Abstract
The theoretical need to study the properties of the Fe-based high-Tc superconductors using reliable manybody
techniques has highlighted the importance of determining what is the minimum number of orbital degrees
of freedom that will capture the physics of these materials. While the shape of the Fermi surface FS obtained
with the local-density approximation LDA can be reproduced by a two-orbital model, it has been argued that
the bands that cross the chemical potential result from the strong hybridization of three of the Fe 3d orbitals.
For this reason, a three orbital Hamiltonian for LaOFeAs obtained with the Slater-Koster formalism by considering
the hybridization of the As p orbitals with the Fe dxz, dyz, and dxy orbitals is discussed here. This
model reproduces qualitatively the FS shape and orbital composition obtained by LDA calculations for undoped
LaOFeAs when four electrons per Fe are considered. Within a mean-field approximation, its magnetic
and orbital properties in the undoped case are here described for intermediate values of J/U. Increasing the
Coulomb repulsion U at zero temperature, four different regimes are obtained: 1 paramagnetic, 2 magnetic
,0 spin order, 3 the same ,0 spin order but now including orbital order, and finally 4 a magnetic and
orbital ordered insulator. The spin-singlet pairing operators allowed by the lattice and orbital symmetries are
also constructed. It is found that for pairs of electrons involving up to diagonal nearest-neighbors sites, the only
fully gapped and purely intraband spin-singlet pairing operator is given by k= fkdk,,ādāk,,ā with
fk=1 or cos kx cos ky which would arise only if the electrons in all different orbitals couple with equal
strength to the source of pairing.
