Search for inhomogeneous Meissner screening in Nb induced by low-temperature surface treatments

Empirical surface treatments, such as low-temperature baking (LTB) in a gaseous atmosphere or in vacuum, are important for the surface preparation of Nb superconducting radio frequency cavities. These treatments inhomogeneously dope approximately the first 50 nm of Nb’s subsurface and are expected to impart depth-dependent characteristics to its Meissner response; however, direct evidence supporting this remains elusive, suggesting the effect is subtle. In this work, we revisit the Meissner profile data for several LTB treatments obtained from low-energy muon spin rotation (LE-μSR) experiments [Romanenko et al., Appl. Phys. Lett. 104, 072601 (2014) and McFadden et al., Phys. Rev. Appl. 19, 044018 (2023)] and search for signatures of inhomogeneous field screening. Using a generalized London expression with a recently proposed empirical model for depth-dependent magnetic penetration lengths λ(z), we obtain improved fits to the Meissner data, revealing that the presence of a non-superconducting surface “dead layer” d ≳ 25 nm is a strong indicator of a reduced supercurrent density at shallow subsurface depths. Our analysis supports the notion that vacuum annealing at 120 °C for 48 h induces a depth-dependent Meissner response, which has consequences for Nb’s ability to maintain a magnetic-flux-free state. Evidence of similar behavior from a “nitrogen infusion” treatment is less compelling. Suggestions for further investigation into the matter are provided.