Effects of successive annealing on high-field electrical transport and the upper critical field in S-substituted FeTe

Since iron-based superconductors have been discovered, many scientists have focused on their characteristic properties, such as an unconventional mechanism and a high upper critical field. Sulphur-substituted FeTe compounds are one of the members of the iron-based superconductors; however, chemical processes, such as O${}_2$ annealing, are needed to induce superconductivity because of the existence of excess iron in as-grown crystals. Thus, the removal of excess iron and the obtaining of clean sulphur-substituted FeTe can play a key role in the understanding of the superconducting properties and the application to the superconducting devices. In this study, we present the successive annealing effects on sulphur-substituted FeTe compounds to investigate the electrical transport properties under high magnetic fields. Our measurements show that successive annealing processes improve the electrical transport properties in the superconducting states under magnetic fields. The removal of excess iron acting as magnetic impurities is indicated by the improvement of the upper critical field and its analysis.