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

Abstract

We report direct, simultaneous measurements of the London penetration depth ($\hat{I}{»}_L$) and Bardeen-Cooper-Schrieffer (BCS) coherence length ($\hat{I}{¾}_0$) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques - low-energy muon spin spectroscopy (LE-$\hat{I}¼$SR) and secondary-ion mass spectrometry (SIMS) - 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 $\hat{I}{»}_L=29.1(10)$ nm and $\hat{I}{¾}_0=39.9(25)$ nm, corresponding to a Ginzburg-Landau (GL) parameter of $\hat{I}º=0.70(5)$. The obtained $\hat{I}{»}_L$ and $\hat{I}º$ values, accurately quantified at the nanoscale, are smaller than values 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.

Main manuscript: 8 pages, 3 figures, 2 tables. Supporting material: 17 pages, 6 figures, 7 tables