Dichotomy of superconductivity between monolayer FeS and FeSe.

The discovery of high-temperature ( ) superconductivity in monolayer FeSe on SrTiO raised a fundamental question: Whether high is commonly realized in monolayer iron-based superconductors. Tetragonal FeS is a key material to resolve this issue because bulk FeS is a superconductor with comparable to that of isostructural FeSe. However, difficulty in synthesizing tetragonal monolayer FeS due to its metastable nature has hindered further investigations.