Ferromagnetism and superconductivity are two antagonistic macroscopic orderings. When the singlet supercurrent enters the ferromagnet, rapid decoherence of the Cooper pairs will be triggered. However, spin-triplet supercurrents induced in the vicinity of superconductor/ferromagnet interfaces enable transport without energy dissipation over long distances in ferromagnets, which has been proved theoretically and experimentally in recent years. Therefore, it provides a more desirable method for constructing quantum devices without dissipation. Earlier research focused on the construction of superconducting Josephson junctions with coupled bulk ferromagnets, to achieve the observation of spin-triplet currents and the control of spin and charge degrees of freedom. However, there are few reports on the observation of spin-triplet supercurrents and related studies of interfacial properties based on heterojunctions of two-dimensional (2D) van der Waals (vdW) materials.
The research team constructed lateral vdW Josephson junctions of S/F/S by bridging two singlet vdW superconductors NbSe2 with vdW ferromagnet Fe3GeTe2 (F). The electrical properties of the S/F/S with different junction channel lengths have been studied by low-temperature electrical tests. The results showed a zero-resistance state of the S/F/S and also a long-range Josephson supercurrent (~ 300 nm). The zero-temperature superconducting critical current tends to decay with increasing channel length and disappears completely when the channel length increases to 450 nm.
More interestingly, the response of the long-range superconducting critical current to an external magnetic field perpendicular to the supercurrent channel presents a periodic oscillation pattern, which is similar to double-slit interference, rather than the conventional Fraunhofer periodic oscillation stripe. This result confirms the existence of a Josephson supercurrent with a long-range skin feature in S/F/S, that is distinctive from the Josephson superconducting current of conventional bulk channels.
Then the research team proposed two possible mechanisms for the skin feature of the long-range supercurrent. First, the Rashba spin-orbit coupling induced by the mirror symmetry breaking on the Fe3GeTe2 surface, when interacting with ferromagnetism and the s-wave superconductivity of NbSe2, may lead to 2D topological superconductivity on the Fe3GeTe2 surface. Second, the magnetic inhomogeneity caused by the non-coplanar structure of Fe atoms in Fe3GeTe2 promotes the transformation of spin-singlet Cooper pairs into spin-triplet pairs at the surface through spin-rotation and spin-mixing, and then forms a long-range Josephson supercurrent.
Skin feature of S/F/S Josephson junction supercurrent density. a Schematic of the S/F/S with the magnetic field along the z-axis direction. b Differential resistance map across the junction at 3 K, showing a double-slit interference pattern. c Distribution of supercurrent density along the y-axis obtained from the inverse Fourier transform of the data in b. The Fe3GeTe2 layer thickness is 22 nm. d Schematic of the S/F/S junction with the magnetic field along the y-axis direction. e Differential resistance map across the junction at 3 K, showing a mixed single-slit and double-slit interference pattern. f Distribution of supercurrent density along the z-axis obtained from the inverse Fourier transform of the data in e, showing that the supercurrent can flow relatively more uniformly across the junction along the x-axis, but still peaked at the surfaces (now the two edges). The channel width w is 2 μm. (Image by HU Guojing et al.)
The S/F/S design of the noncoplanar structure provides a new perspective to explore the interaction between ferromagnetism and superconductivity. The novel physical properties presented by this noncoplanar structure provide a platform for potential applications of new quantum functional devices in 2D superconducting spintronics and the realization of topological superconductivity.
Paper link: https://doi.org/10.1038/s41467-023-37603-9
(Written by LI Rui, edited by WANG Zhaokun, USTC News Center)