Electronic structure, magnetic order and Lifshitz transition in electron doped new structure 12442 type Fe-based superconductors

We investigate in detail the electronic structure of very recently discovered electron doped 12442-type iron based BaTh₂ Fe ₄ As₄(N₁ −_x O_x )₂ compounds (_x = 0.1–0.6), using Density Functional Theory (DFT) based first principles calculations. Unlike in many iron-based superconducting compounds, absence of As-4p_Z orbital character, and dominant contributions from Fe-3d orbitals close to the Fermi level are noted. Formation energy calculations indicate x = 0.3 structure is the most stable which is consistent with experimental observation. Incorporating magnetic order at Fe sites in compound x = 0.3, our calculations predict stripe AFM as ground state magnetic structure. Calculated Arsenic height from the Fe square plane (h_As) for striped AFM configuration agrees well with the experiment. An important outcome of this calculation is the prediction of occurrence of Lifshitz transition at the same doping level (_x = 0.6) for which the highest critical temperature is reported in literature. Detailed analysis of electronic structure results like estimation of radii of various Fermi pockets, orbital characters of Fermi surfaces lead to prediction of mixed _d and _s pairing symmetry.