Direct observation of a heavy fermion state opens a new route to designing quantum materials through their interfaces

Osaka, Japan – A research team led by the University of Osaka has directly observed an unusual heavy-fermion state forming at the boundary between a one-atom-thick material and a metal for the first time. Such states are closely linked to exotic quantum phenomena, including unconventional superconductivity, and the finding opens new possibilities for designing quantum materials through their interfaces.

The researchers created a high-quality, one-atom-thick layer of ytterbium–copper (YbCu₂) on a copper crystal and examined how electrons behaved across the interface using intense synchrotron light. Their measurements showed that electrons localized in the atomic layer interact with mobile electrons in the underlying copper to form the heavy-fermion state.

The measurements revealed two distinct heavy-fermion states. One was confined mainly to the two-dimensional YbCu₂ layer, while the other extended into the three-dimensional copper substrate. Crucially, the latter arose from hybridization between localized Yb 4f electrons in the atomic layer and mobile conduction electrons in the underlying copper, providing direct evidence of an interfacial heavy-fermion state.

“This achievement was made possible by our continued efforts to create high-quality materials and measure their electronic states as precisely as possible,” says senior author Professor Shin-ichi Kimura. “Our next goal is to engineer and control such heavy-electron states, opening the way to previously unexplored quantum states, including unconventional superconductivity.”

The findings suggest that carefully combining atomic layers and substrates could provide a new approach to designing quantum materials. By precisely controlling interfacial structures, electronic orbitals, and moiré patterns, researchers may be able to create and tune new low-dimensional quantum phenomena that cannot be realized in conventional materials.
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The article, “Interfacial heavy fermion formation in a two-dimensional Kondo lattice YbCu₂ on Cu(111) substrate,” will be published in Communications Materials at DOI: https://doi.org/10.1038/s43246-026-01332-5

About The University of Osaka
The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world. Now, The University of Osaka is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.
Website: https://resou.osaka-u.ac.jp/en