Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-Based New Superconductor (Ca(4)Al(2)O(6-y))(Fe(2)As(2)): (75)As Nuclear Quadrupole Resonance Study

[摘要]：We report (75)As nuclear quadrupole resonance studies on (Ca(4)Al(2)O(6-y))(Fe(2)As(2)) 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 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.

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