Exotic heavy fermion superconductivity in atomically thin CeCoIn${}_5$ films

Abstract

We report an {\it in-situ} scanning tunneling microscopy study of atomically thin films of CeCoIn${}_5$, a $d$-wave heavy-fermion superconductor. Both hybridization and superconducting gaps are observed even in monolayer CeCoIn${}_5$, 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 CeCoIn${}_5$/YbCoIn${}_5$ superlattices containing CeCoIn${}_5$ layers with the same thickness as the thin films. Remarkably, the out-of-plane upper critical field $\hat{I}{¼}_0{H}_{c2\perp }$ at zero temperature is largely enhanced from those of bulk and superlattices. The enhanced $H_{c2\perp }$ well exceeds the Pauli and bulk orbital limits, suggesting the possible emergence of unusual superconductivity with parity mixing caused by the inversion symmetry breaking.