Orbital Order, Structural Transition, and Superconductivity in Iron Pnictides

We investigate the 16-band d-p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below T-Q which induces the tetragonal-orthorhombic structural transition at T-s = T-Q, together with the stripe-type antiferro-magnetic order below T-N. Near the phase transitions, the system shows the s(++)-wave superconductivity due to the orbital fluctuation for a large g case with T-Q > T-N, while the s(+/-)-wave due to the magnetic fluctuation for a small g case with T-Q <= T-N. The former case is consistent with the phase diagram of doped iron pnictides with T-s > T-N.