Heat-Capacity Measurements of Energy-Gap Nodes of the Heavy-Fermion Superconductor CeIrIn5 Deep inside the Pressure-Dependent Dome Structure of Its Superconducting Phase Diagram

We use heat-capacity measurements as a function of field rotation to identify the nodal gap structure of CeIrIn5 at pressures to 2.05 GPa, deep inside its superconducting dome. A fourfold oscillation in the heat capacity at 0.3 K is observed for all pressures, but with its sign reversed between 1.50 and 0.90 GPa. On the basis of recent theoretical models for the field-angle-dependent specific heat, all data, including the sign reversal, imply a d(x2-y2) order parameter with nodes along [110], which constrains theoretical models of the pairing mechanism in CeIrIn5.