NMR and NQR studies of the heavy fermion superconductors CeTIn5 (T=Co and Ir)

We have carried out ¹¹⁵In and ⁵⁹Co nuclear quadrupole resonance and nuclear magnetic resonance measurements on CeCoIn₅ and CeIrIn₅. The temperature T dependence of the nuclear spin-lattice relaxation rate 1/T₁ of ¹¹⁵In in the normal state indicates that CeCoIn₅ is located just at an antiferromagnetic instability, and CeIrIn₅ is in the nearly antiferromagnetic region. In the superconducting state, 1/T₁ has no Hebel-Slichter coherence peak just below T_C and a power-law T dependence (close to T³) at very low temperatures, which indicates the existence of line nodes in the superconducting energy gap. The ¹¹⁵In (Ce-In plane) Knight shift in CeCoIn₅ decreases for both parallel and perpendicular directions to the tetragonal c axis in the superconducting state, which shows that the spin susceptibility decreases in all directions. These results indicate that CeCoIn₅ and CeIrIn₅ exhibit non-s-wave even parity (probably d-wave) superconductivity.