Multiple Superconducting Gaps, Anisotropic Spin Fluctuations and Spin-Orbit Coupling in Iron-Pnictides

TL;DR

This review discusses NMR/NQR studies on iron-pnictide superconductors, highlighting multiple energy gaps, the role of anisotropic spin fluctuations, and the influence of spin-orbit coupling on their superconducting properties.

Contribution

It synthesizes experimental findings on the electronic and magnetic properties of iron-pnictides, emphasizing the impact of spin-orbit coupling and multiband effects on superconductivity.

Findings

Superconducting electron pairs are in the spin-singlet state with multiple gaps.

Antiferromagnetic spin fluctuations are correlated with superconductivity.

Spin fluctuations are anisotropic due to spin-orbit coupling.

Abstract

This article reviews the NMR and NQR studies on iron-based high-temperature superconductors by the IOP/Okayama group. It was found that the electron pairs in the superconducting state are in the spin-singlet state with multiple fully-opened energy gaps. The antiferromagnetic spin fluctuations in the normal state are found to be closely correlated with the superconductivity. Also the antiferromagnetic spin fluctuations are anisotropic in the spin space, which is different from the case in copper oxide superconductors. This anisotropy originates from the spin-orbit coupling and is an important reflection of the multiple-bands nature of this new class of superconductors.