Planar tunneling spectroscopy (PTS) measurements on the 115 heavy fermion superconductor CeCoIn₅ (T_c = 2.3 K) suggest the direct involvement of local moments in the Cooper pairing process. [1] Our high-quality, reproducible PTS data on junctions along the three major crystallographic orientations with AlO_x barriers consistent with the STM measurements [2,3] identifying preformed pairs persisting to T_p ≈ 5 K. Further support of the existence of pre-formed pairs is reported using Shot noise spectroscopy. [4] Another interpretation is that the excess conductance arises from a hybridization-controlled pseudogap. [5]. Our PTS results at low temperature, either below T_c or below T_p, as a function of applied magnetic field reveal a field induced splitting or gap (FIG), that grows linearly up to the highest field measured, 18 T. That this splitting only occurs below T_p supports the pre-formed pair interpretation, noting that there is hybridization naturally occurring at these temperatures. To understand any barrier effect and the reproducibility of these features, we extend our PTS investigations with MgO as a tunnel barrier.

[1] Shrestha et al., Phys. Rev. B 103, 224515 (2021).

[2] Ernst et al., Phys. Stat. Sol. B (2010).

[3] Y. Fasano et al., Physica B (2018)​.

[4] Koen M. Mastiaans et al., Science 374 608 (2021)

[5] Harim Jang et al., Phys Rev B 130, 076301 (2023)

*This work was supported by the NSF/DMR-2003405, NSF/DMR- 2128556, and the State of Florida.