We report an in situ scanning tunneling microscopy study of atomically thin films of ${\mathrm{CeCoIn}}_5$, a $d$-wave heavy-fermion superconductor. Both hybridization and superconducting gaps are observed even in monolayer ${\mathrm{CeCoIn}}_5$, 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 ${\mathrm{CeCoIn}}_5/{\mathrm{YbCoIn}}_5$ superlattices containing ${\mathrm{CeCoIn}}_5$ layers with the same thickness as the thin films. Remarkably, the out-of-plane upper critical field ${\mu }_0{H}_{c2\perp }$ 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.

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• Superconductivity
• Heavy-fermion systems
• Superconductors
• Thin films
• Molecular beam epitaxy
• Scanning tunneling microscopy
• Scanning tunneling spectroscopy