Electron Tunneling and X-Ray Photoelectron Spectroscopy Studies of the Superconducting Properties of Nitrogen-Doped Niobium Resonator Cavities
Electron Tunneling and X-Ray Photoelectron Spectroscopy Studies of the Superconducting Properties of Nitrogen-Doped Niobium Resonator Cavities
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DOI：
10.1103/physrevapplied.13.044044
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发表时间：
2020-04-16
影响因子：
4.6
通讯作者：
Iavarone, Maria
中科院分区：
文献类型：
--
作者：
Lechner, Eric M.;Oli, Basu Dev;Iavarone, Maria
利用扫描隧道显微镜（STM）、能谱仪（STS）和X射线光电子能谱仪（XPS）研究了氮掺杂对超导Nb射频腔切口表面电子结构和化学结构的影响。这项工作的目标是深入了解在射频磁场下表面电阻显著降低背后的基本物理和材料机制，这已经在N掺杂Nb腔体上观察到。我们的XPS测量揭示了显着更多的氧化铌3d状态和较薄的金属低氧化物层上的N掺杂的Nb表面，这也证实了隧道光谱测量。反过来，隧穿测量进行原生表面上，以及Ar离子溅射表面上，使我们能够分离的N掺杂的表面氧化层的影响，在体的状态密度。在表面邻近耦合正常层模型的框架下分析我们的隧穿谱[A. Gurevich和T. Kubo，B 96，184515（2017）]与N掺杂改善表面上超导性质的横向不均匀性并收缩金属低氧化物层的假设一致。对于Ar溅射的表面，我们还发现的证据表明，N掺杂的金属低价氧化物和散装铌之间的接触电阻朝着最佳值对应于最小的表面电阻的变化。我们的实验数据的整体表明，N掺杂提供了一个有效的调谐的状态密度，以这样一种方式，它可以导致在一个减少的表面电阻与射频场，预测通过计算的非线性低频电磁响应的脏超导体。此外，STM成像的涡核显示出略有减少的平均超导间隙和较短的相干长度在N掺杂的Nb样品相比，通常制备的Nb样品，表明较强的杂质散射所造成的氮掺杂在适度无序的材料。
We use scanning tunneling microscopy (STM) and spectroscopy (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 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 significantly more oxidized Nb 3d 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, Phys. Rev. B 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 predicted by calculations of the nonlinear lowfrequency 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.