Determination of the BCS material parameters of the HIE-ISOLDE superconducting resonator

Superconducting material parameters of the Nb film coating on the Quarter-Wave Resonator (QWR) for the HIE-ISOLDE project were studied by fitting experimental results with the Mattis-Bardeen theory. We pointed out a strong correlation among fitted estimators of material parameters in the BCS theory, and proposed a procedure to remove the correlation by simultaneously fitting the surface resistance and effective penetration depth. Unlike previous studies, no literature values were assumed in the fitting. As surface resistance and penetration depth had a similar dependence on coherence length and mean free path, the correlation between these two parameters could not be eliminated by this fitting. The upper critical field measured by SQUID magnetometry showed complementary constraint to the RF result, and this allowed all the material parameters to be determined.