Pressure induced Lifshitz transition and anomalous crystal field splitting in AFeAs (A = Li/Na) Fe-based superconducting compounds: A first principles study

Abstract

The effect of hydrostatic pressure on the electronic structures of iron pnictide superconducting compounds LiFeAs and NaFeAs through density functional theory based first principles studies are presented, using the predicted crystal structures at very high pressure by Zhang et al. (2012). The orbital selective pressure induced modifications in the partial density of states and electronic band structures reveal mixed multi-band multi-orbital nature of these compounds, with energetically degenerate dxz/dyz orbitals at ambient pressure. Due to larger hydrostatic pressures some of the electron/hole bands crosses the Fermi level, leading to significant topological modifications in Fermi surfaces known as Lifshitz transition. Interrelation between Lifshitz transition and superconductivity—an important subject matter of current research are described. Based on such interconnection the current study predicts the superconducting-Tc in 111 compounds cannot be raised at such high pressures. Wannier functions based electronic structure investigation reveal a relatively larger hybridization between the Fe-3d and As-4p orbitals at higher pressures, as an effect of which the orbital degeneracy of dxz and dyz orbitals are lifted. Different hybridization contributions in the crystal-field splitting are separated using the Wannier functions based formalism, by incorporating different bands with a particular orbital character in the Wannier function construction. Pressure dependence of the intra/inter orbital hopping amplitudes between Fe-d orbitals has been discussed using low energy tight binding model.