Orbital Order, Structural Transition, and Superconductivity in Iron Pnictides

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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 antiferromagnetic 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 $s_{\pm}$-wave due to the magnetic fluctuation for a small $g$ case with $T_{Q} \leq T_{N}$. The former case is consistent with the phase diagram of doped iron pnictides with $T_{s}>T_{N}$.
2010-12-15