Figure 3: The Fermi surface and superconducting gap ($Δ$) of $Ba0.6K0.4Fe2As2$ as determined from angle-resolved photoemission spectroscopy (from Ding et al. [27]). The Fermi surface is the surface of constant energy in momentum space that separates the filled from the unfilled states. The 3D plot shows the gap at 15 K as a function of the $x$ and $y$ components of the momentum ($Γ$, $M$, and $X$ label high symmetry points of the two-dimensional Brillouin zone), with the colors indicating the gap magnitude (the gap amplitude vs temperature is shown in the inset). The gap anisotropy in momentum space is weak, though the gap magnitude differs between the various Fermi surfaces ($α$, $β$, $γ$). The image at the bottom is the photoemission intensity near the Fermi energy.