Magnetism, superconductivity, and spontaneous orbital order in iron-based superconductors: who comes first and why?

TL;DR

This paper develops an unbiased approach to study the interplay of magnetism and orbital order in iron-based superconductors, revealing how Fermi energy influences whether orbital order or magnetism dominates.

Contribution

It introduces a new renormalization group method that treats magnetic and orbital fluctuations equally, unifying the understanding of different iron-based superconductors.

Findings

Orbital order is induced by stripe magnetism in systems with large Fermi energies.

Spontaneous orbital order occurs in systems with small Fermi energies without magnetic order.

The approach explains the diversity of behaviors in various iron-based superconductors.

Abstract

Magnetism and nematic order are the two non-superconducting orders observed in iron-based superconductors. To elucidate the interplay between them and ultimately unveil the pairing mechanism, several models have been investigated. In models with quenched orbital degrees of freedom, magnetic fluctuations promote stripe magnetism which induces orbital order. In models with quenched spin degrees of freedom, charge fluctuations promote spontaneous orbital order which induces stripe magnetism. Here we develop an unbiased approach, in which we treat magnetic and orbital fluctuations on equal footing. Key to our approach is the inclusion of the orbital character of the low-energy electronic states into renormalization group analysis. Our results show that in systems with large Fermi energies, such as BaFe2As2, LaFeAsO, and NaFeAs, orbital order is induced by stripe magnetism. However, in systems with small Fermi energies, such as FeSe, the system develops a spontaneous orbital order, while magnetic order does not develop. Our results provide a unifying description of different iron-based materials.