Effect of metal encapsulation on bulk superconducting properties of niobium thin films used in qubits
Ames National Laboratory（艾姆斯国家实验室）；Iowa State University（爱荷华州立大学）；Fermi National Accelerator Laboratory（费米国家加速器实验室）
Amlan Datta, Kamal R. Joshi, Sunil Ghimire, Bicky S. Moirangthem, Makariy A. Tanatar, Mustafa Bal, Zuhawn Sung, Sabrina Garattoni, Francesco Crisa, Akshay Murth… 展开作者
Amlan Datta, Kamal R. Joshi, Sunil Ghimire, Bicky S. Moirangthem, Makariy A. Tanatar, Mustafa Bal, Zuhawn Sung, Sabrina Garattoni, Francesco Crisa, Akshay Murthy, David A. Garcia-Wetten, Dominic P. Goronzy, Mark C. Hersam, Michael J. Bedzyk, Shaojiang Zhu, David Olaya, Peter Hopkins, Matthew J. Kramer, Alexander Romanenko, Anna Grassellino, Ruslan Prozorov
铌金属占据了典型二维 transmon 设备体积的近 100%。虽然铝约瑟夫森结至关重要，但维持整个设备的量子相干性意味着 Nb 导线、电容垫和读出谐振器中的成对断裂可能是主要的退相干源。已确定的贡献者是表面氧化物和氢氧化物，以及吸附的氢和氧。用非氧化金属封装新鲜生长的表面， preferably 不破坏真空，是缓解这些问题的成功策略。尽管封装的积极影响不容置疑，但理解其对铌薄膜宏观行为的影响是重要的。我们对封装了金和钯/金的 Nb 薄膜的 bulk 超导性质进行了全面研究，并将其与裸 Nb 薄膜的性质进行了比较。磁光成像、磁化、电阻率以及伦敦和坎贝尔穿透深度测量揭示了封装样品中的显著差异。溅射和 epitaxial Au 覆盖的薄膜表现出最高的残余电阻率比和超导转变温度，以及最低的上临界场、伦敦穿透深度和临界电流。这些结果与 Nb 的各向异性正常和超导态的微观理论一致。我们得出结论，由薄膜中的整体杂波而非仅仅表面驱动的 Nb 薄膜中的成对断裂是 transmon 中重要的量子退相干源。我们还得出结论，金覆盖不仅钝化了表面，还影响了整个薄膜的性质，显著降低了由于表面扩散可能引起的缺陷导致的散射率，如果薄膜在制造后没有立即保护的话。
英文摘要：
Niobium metal occupies nearly 100\% of the volume of a typical 2D transmon device. While the aluminum Josephson junction is of utmost importance, maintaining quantum coherence across the entire device means that pair-breaking in Nb leads, capacitive pads, and readout resonators can be a major source of decoherence. The established contributors are surface oxides and hydroxides, as well as absorbed hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with non-oxidizing metals, preferably without breaking the vacuum, is a successful strategy to mitigate these issues. While the positive effects of encapsulation are undeniable, it is important to understand its impact on the macroscopic behavior of niobium films. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium/gold, and compare them to those of bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered, and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature, as well as the lowest upper critical field, London penetration depth, and critical current. These results are in good agreement with the microscopic theory of anisotropic normal and superconducting states of Nb. We conclude that pair-breaking in the bulk of niobium films, driven by disorder throughout the film rather than just at the surface, is a significant source of quantum decoherence in transmons. We also conclude that gold capping not only passivates the surface but also affects the properties of the entire film, significantly reducing the scattering rate due to defects likely induced by surface diffusion if the film is not protected immediately after fabrication.
Transforming Native Oxide into a Metallic Platinum--Niobium Alloy for Passivation of Superconducting Niobium Films
Center for Functional Nanomaterials, Brookhaven National Laboratory（布鲁克海文国家实验室功能纳米材料中心）；Material Measurement Laboratory, National Institute of Standards and Technology（美国国家标准与技术研究院材料测量实验室）；National Synchrotron Light Source II, Brookhaven National Laboratory（布鲁克海文国家实验室第二代同步辐射光源）
Massive suppression of proximity pairing in topological (Bi$_{1-x}$Sb$_{x})_2$Te$_3$ films on niobium
将体导电拓扑Bi $_2$ Se $_3$薄膜与s波超导体相连接，在非平凡的表面态中产生强超导有序.然而，在体Nb上的体绝缘拓扑（Bi $_{1-x}$ Sb $_{x})_2$ Te $_3$）膜反而表现出表面超导性的巨大衰减，即使对于仅两层厚的膜也是如此。这种对邻近配对的大规模抑制通过超高分辨率能带图和通过将量化的超导能隙与重n掺杂拓扑Bi $_2$ Se $_3$ /Nb的超导能隙进行对比而得到证明。结果强调了利用超导邻近效应在近本征系统中实现拓扑超导性的局限性。
Magnetic force microscopy versus scanning quantum-vortex microscopy: Probing pinning landscape in granular niobium films
Moscow Institute of Physics and Technology（莫斯科物理技术学院）；Institute for Physics of Microstructures RAS（俄罗斯科学院微结构物理研究所）；Lobachevsky State University of Nizhny Novgorod（下诺夫哥罗德罗巴切夫斯基国立大学）；Osipyan Institute of Solid State Physics Russian Academy of Sciences（俄罗斯科学院奥西皮扬固体物理研究所）；ESPCI Paris -- PSL, CNRS, Sorbonne University（巴黎高等物理化工学院-PSL、法国国家科学研究中心、索邦大学）
Encapsulation epitaxy of air-stable monolayer superconducting films for quantum circuits and qubits
Massachusetts Institute of Technology（麻省理工学院）；Yale University（耶鲁大学）；Harvard University（哈佛大学）；Pohang University of Science and Technology（浦项科技大学）；New York University（纽约大学）
Elucidating the local atomic and electronic structure of amorphous oxidized superconducting niobium films
由超导材料制成的量子比特是用于量子计算等量子信息科学应用的成熟平台。然而，基于材料的损耗现在是达到应用所需的相干时间的一个限制因素。特别是，需要电路材料的原子结构和性质的知识来识别、理解和减轻基于材料的退相干通道。在这项工作中，我们通过比较波动电子显微镜实验的密度泛函理论计算，发现一个非晶层符合Nb $_2$ O $_5$化学计量的Nb谐振器上形成的原生氧化膜的原子结构的特征。比较X射线吸收测量在氧K边缘与第一性原理计算，我们发现的d-型磁性杂质在我们的样品中，已知在近端超导体中引起阻抗的证据。这项工作确定了Nb超导体上生长的氧化物层的结构和化学成分，并表明软X射线吸收可以识别这些超导系统中的磁性杂质。
Control of thin NbN film superconducting properties by ScN buffer layer
Institute of Nanotechnology of Microelectronics of Russian Academy of Science（俄罗斯科学院微电子纳米技术研究所）；M.N. Miheev Institute of metal physics UB RAS（俄罗斯科学院乌拉尔分支米海耶夫金属物理研究所）；Higher School of Economics - National Research University（高等经济大学-国立研究大学）
