Towards 2D superconducting spintronics

• Applicants: Professor Dr. Wolfgang Belzig; Professor Dr. Angelo Di Bernardo; Professorin Dr. Elke Scheer
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 443404566

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.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2244: 2D Materials – Physics of van der Waals [hetero]structures (2DMP)
• International Connection: Israel
• International Co-Applicant: Professor Dr. Hadar Steinberg