Evidence of point nodes in superconducting gap of borocarbide superconductor YNi2B2C

In order to determine the superconducting gap structure of the borocarbide superconductor YNi2B2C, we have measured the angular variation of the c-axis thermal conductivity kappa(zz) in magnetic field rotated within the ab-planes. A clear fourfold symmetry with narrow cusps was observed in the angular variation of kappa(zz). These results provide a strong evidence that the gap function has point nodes which are located along the a- and b-axes of the crystal. (C) 2003 Elsevier Science B.V. All rights reserved.