Superconducting radio frequency (SRF) niobium cavities comprise the accelerating components of today’s linear particle accelerators. The goal for next-generation accelerators is to build higher performance SRF cavities using more highly perfected, doped, or alloyed materials to push the limits of operating temperatures and gradient fields. Several next-generation materials do not achieve their potential due to compositional inhomogeneities and structural defects. To overcome these challenges, it is crucial to correlate and understand how atomic-scale structure and composition influence superconducting properties. Low-energy neutral helium atom scattering (HAS) employing supersonic beams is a precise and surface-sensitive probe well matched to this challenge.  HAS can probe atomic-scale surface structure and surface phonon band structure, and through recent theoretical developments, surface electron-phonon coupling (EPC). Surface EPC correlates with a variety of factors determining superconducting performance, including but not limited to superconducting critical temperature and superheating field.  We find that the (3x1)-O niobium surface oxide reconstruction has a surface EPC that is reduced to 40% of pure niobium’s surface EPC. These results were supported by the consistency between HAS measurements and ab initio density-functional theory predictions. This study not only confirms that the oxide has a deleterious effect on niobium’s superconducting performance, but it also paves the way to understand how a variety of specific atomic-scale features for candidate SRF materials like nitrogen-doped niobium and Nb3Sn quantitatively affect surface EPC.

Reference:

C. J. Thompson, M. Van Duinen, C. Mendez, S. A. Willson, V. Do, T. A. Arias, and S. J. Sibener, “Distinguishing the Roles of Atomic-Scale Surface Structure and Chemical Composition in Electron Phonon Coupling of the Nb(100) Surface Oxide Reconstruction,” J. Phys. Chem. C, vol. 128, no. 25, pp. 10714–10722, Jun. 2024, doi: 10.1021/acs.jpcc.4c02430. Available: https://doi.org/10.1021/acs.jpcc.4c02430