Towards 2D superconducting spintronics

传统的(S波)三维超导体(S)和铁磁体(F)之间的邻近效应可以导致库珀电子对从传统的反平行自旋(自旋-单态)态转变为平行-自旋(自旋-三重态)态。S/F系统中自旋三重态产生的证明导致了超导自旋电子学(超自旋电子学)领域的建立，该领域旨在发展超导器件，其中自旋三重态超导电流被用来以非常高的能量效率进行自旋电子操作。最近层状范德华异质结的出现，以及层状范德华材料剥离到单层极限的可能性，为在二维区研究S/F体系铺平了道路。因此，本研究方案旨在探索二维S/F系统。我们的目标不仅是探索2D超导体/2D铁磁(2DS/2DF)异质结中由邻近效应产生的非传统超导状态，而且还希望利用2DS/2DF系统来制备具有与已提出的那些新功能相比具有新功能的超导自旋电子器件。我们的研究项目采取了一种系统的方法，包括材料表征、理论建模、光谱性质的实验研究和器件制造。2DF/2DS系统是在惰性气氛中采用机械剥离和干法转移制备的，其光谱特性将使用隧道器件(在2DF/2DS杂化材料上放置2D绝缘势垒)和低温扫描隧道显微镜和光谱相结合的方法来研究。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，不仅为设备应用确定最合适的2DS/2DF组合和几何结构，而且还帮助理解电子光谱和电子传输数据。在第一个资助期，我们发展了方法学，并获得了由2D超导体和螺旋VDW金属Cr_(1/3)NbSe_2组成的双层系统中产生三重态的间接证据。在此经验的基础上，我们将研究约瑟夫森结和自旋阀结构，并将其他螺旋F金属和/或半导体F与NbSe_2和NbS_2相结合作为超导体。

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.