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 TQ which induces the tetragonal-orthorhombic structural transition at Ts=TQ, together with the stripe-type antiferromagnetic order below TN. Near the phase transitions, the system shows the s++-wave superconductivity due to the orbital fluctuation for a large g case with TQ>TN, while the s±-wave due to the magnetic fluctuation for a small g case with TQ≤TN. The former case is consistent with the phase diagram of doped iron pnictides with Ts>TN. (author)