Strong anisotropy in nearly ideal tetrahedral superconducting FeS single crystals

Mackinawite, the tetragonal form of FeS, is the newest iron-based superconductor that generates excitement in the field, as it represents a direction away from the purely arsenide- and selenide-based systems. Other sulfides have been shown recently to have superconducting properties, including BaFe₂S₃ under high pressures and H₂S under extreme pressures, but tetragonal FeS is perhaps the simplest sulfide to exhibit superconductivity behavior at ambient pressure. The authors of this work present a novel method for preparing high-quality single crystals of FeS and investigate its superconducting properties. They find that the electronic and magnetic properties of FeS are highly anisotropic. In particular, the level of anisotropy characterized by the ratio of the upper critical fields in two orthogonal directions was found to be the largest of all the iron-based superconductors.