Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

Abstract

The recently discovered high-Tc superconductor Ca1−xLaxFeAs2 is a unique compound not only because of its low symmetry crystal structure, but also because of its electronic structure which hosts Dirac-like metallic bands resulting from (spacer) zig-zag As chains. We present a comprehensive first principles theoretical study of the electronic and crystal structures of Ca1−xLaxFeAs2. After discussing the connection between the crystal structure of the 112 family, which Ca1−xLaxFeAs2 is a member of, with the other known structures of Fe pnictide superconductors, we check the thermodynamic phase stability of CaFeAs2, and similar hyphothetical compounds SrFeAs2 and BaFeAs2 which, we find, are slightly higher in energy. We calculate the optical conductivity of Ca1−xLaxFeAs2 using the DFT + DMFT method, and predict a large in-plane resistivity anisotropy in the normal phase, which does not originate from electronic nematicity, but is enhanced by the electronic correlations. In particular, we predict a 0.34 eV peak in the yy component of the optical conductivity of the 30% La doped compound, which correponds to coherent interband transitions within a fast-dispersing band arising from the zig-zag As-chains which are unique to this compound. We also study the Landau free energy for Ca1−xLaxFeAs2 including the order parameter relevant for the nematic transition and find that the free energy does not have any extra terms that could induce ferro-orbital order. This explains why the presence of As chains does not broaden the nematic transition in Ca1−xLaxFeAs2.

Correlated materials design: prospects and challenges

“…This reaction is found by GGA to be CaAs + FeAs → CaFeAs 2 . The onsite Coulomb interaction U = 5 eV and Hund's coupling constant J = 0.8 eV were used in the Gutzwiller calculations 158,178 . We found that CaFeAs 2 lies 7 meV/atom above the hull.…”

“…88 La 0.12 Fe(As 0.9 Sb 0.1 ) Tc vs anion height in iron pnictide and chalcogenide superconductors. Adapted from Ref 178…”

Superconductivity of KFe2As2 Under Pressure: Ab Initio Study of Tetragonal and Collapsed Tetragonal Phases

“…1c). For known IBSC, the apparent correlation between the anion (As, Se, and others) height from the iron layer and the superconducting critical temperature was already reported [88]. Figure 4 shows the critical temperature T c for several IBSC in relation to the distance between anions and Fe layers using data presented in Ref.…”

“…Figure 4 shows the critical temperature T c for several IBSC in relation to the distance between anions and Fe layers using data presented in Ref. [88]. We supplemented this plot by the new data for K122 at zero and 30 GPa pressures (blue open circles in Fig.…”

“…Experimental data are adopted from Ref. [88] To compare the K122 compound with other IBSC, we analyzed the dependence of h As on pressure (Fig. 1c) and the experimental value of T c [23,24,[29][30][31][32].…”