Title: The Anomalous Resonant Frequency Variation of Microwave Superconducting Niobium Cavities Near $T_c$ Show Chinese title

Title: 微波超导铌腔在$T_c$附近的异常共振频率变化

Abstract: Superconducting radio-frequency (SRF) niobium cavities are the modern means of particle acceleration and an enabling technology for record coherence superconducting quantum systems and ultra-sensitive searches for new physics. Here we report a systematic effect observed on a large set of bulk SRF cavities - an anomalous decrease of the resonant frequency at temperatures just below the superconducting transition temperature - which opens up a new means of understanding the physics behind nitrogen doping and other modern cavity surface treatments relevant for future quality factor and coherence improvements. The magnitude of the frequency change correlates systematically with the near-surface impurity distribution in studied cavities and with the observed $T_c$ variation. We also present the first demonstration of the coherence peak in the real part of the AC complex conductivity in Nb SRF cavities and show that its magnitude varies with impurity distribution. Show Chinese abstract

Abstract: 超导射频（SRF）铌腔是现代粒子加速器的手段，也是创纪录的相干超导量子系统和对新物理敏感搜索的一项关键技术。 在这里，我们报告了一组大口径块状SRF腔体上观察到的一种系统效应——在超导转变温度以下的低温下，共振频率出现异常下降——这为理解氮掺杂以及未来品质因数和相干性改进相关的现代腔体表面处理背后的物理机制开辟了新的途径。 频率变化的幅度与所研究腔体中的近表面杂质分布以及观测到的$T_c$变化存在系统相关性。 我们还首次展示了铌SRF腔体交流复数电导率实部中的相干峰，并表明其幅度随杂质分布而变化。