Magnetic and Metallic State at Intermediate Hubbard U Coupling in   Multiorbital Models for Undoped Fe Pnictides
  Multi-orbital Hubbard model Hamiltonians for the undoped parent compounds of the Fe-pnictide superconductors are here investigated using mean-field techniques. For a realistic four-orbital model, our results show the existence of an intermediate Hubbard U coupling regime where the mean-field ground state has spin stripe magnetic order, as in neutron scattering experiments, while remaining metallic, due to the phenomenon of band overlaps. The angle-resolved photoemission intensity and Fermi surface of this magnetic and metallic state are discussed. Other models are also investigated, including a two orbital model where not only the mean-field technique can be used, but also Exact Diagonalization in small clusters and the Variational Cluster Approximation in the bulk. The combined results of the three techniques point toward the existence of an intermediate-coupling magnetic and metallic state in the two-orbital model, similar to the intermediate coupling mean-field state of the four-orbital model. We conclude that the state discussed here is compatible with the experimentally known properties of the undoped Fe-pnictides. 
Rong Yu, Kien T. Trinh, Adriana Moreo, Maria Daghofer, Jos\'e A.   Riera, Stephan Haas, Elbio Dagotto