Abstract
In most existing theories for iron-based superconductors, spin-orbit coupling (SOC) has been assumed to be insignificant. Here, we use spin-polarized inelastic neutron scattering to show that collective low-energy spin excitations in the orthorhombic (or “nematic”) phase of FeSe possess nearly no in-plane component. Such spin-space anisotropy is present over an energy range greater than the superconducting gap and gets fully inherited in the superconducting state, resulting in a -axis polarized “spin resonance” without any noticeable isotropic spectral-weight rearrangement related to the superconductivity, which is distinct from observations in the superconducting iron pnictides. The contrast between the strong suppression of long-range magnetic order in FeSe and the persisting large spin-space anisotropy, which cannot be explained microscopically by introducing single-ion anisotropy into local-moment spin models, demonstrates the importance of SOC in an itinerant-electron description of the low-energy spin excitations. Our result helps to elucidate the nearby magnetic instabilities and the debated interplay between spin and orbital degrees of freedom in FeSe. The prominent role of SOC also implies a possible unusual nature of the superconducting state.
- Received 14 October 2016
DOI:https://doi.org/10.1103/PhysRevX.7.021025
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Superconductors are a material that, once cooled below a certain temperature, can conduct electricity with zero resistance. This remarkable effect has led to the development of powerful electromagnets and offers the promise of a range of applications from high-performance power transmission to levitating trains. Research into iron-based superconductors, a relatively new type of compound that contains layers of iron, is largely focused on developing a theory that can explain ubiquitous electronic behavior at the microscopic level. This requires researchers to correctly identify, based on experimental facts, all indispensable ingredients for the proper theory, such as the relevant degrees of freedom and interactions. As a tempting shortcut, the electrons’ spin and orbital degrees of freedom have been assumed in most theories to be largely independent. Here, we report on experimental results that challenge this assumption.
We use a spin-polarized, inelastic neutron-scattering experiment to study low-energy magnetic excitations in a sample of FeSe, the structurally simplest iron-based superconductor. The excitations are nearly 100% polarized along the axis (the longest axis in a crystal) whether or not the sample is in a superconducting state. This polarization can occur only in the presence of strong spin-orbit coupling. Our data also suggest that the formation of Cooper pairs (loosely bound pairs of electrons thought to be responsible for superconductivity) is actively influenced by the spin-orbit interactions.
These results suggest that entanglement between spin and orbital degrees of freedom is an indispensable ingredient in any proper microscopic theory for iron-based superconductors.