Superconductivity Tuned by the Iron Vacancy Order in K _x Fe _{2− y} Se ₂

Abstract

Combining in-depth neutron diffraction and systematic bulk studies, we discover that the √ 5 × √ 5 Fe vacancy order, with its associated block antiferromagnetic order, is the ground state with varying occupancy ratios of the iron 16𝑖 and vacancy 4𝑑 sites across the phase-diagram of K𝑥Fe2−𝑦Se2. The orthorhombic order, with one of the four Fe sites vacant, appears only at intermediate temperatures as a competing phase. The material experiences an insulator to metal crossover when the √ 5 × √ 5 order is highly developed. Superconductivity occurs in such a metallic phase.

High-Pressure Single-Crystal Neutron Scattering Study of Magnetic and Fe Vacancy Orders in (Tl,Rb) 2 Fe 4 Se 5 Superconductor

“…Detected in the 245 superconductors is the alkaline metal deficient A x Fe 2 Se 2 (x ∼ 0.3-0.6) phase embedded in √ 5 × √ 5 iron vacancy ordered superstructure [12][13][14], forming various microstructure patterns in plane [15,16] or heterostructure along the c-axis [9,13,17] depending on sample preparation procedures. The average sample compositions of these superconductors are consistent with the phase diagram in [8]. The question is what role the A x Fe 2 Se 2 (x ∼ 0.3-0.6), the √ 5 × √ 5 superstructure and the AFM order play in the 245 superconductors.…”

“…The KFe 1.5 Se 2 (234) of the orthorhombic Fe vacancy order has been proposed as the parent compound [10]. However, this phase is not even the ground state for KFe 1.5 Se 2 , and a partially ordered √ 5 × √ 5 vacancy superlattice is more stable at low temperature [8]. The KFe 2 Se 2 (122) of I4/mmm symmetry has also been proposed as the superconducting phase [11].…”

“…High pressure adds an additional dimension to the complex composition phase-diagram of 245 superconductors [8], offering a "clean" way to investigate the relation among various phases [18,19]. The T c has been suppressed to zero at critical pressure P c ≈ 6 GPa for A= Rb [19,20], 8 GPa for A= Cs [21], and 9 GPa for A= K and Tl-Rb superconductors [22].…”

“…The metal-like resistivity behavior is a precursor to superconductivity and corresponds to a highly ordered √ 5 × √ 5 Fe vacancy superlattice [4,8]. The nonsuperconducting samples, on the other hand, contain imperfect Fe vacancy order which introduces substantial site disorder.…”

Superconductivity in LiFeO2Fe2Se2with anti-PbO-type spacer layers

“…However, the superconducting phase can be observed only in single crystals and is always inter-grown with the insulating phase, and there is no evidence of this phase in polycrystalline samples. It seems that the insulating phase A 2 Fe 4 Se 5 closely correlates with the superconductivity in A x Fe 2-y Se 2 , and even is believed to be the parent compound 17 . In order to acquire a better understanding of the underlying physics in iron-based high-T c superconductors, it is of significance to find novel spacer layer to expand the category of the iron-based superconductors.…”