Surface Stability, Phonon Band Structure, and Vibrational Dynamics of the Nb(100) Surface Oxide Reconstruction.

다국어 초록 (Multilingual Abstract)

Niobium is the current industry standard for modern superconducting radiofrequency cavities in particle accelerators, but technology has pushed these cavities to niobium’s fundamental limits. Future progress rests on improved growth procedures for existing materials and the development of new materials, and a complete, detailed, atomic-scale characterization of the niobium surfaces used in accelerators is a prerequisite for this progress. Towards this end, this thesis contains a set of experiments that employ helium atom scattering to describe the (3 x 1)-O reconstruction of the Nb(100) surface. Elastic helium diffraction from the (3 x 1)-O Nb(100) surface is used to characterize the structure of the surface over a wide range of high temperatures. High-resolution helium diffraction and line-shape analysis, confirmed by Auger electron spectroscopy, reveal that the (3 x 1)-O reconstruction is stable up to at least 1130 K. The atomic-scale surface structure, composition, and coherence do not change up to this temperature, which exceeds the temperature at which niobium is held during typical tin nucleation procedures. Inelastic helium time-of-flight measurements are used to map out the phonon band structure of the Nb(100) oxide and determine the nature of the surface’s vibrational dynamics and force constants. Density-functional theory calculations correspond with measured phonon dispersions and elucidate the atomic displacement patterns for each measured phonon resonance. The difference between the calculated bare and oxidized Nb(100) surfaces show that the oxide disperses electron-phonon coupling strengths to higher energies and significantly increases force constants at the surface, potentially affecting surface superconductivity and superconducting radiofrequency cavity behavior.