Structural transition, ferro-orbital order and its fluctuation-mediated -wave superconductivity in iron pnictides

We investigate the electronic states and the superconductivity in the two-dimensional 16-band d – p model extracted from a tight-binding fit to the band structure of iron pnictides, in the presence of both the Coulomb interaction between Fe d -electrons and the electron–lattice coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening of the elastic constant C 66 . Due to the cooperative effects of these interactions, the ferro-orbital order with different occupations of d y z and d z x orbitals occurs and induces the tetragonal-orthorhombic structural transition at T s , together with the stripe-type antiferromagnetic (AFM) order below T N . For a large g case, we obtain the phase diagram consistent with the doped iron pnictides with T s > T N for x > 0 , where the s + + -wave superconductivity is mediated by the ferro-orbital fluctuation which is largely enhanced near the ferro-orbital QCP at x c with T s → 0 . On the other hand, for a small g case, the simultaneous phase transition occurs at T s = T N even for x > 0 , where the s ± -wave superconductivity is mediated by the AFM fluctuation. Both the s -wave states with full superconducting gaps are consistent with most of the experiments but only the former is considered to account for the small T c -suppression against nonmagnetic impurities.