Measurement of the coherence distance of a pure type-I superconductor

The coherence distance (${\xi }_0$) of the pure type-I superconductors (Al, Pb, and Sn) was measured using the proximity effect with a ferromagnetic Ni film. The experimental values of ${\xi }_0$ for Al, Pb, and Sn, which are, respectively, 1.36 μm, 510 \AA{}, and 940 \AA{}, are found to disagree with the BCS theoretical prediction based on the free-electron-like model, the disagreement being worse the less free-electron-like the metal is. These experimental values disagree also with previous determinations of the coherence length obtained by supercooling and superheating experiments and by the magnetic transition of thin films.