Recently, the research group of Prof. Xiu Faxian from the Department of Physics at Fudan University, in collaboration with other researchers, constructed high-quality proximity Josephson junction devices based on the antiferromagnetic topological insulator MnBi₂Te₄. They achieved asymmetric edge supercurrent and a zero-field tunable Josephson diode effect. On May 14, 2025, the related findings were published online in the journal Science Advances under the title "Observation of Edge Supercurrent in Topological Antiferromagnet MnBi₂Te₄-based Josephson Junctions" [Sci. Adv. 11, eads8730 (2025)]. This work was a collaborative effort involving Prof. Xiu Faxian's group at Fudan University's Department of Physics, Prof. Lo Kam Tuen's group at The Hong Kong University of Science and Technology (HKUST), the team of Academician Dong Shaoming and Researcher Yang Jinshan at the Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), Prof. Ni Ni's group at the University of California, Los Angeles (UCLA), Prof. Kou Xufeng's group at ShanghaiTech University, and Prof. Zou Jin's group at The University of Queensland (UQ).
With the rapid development of topology, magnetism, and superconductivity, the novel quantum phenomena arising from the coupling of these three properties have become a frontier hotspot in condensed matter physics research. In such systems, the Josephson junction structure serves as a key platform for coupling two superconductors via a non-superconducting material. It not only exhibits coherent transport behaviors like DC and AC Josephson effects but may also manifest correlated transport phenomena such as Majorana zero modes, 4π-periodic superconducting effects, and superconducting diode effects. This offers possibilities for developing novel low-power quantum devices and topological quantum computing platforms.
MnBi₂Te₄ is an antiferromagnetic topological insulator extensively studied in recent years. It combines a van der Waals layered structure with a topologically non-trivial band structure, exhibiting various quantum topological phases (such as the quantum anomalous Hall effect and axion insulator state) in odd/even layers. It is considered an ideal material for constructing "superconducting-magnetic-topological" triple-coupled devices. Although theories predict that Josephson structures in MnBi₂Te₄ could support topological edge supercurrents and even Majorana modes, experimental studies have remained exploratory due to long-standing technical challenges such as material stability, device fabrication, and interface control. Therefore, realizing high-quality Josephson junctions in this system and further observing edge-dominated superconducting transport phenomena and functional device behavior hold significant theoretical value and application prospects.
Based on high-quality MnBi₂Te₄ single crystals, the research team constructed Nb/MnBi₂Te₄/Nb proximity effect Josephson junction devices. Experiments revealed that when the MnBi₂Te₄ thickness was approximately 110 nm, the device exhibited a Fraunhofer interference pattern with an anomalously enlarged period at low temperatures, indicating strong coupling behavior of the supercurrent within the junction region. Upon further thinning to 55 nm, the current-magnetic field relationship of the device showed an interference pattern resembling that of a Superconducting Quantum Interference Device (SQUID), and the junction magnetoresistance also exhibited corresponding periodic oscillations. Notably, the critical currents in the positive and negative current directions and their response to magnetic fields were asymmetric, revealing highly asymmetric superconducting transport behavior dominated by topological edge states.
To investigate the origin of this asymmetric supercurrent, the team employed Fourier transform techniques to perform inversion analysis on the interference patterns. They discovered that the supercurrent was primarily distributed along the two edges of the device and exhibited significant asymmetry. Theoretical calculations indicate that this phenomenon stems from differences in the lattice structure and scattering environment at the two edges of MnBi₂Te₄, leading to distinct effective Fermi velocities in their topological edge states. This breaks inversion symmetry and introduces asymmetric edge channels. This asymmetry endows the MnBi₂Te₄ Josephson junction with the capability to rectify supercurrents, laying the physical foundation for subsequent non-reciprocal transport functional devices.
Fig. 1: a, Left: Schematic of the gapped top and bottom surface states and the gapless side surface states in MnBi₂Te₄. Top right: Schematic of the asymmetric SQUID device. Bottom right: Schematic of the two asymmetric edges (with unequal effective Fermi velocities) of the MnBi₂Te₄-based Josephson junction. This device is equivalent to an asymmetric SQUID. b, Asymmetric SQUID pattern observed in the MnBi₂Te₄ Josephson junction. c, Supercurrent density distribution map extracted via Fourier transform, showing the supercurrent is mainly concentrated at the two edges, with significantly asymmetric current densities on the two sides. d, Theoretical calculation showing the surface states on the (010) plane of a MnBi₂Te₄ flake exhibit asymmetric Fermi velocities at the two device edges. The different positions where the red and green solid lines cross the Fermi level indicate different Fermi velocities for these two edge states. e, Periodic oscillation in the junction magnetoresistance of the MnBi₂Te₄ Josephson junction under DC current bias. f, Polarity-tunable Josephson diode effect achieved via out-of-plane magnetic field "training".
Based on this mechanism, the research team further constructed a MnBi₂Te₄ Josephson junction device with a thickness of about 70 nm and a channel length of 90 nm, and systematically studied its Josephson diode effect. Experiments found that after "training" the device with a perpendicular magnetic field, it could achieve unidirectional flow of supercurrent under zero magnetic field conditions after removing the field. This demonstrates a "write-and-hold" non-volatile rectification characteristic. This phenomenon indicates that the device retains its spontaneous magnetization direction even after the external magnetic field is removed. Under the combined action of time-reversal symmetry breaking and edge coupling asymmetry, a steady-state unidirectional superconducting transport behavior is formed. At 250 mK, the device's zero-field rectification efficiency reached 23%; even at 3.5 K, it maintained significant rectification characteristics, showcasing unique low-temperature device performance and application potential.
This work is the first to achieve Josephson device construction and functional demonstration in the antiferromagnetic topological insulator MnBi₂Te₄, providing a new experimental platform for exploring quantum phenomena such as topological superconductivity, non-reciprocal transport, and Majorana modes. The research team points out that such superconducting rectifying devices based on edge states hold promise for future development in low-power superconducting logic devices, asymmetric quantum circuits, and controllable topological qubits.
This research received substantial support and funding from the Department of Physics at Fudan University, the State Key Laboratory of Surface Physics, the National Natural Science Foundation of China (Young Scientists Fund A, Original Exploration Program Project), and the Shanghai Basic Research Special Zone Program. The Department of Physics at Fudan University is the first affiliation of the paper. Prof. Xiu Faxian from Fudan University and Prof. Lo Kam Tuen from HKUST are the corresponding authors. Dr. Zhang Enze (now a Research Fellow at the National University of Singapore), a former postdoctoral fellow in Prof. Xiu's group, Ph.D. student Jia Zehao (Fudan University), Ph.D. student Sun Ziting (Prof. Lo's group, HKUST), and Researcher Yang Jinshan (SICCAS) are the co-first authors.
Prof. Xiu Faxian's research group primarily focuses on the growth and quantum control of topological materials, as well as device research on novel low-dimensional atomic crystal materials. In the field of topological Dirac materials, the group is dedicated to the growth of novel quantum materials, property measurements, and the fabrication and characterization of quantum devices. For novel low-dimensional atomic crystal materials, research concentrates on their electrical, magnetic, and optoelectronic properties.
Paper Link:https://www.science.org/doi/10.1126/sciadv.ads8730
