Comment on “Strong Meissner screening change in superconducting radio frequency cavities due to mild baking” [Appl. Phys. Lett. 104, 072601 (2014)]

In a recent Letter by Romanenko et al., 1 the authors used lowenergy muon spin rotation (LE-lSR) 2,3 to measure the Meissner screening profile in cutouts from Nb superconducting radio frequency (SRF) cavities, systematically comparing how different surface treatments affect the screening properties of the elemental type-II superconductor.They reported a "strong" modification to the character of the screening profile upon mild baking at 120 C for 48 h, 4 which was interpreted as a depth-dependent carrier mean-free-path resulting from a "gradient in vacancy concentration" near the surface. 1 While this observation led to speculation that this surface treatment yields an "effective" superconducting bilayer (see, e.g., Ref. 5), we suggest that its likeness to such (see Ref. 6) is accidental and that the behavior is an artifact from the analysis.Issues with the reported analysis are apparent upon scrupulous inspection of Figs. 3 and4 in the Letter, 1 where the mean field hBi below the sample surface (obtained through different analysis models) is plotted against the mean muon stopping depth hzi.In Fig. 3, where hBi was determined from fits to a Gaussian model (i.e., a Gaussian-damped cosine function), 7 Romanenko et al. 1 correctly draw attention to the strikingly different hBiðhziÞ dependence found upon mild baking; 4 however, no mention is made as to why the data deviate from the exponential decay (i.e., the well-known form of Meissner screening profiles for thick slabs 8 ) for all surface treatments.Instead, the discussion shifts to accounting for non-local electrodynamics 9,10 and strong electron-phonon coupling 11 in the formulation of B(z).While the former can cause B(z) to decay nonexponentially, its effect is known to be subtle for Nb, 12 and it is unclear how such corrections could account for the abrupt discontinuity observed at hzi % 60 nm.