Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci Electron correlation induced orbital selective Lifshitz transition in new hybrid 12442 iron based superconductors Abyay Ghosh a,b , Soumyadeep Ghosh a,b , Haranath Ghosh a,b a Human Resources Development Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India b Homi Bhabha National Institute, BARC training school complex, Anushakti Nagar, Mumbai 400094, India ARTICLEINFO Keywords: Fe-based 12442 materials Electronic structure calculations Fermi surface Orbital selectivity Wannier function Tight binding fitting ABSTRACT A systematic detailed electronic structure study of the newly discovered ACa 2 Fe 4 As 4 F 2 i.e., 12442 iron based compounds (where A = K, Rb, Cs) within density functional theory (DFT) based first principles calculations is presented. The orbital projected band structures reveal mixed multi-orbital multi-band nature of the 12442 compounds having contributions mostly from Fe-3d orbitals. Unlike other hybrid iron based superconductors, contribution of As-4p z orbital is negligible in these compounds. Substitution of alkali atoms of gradually higher atomic radius exert chemical pressure inside the compound and induce orbital selective evolution of band structure as well as density of states. Orbital projected Fermi surfaces divulge orbital selective modification in intra orbital, inter-band nesting condition of the d x y z 2 2 / 2 and d xz yz / orbital derived FSs. Correlation induced orbital selective Lifshitz transition is revealed through GGA+U calculations. The tight binding band structure fitted with low energy DFT bands using maximally localized Wannier functions divulge a unique feature of the nearest neighbor hopping parameter. In contrast to many iron based compounds, intra-orbital hopping in d z 2 is larger than intra-orbital hopping in d x y 2 2. Orbital selective decrease in intra-orbital hopping of d xy and d yz due to change in chemical pressure induced by substitution of alkali atoms Rb or Cs is revealed in these compounds. 1. Introduction Theresearchonironbasedsuperconductors(IBSC)hasprogresseda long way since its discovery in LaFeAsO 1-x F x [1]. Although many new families of IBSCs (e.g, 122, 111, 11, 112 etc.) with very different in- teresting properties were discovered in subsequent years [2–6], re- centlyadistinctlyinterestingtrendisemerginginthisfield.Inthequest of raising transition temperature and improving physical properties, synthesis of new hybrid family of superconductors comprising a com- bination of two structures from two different or same previously ex- isting families of IBSCs has received significant attention. For example, CaKFe 4 As 4 which is a member of 1144 family is derived from two different Fe based compounds of the same 122 family – CaFe 2 As 2 and KFe 2 As 2 [7]. Similarly other compounds from 1144 family like CaRb- Fe 4 As 4 , CaCsFe 4 As 4 , SrKFe 4 As 4 , SrRbFe 4 As 4 , SrCsFe 4 As 4 , EuKFe 4 As 4 , EuRbFe 4 As 4 , EuCsFe 4 As 4 are also the hybrid of two different com- pounds from same 122 family [8]. More recently, a copious amount of IBSCs were discovered from concoction of two different families of iron based compounds – 1111 and 122 with lattice mismatch tunning. The KCa 2 Fe 4 As 4 F 2 compound is the inter-growth of CaFeAsF and KFe 2 As 2 [9]. Soon after the discovery of KCa 2 Fe 4 As 4 F 2 , two more 12442 iron based fluoride superconductors are synthesized – RbCa 2 Fe 4 As 4 F 2 , CsCa 2 Fe 4 As 4 F 2 whicharehybridmixtureofCaFeAsFwithRbFe 2 As 2 and CaFeAsF with CsFe 2 As 2 respectively [10]. The superconducting transition temperature in KCa 2 Fe 4 As 4 F 2 , RbCa 2 Fe 4 As 4 F 2 and CsCa 2 Fe 4 As 4 F 2 was observed to be 33K, 30.5K, 28.2K respectively. The crystalstructureof12442fluoridesconsistofFe 2 As 2 double layers with alkali metal (A = K, Rb, Cs) atoms sandwiched and separated by Ca 2 F 2 layers (Fig. 1). This resembles the CuO 2 double planes in high T c cup- rate superconductors. All the 12442 iron based compounds crystallize in body centered tetragonal structure with space group I4/mmm [9,10] incontrastto1144compoundswhichcrystallizeinprimitivetetragonal structure with space group P4/mmm [7]. As atoms occupy two in- equivalentcrystallographicsitesintheFeAs-layerverysimilartothatin 1144 compounds. Thus, it is a legitimate issue to verify whether As atoms play a significant contribution to the electronic structure in these compounds like that in 1144 compounds or not. The exotic phases like spin density wave (SDW) [11,12], nematicity [13,14], tetragonal to orthorhombic structural transition [15] etc. are absent in both the hy- brid family compounds, 1144 and 12442 although they are derived from 122 compounds that exhibit various exotic phases. To explore the nature of superconducting gap in these compounds μSR experiments were performed which revealed multi-gap nodal superconductivity with (s + d)-wave nature in both KCa 2 Fe 4 As 4 F 2 and CsCa 2 Fe 4 As 4 F 2 compounds [16,17]. This is in sharp contrast to the other hybrid 1144 compounds where multi-gap nodeless superconductivity with ± s sym- metry just like that observed in 112 compounds was found from neu- tron spin resonance measurements [18,19]. However, the gap probed https://doi.org/10.1016/j.commatsci.2020.109802 Received 18 March 2020; Accepted 14 May 2020 Computational Materials Science 183 (2020) 109802 0927-0256/ © 2020 Elsevier B.V. All rights reserved. T