Effect of annealing on the structural and superconducting properties of FeSe1−xTex

The term 'high-temperature superconductivity' has long been synonymous with copper oxide-based superconductors (cuprates) up until the recent discovery of the iron-based superconductors in 2008. This new family of superconductors exhibits fundamentally interesting properties such as the interplay between magnetism and superconductivity as well as the very recently discovered topological properties of FeSe1−xTex. Furthermore, from an application point of view, iron-based superconductors have the potential to become the new norm for low-temperature, high-field applications such as MRI and nuclear fusion.

However, some of the post-processing procedures required to obtain high-quality samples, like the annealing process of FeSe1−xTex, are yet to be fully understood.

This thesis reports on the effect of annealing on the structure and composition of FeSe1−xTex and how they manifest as changes in the superconducting properties.

Overall, air annealing is shown to improve the critical temperature and critical current density of FeSe1−xTex for almost all investigated doping concentrations.

These improvements are the result of a decrease in excess iron driven by the formation of thin iron oxide layers on exposed surfaces of the crystal.

Further analysis suggests that the reduction in the excess iron concentration is largest in the region right underneath the oxide layers. Consequently, the improvement in the superconducting properties is also found to be largest in these regions. In terms of the annealing atmosphere, even in nitrogen and low-vacuum atmospheres, annealing still leads to the formation of an iron oxide layer and an improvement in the superconducting properties due to the presence of residual oxygen. In rare cases, annealing was found to induce asymmetric magnetic hysteresis loops as a result of weak bulk pinning and strong surface pinning. Whilst asymmetric hysteresis loops have occasionally been reported in the cuprates and polycrystalline iron-based superconductors, this work reports the first observation of such behaviour in FeSe1−xTex single crystals. This work has deepened the understanding of the annealing process on the intrinsic properties of FeSe1−xTexand facilitates the study of additional post-processing procedures that will further improve the properties of this family of superconductors.