*Review* **Highly-Tunable Crystal Structure and Physical Properties in FeSe-Based Superconductors**

**Kaiyao Zhou 1,2, Junjie Wang 1,2, Yanpeng Song 1,2, Liwei Guo 1 and Jian-gang Guo 1,3,\***


Received: 12 September 2019; Accepted: 22 October 2019; Published: 25 October 2019

**Abstract:** 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.

**Keywords:** FeSe; superconductivity; high pressure; chemical intercalation; interfacial coupling
