Spin-state Crossover Model for the Magnetism of Iron Pnictides

TL;DR

This paper introduces a minimal model for the magnetism of Fe-based superconductors, emphasizing the dynamical mixing of spin states and resulting in universal magnon spectra and temperature-dependent magnetic properties.

Contribution

It presents a novel minimal model that captures the magnetic behavior of Fe-based superconductors through spin state mixing and emergent biquadratic exchange couplings.

Findings

Magnon spectra scale universally with effective local spin Seff.

The model explains large but fluctuating Fe magnetic moments.

Temperature dependence of local moments and susceptibility is accounted for.

Abstract

We propose a minimal model describing magnetic behavior of Fe-based superconductors. The key ingredient of the model is a dynamical mixing of quasi-degenerate spin states of Fe2+ ion by intersite electron hoppings, resulting in an effective local spin Seff. The moments Seff tend to form singlet pairs, and may condense into a spin nematic phase due to the emergent biquadratic exchange couplings. The long-range ordered part m of Seff varies widely, 0<=m<=Seff, but magnon spectra are universal and scale with Seff, resolving the puzzle of large but fluctuating Fe-moments. Unusual temperature dependences of a local moment and spin susceptibility are also explained.