Strong anisotropy in nearly ideal-tetrahedral superconducting FeS single crystals

Abstract

We report the novel 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) ${}^{\circ }$ and 108.1(2) ${}^{\circ }$. Grown via hydrothermal de-intercalation of K${}_x$Fe${}_{2-y}$S${}_2$ crystals under basic and reducing conditions, the silver, plate-like crystals of FeS remain stable up to 200 ${}^{\circ }$C under air and 250 ${}^{\circ }$C under inert conditions, even though the mineral "mackinawite" (FeS) is known to be metastable. FeS single crystals exhibit a superconducting state below $T_c=4$ 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-K${}^2$, and paramagnetic, metallic behavior with a low residual resistivity of 250~$\hat{I}¼\hat{I}\text{c◯}\cdot$cm. 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 $H_{c2}$ anisotropy of all iron-based superconductors, with $H_{c2}^{||ab}(0)/H_{c2}^{||c}(0)=$(2.75~T)/(0.275~T)=10. Comparisons to theoretical models for 2D and anisotropic-3D 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.