Microscopic Investigation of rf Vortex Nucleation in Nb3Sn Films Using a Near-Field Magnetic Microwave Microscope

TL;DR

Nb3Sn films are promising for SRF cavities but surface defects can trigger rf vortex nucleation, degrading performance. The authors use a near-field magnetic microwave microscope to apply a localized rf field and measure the defect-sensitive third-harmonic response $P_{3f}$, comparing vapor-diffused and electrochemically plated Nb3Sn films. Both films show nontrivial $P_{3f}(T)$ below $\sim 7$ K indicative of surface-defect–driven vortex nucleation, with the electrochemical film exhibiting additional structures at 14–16 K, reflecting different defect populations. This work demonstrates a powerful local diagnostic for surface defects and shows fabrication method influences the rf vortex penetration landscape, offering guidance for optimizing Nb3Sn films for SRF applications, including potential stoichiometry and roughness control.

Abstract

We use a near-field magnetic microwave microscope to investigate and compare rf vortex nucleation in two superconducting radio-frequency (SRF)-quality Nb3Sn films fabricated by different methods: a conventional vapor-diffused film and an electrochemically plated film followed by thermal annealing, both of which are deposited on Nb substrates. The microscope applies a localized rf magnetic field to the sample surface and measures the resulting third-harmonic response P3f, which is particularly sensitive to rf vortex nucleation triggered by surface defects. Both Nb3Sn films exhibit nontrivial P3f(T) structures below 7 K that display the key signatures associated with rf vortex nucleation at local defects. The electrochemical film additionally shows multiple P3f(T) structures between 14 K and 16 K that are absent in the vapor-diffused sample. Our results highlight the influence of fabrication method on rf vortex penetration properties and demonstrate the utility of third-harmonic response as a local diagnostic tool for surface defects in Nb3Sn films.

Figures (8)

• Figure 1: Photographs of the two Nb3Sn films used in this study, both prepared at Cornell University. (a) Vapor-diffused Nb3Sn film. (b) Electrochemical Nb3Sn film. Each sample has lateral dimensions of 1 cm $\times$ 1 cm. The red dashed circle in (b) highlights a scar on the surface caused by an aggressive probe contact during a later measurement, which did not affect the data presented in the main text.
• Figure 2: Representative data for $P_\mathrm{3f}$ as a function of temperature for the vapor-diffused Nb3Sn film. The input frequency is 2.08 GHz and the input power is -8 dBm. The blue dots are the raw data, and the red curve is the $P_\mathrm{3f}$ averaged over 0.04 K range bins. (a) Full temperature range from 3.6 K to 20 K. (b) Expanded view of the low-temperature region below 7 K.
• Figure 3: Measured $P_\mathrm{3f}(T)$ on a linear power scale for the vapor-diffused Nb3Sn film at an input frequency of 2.08 GHz, with input power decreasing from strong (red) to weak (purple). Compared to the raw data shown in Fig. \ref{['fig:vapordiffuseTsweepsingleP']}, here the probe background is subtracted. The input power levels are not evenly spaced in power but rather in rf field amplitude. Note that the input power is proportional to the square of the rf field amplitude.
• Figure 4: Color map of the measured third-harmonic power $P_\mathrm{3f}$ (in dBm, log scale) as a function of temperature $T$ and input power $P$ for the vapor-diffused Nb3Sn film. The input frequency is 2.08 GHz. The input-power-dependent probe background is not subtracted.
• Figure 5: Measured linear power scale $P_\mathrm{3f}(T)$ for the electrochemical Nb3Sn film at an input frequency of 1.21 GHz. Data are shown for two different input powers: $P = -18$ dBm (red) and $P = -19$ dBm (blue). The probe background has been subtracted.