R. D. Veit, N. A. Kautz, R. G. Farber, and S. J. Sibener

Niobium (Nb) superconducting radio frequency (SRF) cavity surfaces are terminated by a native bulk oxide layer whose composition and structure directly influences SRF cavity performance. This work demonstrated that the surface crystallographic orientation of Nb determines the structure of this oxide layer; Nb(100) formed well-ordered (3x1)-O ladder structures while NbO on Nb(111) had no discernible long-range order. The energy barrier for oxygen dissolution from NbO to the bulk was determined to be 0.6 eV, and structural evolution of the (3x1)-O surface as a function of oxygen dissolution was investigated using scanning tunneling microscopy (STM). It was shown that oxygen dissolution occurred preferentially at the midpoint of the (3x1)-O ladder structure, which may indicate a preferred surface site for oxygen dissolution.

Two orange STM images. The left shows Nb(111) t 20 nm with fuzzy clumps. The right shows Nb(100) at 5 nm with organized loops.

Relation to CBB goals:

This work may have implications for SRF cavity fabrication procedures, resulting in higher cavity quality factors.

Potential applications:

SRF cavity fabrication procedures, such as nitrogen doping and Nb₃Sn thin film growth, may be optimized with this understanding of how increased surface temperature influences the chemical and structural composition of Nb surfaces.

DOI:

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