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

TL;DR

This paper combines ab-initio calculations and spin-fluctuation theory to clarify how doping and layer separation influence the superconducting transition temperature in alkali metal/ammonia intercalated FeSe, explaining the observed Tc limits.

Contribution

It provides a unified theoretical framework linking doping levels and layer separation to Tc in intercalated FeSe superconductors, using advanced modeling techniques.

Findings

Electron doping enhances superconducting pairing.

Superconductivity has s±-symmetry.

Explains the Tc limit in intercalated FeSe.

Abstract

In the recently synthesized Li$_x$(NH$_2$)$_y$(NH$_3$)$_z$Fe$_2$Se$_2$ family of iron chalcogenides a molecular spacer consisting of lithium ions, lithium amide and ammonia separates layers of FeSe. It has been shown that upon variation of the chemical composition of the spacer layer, superconducting transition temperatures can reach $T_c\sim 44 \mathrm{K}$, but the relative importance of the layer separation and effective doping to the $T_c$ 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 $s_\pm$-symmetry and explain the experimentally observed limit to $T_c$ in the molecular spacer intercalated FeSe class of materials.