Gap Function with Point Nodes in Borocarbide Superconductor YNi_2B_2C
  To determine the superconducting gap function of a borocarbide superconductor YNi_2B_2C, the c-axis thermal conductivity kappa_zz was measured in a magnetic field rotated in various directions relative to the crystal axes. The angular variation of kappa_zz in H rotated within the ab-plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when H is rotated conically around the c-axis with a tilt angle of 45 degrees. Based on these results, we provide the first compelling evidence that the gap function of YNi_2B_2C has POINT NODES, which are located along the [100] and [010]-directions. This unprecedented gap structure challenges the current view on the pairing mechanism and on the unusual superconducting properties of borocarbide superconductors. 
K. Izawa, K. Kamata, Y. Nakajima, Y. Matsuda, T. Watanabe, M. Nohara,   H. Takagi, P. Thalmeier, and K. Maki