Ab initio lattice dynamics simulations and inelastic neutron scattering spectra for studying phonons in BaFe₂As₂: Effect of structural phase transition, structural relaxation, and magnetic ordering

We have performed extensive ab initio calculations to investigate phonon dynamics and their possible role in superconductivity in BaFe₂As₂ and related systems. The calculations are compared to inelastic neutron scattering data that offer improved resolution over published data [ Mittal et al. Phys. Rev. B 78 104514 (2008)], in particular at low frequencies. Effects of structural phase transition and full and/or partial structural relaxations, with and without magnetic ordering, on the calculated vibrational density of states are reported. Phonons are best reproduced using either the relaxed magnetic structures or the experimental cell. Several phonon branches are affected by the subtle structural changes associated with the transition from the tetragonal to the orthorhombic phase. Effects of phonon-induced distortions on the electronic and spin structure have been investigated. It is found that for some vibrational modes, there is a significant change in the electronic distribution and spin populations around the Fermi level. A peak at 20 meV in the experimental data falls into the pseudogap region of the calculation. This was also the case reported in our recent work combined with an empirical parametric calculation [ Mittal et al. Phys. Rev. B 78 104514 (2008)]. The combined evidence for the coupling of electronic and spin degrees of freedom with phonons is relevant to the current interest in superconductivity in BaFe₂As₂ and related systems.