Extremely strong-coupling superconductivity in artificial two-dimensional Kondo lattices

When interacting electrons are confined to low dimensions, the electron-electron correlation effect is enhanced dramatically, which often drives the system into exhibiting behaviours that are otherwise highly improbable. Superconductivity with the strongest electron correlations is achieved in heavy-fermion compounds, which contain a dense lattice of localized magnetic moments interacting with a sea of conduction electrons to form a three-dimensional Kondo lattice. It had remained an unanswered question whether superconductivity would persist on effectively reducing the dimensionality of these materials from three to two. Here we report on the observation of superconductivity in such an ultimately strongly correlated system of heavy electrons confined within a two-dimensional square lattice of Ce atoms (two-dimensional Kondo lattice), which was realized by fabricating epitaxial superlattices built of alternating layers of heavy-fermion CeCoIn 5 (ref.) and conventional metal YbCoIn 5. The field-temperature phase diagram of the superlattices exhibits highly unusual behaviours, including a striking enhancement of the upper critical field relative to the transition temperature. This implies that the force holding together the superconducting electron pairs takes on an extremely strong-coupled nature as a result of two- dimensionalization.