES). ARPES measures the momentum distribution of electrons ejected from a solid exposed to UV light/soft X-rays (typically the energy of the incident beam is around 20 eV; note, this is around 1000 times bigger than the energy of the atoms in a helium scattering beam).…”

Surface electronic corrugation of a one-dimensional topological metal: Bi(114)

“…1,31 This (2 × 2) reconstruction is caused by the intervalley electron–phonon coupling. 35 From the temperature dependence of the Debye–Waller exponent of the specular intensity in theoretical calculations determined the electron–phonon coupling constant to be λ = 0.45 when treating the system as a 1D free-electron gas. 35…”

“…35 From the temperature dependence of the Debye-Waller exponent of the specular intensity in GX; theoretical calculations determined the electron-phonon coupling constant to be l = 0.45 when treating the system as a 1D free-electron gas. 35 Scattering normal to the steps GY À Á as shown in Fig. 2(b) on the other hand, gives rise to a multitude of diffraction peaks due to the large unit cell.…”

The role of high-energy phonons in electron–phonon interaction at conducting surfaces with helium-atom scattering

“…2 and this provides an interesting view to illustrate the way that high energy phonon modes enter into the contributions to the e-ph coupling constant for a polyatomic conducting surface. 10,22 The measurements were made at an incident He atom energy E i = 17.42 meV and at a specular angle of θ i = 45.75 • . In this case, the surface phonon spectrum as obtained from ab-initio DFPT calculations 23 Fig.…”

“…In the last decade, measurements of the HAS-DW factor 2W (T ) = ln[I 00 (T )/I 00 (0)] associated with the specular scattering intensity I 00 (T ) have provided, through the temperature dependence, reliable values of λ for a variety of surfaces, ranging from those of ordinary metals 3 and metal overlayers 4 to the surface of topological [5][6][7] and multidimensional 8 materials, 2D superconductors 9,10 and graphene. 11 The new potentialities of HAS spectroscopy have been reviewed in a recent Perspective.…”