The development of the superconducting PbO-type β-FeSe and related compounds

An overview of the recent development of the superconducting FeSe1−x and related compounds is presented. Methods to synthesize high purity polycrystalline samples, single crystals and thin films with preferred orientation are described. In addition to the synthesis of FeSe and FeSeTe, the effects of various partial chemical substitutions on Fe and Se/Te site are described. It was found that the effects of chemical doping to the Se-site or Fe-site are rather different. Ionic size of the dopant is found to play a critical role on the occurrence of superconductivity. We also review the physical properties, including transport, magnetic, and thermal properties. There exist interesting transport anomalies in the resistivity at low temperature, and it was found that a structural distortion at low temperature is critical to the occurrence of superconductivity in these materials. However, the exact origin of these observed anomalies is not clear, and the exact pairing symmetry in FeSe-based superconductors is also still in question.