Abstract
We investigate the electronic structures of iron-based superconductors having perovskite-like blocking layers, %Ca$_4$Al$_2$O$_6$Fe$_2$(As$_{1-x}$P$_x$)$_2$ from first principles. Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ and Ca$_4$Al$_2$O$_6$Fe$_2$As$_2$ from first principles. Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ is found to have two hole-like Fermi surfaces around $\Gamma$, and one hole-like Fermi surface around M in the unfolded Brillouin zone. This is in contrast with LaFePO, where no Fermi surface is found around M. The relationship of their band structures and measured transition temperatures of superconductivity is discussed. The number of Fermi surfaces in Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ is also different from that of Ca$_4$Al$_2$O$_6$Fe$_2$As$_2$, in which only one Fermi surface is formed around $\Gamma$. Analysis using maximally localized Wannier functions clarifies that the differences between their band structures originate mainly from the pnictogen height. We then analyze the alloying effect on the electronic structure of Ca$_4$Al$_2$O$_6$Fe$_2$AsP. It is found that its electronic structure is similar to that of Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ and Ca$_4$Al$_2$O$_6$Fe$_2$As$_2$ with the average crystal structure, though Ca$_4$Al$_2$O$_6$Fe$_2$AsP contains the pnictogen height disorder. We calculate the generalized susceptibility for Ca$_4$Al$_2$O$_6$Fe$_2$(As$_{1-x}$P$_x$)$_2$ and clarify the factors determining its tendency.
Highlights
Since the discovery of iron-based superconductors in LaFePO with its transition temperature Tc of 5 K1) and in LaFeAsO with Tc 1⁄4 26 K,2) many attempts to clarify the material properties and to realize higher Tc have been continued, and Fe-based superconductors with various chemical formulae and crystal structures have been reported, among which SmFeAsO has shown the highest Tc of 55 K to date is realized.3)An Fe-based superconductor comprises two-dimensional iron (Fe)-anion (A) layers
We demonstrate that electronic structure calculation for Ca4 Al2 O6 Fe2 AsP corroborates this idea by unfolding its band structure
We found that the systems of the same crystal structure but different chemical compositions give rise to similar band structures
Summary
Since the discovery of iron-based superconductors in LaFePO with its transition temperature Tc of 5 K1) and in LaFeAsO with Tc 1⁄4 26 K,2) many attempts to clarify the material properties and to realize higher Tc have been continued, and Fe-based superconductors with various chemical formulae and crystal structures have been reported, among which SmFeAsO has shown the highest Tc of 55 K to date is realized.). It is important to investigate the electronic properties of such systems with the crystal structure and/or formula varied, focusing on the behavior of Fermi surfaces and electronic orbitals in the vicinity of the Fermi level. We extract the transfer integrals by constructing maximally localized Wannier functions,26) which provide a picture of localized electronic orbitals in solid. The results of this analysis suggest that the band structures of Ca4 Al2 O6 Fe2 (As1x Px )[2] are determined mainly by crystal structure, 014701-1. The electronic band structure in the vicinity of the Fermi level is analyzed in detail using the maximally localized Wannier function technique.). Its expression for a system with time reversal symmetry is given by
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