Josephson nano-junctions fabricated with a focused He-ion beam

最近开发的和商业上可获得的He离子显微镜（HIM）提供了具有约0.5nm的斑点尺寸的高度聚焦的He离子束（He-FIB）。这不仅可以实现超高分辨率成像，而且还为纳米级薄膜的可控改性提供了令人兴奋的前景。在这个项目中，我们将探索通过使用He-FIB对由铜酸盐超导体制成的薄膜进行可控纳米级改性的潜力。选择铜酸盐超导体是出于其独特的性质：（i）超短相干长度允许人们在原子尺度上改变它们的性质，（ii）超导序参量的d波对称性使得能够实现非常规约瑟夫森结（JJ），例如几何π JJ，以及（iii）在掺杂的变化下，可以将这些材料从超导状态驱动到绝缘状态。该项目主要集中在研究得很好的铜酸盐超导体YBCO，其中掺杂（氧含量）可以通过He-FIB照射局部改变。通过使用适当剂量的He-FIB在YBCO膜上“画”一条线，可以创建约瑟夫森势垒。这样的JJ可以自由地分布在芯片上，并且可以具有不同的取向。通过调整He-FIB剂量，可以在芯片上可控地改变临界电流密度。使用高剂量的He-FIB照射，可以“写入”高电阻的壁和区域。这将用于实现超窄收缩JJ，超导量子干涉器件（SQUID）和更先进的约瑟夫森器件。利用可调节的辐照剂量和超高的空间分辨率，创造出目前尚不可行的新型超导纳米电子器件和电路。该项目分为三个主要部分：用于铜酸盐超导体纳米改性的He-FIB技术的优化; He-FIB诱导的JJ和SQUID的基本性质和先进器件的研究。总而言之，我们打算研究由He-FIB在铜酸盐超导体中制成的JJ的基本特性，了解这种新技术的局限性，并探索实现更复杂和更先进的JJ器件和电路的前景。

The recently developed and commercially available He ion microscope (HIM) provides a highly focused He ion beam (He-FIB) with spot size ~0.5 nm. This enables not only ultra-high-resolution imaging, but also offers exciting perspectives for the controlled modification of thin films on the nm scale. Within this project, we will explore the potential of controlled nanoscale modification of thin films made of cuprate superconductors by using a He-FIB. The choice of cuprate superconductors is motivated by their peculiar properties: (i) the ultra-short coherence length allows one to change their properties on the atomic scale, (ii) the d-wave symmetry of the superconducting order parameter enables the realization of unconventional Josephson junctions (JJs), e.g. geometric pi JJs, and (iii) upon variation of doping one can drive these materials from the superconducting to the insulating state.The project mainly focuses on the well-studied cuprate superconductor YBCO, where doping (oxygen content) can be modified locally by He-FIB irradiation. By “drawing” a line using a He-FIB of proper dose across the YBCO film one can create a Josephson barrier. Such JJs can be freely distributed on a chip and can have different orientations. The critical current densities can controllably vary on a chip by adjusting the He-FIB dose. Using He-FIB irradiation with high dose one can “write” highly resistive walls and areas. This will be used to realize ultra-narrow constriction JJs, superconducting quantum interference devices (SQUIDs) and more advanced Josephson devices. Adjustable irradiation dose and ultra-high spatial resolution shall be exploited to create novel superconducting nano-scale electronic devices and circuits that were not feasible so far.The project is divided into three major parts: optimization of the He-FIB technology for nanoscale modification of cuprate superconductors; studies on basic properties of He-FIB induced JJs and SQUIDs and advanced devices. Altogether, we intend to examine the fundamental properties of the JJs made by a He-FIB in cuprate superconductors, understand the limitations of this new technology and explore the perspectives for the realization of more complex and advanced JJ devices and circuits.