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.
@article{arxiv.cond-mat/0205178,
title = {Gap Function with Point Nodes in Borocarbide Superconductor YNi_2B_2C},
author = {K. Izawa and K. Kamata and Y. Nakajima and Y. Matsuda and T. Watanabe and M. Nohara and H. Takagi and P. Thalmeier and K. Maki},
journal= {arXiv preprint arXiv:cond-mat/0205178},
year = {2009}
}