Hybrid gap structure in the heavy-fermion superconductor CeIrIn${}_5$

The thermal conductivity $\kappa$ of the heavy-fermion superconductor CeIrIn${}_5$ was measured as a function of temperature down to $T_c$/8, for current directions perpendicular ($J\parallel a$) and parallel ($J \parallel c$)tothetetragonalcaxis\mathrm{.}For$J \parallel a$, a sizable residual linear term ${\kappa }_0\mathrm{/}T$ is observed, as previously, which confirms the presence of line nodes in the superconducting gap. For $J\parallel c$, on the other hand, $\kappa \mathrm{/}T\to 0$ as $T\to 0$. The resulting precipitous decline in the anisotropy ratio ${\kappa }_c\mathrm{/}{\kappa }_a$ at low temperature rules out a gap structure with line nodes running along the c-axis, such as the d-wave state favoured for CeCoIn${}_5$, and instead points to a hybrid gap of $E_g$ symmetry. It therefore appears that two distinct superconducting states are realized in the Ce$M$In${}_5$ family.