Karl Franzens University Graz
Graz University of Technology
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Karl Franzens University Graz | Graz University of Technology | |
Unveiling the Phonon‐Selected Electron‐Phonon Coupling Constants in Nanometer Superconducting Lead Films by Helium Atom Scattering The recent discovery of superconductivity in single monolayer lead films raises intriguing questions about the electron-phonon coupling in quasi-two-dimensional systems. Recent calculations carried out at the Donostia International Physics Center and at Karlsruhe show that the electron-phonon coupling strength for each phonon mode in a superconducting film can be measured by inelastic helium atom scattering (HAS)[1]. This surprising ability of HAS relies on two circumstances. First, in ultra-thin metal films the e-p coupling range exceeds the film thickness, enabling HAS to detect all the film phonons, even those one nanometer below the surface (quantum sonar). This result explains recent HAS experiments in which many more than the usual one or two phonon dispersion curves were detected by HAS[2]. More important, the ab-initio theoretical analysis shows that HAS scattering amplitudes are proportional to the phonon-induced surface charge density oscillations, and therefore to the e-p coupling for that particular phonon (mode-lambda). Thus He-atom scattering is the first experiment providing mode-selected e-p coupling strengths (mode-lambda spectroscopy). Besides superconductivity in ultra-thin films, the mode-lambdas are relevant for inelastic photoemission, phonon-induced surface reactions, scanning tunnel spectroscopy (STS) and other thin-film properties. |