Enhancement of Superconducting Transition Temperature due to the Strong Antiferromagnetic Spin Fluctuations in the Noncentrosymmetric Heavy-Fermion Superconductor CeIrSi3 : A 29Si NMR Study under Pressure

We report a 29 Si NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CelrSi 3 without inversion symmetry. In the SC state at P= 2.7-2.8 GPa, the temperature (T) dependence of the nuclear-spin lattice relaxation rate 1/T 1 below T c exhibits a T 3 behavior without any coherence peak just below T c , revealing the presence of line nodes in the SC gap. In the normal state, 1/T 1 follows a √T-like behavior, suggesting that the SC emerges under the non-Fermi-liquid state dominated by AFM spin fluctuations enhanced around a quantum critical point. The reason why the maximum T c in CeIrSi 3 is relatively high among the Ce-based heavy-fermion superconductors may be the existence of the strong AFM spin fluctuations. We discuss the comparison with the other Ce-based heavy-fermion superconductors.