Itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors

TL;DR

This paper explores how itinerant carriers influence magnetic moment fluctuations in Fe-based superconductors, revealing that nesting-induced quantum fluctuations reduce ordered moments and challenge previous ferromagnetic coupling assumptions.

Contribution

It demonstrates that nesting-related long-range interactions, not ferromagnetic double-exchange, drive quantum fluctuations and magnetic order stabilization in Fe-based superconductors.

Findings

Itinerant carriers induce significant quantum fluctuations reducing magnetic order.

Long-range nesting interactions, not ferromagnetic exchange, dominate magnetic dynamics.

Ferro-orbital order explains the observed magnetic coupling and order stability.

Abstract

We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($\pi$,0) order, itinerant carriers with well nested Fermi surfaces is found to induce significant amount of \textit{spatial} and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be intra-pocket nesting-associated long-range coupling, rather than the previously believed ferromagnetic double-exchange effect. This challenges the validity of ferromagnetically compensated first-neighbor coupling reported from short-range fitting to the experimental dispersion, which turns out to result instead from the ferro-orbital order that is also found instrumental in stabilizing the magnetic order.