Microscopic Investigation of Nonlinear Microwave Response of Nb₃Sn in the Superconducting State

The microscopic origins of Superconducting Radio Frequency (SRF) cavity breakdown by surface defects are 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. Results on a Nb₃Sn film with T_c = 18.3 K show evidence for multiple superconducting transitions (around 4.8 K and 10.1 K), probably because there are different Sn concentrations in the sample. Moreover, for the 10.1 K impurity, the 3rd harmonic response exhibits a periodic feature in both the rf field amplitude-dependence, and the temperature-dependence. Such a signal can be understood in terms of vortex dynamics in an inhomogeneous medium, perhaps associated with grain boundaries. In particular, time-dependent Ginzburg-Landau modeling of the probe/sample interaction is used to better understand the origins of the measured nonlinear signals.



Acknowledgement:

This work is funded by US Department of Energy / High Energy Physics through grant # DE-SC0017931 and the Maryland Quantum Materials Center.