Cooper Pairing in Insulating Valence Band in Fe-Based Superconductors

TL;DR

This paper investigates how superconductivity can occur in the insulating valence band of Fe-based superconductors, specifically in Co-doped LiFeAs, through a theoretical model showing the necessity of finite pairing interaction.

Contribution

It provides a theoretical explanation for superconductivity in an insulating band, highlighting the role of finite pairing strength and predicting chemical potential shifts.

Findings

Superconductivity in the insulating band requires finite pairing interaction.

Strong coupling leads to robust superconductivity despite the band sinking below the Fermi level.

The model predicts an upward shift of the chemical potential during the transition.

Abstract

Conventional Cooper pairing arises from attractive interaction of electrons in the metallic bands. Recent experiment on Co-doped LiFeAs shows superconductivity in the insulating valence band, which is evolved from a metallic hole band upon doping. Here we examine this phenomenon by studying superconductivity in a three-orbital Hamiltonian relevant to the doped LiFeAs. We show explicitly that Cooper pairing of the insulating hole band requires a finite pairing interaction strength. For strong coupling, the superconductivity in the hole band is robust against the sink of the hole band below the Fermi level. Our theory predicts a substantial upward shift of the chemical potential in the superconducting transition for Co-doped LiFeAs.