  The symmetry of the order parameter is the defining property of a
superconductor and a strong clue to its pairing mechanism. While d-wave
symmetry is firmly established in high-Tc superconductors, in no heavy-fermion
superconductor is the symmetry known definitively. One way to elucidate the
order parameter is to locate the symmetry-imposed nodes in the energy gap by
measuring the heat carried by the zero-energy quasiparticles associated with
those nodes as a function of direction. As verified in high-Tc superconductors
a line of nodes yields universal heat transport independent of impurity
scattering. Here we show that heat conduction in the heavy-fermion
superconductor CeIrIn5 is also universal, but only for a current perpendicular
to the tetragonal axis of its crystal structure, not parallel to it. This
uniaxial anisotropy is strong evidence for a line of nodes in the basal plane,
which is inconsistent with the d-wave symmetry proposed for the isostructural
superconductor CeCoIn5. Different symmetries in the two materials would explain
why the phase diagram of these heavy-fermion compounds consists of two separate
superconducting domes.
