Universal understanding of nematicity and magnetism in Fe-pnictides and FeSe

By employing the mean field and variational Monte Carlo methods, we investigate the iron-based superconductors (FeSCs) based on a realistic five-orbital Hubbard model in which an off-site Coulomb interaction [Formula: see text] is explicitly included. Our results demonstrate that [Formula: see text] plays an important role in stabilizing the nematic state in both Fe-pnictides and FeSe. Below a critical [Formula: see text], the model is shown to lie in the striped antiferromagnetic ground state, and an increasing of [Formula: see text] leads to an energy degeneracy between different magnetic configurations. This finding provides a natural explanation for the magnetism in Fe-pnictides with relatively small [Formula: see text], and more importantly, unveils the microscopic mechanism behind the absence of magnetic order in FeSe with larger [Formula: see text]. Simultaneously, the common anisotropy of [Formula: see text] and [Formula: see text] orbital occupations in different magnetic configurations accounts for the similar orbital ordering observed in Fe-pnictides and FeSe. In addition, the unusual smallness of the Fermi surfaces in FeSe can be obtained when the renormalization effect of [Formula: see text] on the band structure is taken into account.