Coexistence of antiferromagnetic order and unconventional superconductivity in heavy-fermion CeRh_1-xIr_xIn₅ compounds: Nuclear quadrupole resonance studies

We present a systematic ¹¹⁵In NQR study on the heavy-fermion compounds CeRh_1-xIr_xIn₅ (x=0.25, 0.35, 0.45, 0.5, 0.55, and 0.75). The results provide strong evidence for the microscopic coexistence of antiferromagnetic (AF) order and superconductivity (SC) in the range of 0.35≤x≤0.55. Specifically, for x=0.5, T_N is observed at 3 K with a subsequent onset of superconductivity at T_c=0.9 K. T_c reaches a maximum (0.94 K) at x=0.45 where T_N is found to be the highest (4.0 K). Detailed analysis of the measured spectra indicate that the same electrons participate in both SC and AF order. The nuclear spin-lattice relaxation rate 1/T₁ shows a broad peak at T_N and follows a T³ variation below T_c, the latter property indicating unconventional SC as in CeIrIn₅ (T_c=0.4 K). We further find that, in the coexistence region, the T³ dependence of 1/T₁ is replaced by a T-linear variation below T ∼0.4 K, with the value (T₁)T_c/(T₁)_{low T} increasing with decreasing x, likely due to low-lying magnetic excitations associated with the coexisting magnetism.