Direct observation of superconducting gap anisotropy in YNi$_{2}$B$_{2}$C : Angle-resolved photoemission spectroscopy

In a borocarbide superconductor YNi$_{2}$B$_{2}$C(T$_{c}$=15K), various experimental results have shown the existence of a gap anisotropy with node. More recently even the direction and type of node have been reported. However, the type of the nodal structure (point or line) seems to be controversial. Also, the position of the node on Fermi surfaces, which is essential for determining the origin of the large anisotropy, has not been clarified, yet. Therefore, we have performed low-temperature ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) to clarify the origin of the large anisotropy. We successfully observed experimental valence band dispersions, Fermi surfaces, and momentum-dependent superconducting gap of YNi$_{2}$B$_{2}$C (001). Superconducting gaps have shown large anisotropy. From these results, we discuss possible orgins of the large anisotropy in YNi$_{2}$B$_{2}$C.