Abstract

Andreev reflection spectroscopy results are obtained with nanoscale normal-metal Au tips on single crystals of the pure and Cd-doped heavy-fermion superconductor CeCoIn5. Contacts are shown to be in the Sharvin (ballistic) limit. Asymmetry observed in the background conductance starting at T* (the heavy-Fermion condensation temperature) increasing with decreasing temperature to Tc (the superconducting condensation temperature), signifies the emerging heavy-fermion liquid. Below Tc, the enhancement of the sub-gap conductance arises from Andreev reflection. According to standard theory, the Fermi velocity mismatch between these materials 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 (001), (110) and (100) orientations provide consistent and reliable spectroscopic evidence of a dx2-y2 superconducting order parameter. Measurements on Cd-doped CeCoIn5, at the 10% doping level, show characteristics consistent with antiferromagnetic and subsequent superconducting transitions with decreasing temperature. The roles of competing phases in this material will be discussed.

This work is in collaboration with Wan Kyu Park, J. L. Sarrao, J. N. Thompson, L. Pham, T. Park, S. Maquilon and Z. Fisk.