Surface oxides, carbides, and impurities on RF superconducting Nb and Nb3Sn: A comprehensive analysis

Kavli Affiliate: David A. Muller

| First 5 Authors: Zeming Sun, Zhaslan Baraissov, Catherine A. Dukes, Darrah K. Dare, Thomas Oseroff

| Summary:

Surface structures on radio-frequency (RF) superconductors are crucially
important in determining their interaction with the RF field. Here we
investigate the surface compositions, structural profiles, and valence
distributions of oxides, carbides, and impurities on niobium (Nb) and
niobium-tin (Nb3Sn) in situ under different processing conditions. We establish
the underlying mechanisms of vacuum baking and nitrogen processing in Nb and
demonstrate that carbide formation induced during high-temperature baking,
regardless of gas environment, determines subsequent oxide formation upon air
exposure or low-temperature baking, leading to modifications of the electron
population profile. Our findings support the combined contribution of surface
oxides and second-phase formation to the outcome of ultra-high vacuum baking
(oxygen processing) and nitrogen processing. Also, we observe that
vapor-diffused Nb3Sn contains thick metastable oxides, while electrochemically
synthesized Nb3Sn only has a thin oxide layer. Our findings reveal fundamental
mechanisms of baking and processing Nb and Nb3Sn surface structures for
high-performance superconducting RF and quantum applications

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