We look for possible topological terms in a mean-field Lagrangian for ferromagnets and antiferromagnets inspired by the Hubbard model and study their physical effects. A topological term corresponding to Berry's phase causes the order parameter to behave like a spin moment. We find that the Hopf term is not induced and thus there is no evidence for a neutral fermion in this theory.
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Wen et al. (1988) studied this question.
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