Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-Based New Superconductor (Ca4Al2O6-y)(Fe2As2): 75As Nuclear Quadrupole Resonance Study

We report As-75 nuclear quadrupole resonance studies on (Ca4Al2O6-y)(Fe2As2) with T-c = 27 K. Measurement of nuclear-spin-relaxation rate 1/T-1 has revealed a significant development of two-dimensional antiferromagnetic spin fluctuations down to Tc in association with the smallest As-Fe-As bond angle. Below Tc, the temperature dependence of 1/T-1 without any trace of the coherence peak is well accounted for by a nodeless s(+/-)-wave multiple-gaps model. From the fact that its T-c is comparable to T-c = 28 K in the optimally doped LaFeAsO1-y in which antiferromagnetic spin fluctuations are not dominant, we remark that antiferromagnetic spin fluctuations are not a unique factor for enhancing T-c among Fe-based superconductors, but a condition for optimizing superconductivity should be addressed from the lattice structure point of view.