In a conventional Josephson junction, two superconductors separated by a thin barrier share their superconducting state so that paired electrons move coherently between them without energy loss. In the new study, the team observed similar synchronized behavior even though only one side of the junction was a true superconductor, confirming long-standing theoretical predictions that such a configuration could function using a single superconducting electrode.
The measurements, reported in Nature Communications, indicate that superconductivity from vanadium leaked through the magnesium oxide barrier and generated electron pairing inside the iron layer. While superconductors can induce weak superconducting correlations in nearby materials, the induced behavior in iron was strong enough to create collective charge motion that mimicked the dynamics of a standard two-superconductor Josephson junction.
Study co-corresponding author Igor Zutic, SUNY Distinguished Professor in the Department of Physics in the University at Buffalo College of Arts and Sciences, likened a typical Josephson junction to "two army battalions marching in step along opposite banks of a river." He explained that in the new device "there was only one battalion - yet it's as if its marching caused citizens on the other side to form a militia and begin marching to the beat of a different drum."
The experiments were carried out in the laboratory of co-corresponding author Farkhad Aliev, professor of condensed matter physics at the Autonomous University of Madrid in Spain, with additional collaborators from Comillas Pontifical University in Spain, the University of Lorraine in France, Babes-Bolyai University in Romania, and the Eastern Institute for Advanced Study in China. The work was supported by the U.S. Department of Energy Office of Science, Basic Energy Sciences program.
To probe how charge moved through the structure, the researchers analyzed tiny fluctuations in the electrical current known as noise, which reveal how electrons traverse a material. By monitoring this noise in the vanadium-magnesium oxide-iron stack, they detected electrons in the iron moving in large, coordinated groups, a signature usually associated with two superconductors coupled through a Josephson junction.
This behavior surprised the team because ferromagnetism and superconductivity typically oppose each other: paired electrons in superconductors have opposite spins, whereas electrons in ferromagnets tend to align their spins in the same direction. In the experiment, however, the iron formed same-spin electron pairs that still behaved collectively enough to synchronize with the vanadium layer across the barrier.
"The iron essentially created a different type of superconductivity from vanadium," Zutic said. "In other words, the citizens organized in their own way but kept time well enough to march as an army and send their own rhythm back across the river."
The team is now exploring how iron was able to generate same-spin pairs that were robust enough for the iron to act as if it were an independent superconductor. They suggest that this same-spin pairing may be relevant for topological superconductors, which are designed to protect quantum information - often encoded in electron spin - against local disturbances by storing it in non-local, knot-like configurations.
In conventional quantum computing systems, small changes in the environment can disturb electron spins and disrupt calculations, creating a major challenge for scaling devices. Zutic noted that researchers want to "find a way to essentially lock an electron's spin into place," and that pairing electrons with the same spin may offer a pathway toward more stable quantum states that are less sensitive to noise.
Another implication of the work is that future Josephson-like devices might be constructed from widely used industrial materials rather than exotic components. Both iron and magnesium oxide already appear in commercial magnetic hard drives and magnetic random-access memory, raising the prospect of integrating superconducting functions into existing device architectures.
"We have added a superconducting twist to commercially viable devices," Zutic said, suggesting that leveraging common ferromagnetic and oxide materials in this way could broaden the design space for quantum and superconducting electronics. The findings point toward simpler junction geometries that still deliver the coherent behavior needed for quantum technologies.
Research Report:Giant shot noise in superconductor/ferromagnet junctions with orbital-symmetry- controlled spin-orbit coupling
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