Ultrahigh-resolution photoemission spectroscopy of Ni borocarbides: Directobservation of the superconducting gap and a change in gap anisotropy by impurity

We have performed ultrahigh-resolution photoemission spectroscopy of Y(Ni1- xPtx)(2)B2C (x = 0.0 and 0.2) in order to study the changes in the density of states across the superconducting transition. Because of a drastic incre ase in energy resolution, we clearly observe the opening of superconducting gaps across T-c in both compounds. Furthermore, we find a small but signif icant difference in the superconducting-state spectral shape. This can be e xplained in terms of reduction in gap anisotropy by introducing impurities and provides spectroscopic evidence for an anisotropic s-wave Sap in YNi2B2 C.