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

We report measurements of the first-flux-penetration field in surface-treated and coated Nb samples using muon spin rotation (μSR). 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 μm to ∼ 100 μm. In a coated sample [Nb_3Sn(2 μm)/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_0H_c1≈ 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 (SRF) cavities, is discussed.