Measurements of the first-flux-penetration field in surface-treated and coated Nb: distinguishing between near-surface pinning and an interface energy barrier

We report measurements of the first-flux-penetration field in surface-treated and coated Nb samples using . Using thin Ag foils as energy moderators for the implanted muon spin-probes, we ‘profile’ the vortex penetration field at sub-surface depths on the order of m to m. In a coated sample [Nb3Sn( )/Nb], we find that is depth-independent with a value of 234.5(35) mT, consistent with Nb’s metastable superheating field and suggestive of surface energy barrier for flux penetration. Conversely, in a surface-treated sample [Nb baked in vacuum at 120 ∘ for ], vortex penetration onsets close to pure Nb’s lower critical field , but increases with increasing implantation depth, consistent with flux-pinning localized at the surface. The implication of these results for technical applications of superconducting Nb, such as superconducting radio frequency cavities, is discussed.