Depth-resolved measurements of the Meissner screening profile in surface-treated Nb

TL;DR

This study uses low-energy muon spin rotation to measure how different surface treatments affect the Meissner screening profile in niobium, finding no anomalous effects and explaining differences through changes in carrier mean-free-paths.

Contribution

First depth-resolved measurements of the Meissner profile in surface-treated Nb using LE-μSR, confirming the London model and clarifying effects of surface treatments.

Findings

No evidence of anomalous Meissner profile modifications.

Screening differences explained by dopant-induced changes in mean-free-path.

All data consistent with the London model.

Abstract

We report depth-resolved measurements of the Meissner screening profile in several surface-treated Nb samples using low-energy muon spin rotation (LE-$\mu$SR). In these experiments, implanted positive muons, whose stopping depths below Nb's surface were adjusted between ~10 nm to ~150 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 employed to prepare superconducting radio frequency (SRF) 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.