Microscopic Investigation of Nb Film Impurities by Nonlinear Microwave Response

The microscopic origins of Superconducting Radio Frequency (SRF) cavity breakdown by surface defects are not completely understood. In particular, a clear indication of which defects are harmful for SRF operation, and which are not, is important to establish. One approach is to compare cavities/samples made by various fabrication methods. Here we examine several Nb films on Cu substrates fabricated by different methods. 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. By studying 3rd harmonic response of these Nb films, we find that some of them show evidence for superconducting transitions with transition temperatures between 6 K and 7 K. One possible explanation of such impurities is that they come from surface oxides of Nb. Time-dependent Ginzburg-Landau modeling of the probe/sample interaction is used to better understand the origins of the measured nonlinear signals.