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The performance of superconducting radio-frequency (SRF) cavities issometimes limited by local defects. To investigate the RF properties of theselocal defects, a near-field magnetic microwave microscope is employed. Localthird harmonic response (P3f) and its temperature-dependence and RFpower-dependence are measured for one Nb/Cu film grown by Direct CurrentMagnetron Sputtering (DCMS) and six Nb/Cu films grown by High Power ImpulseMagnetron Sputtering (HiPIMS) with systematic variation of depositionconditions. Five out of the six HiPIMS Nb/Cu films show a strong third harmonicresponse that is likely coming from a low-Tc surface defect with a transitiontemperature between 6.3 K and 6.8 K, suggesting that this defect is a genericfeature of air-exposed HiPIMS Nb/Cu films. One possible origin of such a defectis grain boundaries hosting a low-Tc impurity such as oxidized Nb.Time-Dependent Ginzburg-Landau (TDGL) simulations are performed to betterunderstand the measured third harmonic response. The simulation results showthat the third harmonic response of RF vortex nucleation caused by surfacedefects can qualitatively explain the experimental data. Moreover, the densityof surface defects that nucleate RF vortices, and how deep an RF vortex travelsthrough these surface defects, can be extracted qualitatively from thirdharmonic response measurements. From the point of view of these two properties,the best Nb/Cu film for SRF applications can be identified.

Microscopic Examination of SRF-quality Nb Films through Local Nonlinear Microwave Response