A new layered iron arsenide oxide (Fe 2 As 2 )(Ca 4 (Mg,Ti) 3 O y ) was discovered. Its crystal structure is tetragonal with a space group of I4=mmm consisted of the anti-fluorite type FeAs layer and blocking layer of triple perovskite cells and is identical with (Fe 2 As 2 )(Sr 4 (Sc,Ti) 3 O 8 ) discovered in our previous study. The lattice constants of (Fe 2 As 2 )(Ca 4 (Mg,Ti) 3 O y ) are a ¼ 3:877A and c ¼ 33:37 A. This compound exhibited bulk superconductivity up to 43 K in susceptibility measurement without intentional carrier doping. A resistivity drop was observed at 47 K and zero resistance was achieved at 42 K. These values correspond to the second highest T c among the layered iron-based superconductors after REFeAsO system. #

We have discovered a new iron pnictide oxide superconductor (Fe 2 As 2 )(Sr 4 (Mg, Ti) 2 O 6 ). This material is isostructural with (Fe 2 As 2 )(Sr 4 M 2 O 6 ) (M = Sc, Cr, V), which were found in previous studies. The structure of this compound is tetragonal with a space group of P4/nmm and consists of the anti-fluorite type FeAs layer and perovskite-type block layer. The lattice constants are a = 3.935 Å and c = 15.952 Å for (Fe 2 As 2 )(Sr 4 MgTiO 6 ). Bulk superconductivity with a T c(onset) of ∼26 K was observed for a partially Co-substituted sample. Moreover, Co-free and Ti-rich samples exhibited higher T c(onset) s above 35 K, which were further enhanced by applying high pressures up to ∼43 K.

We synthesized new layered iron arsenide oxides (Fe2As2)(Sr4(Sc,Ti)3O8), (Fe2As2)(Ba4Sc3O7.5), and (Fe2As2)(Ba3Sc2O5). The crystal structures of these compounds are tetragonal with a space group of I4/m m m. The structure of (Fe2As2)(Sr4(Sc,Ti)3O8) and (Fe2As2)(Ba4Sc3O7.5) consists of the alternate stacking of antifluorite Fe2As2 layers and triple perovskite-type oxide layers. The interlayer distance between the Fe planes of (Fe2As2)(Ba4Sc3O7.5) is ∼18.7 Å. Moreover, the a-axis of (Fe2As2)(Ba3Sc2O5) is the longest among the layered iron pnictides, indicating the structural flexibility of the layered iron pnictide containing perovskite-type layers. The bulk sample of (Fe2As2)(Sr4(Sc0.6Ti0.4)3O8) exhibited diamagnetism up to 28 K in susceptibility measurements.

We studied the quantum critical behavior of the second antiferromagnetic (AF) phase in the heavily electron-doped high-Tc pnictide, LaFeAsO1−xHx by using 75 As and 1 H nuclear-magneticresonance (NMR) technique. In the second AF phase, we observed a spatially modulated spindensity-wave-like state up to x=0.6 from the NMR spectral lineshape and detected a low-energy excitation gap from the nuclear relaxation time T1 of 75 As. The excitation gap closes at the AF quantum critical point (QCP) at x ≈ 0.49. The superconducting (SC) phase in a lower-doping regime contacts the second AF phase only at the AF QCP, and both phases are segregated from each other. The absence of AF critical fluctuations and the enhancement of the in-plane electric anisotropy are key factors for the development of superconductivity.