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

Abstract

${}^{75}$As and ${}^{45}$Sc NMR measurements unravel the electronic state for Fe-based superconductors with perovskite-type blocking layers Ca${}_4$(Mg,Ti)${}_3$Fe${}_2$As${}_2$O${}_{8-y}$ ($T_c^{onset}=47$ K) and Ca${}_5$(Sc,Ti)${}_4$Fe${}_2$As${}_2$O${}_{11-y}$ ($T_c^{onset}=41$ K). In Ca${}_5$(Sc,Ti)${}_4$Fe${}_2$As${}_2$O${}_{11-y}$, the nuclear spin relaxation rate $1/T_1$ shows pseudogap behavior below $\sim 80$ K, suggesting that the electronic state is similar to that of LaFeAs(O,F) system with moderate electron doping. The presence of the pseudogap behavior gives an interpretation that the hole-like band (so-called $\gamma$ pocket) is located just below the Fermi 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. On the other hand, in Ca${}_4$(Mg,Ti)${}_3$Fe${}_2$As${}_2$O${}_{8-y}$ satisfying the structural optimal condition for higher $T_c$ among the perovskite systems, the extrinsic contribution, which presumably originates in the Ti moment, is observed in $1/T_{1}T$; however, the moderate temperature dependence of $1/T_{1}T$ appears by its suppression under high magnetic field. In both systems, the high $T_c$ of $\sim 40$ K is realized in the absence of the strong development of the low-energy spin fluctuations. The present results reveal that the structural optimization does not induce the strong development of the low-energy spin fluctuations. If we consider that superconductivity is mediated by spin fluctuations, the structural optimization is conjectured to provide a benefit to the development of the high-energy spin fluctuations irrespective to the low-energy part.

7 pages, 7 figures, accepted for publication in Phys. Rev. B

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