  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. [1]. 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 can not 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.
