Ab Initio Surface Electron-Phonon Coupling Theory of Elastic Helium Atom Scattering

Helium Atom Scattering (HAS) has emerged as a powerful and versatile technique used to investigate the structural and dynamical characteristics of material surfaces. Utilizing the interaction between low-energy helium atoms and surfaces, it offers non-destructive, atomic-scale resolution measurements of surface properties. In HAS, the incoming helium atoms primarily interact with the surface through the electron density, thereby revealing essential details about electron-phonon coupling and thus superconductivity at material surfaces.<br/>This presentation will describe a novel ab initio approach which can help extract electron-phonon coupling information directly from HAS data. Specifically, recent studies have suggested using elastic HAS to study surface electron-phonon coupling through a Debye-Waller type effect. Current theories of this process, however, only consider fluctuations in the atomic positions and do not explicitly include the electrons, thereby radically limiting the information that can be extracted regarding the electron-phonon coupling. These models also require ad hoc assumptions regarding the range of interaction of the incoming helium atoms.<br/>To address these defects, our approach employs the hard corrugated surface model (HCS) [1,2], in which helium atoms scatter at a specific surface electron-density contour, in an entirely new, ab initio context. After explicitly confirming the accuracy of this approach through ab initio calculations, we demonstrate how the resulting theory probes the electron-phonon coupling directly. We then apply the new approach to the metallic Nb(100) surface and the oxidized (3x1)-O/Nb(100) surface, finding excellent agreement with our experimental results and confirming recent conjectures regarding observed non-linear effects in the associated Debye-Waller exponents. The ability to accurately model elastic HAS directly from first principles opens new possibilities for studying a wide range of surface phenomena, potentially leading to novel insights into surface dynamics and interactions.<br/>Bibliography<br/>[1] N. Garcia, J. Ibáñez, J. Solana and N. Cabrera, Surface Science 60 385–396 (1976)<br/>[2] G. Benedek and N. Garcia, Surface Science 80 543–549 (1979)