Minimal model for study on Superconductivity in LaFeAsO_1-xF _x based on ab-initio downfolding

We have constructed an effective model for the newly discovered iron-based superconductor LaFeAsO_1-xF_x by downfolding an ab-initio electronic structure around the Fermi energy with the maximally localized Wannier functions. The model has turned out to involve all the five Fe 3d orbitals. We have then used the model to calculate the spin and charge susceptibilities with the five-band random phase approximation. The stripe-type antiferromagnetic spin instability observed in neutron scattering experiments is successfully reproduced. Superconductivity is examined by solving the linearized Eliashberg equation to identify the dominant pairing gap function. We found that the gap function has an extended-s symmetry.