Point-contact spectroscopy of CeCoIn 5: Andreev reflection studies of the normal-metal–heavy-fermion superconductor interface

Abstract

Point-contact spectroscopy (PCS) results are obtained with normal-metal (N) Au tips on single crystals of the heavy-fermion superconductor (HFS) CeCoIn 5. Our exceptionally clean, reproducible data are taken over a temperature range of 60 K to 400 mK. Contacts are shown to be in the Sharvin limit. Asymmetry observed in the background conductance starting at T* (∼45 K), increasing with decreasing temperature to T c (2.3 K), signifies the emerging heavy-fermion liquid. Below T c, the enhancement of the subgap conductance (∼13.3% at 400 mK) arises from Andreev reflection (AR). According to standard Blonder–Tinkham–Klapwijk (BTK) theory, the Fermi velocity mismatch between an N and a HFS should yield no AR. The AR we do observe is several times lower than that observed in N/conventional superconductor PCS, but consistent with other N–HFS data reported. Our zero-bias conductance data are best fit with an extended d-wave BTK model. A fit to the full conductance curve at 400 mK indicates strong coupling (2Δ/ k B T c=4.6), but the suppression of both the AR signal and the energy gap cannot be accounted for with existing models.