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The heavy-fermion compound CeCoIn₅ shows unconventional superconductivity (T_c=2.3 K) with d_x²_-y²–wave pairing symmetry[1]. Despite evidence for pseudogap in the normal state[2], their spectroscopic nature remains to be unraveled. Here, we present results from planar tunneling spectroscopy measurements on CeCoIn₅ along [001], [100], and [110] directions. While the nodal junction exhibits only a zero-bias conductance peak, the non-nodal junctions show sharp double peaks corresponding to the superconducting gap. Interestingly, they evolve continuously crossing the T_c, merging into a single broad peak at T_p=5K. From quantitative analyses of the conductance spectra, we have found that the gap persistent in the normal state originates from the formation of heavy electron pairs that condense into a coherent state below T_c. We will discuss the implications of our findings in the context of other measurement results in the literature along with the underlying pairing mechanism.

[1]Park,et al,PRL100,177001(2008);Thompson,et al,JPSJ 81,011002(2012).
[2]Ernst,et al,Phys. Stat. Sol. B 247,624(2010);Fasano,et al,Physica B 536,798(2018).