Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1-x LaxFeAs2

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) zigzag 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 corresponds to coherent interband transitions within a fast-dispersing band arising from the zigzag 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.