Observation of strong electron pairing on bands without Fermi surfaces in LiFe1-xCoxAs

TL;DR

This study reveals strong electron pairing in LiFe1-xCoxAs without Fermi surface involvement, challenging traditional BCS theory and suggesting alternative pairing mechanisms in high-Tc superconductors.

Contribution

It demonstrates superconducting gaps on bands without Fermi surfaces, indicating a non-Fermi surface instability origin of pairing in iron-based superconductors.

Findings

Superconducting gap observed on a band below the Fermi energy.

Fermi surface independence of the superconducting order.

Largest Fermi surface-free superconducting order in the studied material.

Abstract

In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface (FS) instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally different picture of the electron pairs, which are believed to be formed locally by repulsive interactions, may prevail. Here we report a high-resolution angle-resolved photoemission spectroscopy study on LiFe1-xCoxAs. We reveal a large and robust superconducting (SC) gap on a band sinking below the Fermi energy upon Co substitution. The observed FS-free SC order is also the largest over the momentum space, which rules out a proximity effect origin and indicates that the SC order parameter is not tied to the FS as a result of a FS instability.