Orbital Order, Structural Transition, and Superconductivity in Iron Pnictides

Abstract

We investigate the 16-band d-p model for iron pnictide superconductors in the\npresence of the electron-phonon coupling g with the orthorhombic mode which is\ncrucial for reproducing the recently observed ultrasonic softening. Within the\nRPA, we obtain the ferro-orbital order below TQ which induces the\ntetragonal-orthorhombic structural transition at Ts = TQ, together with the\nstripe-type antiferromagnetic order below TN. Near the phase transitions, the\nsystem shows the s++ wave superconductivity due to the orbital fluctuation for\na large g case with TQ > TN, while the s+- wave due to the magnetic fluctuation\nfor a small g case with TQ < TN. The former case is consistent with the phase\ndiagram of doped iron pnictides with Ts > TN.\n