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

TL;DR

This study accurately determined the superconducting material parameters of Nb film coatings on HIE-ISOLDE resonators by fitting experimental data with the Mattis-Bardeen theory, addressing parameter correlation issues without relying on literature values.

Contribution

It introduces a novel fitting procedure that removes parameter correlation in determining BCS parameters without assuming literature values, using combined RF and SQUID measurements.

Findings

Successfully determined all BCS parameters of Nb film coating.

Identified and addressed correlation issues among fitted parameters.

Combined RF and SQUID data provided comprehensive material characterization.

Abstract

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.