Sn in the Superconducting State*

The microscopic origins of Superconducting Radio Frequency (SRF) cavity breakdown by surface defect are still not completely understood. To locally study the electrodynamics of superconductors, a near-field magnetic microwave microscope was built. We study the 3rd harmonic response as a function of rf field amplitude and temperature. In previous experiments on Nb, two different types of nonlinearity were observed, which we call Low-field and Periodic. The Low-field response can be explained by the intrinsic response of the sample due to vortex semiloops created by the magnetic writer probe. As for the Periodic case, the response can be linked to the Josephson effect at or near the surface. New results on a Nb₃Sn film show evidence for multiple superconducting transitions, probably because there are different Sn concentrations in the sample. In particular, there is a clear phase with a transition temperature of about 5.5 K. The most prominent nonlinear response appears to be due to the intrinsic Low-field mechanism. We summarize our results with the materials properties of Nb₃Sn, and compare to those previously obtained on Nb materials.