Direct observation of spin-orbit coupling in iron-based superconductors

TL;DR

This study provides direct experimental evidence of significant spin-orbit coupling in iron-based superconductors, revealing its crucial influence on electronic structure and pairing mechanisms, challenging previous assumptions of its insignificance.

Contribution

The paper presents the first direct observation of sizeable spin-orbit splitting in all main families of iron-based superconductors using ARPES, highlighting its dominant role over nematic ordering.

Findings

Spin-orbit splitting is sizeable in all main IBS families.

SOC significantly alters low-energy electronic structure and Fermi surface topology.

Largest pairing gap correlates with SOC-induced Fermi surfaces.

Abstract

Spin-orbit coupling (SOC) is a fundamental interaction in solids which can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin- or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOC-induced Fermi surfaces.