Towards 2D superconducting spintronics

传统的（S波）三维超导体(S)和铁磁体(F)之间的接近效应可以导致库珀电子对从传统的反平行自旋（自旋-单重态）状态转变为平行自旋（自旋-三重态）状态。自旋三重态在S/F系统中产生的证明导致了超导自旋电子学（superspintronics）领域的建立，该领域旨在开发超导器件，利用自旋三重态超电流以非常高能量的效率进行自旋电子操作。近年来，层状范德瓦尔斯异质结构（vdW）的出现，以及将vdW材料剥离至单层极限的可能性，为二维（2D）体系的研究铺平了道路。因此，本研究计划旨在探索二维S/F系统。我们的目标不仅是探索2D超导体/2D铁磁体（2DS/2DF）异质结构中由邻近效应产生的非常规超导态，这从基本角度来看是有趣的，而且还使用2DS/2DF系统制造具有新功能的超导自旋电子器件。我们的研究项目采用系统的方法，包括材料表征、理论建模、光谱特性的实验研究和器件制造。2DF/2DS系统是在惰性气氛中使用机械剥离和干燥转移制造的，其光谱特性将使用隧道装置（在2DF/2DS混合物的顶部放置2D绝缘屏障）和低温扫描隧道显微镜和光谱的组合来研究。基于Bogolubov-de Gennes和准经典格林函数技术的理论模型将伴随实验，不仅可以确定最适合器件应用的2DS/2DF组合和几何形状，还可以帮助理解电子光谱和电子输运数据。在第一个资助期内，我们发展了方法，并获得了二维超导体和螺旋vdW金属Cr_(1/3)NbSe_2组成的双层体系中三重态产生的间接证据。基于这一经验，我们现在将研究约瑟夫森结和自旋阀结构，也使用其他螺旋F金属和/或半导体F与NbSe_2和NbS_2结合作为超导体。

The proximity effect between a conventional (s-wave) three-dimensional superconductor (S) and a ferromagnet (F) can lead to the conversion of the Cooper pairs of electrons from a conventional antiparallel-spin (spin-singlet) state into a parallel-spin (spin-triplet) state. The demonstration of spin-triplet generation in S/F systems has led to the establishment of the field of superconducting spintronics (superspintronics) which aims at developing superconducting devices, where spin-triplet supercurrents are used to perform spintronic operations with very high-energy efficiency. The recent advent of layered van der Waals (vdW) heterostructures combined with the possibility of exfoliating vdW materials down to the monolayer limit, pave the way to the investigation of S/F systems in the two-dimensional (2D) regime. This research proposal thus aims at exploring 2D S/F systems. Our ambition is not only to explore unconventional superconducting states emerging from the proximity effect in 2D superconductor/2D ferromagnet (2DS/2DF) heterostructures, which is interesting from a fundamental point of view, but also to use 2DS/2DF systems to fabricate superconducting spintronic devices with novel functionalities compared to those proposed to date. Our research program takes a systematic approach involving materials characterization, theoretical modeling, experimental investigation of spectroscopic properties, and device fabrication. The 2DF/2DS systems are fabricated using mechanical exfoliation and dry transfer in inert atmosphere, and their spectroscopic properties will be studied using a combination of tunneling devices (with a 2D insulating barrier placed on top of the 2DF/2DS hybrid) and low-temperature scanning tunneling microscopy & spectroscopy. Theoretical models based on Bogolubov-de Gennes and quasiclassical Green’s function techniques will accompany the experiment, not only to identify the most suitable 2DS/2DF combinations and geometries for device applications, but also to help understand electronic spectroscopic and electronic transport data. In the first funding period we developed the methodology and obtained indirect evidence for triplet generation in bilayer systems composed of 2D superconductors and the helical vdW metal Cr_(1/3)NbSe_2. Based on this experience we will now study Josephson junctions and spin valve structures using also other helical F metals and/or semiconducting Fs in combination with NbSe_2 and NbS_2 as superconductors.