Electronic structure and superconductivity of FeSe-related superconductors

FeSe superconductors and their related systems have attracted much attention in the study of iron-based superconductors owing to their simple crystal structure and peculiar electronic and physical properties. The bulk FeSe superconductor has a superconducting transition temperature (T_c) of ∼8 K and it can be dramatically enhanced to 37 K at high pressure. On the other hand, its cousin system, FeTe, possesses a unique antiferromagnetic ground state but is non-superconducting. Substitution of Se with Te in the FeSe superconductor results in an enhancement of T_c up to 14.5 K and superconductivity can persist over a large composition range in the Fe(Se,Te) system. Intercalation of the FeSe superconductor leads to the discovery of the A_xFe_2−ySe₂ (A = K, Cs and Tl) system that exhibits a T_c higher than 30 K and a unique electronic structure of the superconducting phase. A recent report of possible high temperature superconductivity in single-layer FeSe/SrTiO₃ films with a T_c above 65 K has generated much excitement in the community. This pioneering work opens a door for interface superconductivity to explore for high T_c superconductors. The distinct electronic structure and superconducting gap, layer-dependent behavior and insulator–superconductor transition of the FeSe/SrTiO₃ films provide critical information in understanding the superconductivity mechanism of iron-based superconductors. In this paper, we present a brief review of the investigation of the electronic structure and superconductivity of the FeSe superconductor and related systems, with a particular focus on the FeSe films. (topical review)