Enhancing the Superconducting Transition Temperature due to Strong-Coupling Effect under Antiferromagnetic Spin Fluctuations in CeRh1-xIrxIn5 : 115In-NQR Study

We report on systematic evolutions of antiferromagnetic (AFM) spin fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF) compounds CeRh${}_{1-x}$Ir${}_x$In${}_5$ via ${}^{115}$In nuclear-quadrupole-resonance (NQR) experiment. The measurements of nuclear spin-lattice relaxation rate $1\mathrm{/}{T}_1$ have revealed the marked development of AFM spin fluctuations as a consequence of approaching an AFM ordered state with increasing Rh content. Concomitantly the superconducting transition temperature $T_{\mathrm{c}}$ and the energy gap ${\mathrm{\Delta }}_0$ increase drastically from $T_{\rm c} = 0.4$Kand$2\Delta_0/k_{\rm B}T_{\rm c} = 5$inCeIrIn$5$upto$T{\rm c} = 1.2$Kand$2\Delta_0/k_{\rm B}T_{\rm c} = 8.3$ in CeRh${}_{0.3}$Ir${}_{0.7}$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 onset of strong-coupling unconventional SC with the line node in the gap function in HF compounds.