Crystal structures of some of iron-based superconductors, after Ref. 8.
(a) FeAs lattice indicating As above and below the Fe plane. Dashed green and solid blue squares indicate 1- and 2-Fe unit cells, respectively. (b) Schematic 2D Fermi surface in the 1-Fe BZ whose boundaries are indicated by a green dashed square. (c) Fermi sheets in the folded BZ whose boundaries are now shown by a solid blue square. After Ref. 11.
Examples of the FeSC phase diagrams: a schematic one,12 and the diagrams measured for (Ba1−x K x )Fe2As2,9 Ba(Fe1− x Co x )2As2,19 La(O1− x F x )FeAs,20 Fe1+ y Se x Te1− x ,21 BaFe2(As1− x P x )2.22
Fermi surface (FS) maps measured by ARPES for LiFeAs (Ref. 62) (left) and optimally doped Ba1− x K x Fe2As2 (BKFA).40
In-plane magnetic structure common for the 1111 and 122 parent compounds (left) and for parent 11 compound (FeTe, right). The shaded areas indicate the magnetic unit cells. After Ref. 94.
Superconducting gap symmetry in LiFeAs. Experimental Fermi surface (left). The experimental dispersions (center) measured along the cuts A and B. A sketch of the distribution of the superconducting gap magnitude over the Fermi surfaces (right). After Ref. 11.
Three-dimensional distribution of the superconducting gap and orbital composition of the electronic states at the Fermi level of Ba1− x K x Fe2As2 (BKFA). (a) Distribution of the superconducting gap (plotted as height) and distribution of the orbital composition for the states at the Fermi level (shown in color: dxz , yz —red, dxy —green, dxz , yz with admixture of other orbitals—orange) as function of kx and ky at constant kz = 0; (b) the same, only for kz = π; (c) the same distributions as functions of in-plane momentum, directed along BZ diagonal, and kz . Note the unambiguous correlation between the color and height, i.e., there is strong correlation between the orbital composition and superconducting gap magnitude. From Ref. 146.
Electronic band structure of LiFeAs (a)–(c), a representative 111 compound, and BaFe2As2 (BFA)/Ba0.6K0.4Fe2As2 (BKFA) (d)–(f), the parent/optimally doped 122 compound: the electronic bands, calculated (a), (d) and derived from ARPES data (b), (e), and the Fermi surfaces of LiFeAs (c) and BKFA (f), as seen by ARPES. The bands and FS contours are colored according to their most pronounced orbital character: Fe 3dxy , 3dxz , and 3dyz .
Phase diagram of the 122 family of ferro-pnictides complemented by the 122(Se) family as a generalized band structure-driven diagram for the iron-based superconductors. The insets show that the Fermi surfaces for every compound close to Tc max are in the proximity of Lifshitz topological transitions: the corresponding FS sheets are indicated in color (blue for hole- and red for electron-like).
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