Discovery of long-range skin Josephson supercurrent in van der Waals ferromagnet

A recent study published in Nature Communications by Prof. Xiang Bin’s group from the University of Science and Technology of China, in collaboration with Assoc. Prof. Wang Zhi from Sun Yat-sen University, has made a significant discovery in the field of superconductivity. The researchers successfully observed a long-range skin Josephson supercurrent across a van der Waals ferromagnet.

Traditionally, ferromagnetism and superconductivity have been considered incompatible phenomena. When a singlet supercurrent enters a ferromagnet, it typically leads to the rapid decoherence of the Cooper pairs. However, recent theoretical and experimental studies have shown that spin-triplet supercurrents induced near superconductor/ferromagnet interfaces can enable dissipation-free transport over long distances in ferromagnets. This opens up exciting possibilities for the development of quantum devices without energy loss.

Previous research efforts have mainly focused on creating superconducting Josephson junctions with bulk ferromagnets to observe and control spin-triplet currents and related interfacial properties. However, there have been limited reports on studying spin-triplet supercurrents in two-dimensional (2D) van der Waals (vdW) materials.

In this study, the researchers constructed lateral vdW Josephson junctions by connecting two 2D singlet superconductors, NbSe2, with a vdW ferromagnet, Fe3GeTe2. They then investigated the electrical properties of these junctions with different channel lengths through low-temperature electrical tests. The results revealed a zero-resistance state and the presence of a long-range Josephson supercurrent (~ 300 nm) in the S/F/S junctions.

Interestingly, as the channel length increased, the zero-temperature superconducting critical current tended to decay and eventually disappeared completely at a channel length of 450 nm. Furthermore, the response of the long-range superconducting critical current to an external magnetic field displayed a periodic oscillation pattern resembling double-slit interference, rather than the conventional Fraunhofer periodic oscillation stripe. This finding confirmed the existence of a unique Josephson supercurrent with a long-range skin characteristic in S/F/S junctions, distinct from the Josephson superconducting current observed in conventional bulk channels.

The researchers proposed two potential mechanisms to explain the skin feature of the observed long-range supercurrent. The first mechanism involves Rashba spin-orbit coupling induced by mirror symmetry breaking on the Fe3GeTe2 surface. This coupling, when interacting with ferromagnetism and the s-wave superconductivity of NbSe2, could give rise to 2D topological superconductivity on the surface of Fe3GeTe2.

The second mechanism is related to the magnetic inhomogeneity resulting from the non-coplanar structure of Fe atoms in Fe3GeTe2. This non-coplanarity promotes the conversion of spin-singlet Cooper pairs into spin-triplet pairs at the surface through spin rotation and mixing, leading to the formation of a long-range Josephson supercurrent.

The design of the S/F/S junction with a noncoplanar structure offers a fresh perspective for investigating the interplay between ferromagnetism and superconductivity. The novel physical properties demonstrated by this noncoplanar structure provide a platform for potential applications in 2D superconducting spintronics and the realization of topological superconductivity.

Source: Chinese Academy of Sciences

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