Universal heat conduction and nodal gap structure of the heavy-fermion superconductor CeIrIn5

The effect of impurity scattering on the thermal conductivity kappa of the heavy-fermion superconductor CeIrIn5 was studied for a current parallel (J parallel to c) and perpendicular (J parallel to a) to the tetragonal c axis. For J parallel to a, adding La impurities does not change the residual linear term kappa(0a)/T, showing that heat conduction in the basal plane is universal, compelling evidence that the superconducting gap vanishes along a symmetry-imposed line. By contrast, for J parallel to c, La impurities greatly enhance the residual linear term kappa(0c)/T. This is strong evidence that the line of nodes lies within the basal plane, a gap structure which is inconsistent with the d-wave symmetry proposed for the isostructural superconductor CeCoIn5. Different symmetries in the two materials could explain why the phase diagram of this heavy-fermion family consists of two separate superconducting domes. We also compare our data on CeIrIn5 to corresponding data on the heavy-fermion superconductor UPt3, where no universal conduction is observed.