[论文解读] 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 Magnetism被引用 5

一句话总结

本研究采用点接触隧道效应谱学方法，对比了体材料铌金属射频腔的超导性质，发现氮掺杂腔体表现出均匀、接近理想的超导能隙（Δ ≈ 1.5–1.6 meV）和较低的成对破坏速率（Γ/Δ ≈ 4.8%），与抗-Q斜率性能呈正相关。相比之下，常规腔体表现出不均匀的能隙和较高的Γ/Δ，伴有Kondo峰，表明存在磁性杂质和缺陷氧化物，导致高场下出现热点并引起Q值下降。

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.

研究动机与目标

• 理解常规体材料铌金属超导射频腔在高场下Q值退化（HFQS）的微观起源。
• 研究氮掺杂如何改变表面超导性质，并与常规处理工艺相比提升射频性能。
• 将态密度和氧化物势垒性质的隧道效应谱测量结果与观测到的射频腔性能相关联。
• 明确磁性杂质、氢化物相及氧化物质量在限制腔体性能中的作用。
• 确定表面均匀性提升和非弹性散射减少是否是掺杂腔体呈现抗-Q斜率的原因。

提出的方法

• 在1.3 GHz铌金属射频腔的厘米级切片样品上进行点接触隧道效应谱学（PCTS）测量，包括一个常规的缓冲化学抛光（BCP）腔和一个氮掺杂（N-doped）腔。
• PCTS测量了超导态密度（DOS），从隧道谱中提取出超导能隙Δ和成对破坏速率Γ。
• 基于McMillan模型并拟合至BCS型DOS，估算氧化物隧道势垒厚度和功函数。
• 结合互补的表面表征技术，将电子性质与表面化学和结构相关联。
• 通过射频测试和热成像识别热点和冷点区域，随后进行针对性的PCTS测量。
• 分析谱中的Kondo峰，以检测局域磁矩，表明存在表面磁性杂质。

实验结果

研究问题

• RQ1 在常规BCP铌腔中，热点与冷点区域的超导能隙（Δ）和成对破坏速率（Γ）有何差异？
• RQ2 氮掺杂如何改变超导能隙的均匀性以及Γ/Δ在腔体表面的分布？
• RQ3 表面磁性杂质和缺陷氧化物在导致常规腔体高场Q值退化中起什么作用？
• RQ4 为何氮掺杂腔体表现出抗-Q斜率，而常规腔体在高场下出现Q值下降？
• RQ5 隧道效应谱测量结果在多大程度上与观测到的射频腔性能及表面氧化物质量相关？

主要发现

• 常规BCP腔的冷点区域表现出接近理想的BCS态密度，其超导能隙Δ高达1.62 meV，为铌材料中报道的最高值之一。
• BCP腔的热点区域显示出从Δ ≈ 1.0 meV起的宽分布能隙值和较高的成对破坏速率Γ，表明超导性显著降低。
• 在热点区域观察到Kondo隧道峰，表明存在局域磁矩，归因于缺陷表面氧化物。
• 氮掺杂腔体表现出以Δ ≈ 1.5 meV为中心的均匀能隙分布，且Γ/Δ比值低至4.8 ± 1.6%，与性能提升一致。
• 氮掺杂腔体中未发现Δ低且Γ/Δ高的区域，表明氮间隙原子可抑制氢化物形成和缺陷相关的非均匀性。
• 氮掺杂腔体表现出更厚、更绝缘的本征氧化物层，具有更高的隧道势垒高度，有助于降低能量耗散并解释所观测到的抗-Q斜率。