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Air annealing has been found to increase both Tc and Jc 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, Tc, and Jc 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. [ABSTRACT FROM AUTHOR]

We report the results from a comprehensive study of the effect that annealing FeSe0.35Te0.65 single crystals in different atmospheres has on the surface chemistry, the superconducting transition temperature, Tc, and the critical current density, Jc. A review of the literature and our data shows that annealing in oxygen, nitrogen, air or vacuum atmosphere leads to a universal superconducting phase diagram that is independent of the variance in the as-grown properties. We show that atmospheres with an oxygen partial pressure of p > 10−3 hPa during annealing are necessary to improve the superconducting properties in this compound. This is demonstrated by Raman and magnetisation measurements on air, nitrogen, or low vacuum annealed FeSe0.35Te0.65 samples, which show that the improvement in the superconducting properties is strongly correlated to the formation of a thin iron oxide surface layer. However, annealing under high vacuum does not lead to the formation of an iron oxide layer and no improvement in the superconducting properties is observed. Our findings show that the previously reported improvements in the superconducting properties after annealing under vacuum or in nitrogen are thus likely to have been caused by residual oxygen. Furthermore, we propose a diffusion model for the formation of the iron oxide layer as the driving force for the reduction of interstitial excess iron that suppresses superconductivity in this compound. Overall, our results show that the presence of an iron reactant, such as oxygen, during annealing is necessary to improve the superconducting properties of FeSe1−xTex. [ABSTRACT FROM AUTHOR]

We report the results from a comprehensive study of the effect that annealing FeSe0.35Te0.65 single crystals in different atmospheres has on the surface chemistry, the superconducting transition temperature, Tc, and the critical current density, Jc. A review of the literature and our data shows that annealing in oxygen, nitrogen, air or vacuum atmosphere leads to a universal superconducting phase diagram that is independent of the variance in the as-grown properties. We show that atmospheres with an oxygen partial pressure of p > 10−3 hPa during annealing are necessary to improve the superconducting properties in this compound. This is demonstrated by Raman and magnetisation measurements on air, nitrogen, or low vacuum annealed FeSe0.35Te0.65 samples, which show that the improvement in the superconducting properties is strongly correlated to the formation of a thin iron oxide surface layer. However, annealing under high vacuum does not lead to the formation of an iron oxide layer and no improvement in the superconducting properties is observed. Our findings show that the previously reported improvements in the superconducting properties after annealing under vacuum or in nitrogen are thus likely to have been caused by residual oxygen. Furthermore, we propose a diffusion model for the formation of the iron oxide layer as the driving force for the reduction of interstitial excess iron that suppresses superconductivity in this compound. Overall, our results show that the presence of an iron reactant, such as oxygen, during annealing is necessary to improve the superconducting properties of FeSe1−xTex. [ABSTRACT FROM AUTHOR]