Removal of excess iron by annealing processes and emergence of bulk superconductivity in sulfur-substituted FeTe

There are several strategies to discover new superconductors. Growing new materials and applying high pressures have been the classic ways since superconductivity was found. Also, chemical processing, such as annealing, is another way to induce superconductivity in a nonsuperconducting material. Here, we show chemical processing effects in the nonsuperconducting material, sulfur-substituted FeTe. It has been known that superconductivity in S-substituted FeTe is induced by ${\mathrm{O}}_{2}$ annealing. We revealed that hydrochloric acid etching and vacuum annealing for ${\mathrm{O}}_{2}$-annealed samples made the quality of superconductivity higher by several physical property measurements. Furthermore, we visualized the superconducting regions by a magneto-optical imaging technique, indicating that the superconductivity in the processed sample was bulk. In this sample we confirmed that the concentration of excess iron was reduced compared to that in the as-grown state. These results provide an important route to bulk superconductivity in S-substituted FeTe and its related iron-based compounds.