Point-contact Andreev reflection tunneling spectroscopic (PCARTS) study of the superconducting gap structure in LuNi2B2C

We employ the point-contact Andreev reflection tunneling spectroscopy (PCARTS) technique in two sets of LuNi2B2C (Tc ~16.0 K) single crystals along three different crystallographic orientations as a function of temperature. For one set of samples, our data taken on the [001], [110] and [100] faces show gap values at ~2 K of ~2.4 meV, 2.6 meV, and 2.3 meV, respectively, as analyzed by the one-gap Blonder-Tinkham-Klapwijk model. Another set of samples gives ~2.5 meV, 2.8 meV, and 2.7 mev, respectively. Simulated gap values as a function of temperature follow the BCS-like temperature dependence in the weak-coupling limit. The small gap anisotropy observed in both sets of samples is explained by a model assuming an anisotropic s-wave with point-nodes for the gap structure and a large tunneling cone at the interface for the point-contact junctions.