Antiferromagnetic Order and Superconductivity in Sr₄(Mg_0.5-xTi_0.5+x)₂O₆Fe₂As₂ with Electron Doping: ⁷⁵As-NMR Study

Abstract

We report an 75 As-NMR study on iron (Fe)-based superconductors with thick perovskitetype blocking layers Sr4(Mg0.5−xTi0.5+x)2O6Fe2As2 with x=0 and 0.2. We have found that antiferromagnetic (AFM) order takes place when x=0, and superconductivity (SC) emerges below Tc=36 K when x=0.2. These results reveal that the Fe-pnictides with thick perovskitetype blocks also undergo an evolution from the AFM order to the SC by doping electron carriers into FeAs planes through the chemical substitution of Ti +4 ions for Mg +2 ions, analogous to the F-substitution in LaFeAsO compound. The reason why the Tc=36 K when x=0.2 being higher than the optimally electron-doped LaFeAsO with Tc=27 K relates to the fact that the local tetrahedron structure of FeAs4 is optimized for the onset of SC.

Enhancement of superconducting transition temperature due to antiferromagnetic spin fluctuations in iron pnictides LaFe(As1−xPx)(Mukuda1, Engetsu2, Yamamoto3 et al. 2014Phys. Rev. B2810330View full textAdd to dashboardCite

“…It is noteworthy that the T dependence of 1/T 1 T of (Y 0.95 La 0.05 )1111 is saturated below 100 K. A similar saturation and/or broad maximum in 1/T 1 T was observed for Ba 0.6 K 0. 34 , which are characterized by the lattice parameters of the FeAs block being close to the values of the regular tetrahedron. This is in contrast to the T dependence of 1/T 1 T in LaFe(As 1−x P x )(O 1−y F y ) compounds that continues to …”

NMR evidence for an intimate relationship between antiferromagnetic spin fluctuations and extendeds-wave superconductivity in monocrystallineSrFe2(As1−xMiyamoto1, Mukuda2, Kobayashi3 et al. 2015Phys. Rev. B13260View full textAdd to dashboardCite

“…4 47 and the hole doped 122 compound BaK122(T c =38K) 48 are always characterized by h P n = 1.35 ∼ 1.4Å and the bond angle of P n-Fe-P n α ∼ 109. 61 , and so on. As for the 122 series, although the low energy AFM-SFs are significantly observed in BaK122 34,[56][57][58] , the relationship between the T c and QCP in the phase diagram was not clearly revealed in Ba 1−x K x Fe 2 As 2 56 .…”

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“…Since 75 ν Q in proportion to an electric field gradient at As nuclear site is determined by some charge distribution around the 75 As nucleus in the FeAs 4 tetrahedron, the increase in h P n in association with the reduction in a-axis increases 75 ν Q , yielding a monotonous variation in the covalency of Fe-As bond [5]. As-NQR frequencies vs a-axis length for Y0.95La0.051111, along with those for Ln1111 [17,18], M 42622 [13,14], and A111 [15,16]. (c) SC diamagnetic susceptibility from which Tc=50 K was uniquely determined.…”

“…A coarse powder sample was used for the measurements of nuclear spin-lattice relaxation rate 75 (1/T 1 ) of 75 As-NMR at the field µ 0 H ∼ 12 T perpendicular to the c-axis. [13,14], and AFeAs(A=Li,Na) (A111) [15,16], as drawn by the dashed line in Fig. 1(b).…”