Angle-resolved photoemission observation of the superconducting-gap minimum and its relation to the nesting vector in the phonon-mediated superconductor YNi₂ B₂ C

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy to directly study the large superconducting (SC) gap anisotropy of YNi₂ B₂ C. We succeed in measuring momentum (k) dependence of SC gap for individual Fermi surface (FS) sheets, which demonstrates complexity of SC gap in a phonon-mediated superconductor. Within measured k regions on FS sheets, we find a pointlike minimum of SC gap, whose k positions can be connected by the known nesting vector. This shows close correlation between the nesting vector and node formation.