Andreev reflection at the normal-metal/heavy-fermion superconductor CeCoIn₅ interface

The dynamic conductance across nanoscale junctions consisting of a normal-metal (N) and a heavy-fermion superconductor (HFS) is measured over a wide temperature range (60 K to 400 mK). The N/HFS (Au∕CeCoIn₅) contact is shown to be in the Sharvin limit. The background conductance develops a gradual asymmetry with decreasing temperature starting at the heavy-fermion liquid coherence temperature, T^*∼45 K, to the onset of superconducting coherence, T_c=2.3 K. The enhanced subgap conductance observed below T_c arises from Andreev reflection. This enhancement is an order of magnitude smaller (∼13.3% at 400 mK) than that observed for N/conventional superconductors but consistent with other N/HFS data reported. Attempts to fit to the full conductance curve as a function of temperature with extended Blonder-Tinkham-Klapwijk models, including those that account for the breakdown of the Andreev approximation and renormalizations of the Fermi momenta, clearly show that existing models cannot account for our data. We provide a theoretical framework for understanding the N/HFS Andreev conversion process.