We used scanning tunneling microscopy and spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) to investigate the effect of nitrogen doping on the surface electronic and chemical structures of cutouts from superconducting Nb radio frequency (SRF) cavities. The goal of this work is to get insights into the fundamental physics and materials mechanisms behind the striking decrease of the surface resistance with the radio-frequency magnetic field, which has been observed on N-doped Nb cavities. Our XPS measurements reveal a significantly more oxidized Nb $\it{3}$d states and a thinner metallic suboxide layer on the N-doped Nb surfaces which is also confirmed by tunneling spectroscopy measurements. In turn, tunneling measurements performed on native surfaces as well as on Ar ion sputtered surfaces allow us to separate the effect of N-doping on the surface oxide layer from that on the density of states in the bulk. Analysis of our tunneling spectra in the framework of a model of a proximity-coupled normal layer at the surface (A. Gurevich and T. Kubo {} 96 , 184515 (2017)) is consistent with the hypothesis that N-doping ameliorates lateral inhomogeneities of superconducting properties on the surface and shrinks the metallic suboxide layer. For the Ar sputtered surfaces, we also find evidence that N-doping changes the contact resistance between the metallic suboxide and the bulk niobium toward an optimum value corresponding to a minimum surface resistance. The totality of our experimental data suggests that the N doping provides an effective tuning of the density of states in such a way that it can result in a decrease of the surface resistance with the radio frequency field, as it was predicted by calculations of the nonlinear low-frequency electromagnetic response of dirty superconductors. Furthermore, STM imaging of vortex cores shows a slightly reduced average superconducting gap and a shorter coherence length in the N-doped Nb samples as compared to typically prepared Nb samples, indicating a stronger impurity scattering caused by nitrogen doping in a moderately disordered material.

中文翻译：

---

电子隧穿和X射线光电子能谱研究掺氮铌谐振腔的超导性能

我们使用扫描隧道显微镜和光谱（STM / STS）和X射线光电子能谱（XPS）来研究氮掺杂对超导Nb射频（SRF）腔中切口的表面电子和化学结构的影响。这项工作的目的是深入了解在掺杂Nb的Nb腔中观察到的射频磁场引起的表面电阻急剧下降背后的基本物理和材料机理。我们的XPS测量结果显示Nb $ {it} {3}d态和N掺杂Nb表面上较薄的金属亚氧化物层也可以通过隧道光谱法测量得到证实。反过来，在本征表面以及在Ar离子溅射表面上执行的隧穿测量使我们能够将N掺杂对表面氧化物层的影响与对体态密度的影响分开。在地表邻近耦合法线层模型的框架内对隧道光谱的分析（A. Gurevich和T. Kubo {} 96，184515（2017））与以下假设相符：N掺杂改善了横向不均匀性表面上的超导性能降低并且使金属亚氧化物层收缩。对于Ar溅射表面，我们还发现有证据表明，N掺杂将金属亚氧化物和大块铌之间的接触电阻改变为与最小表面电阻相对应的最佳值。我们所有的实验数据表明，N掺杂可以有效地调整态密度，从而可以通过射频场来降低表面电阻，这是通过非线性计算得出的。肮脏的超导体的低频电磁响应。此外，与典型制备的Nb样品相比，涡流核的STM成像显示N掺杂Nb样品中的平均超导间隙略微减小，相干长度更短，这表明中度无序材料中氮掺杂会导致较强的杂质散射。