PNNL

Abstract

We report direct, simultaneous measurements of the London penetration depth (????) and Bardeen-Cooper-Schrieffer coherence length (??0) 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 ????=29.1?(10)?nm and ??0=39.9?(25)?nm, corresponding to a Ginzburg-Landau parameter ??=0.70?(5). The obtained ???? and ?? 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.