Anomalous correlation effects and unique phase diagram of electron-doped FeSe revealed by photoemission spectroscopy

FeSe layer-based superconductors exhibit exotic and distinctive properties. The undoped FeSe shows nematicity and superconductivity, while the heavily electron-doped K$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{\mathrm{2}}$ and single-layer FeSe/SrTiO$_{\mathrm{3}}$ possess high superconducting transition temperatures. However, a comprehensive study on the doping dependence of an FeSe layer-based superconductor is still lacking. Through angle-resolved photoemission spectroscopy studies on K-dosed thick FeSe ?lms and FeSe$_{\mathrm{0.93}}$S$_{\mathrm{0.07}}$ bulk crystals, here we reveal the internal connections between these two types of FeSe-based superconductors, and obtain superconductivity below \textasciitilde 46 K in an FeSe layer under electron doping without interfacial effects. Moreover, we discover an exotic phase diagram of FeSe with electron doping, including a nematic phase, a superconducting dome, a correlation-driven insulating phase and a metallic phase. Such an anomalous phase diagram unveils the remarkable complexity, and highlights the importance of correlations in FeSe layer-based superconductors.