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 μ₀H_c2⊥ at zero temperature is largely enhanced from those of bulk and superlattices. The enhanced H_c2⊥ well exceeds the Pauli and bulk orbital limits, suggesting the possible emergence of unusual superconductivity with parity mixing caused by the inversion symmetry breaking.