[Paper Review] Insight into bulk niobium superconducting RF cavities performances by tunneling spectroscopy

Nickolas Groll, G. Ciovati+4 more|arXiv (Cornell University)|May 16, 2018

Physics of Superconductivity and Magnetism5 citations

TL;DR

This study uses point contact tunneling spectroscopy to compare superconducting properties in bulk niobium RF cavities, revealing that nitrogen-doped cavities exhibit homogeneous, near-ideal superconducting gaps (Δ ≈ 1.5–1.6 meV) and low pair-breaking rates (Γ/Δ ≈ 4.8%), correlating with anti-Q slope performance. In contrast, conventional cavities show inhomogeneous gaps and high Γ/Δ with Kondo peaks, indicating magnetic impurities and defective oxides that cause hot spots and Q degradation at high fields.

ABSTRACT

Point contact tunneling (PCT) spectroscopy measurements are reported over wide areas of cm-sized cut outs from niobium superconducting RF cavities. A comparison is made between a high-quality, conventionally processed (CP) cavity with a high field Q drop for acceleration field E $>$ 20 MV/m and a nitrogen doped (N-doped) cavity that exhibits an increasing Q up to fields approaching 15 MV/m. The CP cavity displays hot spot regions at high RF fields where Q-drop occurs as well as unaffected regions (cold spots). PCT data on cold spots reveals a near ideal BCS density of states (DOS) with gap parameters, $Δ$ as high as 1.62 meV, that are among the highest values ever reported for Nb. Hot spot regions exhibit a wide distribution of gap values down to $Δ\sim$ 1.0 meV and DOS broadening characterized by a relatively large value of pair-breaking rate, $Γ$, indicating surface regions of significantly reduced superconductivity. In addition, hot spots commonly exhibit Kondo tunneling peaks indicative of surface magnetic moments attributed to a defective oxide. N-doped cavities reveal a more homoegeneous gap distribution centered at $Δ\sim$ 1.5 meV and relatively small values of $Γ/Δ$. The absence of regions of significantly reduced superconductivity indicates that the N interstitials are playing an important role in preventing the formation of hydride phases and other macroscopic defects which might otherwise severely affect the local, surface superconductivity that lead to hot spot formation. The N-doped cavities also display a significantly improved surface oxide, i.e., increased thickness and tunnel barrier height, compared to CP cavities. These results help explain the improved performance of N-doped cavities and give insights into the origin of the initial increasing Q with RF amplitude.

Motivation & Objective

• To understand the microscopic origins of the high-field Q degradation (HFQS) in conventional bulk niobium superconducting RF cavities.
• To investigate how nitrogen doping alters surface superconducting properties and improves RF performance compared to conventional processing.
• To correlate tunneling spectroscopy measurements of the density of states and oxide barrier properties with observed RF cavity performance.
• To identify the role of magnetic impurities, hydride phases, and oxide quality in limiting cavity performance.
• To determine whether improved surface homogeneity and reduced inelastic scattering are responsible for the anti-Q slope in doped cavities.

Proposed method

• Point contact tunneling spectroscopy (PCTS) was performed on cm-sized cutouts from 1.3 GHz niobium RF cavities, including a conventional buffered chemical polished (BCP) cavity and a nitrogen-doped (N-doped) cavity.
• PCTS measured the superconducting density of states (DOS), extracting the superconducting gap Δ and pair-breaking rate Γ from the tunneling spectra.
• The oxide tunnel barrier thickness and work function were estimated from the tunneling spectra using the McMillan model and fitting to the BCS-like DOS.
• Complementary surface characterization techniques were used to correlate electronic properties with surface chemistry and structure.
• Hot spot and cold spot regions were identified via RF testing and thermal mapping, followed by targeted PCTS measurements.
• Kondo peaks in the spectra were analyzed to detect localized magnetic moments, indicating surface magnetic impurities.

Experimental results

Research questions

• RQ1 What are the differences in superconducting gap (Δ) and pair-breaking rate (Γ) between hot spots and cold spots in conventional BCP niobium cavities?
• RQ2 How does nitrogen doping alter the homogeneity of the superconducting gap and the distribution of Γ/Δ across the cavity surface?
• RQ3 What is the role of surface magnetic impurities and defective oxides in causing high-field Q degradation in conventional cavities?
• RQ4 Why do N-doped cavities exhibit an anti-Q slope while conventional cavities show a Q drop at high fields?
• RQ5 To what extent do tunneling spectroscopy measurements correlate with observed RF cavity performance and surface oxide quality?

Key findings

• Cold spots in the conventional BCP cavity exhibit a near-ideal BCS density of states with a superconducting gap Δ as high as 1.62 meV, among the highest values reported for niobium.
• Hot spot regions in the BCP cavity show a broad distribution of gap values down to Δ ≈ 1.0 meV and a high pair-breaking rate Γ, indicating significantly reduced superconductivity.
• Kondo tunneling peaks were observed in hot spot regions, indicating the presence of localized magnetic moments attributed to defective surface oxides.
• The N-doped cavity exhibits a homogeneous gap distribution centered at Δ ≈ 1.5 meV with a low Γ/Δ ratio of 4.8 ± 1.6%, consistent with improved RF performance.
• The absence of regions with low Δ and high Γ/Δ in N-doped cavities suggests that nitrogen interstitials suppress hydride formation and defect-related inhomogeneities.
• The N-doped cavities show a thicker, more insulating native oxide layer with higher tunnel barrier height, contributing to reduced dissipation and the observed anti-Q slope.