Differential conductance spectra are taken from metallic point-contact junctions on Cd-doped CeCoIn5. For (100) junctions with nominal 10% doping, we observe signatures for subsequent antiferromagnetic and superconducting transitions in qualitative agreement with bulk measurements. In the superconducting state, two conductance channels compete, both with enhanced subgap conductance, one due to superconductivity and the other antiferromagnetism. The conductance data exhibit variances for different doping levels and crystallographic orientations and junction to junction. This issue will be addressed further in terms of surface inhomogeneity or intrinsic nonuniform phase formation. © 2009 IOP Publishing Ltd.

Andreev reflection conductance spectra are obtained on nanoscale ballistic junctions between Au tips and single crystals of the pure and Cd-doped heavy-fermion superconductor CeCoIn5. Background conductance asymmetry starting at the heavy-fermion coherence temperature T* (∼45 K) and increasing with decreasing temperature down to Tc (2.3 K) signifies the emerging heavy-fermion liquid in CeCoIn5. Below Tc, enhancement of the sub-gap conductance arises from Andreev reflection, but the Blonder-Tinkham-Klapwijk theory dictates that the Fermi velocity mismatch should yield no Andreev reflection. The signal we do observe is several times weaker than that observed in conventional superconductors, but consistent with other heavy-fermion superconductor data reported. Data taken in the (0 0 1), (1 1 0), and (1 0 0) orientations provide consistent and reliable spectroscopic evidence for dx2 - y2 symmetry of the superconducting order parameter. Conductance spectra on the (1 0 0) surface of 10% Cd-doped CeCoIn5 show intriguing behaviors following antiferromagnetic and subsequent superconducting transitions.

Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn5 using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance. To explore their physical origins, we have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn5, CeRhIn5, and YbAl3, each with different electron mass. In contrast with Au/CeCoIn5 junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed.