A research team led by the University of Osaka has directly observed, for the first time, an unusual heavy-fermion state forming at the boundary between a one-atom-thick material and a metal. 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.