Highly-tunable crystal structure and physical properties in FeSe-based superconductors

Here, crystal structure, electronic structure, chemical substitution, pressure-dependent superconductivity, and thickness-dependent properties in FeSe-based superconductors are systemically reviewed. First, the superconductivity versus chemical substitution is reviewed, where the doping at Fe or Se sites induces different effects on the superconducting critical temperature (T_c). Meanwhile, the application of high pressure is extremely effective in enhancing T_c and simultaneously induces magnetism. Second, the intercalated-FeSe superconductors exhibit higher T_c from 30 to 46 K. Such an enhancement is mainly caused by the charge transfer from the intercalated organic and inorganic layer. Finally, the highest T_c emerging in single-unit-cell FeSe on the SrTiO₃ substrate is discussed, where electron-phonon coupling between FeSe and the substrate could enhance T_c to as high as 65 K or 100 K. The step-wise increment of T_c indicates that the synergic effect of carrier doping and electron-phonon coupling plays a critical role in tuning the electronic structure and superconductivity in FeSe-based superconductors.