In iron-based superconductors, high critical temperature (T c ) superconductivity over 50 K has only been accomplished in electron-doped hREFeAsO (hRE is heavy rare earth (RE) element). Although hREFeAsO has the highest bulk T c (58 K), progress in understanding its physical properties has been relatively slow due to difficulties in achieving high-concentration electron doping and carrying out neutron experiments. Here, we present a systematic neutron powder diffraction study of 154 SmFeAsO 1−x D x , and the discovery of a long-range antiferromagnetic ordering with x ≥ 0.56 (AFM2) accompanying a structural transition from tetragonal to orthorhombic. Surprisingly, the Fe magnetic moment in AFM2 reaches a magnitude of 2.73 μ B /Fe, which is the largest in all nondoped iron pnictides and chalcogenides. Theoretical calculations suggest that the AFM2 phase originates in kinetic frustration of the Fe-3d xy orbital, in which the nearest-neighbor hopping parameter becomes zero. The unique phase diagram, i.e., highest-T c superconducting phase adjacent to the strongly correlated phase in electron-overdoped regime, yields important clues to the unconventional origins of superconductivity.high-T c superconductivity | neutron scattering | oxyhydrides | iron-based superconductors | antiferromagnetism C arrier doping is a critical parameter that governs the electronic correlations and ground states in high-T c superconductors. Electronic phase diagrams depicting the evolution of the ground state with doping level not only deepen our understanding of the mechanism of superconductivity but help us extract common trends across different superconducting materials. Until recently, two electronic phases were believed to play a vital role in iron-based superconductivity, i.e., a stripe or double-stripe-type antiferromagnetic (AFM) ordering at a nondoped Fe-3d 6 state and a Mott insulator at a hole-doped Fe-3d 5 state (1-3). The former phase is observed at ambient pressure for the arsenides and telluride, while under high pressures for the selenides, and the AFM fluctuations are a promising candidate for the pairing glue leading to superconductivity (4, 5). On the other hand, the latter picture, confirmed recently in effectively hole-doped Na(Fe 3+ 0.5 Cu + 0.5 )As (6), explains the asymmetric response of the T c and electron correlation to hole and electron doping, where the hole doping (electron doping) into the Fe-3d 6 state tends to enhance (reduce) the T c and the degree of electron correlation, because the system approaches (goes away from) the half-filled Fe-3d 5 state (2).However, these two prevailing scenarios were challenged recently by observations of the behavior of heavily electron-doped FeSe (11-type) and REFeAsO (1111-type), where the T c and electron correlation strength are enhanced rather than weakened with electron doping. In the first of these systems, a quite high T c in the range of 35 K to 41 K was observed, for the electron-doped FeSe using an electric double-layer transistor, which climbs to 46 K to 48 K for ...

A carrier doping by a hydrogen substitution in LaFeAsO1−x H x is known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO1−x H x has a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitride LnFeAsO1−x H x (Ln = La, Sm) with the range of x = 0–0.5 by using x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction. For both compounds, we observed that the pre-edge peaks of x-ray absorption spectra near the Fe-K edge were reduced in intensity on doping. The character arises from the weaker As–Fe hybridization with the longer As–Fe distance in the higher doped region. We can reproduce the spectra near the Fe-K edge according to the Anderson impurity model with realistic valence structures using the local-density approximation (LDA) plus dynamical mean-field theory (DMFT). For Ln = Sm, the integrated-absolute difference (IAD) analysis from x-ray Fe-Kβ emission spectra increases significantly. This is attributed to the enhancement of magnetic moment of Fe 3d electrons stemming from the localized picture in the higher doped region. A theoretical simulation implementing the self-consistent vertex-correction method reveals that the single dome superconducting phase for Ln = Sm arises from a better nesting condition in comparison with Ln = La.

We examine electronic and crystal structures of iron-based superconductors LnFeAsO1−x H x (Ln = La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe-K absorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES for Ln = La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when the T c approaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected for Ln = Sm. We reveal that the superconductivity is closely related to the lower spin state for Ln = La unlike Sm case. We observed that As height from the Fe basal plane and As–Fe–As angle on the FeAs4 tetrahedron for Ln = La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa for Ln = Sm.