Evidence of depth-limited reduction of excess iron in FeSe 1-x Te x after air annealing

Air annealing has been found to increase both T c and J c of FeSe1-xTex single crystals. However, there are indications that this effect might be limited to the surface of the crystals. In this work, we report on the changes in the surface structure and composition, T c, and J c of air-annealed FeSe0.35Te0.65 single crystals. Using XPS depth-profiling and cross-sectional STEM, we found that both the formation of an iron oxide layer and the resulting reduction of interstitial iron in the bulk are confined to exposed surfaces of the single crystals. This is linked to the largest improvement in the superconducting transition width and critical current density close to the surface of the sample.