Orbital Order, Structural Transition and Superconductivity in Iron Pnictides

TL;DR

This study models iron pnictide superconductors to understand how orbital order, structural transitions, and superconductivity are interconnected, highlighting the roles of electron-phonon coupling and fluctuations.

Contribution

It introduces a 16-band d-p model incorporating electron-phonon interactions to explain structural and magnetic transitions and their relation to superconductivity in iron pnictides.

Findings

Ferro-orbital order induces structural transition at TQ.

Superconductivity can be s++ or s+- wave depending on the dominant fluctuation.

Phase diagram aligns with experimental observations of doped iron pnictides.

Abstract

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.