Strong anisotropy in nearly ideal-tetrahedral superconducting FeS single crystals

TL;DR

This study reports the synthesis of nearly ideal tetrahedral FeS single crystals that exhibit strong anisotropic superconductivity with the highest known upper critical field anisotropy among iron-based superconductors.

Contribution

The paper introduces a novel hydrothermal method to produce stable, nearly ideal tetrahedral FeS single crystals with detailed characterization of their superconducting and anisotropic properties.

Findings

FeS exhibits superconductivity below 4 K.

FeS has the largest known Hc2 anisotropy among iron-based superconductors.

FeS shows remarkable two-dimensional behavior in magnetoresistance and superconductivity.

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~$\mu\Omega\cdot$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.