Point contacts are used to investigate the anisotropy of the superconducting energy gap in ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$ in the $ab$ plane and along the $c$ axis. It is shown that the experimental curves should be described assuming that the superconducting gap is nonuniformly distributed over the Fermi surface. The largest and the smallest gaps have been estimated by two-gap fitting models. It is found that the largest contribution to the point-contact conductivity in the $c$ direction is made by a smaller gap and, in the $ab$ plane by a larger gap. The deviation from the one-gap BCS model is pronounced in the temperature dependence of the gap in both directions. The temperature range, where the deviation occurs, is for the $c$ direction approximately 1.5 times more than in the $ab$ plane. The $\Gamma$ parameter, allowing quantitative estimates of the gap anisotropy by one-gap fitting, in the $c$ direction is also about 1.5 times greater than in the $ab$ plane. Since it is impossible to describe satisfactorily such gap distribution either by the one- or two-gap models, a continuous, dual-maxima model of gap distribution over the Fermi surface should be used to describe superconductivity in this material.

• Received 22 April 2004

DOI:https://doi.org/10.1103/PhysRevB.71.014512