Emergent exotic superconductivity in artificially engineered tricolor Kondo superlattices

In the quest for exotic superconducting pairing states, the Rashba effect, which lifts the electron-spin degeneracy as a consequence of strong spin-orbit interaction (SOI) under broken inversion symmetry, has attracted considerable interest. Here, to introduce the Rashba effect into two-dimensional (2D) strongly correlated electron systems, we fabricate noncentrosymmetric (tricolor) superlattices composed of three kinds of f-electron compounds with atomic thickness; d-wave heavy fermion superconductor CeCoIn5 sandwiched by two different nonmagnetic metals, YbCoIn5 and YbRhIn5. We find that the Rashba SOI-induced global inversion symmetry breaking in these tricolor Kondo superlattices leads to profound changes in the superconducting properties of CeCoIn5, which are revealed by unusual temperature and angular dependencies of upper critical fields that are in marked contrast with the bulk CeCoIn5 single crystals. We demonstrate that the Rashba effect incorporated into 2D CeCoIn5 block layers is largely tunable by changing the layer thickness. Moreover, the temperature dependence of in-plane upper critical field exhibits an anomalous upturn at low temperatures, which is attributed to a possible emergence of a helical or stripe superconducting phase. Our results demonstrate that the tricolor Kondo superlattices provide a new playground for exploring exotic superconducting states in the strongly correlated 2D electron systems with the Rashba effect.