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$^{75}$As and $^{45}$Sc NMR measurements unravel the electronic state forFe-based superconductors with perovskite-type blocking layersCa$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ ($T_c^{onset}=47$ K) andCa$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$ ($T_c^{onset}=41$ K). InCa$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$, the nuclear spin relaxation rate$1/T_1$ shows pseudogap behavior below $\sim80$ K, suggesting that theelectronic state is similar to that of LaFeAs(O,F) system with moderateelectron doping. The presence of the pseudogap behavior gives an interpretationthat the hole-like band (so-called $\gamma$ pocket) is located just below theFermi level from the analogy to LaFeAs(O,F) system and the disappearance of the$\gamma$ pocket yields the suppression of the low-energy spin fluctuations. Onthe other hand, in Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ satisfying thestructural optimal condition for higher $T_c$ among the perovskite systems, theextrinsic contribution, which presumably originates in the Ti moment, isobserved in $1/T_1T$; however, the moderate temperature dependence of $1/T_1T$appears by its suppression under high magnetic field. In both systems, the high$T_c$ of $\sim40$ K is realized in the absence of the strong development of thelow-energy spin fluctuations. The present results reveal that the structuraloptimization does not induce the strong development of the low-energy spinfluctuations. If we consider that superconductivity is mediated by spinfluctuations, the structural optimization is conjectured to provide a benefitto the development of the high-energy spin fluctuations irrespective to thelow-energy part.

NMR Investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As2O8-y and Ca5(Sc,Ti)4Fe2As2O11-y