Large-moment antiferromagnetic order in overdoped high- T _c superconductor ¹⁵⁴ SmFeAsO _{1− x} D _x

Abstract

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 ...

High pressure growth and electron transport properties of superconducting SmFeAsO1−xHx single crystals

“…It was our surprise to see a structural transition from the tetragonal to a unique orthorhombic structure in AFM2 phase( Fig. 1b) [13][14][15].…”

High Jc and low anisotropy of hydrogen doped NdFeAsO superconducting thin film

“…Very similar to the partial substitution of fluorine for oxygen in Ln FeAsO, hydrogen also leads to electron doping (O 2− → H − + e − ) 7 , resulting in a T c of up to ~ 55 K. The distinct difference between H- and F-doping is the substitution limit: x ≤ 0.8 for Ln FeAsO 1− x H x 8 in contrast to x ≤ 0.2 for Ln FeAsO 1− x F x 9 . Furthermore, a high T c of ~ 50 K is maintained in the range 0.13 < x < 0.43 for Ln FeAsO 1−x H x 7 .…”

“…The growth of Ln FeAs(O,H) opens new opportunities to explore how heavily electron doping influences the superconducting properties. However, most of the studies have been carried out on polycrystals 7 , 8 or tiny single crystals 10 , on which measurements of the transport critical current density are rather complicated. The successful growth of Ln FeAs(O,H) epitaxial thin films gives a great opportunity to explore the intrinsic physical properties by electrical transport measurements especially for critical current characteristics, since thin films are the ideal platform for such investigations.…”

Gapless magnetic excitation in a heavily electron-doped antiferromagnetic phase of LaFeAsO0.5D0.5

“…Recently, a doping technique for producing a higher electron concentration for 1111 systems has been developed in which H − is used instead of F − [17][18][19][20]. Remarkably, owing to the heavy electron doping, a second antiferromagnetic phase appears in La-and Sm-1111 systems just after disappearance of the superconducting phase [21,22]. Several subsequent studies have all suggested that the heavy electron doping enhances electron correlations and the localized character in the second antiferromagnetic phase [8,19,20,[23][24][25].…”

“…Several subsequent studies have all suggested that the heavy electron doping enhances electron correlations and the localized character in the second antiferromagnetic phase [8,19,20,[23][24][25]. These studies suggested that spin and/or orbital fluctuations from the strongly correlated phase significantly contribute to the high T c in 1111 compounds [15,16,21,22]. Therefore, it is indispensable to understand magnetic excitations in the heavily electron-doped antiferromagnetic phase.…”