Realization of continuous electron doping in bulk iron selenides and identification of a new superconducting zone

It is known that iron selenide superconductors exhibit unique characteristics distinct from iron pnictides, especially in the electron-doped region. However, a comprehensive study of continuous carrier doping and the corresponding crystal structures of FeSe is still lacking, mainly due to the difficulties in controlling the carrier density in bulk materials. Here we report the successful synthesis of a new family of bulk Li_x(C₃N₂H₁₀)_0.37FeSe, which features a continuous superconducting dome harboring Lifshitz transition within the wide range of $0.06⩽x⩽0.68$ . We demonstrate that with electron doping, the anion height of FeSe layers deviates linearly away from the optimized values of pnictides and pressurized FeSe. This feature leads to a new superconducting zone with unique doping dependence of the electronic structures and strong orbital-selective electronic correlation. Optimal superconductivity is achieved when the $Fe3d{t}_{2g}$ orbitals have almost the same intermediate electronic correlation strength, with moderate mass enhancement between 3 ~ 4 in the two separate superconducting zones. Our results shed new light on achieving unified mechanism of superconductivity in iron-based superconductors.