Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-Based New Superconductor (Ca_4Al_2O_6_y)(Fe_2As_2):~ 75As Nuclear Quadrupole Resonance Study

摘要

We report ~75As nuclear quadrupole resonance studies on (Ca_4Al_2O_6-y)(Fe_2As_2) with T_c = 27 K. Measurement of nuclear-spin-relaxation rate 1/T1 has revealed a significant development of two-dimensional antiferromagnetic spin fluctuations down to T_c in association with the smallest As-Fe-As bond angle. Below T_c, the temperature dependence of 1/T1 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 LaFeAsO_1_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.