Superconducting properties of S-substituted FeTe

Iron-chalcogenide superconductors attract us as the simplest iron-based superconductor. Tetragonal FeSe is a superconductor with a transition temperature $T_{c} \quad \sim $10 K. The $T_{c}$ dramatically increases to 37 K under pressure above 4 GPa. While FeSe exhibits superconductivity without any chemical substitution, FeTe that has a structure analogous to FeSe undergoes magnetic and structural phase transition around 70 K and does not show superconductivity. We found that S substitution for the Te site suppresses the magnetic ordering and induces superconductivity at $T_{c}$ $\sim $10 K. Recently we observed a dramatic change of superconducting property for the FeTe$_{1-x}$S$_{x}$ sample exposed to the air. Both the $T_{c}$ and the superconducting volume fraction were enhanced by the air exposure. We will discuss the factor that induces the dramatic change in FeTe$_{1-x}$S$_{x}$.