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

► Electronic state and superconductivity are studied on the basis of the 16 band d-p model. ► Ferro-orbital fluctuation is enhanced by Coulomb and orbital-lattice interactions. ► The enhanced orbital fluctuation mediates the s++-wave pairing without sign change. ► Ferro-orbital order induces the tetragonal-orthorhombic structural transition at Ts. ► Phase diagram with Ts>TN and softening of elastic constant C66 are well reproduced.