Lifshitz transition in new doped 112 superconductors

Abstract

A detailed density functional theory based first principles electronic structure calculations on Ca1−xLaxFe1−yTMy(As1−pSbp)2 for various values of x, y, p and transition metals (TM = Co, Ni, Mn) are presented. We examine the possibility of Lifshitz like electronic topological transition in a variety of recently discovered doped 112 materials and demonstrate its possible connection with the occurrence of superconductivity in these materials. We show that La doping at Ca site, Co or Ni doping at Fe site causes electron doping whereas Mn doping at Fe site causes hole doping. As the level of electron doping is enhanced either through La or TM (= Ni, Co) doping the hole bands drift below the Fermi level causing Lifshitz transition. At the verge of Lifshitz transition, a merger of top of hole bands in the close vicinity of Fermi level i.e, singular band structure are found that occurs at around optimal doping. We show that the hole bands move from above the Fermi level to below it with La doping, and at x = 0.25, all the hole bands move below the Fermi level causing a topological modification in the hole Fermi surfaces, creating disconnected Fermi surfaces at the Γ point where superconductivity is predicted to cease - this is in agreement with experimental observations. We also predict occurrence of Lifshitz transition in more recently discovered Ca1−xLaxFe1−yNiy As2 for various values of doping concentration x and y. In particular for x = 0.18, 0.24 and y = 0.005, 0.010, 0.015, 0.020, the system is on the verge of Lifshitz transition. While the results of Co doping is similar to that of Ni doping, Mn doping has adverse effect on superconductivity causing its absence. Unique modifications in the electronic structures compared to other family of Fe based superconductors are obtained through Sb doping at As belonging to FeAs layers as well as that belonging to chains in between the layers. While Sb doping at As of FeAs layer causes enhancement of electronic Fermi arcs around the high symmetry X point of the Brillouin zone, the same on the chain As atom suppresses its formation at X. Contribution of chain As atoms and their role in Lifshitz transition are also demonstrated. Enhanced contribution to density of states from chain atoms in comparison to that of the layers signify possible role of chain atoms in contributing to superconductivity in these materials.