Impact of Dynamic Orbital Correlations on Magnetic Excitations in the Normal State of Iron-Based Superconductors

TL;DR

This paper demonstrates how orbital correlations influence magnetic excitations in iron-based superconductors, providing insights into their normal state behavior and distinguishing orbital effects from spin physics.

Contribution

It reveals the impact of static and fluctuating orbital correlations on magnetic excitations using a five-orbital model, explaining experimental observations.

Findings

Orbital correlations enhance commensurate magnetic excitations.

Fluctuating orbital correlations modify magnetic spectra beyond RPA.

Incommensurate-to-commensurate transformation explained by orbital effects.

Abstract

We show here that orbital degrees of freedom produce a distinct signature in the magnetic excitation spectrum of iron-based superconductors above the magnetic ordering temperature. Because $d_{xz}$ and $d_{yz}$ orbitals are strongly connected with the Fermi surface topology, the nature of magnetic excitations can be modified significantly due to the presence of either static or fluctuating orbital correlations. Within a five-orbital itinerant model, we show that static orbital order generally leads to an enhancement of commensurate magnetic excitations even when the original Fermi surface lacks nesting at $(\pi,0)$ or $(0,\pi)$. When long-range orbital order is absent, Gaussian fluctuations beyond the standard random-phase approximation (RPA) capture the effects of fluctuating orbital correlations on the magnetic excitations. We find that commensurate magnetic excitations can also be enhanced if the orbital correlations are strong. Our results offer a natural explanation for the incommensurate-to-commensurate transformation observed in a recent neutron scattering measurement (Z. Xu, et. al., arXiv:1201.4404), and we propose that this unusual transformation is an important signature to distinguish orbital from spin physics in the normal state of the pnictides. Implications for the magnetic and superconducting states are discussed.