$d_{x^{2}-y^{2}}$ paring symmetry of heavy fermion CeIrIn$_{5}$ remote from antiferromagnetic quantum critical point

Quasi-two dimensional heavy Fermion CeIrIn$_{5}$ involves two distinct superconducting domes in the phase diagram, which appear as a function of pressure or Rh substitution of Ir. In the analogy to CeCu$_{2}$Si$_{2}$, two distinct superconducting domes with different symmetry has been invoked. We report on the results of low-temperature thermal transport of CeIrIn$_{5}$ in the second dome, which locates away from an antiferromagnetic quantum critical point. The thermal conductivity is measured under a magnetic field rotated with respect to the crystal axes, which give direct evidence for superconducting gap structure. Clear fourfold oscillation with minima at [110] and [1-10] directions is observed as rotating magnetic field within the basal $ab$-plane, while no oscillation is observed within the $bc$-plane. In sharp contrast to previous reports that suggested $E_{g}$ symmetry with horizontal line node within the $ab$-plane [1], our results are most consistent with $d_{x^{2}- y^{2}}$ symmetry with vertical line nodes along the $c$-axis. These results imply that two superconducting domes have the same gap symmetry which appears to be mediated by antiferromagnetic spin fluctuations. \newline [1] H.~Shakeripour et al., Phys. Rev. Lett. 99, 187004 (2007).