We report on systematic evolutions of antiferromagnetic (AFM) spin fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF) compounds ${\mathrm{CeRh}}_{1-x}{\mathrm{Ir}}_x{\mathrm{In}}_5$ via an ${}^{115}\mathrm{In}$ nuclear-quadrupole-resonance experiment. The nuclear spin-lattice relaxation rate $1/T_1$ has revealed the marked development of AFM spin fluctuations as approaching an AFM ordered state. Concomitantly, the superconducting transition temperature $T_c$ and the energy gap ${\Delta }_0$ increase drastically from $T_c=0.4\mathrm{K}$ and $2{\Delta }_0/k_B{T}_c=5$ in ${\mathrm{CeIrIn}}_5$ up to $T_c=1.2\mathrm{K}$ and $2{\Delta }_0/k_B{T}_c=8.3$ in ${\mathrm{CeRh}}_{0.3}{\mathrm{Ir}}_{0.7}{\mathrm{In}}_5$, respectively. The present work suggests that the AFM spin fluctuations in close proximity to the AFM quantum critical point are indeed responsible for the strong-coupling unconventional SC in HF compounds.

• Received 31 October 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.147001