Microscopic examination of rf-cavity-quality niobium films through local nonlinear microwave response

Wang, Chung-Yang; Pereira, Carlota; Leith, Stewart; Rosaz, Guillaume; Anlage, Steven M.

doi:10.1103/PhysRevApplied.22.054010

Title: Microscopic examination of rf-cavity-quality niobium films through local nonlinear microwave response

Journal Article · 05 November 2024 · Physical Review Applied

DOI: https://doi.org/10.1103/PhysRevApplied.22.054010 · OSTI ID:2476260

; Pereira, Carlota;

;

The performance of superconducting radio-frequency (SRF) cavities is sometimes limited by local defects. To investigate the rf properties of these local defects, especially those that nucleate rf magnetic vortices, a near-field magnetic microwave microscope is employed. Local third-harmonic response ( $P_{3 f}$ ) and its temperature dependence and rf power dependence are measured for one $Nb$ / $Cu$ film grown by direct current magnetron sputtering (DCMS) and six $Nb$ / $Cu$ films grown by high-power impulse magnetron sputtering (HiPIMS) with systematic variation of deposition conditions. Five out of the six HiPIMS $Nb$ / $Cu$ films show a strong third-harmonic response that is likely coming from rf vortex nucleation due to a low- $T_{c}$ surface defect with a transition temperature between 6.3 and 6.8 K, suggesting that this defect is a generic feature of air-exposed HiPIMS $Nb$ / $Cu$ films. A phenomenological model of surface-defect grain boundaries hosting a low- $T_{c}$ impurity phase is introduced and studied with time-dependent Ginzburg-Landau (TDGL) simulations of probe-sample interaction to better understand the measured third-harmonic response. The simulation results show that the third-harmonic response of rf vortex nucleation caused by surface defects exhibits the same general features as the data, including peaks in third-harmonic response with temperature, and their shift and broadening with higher microwave amplitude. We find that the parameters of the phenomenological model (the density of surface defects that nucleate rf vortices and the depth an rf vortex travels through these surface defects) vary systematically with film deposition conditions. From the point of view of these two properties, the $Nb$ / $Cu$ film that is most effective at reducing the nucleation of rf vortices associated with surface defects can be identified. Published by the American Physical Society 2024

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Sponsoring Organization:: USDOE Office of Science (SC), High Energy Physics (HEP)

Grant/Contract Number:: SC0017931

OSTI ID:: 2476260

Journal Information:: Physical Review Applied, Journal Name: Physical Review Applied Journal Issue: 5 Vol. 22; ISSN 2331-7019; ISSN PRAHB2

Publisher:: American Physical SocietyCopyright Statement

Country of Publication:: United States

Language:: English

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