Correlation effects on antiferromagnetism in Fe pnictides

Kubo, Katsunori ; Thalmeier, P.*

We investigate the magnetism and orbital state of the ground state of a two-orbital model proposed for Fe pnictides which includes and orbitals of Fe ions. We apply a Hartree-Fock approximation and a variational Monte Carlo method to this model. First, we apply the Hartree-Fock approximation. The antiferromagnetic order with ordering vector is stabilized by the nesting between hole and electron pockets. This antiferromagnetic state accompanies ferro-orbital order, that is, the occupancies of and orbitals become different. Under such ferro-orbital order, the lattice should be distorted from a tetragonal to orthorhombic structure through an electron-lattice interaction. We also apply the variational Monte Carlo method to the model. We find that correlation effects reduce the ordered moment. Even for a large value of the Coulomb interaction comparable to the band width, the ordered moment can be small.