paper

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

arXiv:1210.5914 · doi:10.1103/PhysRevB.86.134527

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 $_{8 - y}$ ( $T_{c}^{o n se t} = 47$ K) and Ca $_{5}$ (Sc,Ti) $_{4}$ Fe $_{2}$ As $_{2}$ O $_{11 - y}$ ( $T_{c}^{o n se t} = 41$ 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 $I^^{3}$ pocket) is located just below the Fermi level from the analogy to LaFeAs(O,F) system and the disappearance of the $I^^{3}$ 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