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 ( μ SR ) . Using thin Ag foils as energy moderators for the implanted muon spin-probes, we ‘profile’ the vortex penetration field μ 0 H vp at sub-surface depths on the order of ∼ 10 μ m to ∼ 100 μ m. In a coated sample [Nb 3 Sn( 2 μ m )/Nb], we find that μ 0 H vp 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 ∘ C for 48 h ], vortex penetration onsets close to pure Nb’s lower critical field μ 0 H c 1 ≈ 170 mT , 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.