Figure 6: Influence of α on the electronic structure and SDW order.

(a) Effect of Δα=−1.2° on the low-energy bands along the Γ–Y momentum cut in the SDW-folded Brillouin zone. The equilibrium bands (solid lines) shift as a result of the change in the Fe–As–Fe bond angle (dashed lines). The dominant (>50%) d-orbital character for each band is colour coded, that is, d_xz (red), d_yz (green) and d_xy (blue). We choose twice the experimentally observed Δα_max to better illustrate the qualitative change. The inset shows the equilibrium Fermi surface and the locations of the Γ- and Y-points. The unshaded area represents the Q=(π, 0) SDW-folded Brillouin zone. (b,c) Enlarged portions of the squares (dashed lines) in a show the effect of Δα=−1.2° on the Fermi surface in the SDW-folded Brillouin zone. Equilibrium Fermi surface pockets (left half of each panel) shift to new positions under the change of α (right half of each panel). Improved nesting of bands with similar orbital character (d_xy) is observed at the Y-point. (d) Results of self-consistent Hartree–Fock mean-field calculations for the relative change of T_N as a function of Δα. The arrow indicates a 6.5% increase in T_N for the experimentally observed Δα_max≈−0.6°. α_tet is the Fe–As–Fe bond angle for a regular FeAs₄ tetrahedron, where superconductivity is found to be maximal in iron-based compounds^21,22. The orange line is a fit through the full data set as obtained from the mean-field calculations.