Anomalous electron-phonon interaction in doped LaFeAsO: First-principles calculations

We present first-principles calculations of the atomic and electronic structure of electron-doped LaOFeAs. We find that whereas the undoped compound has an antiferromagnetic arrangement of magnetic moments at the Fe atoms, the doped system becomes nonmagnetic at a critical electron concentration. We have studied the electron-phonon interaction in the doped paramagnetic phase. For the A(1g) phonon, the separation between the As and Fe planes induces a noncollinear arrangement of the Fe magnetic moments. This arrangement is antiparallel for interactions mediated by As and perpendicular for Fe-Fe direct interactions, thus avoiding frustration. This coupling of magnetism with vibrations induces anharmonicities and an electron-phonon interaction much larger than in the pure paramagnetic case. We propose that such enhanced interactions play an essential role in superconducting compounds close to an atiferromagnetic phase transition.