1144 Fe based superconductors: natural example of orbital selective self-doping and chemical pressure induced Lifshitz transition

Abstract

A systematic density functional theory based first principles electronic structure studies on AeAFe4As4 known as, 1144 iron based superconductors (IBSC) for various Ae = Ca, Sr, Eu and different alkali metals A = K, Rb, Cs are presented. Our study reveals multi-orbital derived multi-band nature of the electronic structure as well as Fermi surfaces for all the compounds. In contrast to the electronic structure of most of the Fe-based superconductors, significant contribution from As-4pz and its mixing with Fe-3d is found at the Fermi level. A unique feature of the electronic structure of these compounds reveal orbital selective electron/hole self-doping. This is due to natural chemical pressure of larger/smaller atomic (Ae/A) sizes of various members of 1144 family. The different chemical potential (self-doping) of different orbital derived electron/hole bands causes crossing of the Fermi level for some bands, leading to orbital selective chemical pressure induced topological Lifshitz transition. This natural pressure induced orbital selective Lifshitz transition of hole bands is a unique characteristic of this family of iron based superconductors. Our conclusions remain robust even in presence of moderate electron correlation and spin-orbit coupling. Different combinations of Ae and A in different 1144 materials causes different bandwidths to different orbital selective bands; bandwidths of different bands that crosses Fermi level around Γ-point are evaluated. A large value of bandwidths for most of the compounds including EuAFe4As4 indicates weak correlation effect in these compounds.