Prominent role of spin-orbit coupling in FeSe

TL;DR

This study demonstrates that spin-orbit coupling significantly influences the low-energy spin excitations and magnetic properties of FeSe, challenging previous assumptions of its insignificance in iron-based superconductors.

Contribution

It provides direct experimental evidence of SOC's role in FeSe's spin dynamics and its inheritance in the superconducting state, highlighting its importance in understanding magnetism and superconductivity.

Findings

Spin fluctuations in FeSe are nearly in-plane isotropic, indicating SOC influence.

SOC causes spin-space anisotropy observed over energies above the superconducting gap.

A $c$-axis polarized spin resonance appears in the superconducting state, inherited from normal state excitations.

Abstract

In most existing theories for iron-based superconductors, spin-orbit coupling (SOC) has been assumed insignificant. Even though recent experiments have revealed an influence of SOC on the electronic band structure, whether SOC fundamentally affects magnetism and superconductivity remains an open question. Here we use spin-polarised inelastic neutron scattering to show that collective low-energy spin fluctuations in the orthorhombic (or "nematic") phase of FeSe possess nearly no in-plane component. Such spin-space anisotropy can only be caused by SOC. It is present over an energy range greater than the superconducting gap 2$\Delta _\mathrm{sc}$ and gets fully inherited in the superconducting state, resulting in a distinct $c$-axis polarised "spin resonance". Our result demonstrates the importance of SOC in defining the low-energy spin excitations in FeSe, which helps to elucidate the nearby magnetic instabilities and the debated interplay between spin and orbital degrees of freedom. The prominent role of SOC also implies a possible unusual nature of the superconducting state.