FeSe is a unique iron-based superconductor in which the superconducting transition temperature (Tc) can be tuned over an exceptionally wide range, i.e. from ~8 K to above 65 K, through various external control parameters, including carrier doping, pressure, strain, reduced dimensionality, and interfacial engineering. This remarkable tunability provides a versatile platform for exploring superconductivity; however, the underlying mechanism responsible for the dramatic enhancement of Tc remains under active debate.

In this talk, I will present our recent progress in tuning superconductivity in FeSe thin films via carrier doping and strain engineering. By introducing epitaxial strain through substrate selection, we demonstrate that Tc can be enhanced from ~8 K in bulk FeSe to approximately 20 K. Alternatively, using ionic liquid gating to modulate carrier concentration, Tc can be continuously tuned over a wide range, reaching values exceeding 45 K. This continuous and reversible control enables a systematic investigation of the relationship between carrier density and superconductivity.

Furthermore, by directly comparing ultra-thin FeSe films grown on SrTiO3 substrates with FeSe films tuned via ionic gating, we find that the Tc enhancement observed in FeSe/SrTiO3 may be predominantly attributed to charge transfer–induced carrier doping. Our results suggest that, while interfacial effects may play a fole, carrier doping is the primary driving force behind the significant Tc enhancement in this system.