Doping effects on electronic states in electron-doped FeSe: Impact of self-energy and vertex corrections

摘要

The pairing glue of high-T_c superconductivity in heavily electron-doped (e-doped) FeSe, in which hole pockets are absent, has been an important unsolved problem. Here, we focus on a heavily e-doped bulk superconductor Li_(1-x)Fe_xOHFeSe (T_c～ 40 K). We construct a multiorbital model beyond the rigid band approximation and analyze the spin and orbital fluctuations by taking both vertex corrections (VCs) and self-energy into consideration. Without e-doping (x = 0), the ferro-orbital order without magnetism in FeSe is reproduced by the VCs. The orbital order quickly disappears when the hole pocket vanishes at x ～ 0.03. With increasing x further, the spin fluctuations remain small, whereas orbital fluctuations gradually increase with x due to the VCs. The negative feedback due to the self-energy is crucial to explain experimental phase diagram. Thanks to both vertex and self-energy corrections, the orbital-fluctuation-mediated .s_(++)-wave state appears for a wide doping range, consistent with experiments.