Discovery of Orbital-Selective Cooper Pairing in FeSe

TL;DR

This study uncovers orbital-selective Cooper pairing in FeSe, showing highly anisotropic, nodeless gaps with opposite signs, indicating a complex pairing mechanism tied to specific electron orbitals.

Contribution

It introduces a novel technique to demonstrate that FeSe's superconducting gaps are orbital-dependent and have opposite signs, revealing a new pairing mechanism.

Findings

Gaps are highly anisotropic and nodeless

Gaps have opposite signs in momentum space

Orbital-selective pairing is based on $d_{yz}$ orbitals

Abstract

The superconductor FeSe is of intense interest thanks to its unusual non-magnetic nematic state and potential for high temperature superconductivity. But its Cooper pairing mechanism has not been determined. Here we use Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the bands surrounding the $\Gamma = (0,0)$ and $X=(\pi / a_{Fe}, 0)$ points of FeSe, and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless, and exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing which, in FeSe, is based preferentially on electrons from the $d_{yz}$ orbitals of the iron atoms.