Depth-Resolved Measurements of the Meissner Screening Profile in Surface-Treated Nb

Abstract

We report depth-resolved measurements of the Meissner screening profile in several surface-treated $\mathrm{Nb}$ samples using low-energy muon spin rotation. In these experiments, implanted positive muons, whose stopping depths below $\mathrm{Nb}$'s surface are adjusted between approximately $10\phantom{\rule{0.2em}{0ex}}\mathrm{nm}$ and $150\phantom{\rule{0.2em}{0ex}}\mathrm{nm}$, reveal the field distribution inside the superconducting element via their spin precession (communicated through their radioactive decay products). We compare how the field screening is modified by different surface treatments commonly used to prepare superconducting radio-frequency cavities used in accelerator beamlines. In contrast to an earlier report [A. Romanenko et al., Appl. Phys. Lett. 104, 072601 (2014)], we find no evidence for any ``anomalous'' modifications to the Meissner profiles, with all data being well described by a London model. Differences in screening properties between surface treatments can be explained by changes to the carrier mean free paths resulting from dopant profiles near the material's surface.