Gap Function with Point Nodes in Borocarbide Superconductor ${\mathrm{Y}\mathrm{N}\mathrm{i}}_2{\mathrm{B}}_2\mathrm{C}$

To determine the superconducting gap function of ${\mathrm{Y}\mathrm{N}\mathrm{i}}_2{\mathrm{B}}_2\mathrm{C}$, the $c$-axis thermal conductivity ${\kappa }_{zz}$ was measured in $\mathit{H}$ rotated in various directions. The angular variation of ${\kappa }_{zz}$ in $\mathit{H}$ rotated within the $ab$ plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when $\mathit{H}$ is rotated conically around the $c$ axis with a tilt angle of 45°. These results provide the first compelling evidence that the gap function has point nodes located along the $a$ and $b$ axes. This unprecedented gap structure challenges the current view on the pairing mechanism.

Figure 1

(a) $T$ dependence of ${\kappa }_{zz}$ inzero field. Inset: Log-log plot of the same data below 1 K.(b) Field dependence of ${\kappa }_{zz}$ atlow temperatures ($\mathit{H}\parallel \left[110\right]$). The solid circles represent the data measuredby sweeping $H$ after zero field cooling, and the open circles represent the datameasured under the field cooling conditions at each temperature.

Figure 2

Angular variation of ${\kappa }_{zz}$ at $H=1\mathrm{T}$ and $T=0.43\mathrm{K}$ ($\mathit{q}\parallel c$). ${\kappa }_{zz}$ aremeasured by rotating $\mathit{H}=H(\sin \theta \cos \varphi ,\sin \theta \sin \varphi ,\cos \theta )$ conically as a function of $\varphi$ at fixed $\theta =90°$, 60°, and45° (see the inset). The crosses represent the data obtained underthe field cooling condition at each angle.

Figure 3

Angular variation of $I_{zz}(\varphi ,\theta )$ normalized by $I_{zz}(45°,\theta )$ as a function of $\varphi$ at $\theta =90°$ (solid lines), $60°$ (dashed lines), and $45°$ (dash-dotted lines), which are calculated by thegap functions with (a) point node and (b) linenode. For the definition of $\theta$ and $\varphi$, see the inset of Fig. 2.The corresponding gap functions are illustrated in the insets. Fordetails, see the text.