Spin-state Crossover Model for the Magnetism of Iron Pnictides [PDF]
Jiří Chaloupka, Giniyat Khaliullin
We introduce 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 spin Seff in the ground state. The moments Seff tend to form singlet pairs, and may condense into a spin nematic phase due to the emergent biquadratic exchange couplings. We show that while the spin length Seff is robust against the variations of physical parameters, its long-range ordered part may take any value, resolving the puzzle of large but fluctuating Fe-moments observed. Underlying singlet correlations explain also the unusual temperature dependence of the paramagnetic spin susceptibility.
View original: http://arxiv.org/abs/1208.1197
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