DATE2025.09.04 #Press Releases
Switching Flux Quanta to Half-Integer in a Topological Superconductor
Summary
A research group led by Professor Yasuhiro Niimi (Department of Physics, Graduate School of Science, Osaka University), in collaboration with Assistant Professor Kazushi Aoyama (Department of Earth and Space Science, Graduate School of Science, Osaka University), Associate Professor Takeshi Mizushima (Division of Materials Physics, Graduate School of Engineering Science, Osaka University), Professor Jun-ichiro Ohe (Department of Physics, Faculty of Science, Toho University), Professor Teruo Ono (Institute for Chemical Research, Kyoto University), Professor Xiaofeng Jin (Fudan University, China), and Professor Kensuke Kobayashi (Department of Physics, School of Science, The University of Tokyo), has discovered a phenomenon in which the magnetic flux quanta trapped inside a superconducting thin-film ring—composed of the semimetal bismuth (with strong spin–orbit interaction) and the ferromagnetic metal nickel—switch from conventional integer values to half-integer values (e.g., 1.5, 2.5). This finding suggests that the Bi/Ni thin films function as a topological superconductor. (Figure)
Figure: (Left) Schematic illustration of the bilayer thin film of ferromagnetic nickel (2 nm) and semimetal bismuth (35 nm) grown on a magnesium oxide substrate. (Right) Schematic of the thin-film ring device. H→ indicates the applied external magnetic field.
Topological superconductors are essential for realizing Majorana quasiparticles, which are attracting significant attention in quantum information science. Majorana quasiparticles are promising candidates for next-generation quantum computers because of their robustness against environmental noise as quantum bits (qubits). Current superconducting quantum computers primarily utilize spin-singlet s-wave superconductors with isotropic superconducting gaps. In contrast, superconductors with anisotropic gaps, including topological superconductors, have not yet been used.
In this study, the researchers fabricated superconducting thin-film rings and applied an electric current while sweeping the magnetic field penetrating the ring. They observed that the number of trapped flux quanta switched from integer to half-integer values across a certain magnetic-field threshold. This phenomenon arises from the interplay of anisotropic superconducting gaps and spin–orbit interaction, revealing a novel physical effect that strongly suggests the film is a topological superconductor.
The ability to tune the trapped magnetic flux quanta from integer to half-integer values with only a slight external magnetic field opens new possibilities for applications in superconducting quantum computers.
These research results were published in the U.S. scientific journal Science Advances on September 4, 2025, at 3:00 a.m. JST.
Links
Osaka University, Toho University, Kyoto University
Journals
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Journal name Science AdvancesTitle of paper