Ultrahigh-Resolution Photoemission Spectroscopy of Ni Borocarbides: Direct Observation of the Superconducting Gap and a Change in Gap Anisotropy by Impurity

We have performed ultrahigh-resolution photoemission spectroscopy of Y(Ni_1-xPtₓ)₂B₂C ( x0ex0ex=0ex0ex0.0 and 0.2) in order to study the changes in the density of states across the superconducting transition. Because of a drastic increase in energy resolution, we clearly observe the opening of superconducting gaps across Tc in both compounds. Furthermore, we find a small but significant difference in the superconducting-state spectral shape. This can be explained in terms of reduction in gap anisotropy by introducing impurities and provides spectroscopic evidence for an anisotropic s-wave gap in YNi₂B₂C.