Hybrid gap structure of the heavy-fermion superconductor CeIrIn5

The thermal conductivity kappa of the heavy-fermion superconductor CeIrIn5 was measured as a function of temperature down to T-c/8, for current directions parallel (J parallel to c) and perpendicular (J parallel to a) to the tetragonal c axis. For J parallel to 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 to c, on the other hand, kappa/T -> 0 as T -> 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 CeCoIn5, and instead points to a hybrid gap of E-g symmetry.