[论文解读] Planar Josephson Junctions Templated by Nanowire Shadowing

Po Zhang, A. Zarassi+17 more|arXiv (Cornell University)|Nov 8, 2022

Physics of Superconductivity and Magnetism被引用 7

一句话总结

本文提出了一种无需光刻的平面约瑟夫森结制备方法，利用半导体纳米线在超导蒸发过程中作为掩膜，实现高质量结的制备。该技术可实现透明界面和自对准的电栅，已在InAs量子阱上的Al和Sn基结中得到验证，临界电流乘积超过超导能隙（0.4 mV 和 0.86 mV），证实了强近邻效应和可调的约瑟夫森行为。

ABSTRACT

More and more materials, with a growing variety of properties, are built into electronic devices. This is motivated both by increased device performance and by the studies of materials themselves. An important type of device is a Josephson junction based on the proximity effect between a quantum material and a superconductor, useful for fundamental research as well as for quantum and other technologies. When both junction contacts are placed on the same surface, such as a two-dimensional material, the junction is called ``planar". One outstanding challenge is that not all materials are amenable to the standard planar junction fabrication. The device quality, rather than the intrinsic characteristics, may be defining the results. Here, we introduce a technique in which nanowires are placed on the surface and act as a shadow mask for the superconductor. The advantages are that the smallest dimension is determined by the nanowire diameter and does not require lithography, and that the junction is not exposed to chemicals such as etchants. We demonstrate this method with an InAs quantum well, using two superconductors - Al and Sn, and two semiconductor nanowires - InAs and InSb. The junctions exhibit critical current levels consistent with transparent interfaces and uniform width. We show that the template nanowire can be operated as a self-aligned electrostatic gate. Beyond single junctions, we create SQUIDs with two gate-tunable junctions. We suggest that our method can be used for a large variety of quantum materials including van der Waals layers, topological insulators, Weyl semimetals and future materials for which proximity effect devices is a promising research avenue.

研究动机与目标

• 开发一种无需光刻的平面约瑟夫森结制备方法，避免对敏感量子材料造成化学和机械损伤。
• 利用遮蔽纳米线作为栅电极，实现结弱连接区的自对准电栅调控。
• 在InAs量子阱中的二维电子气（2DEGs）上，利用Al和Sn超导体实现高质量约瑟夫森结的制备。
• 实现具有电调制结的SQUID器件，用于探测非正弦的电流-相位关系。
• 拓展兼容的量子材料范围，包括范德华异质结构和拓扑绝缘体，用于近邻效应器件。

提出的方法

• 通过范德华力将纳米线转移至2DEG衬底上，在超导蒸发过程中作为遮蔽掩膜。
• 在2DEG上蒸发Al或Sn，纳米线遮蔽区域阻止沉积，从而定义结的宽度。
• 纳米线通过与下方2DEG形成背栅结构，作为自对准电栅使用。
• 结的制备无需刻蚀或光刻，最大限度减少对2DEG的损伤并保持界面透明性。
• 该方法应用于Al/InAs和Sn/InAs体系，其中Sn沉积在80 K下进行以防止互扩散。
• 后处理阶段通过选择性湿法刻蚀去除结外多余的Sn和InAs，保持结的完整性。

实验结果

研究问题

• RQ1 纳米线遮蔽能否实现高界面透明度且加工损伤最小的平面约瑟夫森结？
• RQ2 遮蔽纳米线能否在电调制结中作为自对准电栅？
• RQ3 通过该方法制备的结是否表现出$ I_cR_N $乘积超过超导能隙，表明强近邻耦合？
• RQ4 该方法能否扩展至多种超导体（Al和Sn）和不同量子材料（InAs、InSb）？
• RQ5 非正弦电流-相位关系对这些器件中SQUID和单结行为的影响如何？

主要发现

• Al/InAs结表现出$ I_cR_N $ = 0.4 mV，超过Al的超导能隙0.35 meV，表明界面高透明度和强近邻效应。
• Sn/InAs结显示$ I_cR_N $ = 0.86 mV，显著超过Sn的能隙0.6 meV，证实强近邻耦合。
• 通过$ dI/dV $谱中的多次Andreev反射峰测得Sn结的诱导能隙为Δ = 0.57 meV。
• 具有两个电调制结的SQUID器件表现出类似Fraunhofer的磁通调制，且在半量子通量附近出现拐点，表明电流-相位关系中存在强二次谐波分量。
• 该方法可实现60多个器件的可重复制备，覆盖6块芯片，共收集8次低温稀释制冷机循环的5700组数据。
• 该技术与多种量子材料兼容，包括范德华异质结构、拓扑绝缘体和外尔半金属，为未来探索奇异量子态提供了可能。