Superconducting gap structure of CeIrIn ₅ from field-angle-resolved measurements of its specific heat

In order to identify the gap structure of CeIrIn ₅, we measured field-angle-resolved specific heat C(φ) by conically rotating the magnetic field H around the c axis at low temperatures down to 80 mK. We revealed that C(φ) exhibits a fourfold angular oscillation, whose amplitude decreases monotonically by tilting H out of the ab plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the d _x2-y2-wave gap. These results strongly indicate that CeIrIn ₅ is a d _x2-y2-wave superconductor and suggest the universal pairing mechanism in CeMIn ₅ (M=Co, Rh, and Ir).