Unified picture of the doping dependence of superconducting transition temperatures in alkali metal/ammonia intercalated FeSe

In the recently synthesized Liₓ(NH₂)y(NH₃)zFe₂Se₂ family of iron chalcogenides, a molecular spacer consisting of lithium ions, lithium amide, and ammonia separates the layers of FeSe. It has been shown that upon variation of the chemical composition of the spacer layer, superconducting transition temperatures can reach Tc~440.28em0exK, but the relative importance of the layer separation and effective doping to the Tc enhancement is currently unclear. Using state of the art band structure unfolding techniques, we construct eight-orbital models from ab initio density functional theory calculations for these materials. Within an RPA spin-fluctuation approach, we show that the electron doping enhances the superconducting pairing, which is of s_± symmetry and explain the experimentally observed limit to Tc in the molecular spacer intercalated FeSe class of materials.