Niobium’s intrinsic coherence length and penetration depth revisited using low-energy muon spin spectroscopy and secondary-ion mass spectrometry

We report direct, simultaneous measurements of the London penetration depth (λ_L ) and Bardeen-Cooper-Schrieffer coherence length (ξ₀) in oxygen-doped niobium, with impurity concentrations spanning the “clean”to “dirty” limits. Two depth-resolved techniques—low-energy muon spin spectroscopy and secondary-ion mass spectrometry—were used to quantify the element’s Meissner screening profiles, analyzed within a framework that accounts for nonlocal electrodynamics. 

The analysis indicates intrinsic length scales of λ_L = 29.1(10) nm and ξ0 = 39.9(25) nm, corresponding to a Ginzburg-Landau parameter κ = 0.70(5). The obtained λL and κ values, accurately quantified at the nanoscale, are smaller than those commonly used in applications and modeling, and indicate that clean niobium lies at the boundary between type-I and type-II superconductivity, supporting the contemporary view that its intrinsic state may be type I.