Strong anisotropy in nearly ideal tetrahedral superconducting FeS single crystals

Abstract

We report the preparation of single crystals of tetragonal iron sulfide (FeS) which exhibits a nearly ideal tetrahedral geometry with S–Fe–S bond angles of 110.2(2)∘ and 108.1(2)∘. Grown via hydrothermal de-intercalation of KxFe2−yS2 crystals under basic and reducing conditions, the silver, platelike crystals of FeS remain stable up to 200∘C under air and 250∘C under inert conditions, even though the mineral “mackinawite” (FeS) is known to be metastable. FeS single crystals exhibit a superconducting state below Tc=4 K as determined by electrical resistivity, magnetic susceptibility, and heat capacity measurements, confirming the presence of a bulk superconducting state. Normal state measurements yield an electronic specific heat of 5 mJ/mol K2, and paramagnetic, metallic behavior with a low residual resistivity of 250 μΩcm. Magnetoresistance measurements performed as a function of magnetic field angle tilted toward both transverse and longitudinal orientations with respect to the applied current reveal remarkable two-dimensional behavior. This is paralleled in the superconducting state, which exhibits the largest known upper critical field Hc2 anisotropy of all iron-based superconductors, with Hc2||ab(0)/Hc2||c(0)=(2.75T)/(0.275T)=10. Comparisons to theoretical models for two-dimensional and anisotropic three-dimensional superconductors, however, suggest that FeS is the latter case with a large effective mass anisotropy. We place FeS in context to other closely related iron-based superconductors and discuss the role of structural parameters such as anion height on superconductivity.

Source-reported materials — not catalogue approval

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