Scientists have directly observed an unusual quantum state forming where a material just one atom thick meets a metal. The discovery, made by a research team led by the University of Osaka, demonstrates how interactions between electrons in different materials can produce a state known as a heavy fermion, potentially offering new ways to engineer quantum materials.
Heavy-fermion states occur when strong interactions cause electrons to behave as if they have unusually large effective masses. These states are closely associated with exotic quantum phenomena, including unconventional superconductivity, in which electrical current can flow without resistance through mechanisms that differ from those in conventional superconductors.
To investigate how these states emerge, the researchers created a high-quality, one-atom-thick layer of ytterbium and copper (YbCu₂) on the surface of a copper crystal. They then used intense synchrotron light to examine the electronic structure of the combined materials and identify interactions occurring at their boundary.
The measurements revealed two separate heavy-fermion states. One was concentrated primarily within the two-dimensional YbCu₂ layer, while the other extended into the three-dimensional copper substrate. The second state formed through hybridization, a process in which localized ytterbium 4f electrons interact with mobile conduction electrons in the underlying metal. This provided the first direct experimental evidence of a heavy-fermion state arising from electronic interactions across such an interface.
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