Experimental Demonstration of the Sign Reversal of the Order Parameter in (Li1-xFex)OHFe1-yZnySe

TL;DR

This study demonstrates that in LiOH intercalated FeSe, the superconducting order parameter reverses sign within the electronic pockets, supporting the idea that spin fluctuations are central to pairing even without hole pockets.

Contribution

It provides phase-sensitive evidence of sign reversal of the order parameter within electronic pockets in a FeSe-based superconductor, challenging previous assumptions about pairing mechanisms.

Findings

Order parameter changes sign within electronic pockets

Supports spin fluctuation-driven pairing in FeSe superconductors

Unifies pairing mechanism across iron-based superconductors

Abstract

Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is spin-fluctuation driven and intimately related to their fermiology, specifically, hole and electron pockets separated by the same wave vector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs. This picture was questioned after the discovery of a new family, based on the FeSe layers, either intercalated or in the monolayer form. The critical temperatures there reach ~40 K, the same as in optimally doped bulk FeSe - despite the fact that intercalation removes the hole pockets from the Fermi level and, seemingly, undermines the basis for the spin-fluctuation theory and the idea of a sign-changing OP. In this paper, using the recently proposed phase-sensitive quasiparticle interference technique, we show that in LiOH intercalated FeSe compound the OP does change sign, albeit within the electronic pockets, and not between the hole and electron ones. This result unifies the pairing mechanism of iron based superconductors with or without the hole Fermi pockets and supports the conclusion that spin fluctuations play the key role in electron pairing.