Superconductivity across Lifshitz transition and anomalous insulating state in surface K-dosed (Li0.8Fe0.2OH)FeSe

TL;DR

This study investigates how electron doping via surface potassium dosing affects the electronic structure and superconductivity in (Li0.8Fe0.2OH)FeSe, revealing a Lifshitz transition, a superconducting gap on a new electron pocket, and an eventual insulating state.

Contribution

It demonstrates the continuous tuning of electronic bands across the Fermi level and the resulting effects on superconductivity and insulating behavior in FeSe-based superconductors.

Findings

Lifshitz transition with a new electron pocket at Γ

Superconducting gap observed on the Γ electron pocket

Evolution into an insulating state with further doping

Abstract

In the iron-based superconductors, understanding the relation between superconductivity and electronic structure upon doping is crucial for exploring the pairing mechanism. Recently it was found that in iron selenide (FeSe), enhanced superconductivity (Tc over 40K) can be achieved via electron doping, with the Fermi surface only comprising M-centered electron pockets. Here by utilizing surface potassium dosing, scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission spectroscopy (ARPES), we studied the electronic structure and superconductivity of (Li0.8Fe0.2OH)FeSe in the deep electron-doped regime. We find that a {\Gamma}-centered electron band, which originally lies above the Fermi level (EF), can be continuously tuned to cross EF and contribute a new electron pocket at {\Gamma}. When this Lifshitz transition occurs, the superconductivity in the M-centered electron pocket is slightly suppressed; while a possible superconducting gap with small size (up to ~5 meV) and a dome-like doping dependence is observed on the new {\Gamma} electron pocket. Upon further K dosing, the system eventually evolves into an insulating state. Our findings provide new clues to understand superconductivity versus Fermi surface topology and the correlation effect in FeSe-based superconductors.