Exotic heavy-fermion superconductivity in atomically thin CeCoIn₅ films

We report an in situ scanning tunneling microscopy study of atomically thin films of CeCoIn5, a d-wave heavy-fermion superconductor. Both hybridization and superconducting gaps are observed even in monolayer CeCoIn5, providing direct evidence of superconductivity of heavy quasiparticles mediated by purely two-dimensional bosonic excitations. In these atomically thin films, T-c is suppressed to nearly half of the bulk, but is similar to CeCoIn5/YbCoIn5 superlattices containing CeCoIn5 layers with the same thickness as the thin films. Remarkably, the out-of-plane upper critical field mu H-0(c2&updatedExpOTTOM;) at zero temperature is largely enhanced from those of bulk and superlattices. The enhanced H-c2&updatedExpOTTOM; well exceeds the Pauli and bulk orbital limits, suggesting the possible emergence of unusual superconductivity with parity mixing caused by the inversion symmetry breaking.