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We report 75As-nuclear quadrupole resonance (NQR) studies on(Ca_4Al_2O_{6-y})(Fe_2As_2) with Tc=27K, which unravel unique normal-stateproperties and point to unconventional nodeless superconductivity (SC).Measurement of nuclear-spin-relaxation rate 1/T_1 has revealed a significantdevelopment of two dimensional (2D) antiferromagnetic (AFM) spin fluctuationsdown to Tc, in association with the fact that FeAs layers with the smallestAs-Fe-As bond angle are well separated by thick perovskite-type blocking layer.Below Tc, the temperature dependence of 1/T_1 without any trace of thecoherence peak is well accounted for by an s(+-)-wave multiple gaps model. Fromthe fact that Tc=27K in this compound is comparable to Tc=28K in theoptimally-doped LaFeAsO_{1-y} in which AFM spin fluctuations are not dominant,we remark that AFM spin fluctuations are not a unique factor for enhancing Tcamong existing Fe-based superconductors, but a condition for optimizing SCshould be addressed from the lattice structure point of view.

physical review letters

Antiferromagnetic Spin Fluctuations and Unconventional Nodeless  Superconductivity in an Iron-based New Superconductor  (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study