Gap Function with Point Nodes in Borocarbide Superconductor [Formula presented]

To determine the superconducting gap function of [Formula presented], the [Formula presented]-axis thermal conductivity [Formula presented] was measured in [Formula presented] rotated in various directions. The angular variation of [Formula presented] in [Formula presented] rotated within the [Formula presented] plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when [Formula presented] is rotated conically around the [Formula presented] axis with a tilt angle of 45°. These results provide the first compelling evidence that the gap function has point nodes located along the [Formula presented] and [Formula presented] axes. This unprecedented gap structure challenges the current view on the pairing mechanism.