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

TL;DR

This study investigates how low-temperature surface treatments in Nb superconducting cavities cause inhomogeneous Meissner screening, revealing a depth-dependent response linked to surface modifications and a non-superconducting dead layer.

Contribution

The paper provides evidence for inhomogeneous Meissner response in Nb due to surface treatments, using a generalized London model to fit experimental data and identify a surface dead layer.

Findings

Presence of a non-superconducting surface dead layer ≥25 nm.

Vacuum annealing induces a depth-dependent Meissner response.

Nitrogen infusion shows less clear evidence of inhomogeneous screening.

Abstract

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 (SRF) cavities. These treatments inhomogeneously dope the first $\sim$50 nm of Nb's subsurface and are expected to impart depth-dependent characteristics to its Meissner response; however, direct evidence supporting this remains elusive, suggesting the effect is subtle. In this work, we revisit the Meissner profile data for several LTB treatments obtained from low-energy muon spin rotation (LE-$\mu$SR) experiments [A. Romanenko et al., Appl. Phys. Lett. 104, 072601 (2014) and R. M. L. 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 a depth-dependent magnetic penetration depth $\lambda(z)$, we obtain improved fits to the Meissner data, revealing that the presence of a non-superconducting surface "dead layer" $d \geq 25$ nm is a strong indicator of a reduced supercurrent density at shallow subsurface depths. Our analysis supports the notion that vacuum annealing at 120 $^{\circ}$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.