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 $\mathrm{Y}({\mathrm{Ni}}_{1-x}{\mathrm{Pt}}_x{)}_2{\mathrm{B}}_2\mathrm{C}$ ( $x=0.0\mathrm{}$ 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 $T_c$ 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 ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$.