IFIMAC+ICMM Joint Seminars. Giorgio Benedek. May 19th, 2022. 12:00 CEST

The development of supersonic monochromatic (DE < 0.5 meV) He-atom scattering (HAS) and ³He spin-echo (3He-SE, DE < 0.5 meV) spectrometers has paved the way to high-resolution momentum-resolved spectroscopy of surface dynamics on the atomic scale [1]. Unlike thermal neutrons and X-rays, in HAS atoms at thermal energy only tickle the surface, ~0.3 nm away from the first atomic plane, and exchange energy and momentum with the solid atoms only via the interposed electrons. Thus, phonons are detected via the electron-phonon (e-ph) interaction, and not just at the surface, but possibly as deep beneath the surface as the range of that interaction. This sort of quantum sonar allows for the direct measurement of the e-ph coupling strength for each individual phonon (mode-l spectroscopy), and for ultimately knowing which phonons support phonon-mediated pairing in 2D-superconductors [2]. Moreover, the linear decrease with temperature of the Debye-Waller (DW) exponent for HAS (or 3He-SE) from a conducting surface presently allows for a direct measurement of the e-ph mass-enhancement factor l [3]. In particular, much can be learnt with HAS and 3He-SE about the e-ph coupling as function of the dimensionality and in 2D topological superconductors, as it is shown with a few recent examples [4-6]. When deviations from linearity of the DW exponent can be resolved, it is here suggested that the specific e-ph contributions to l. of acoustic and optical phonons can be singled out. It is finally shown that surface electrons, when hit by a He atom, may however retain the received energy and momentum, rather than delivering them to phonons. Thus, HAS also qualifies as a possible probe of surface electron collective excitations in the THz domain [7].