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

• Received 26 September 2006

DOI:https://doi.org/10.1103/PhysRevLett.99.187004