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

Superconductivity with the strongest electron correlations is realized in heavy-fermion system, where almost all of the compounds have three-dimensional nature. It had remained an unanswered question whether superconductivity would persist on reducing the dimensionality of these materials. We succeeded in observing superconductivity in the system of heavy electrons confined within a two dimensional square lattice of Ce atoms, which was realized by fabricating epitaxial superlattices built of alternating layers of heavy-fermion CeCoIn$_{5}$ and conventional metal YbCoIn$_{5}$[1]. The field-temperature phase diagram of the superlattices exhibits 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. [1]Mizukami \textit{et al}., Nature Phys. \textbf{7}, 849 (2011).