Illustration: Alan Stonebraker

Figure 1: Schematic representation of the variation of the energy gap (order parameter) plotted as function of the local angle circling the hole ($α$) and electron ($β$) Fermi surfaces of the material. Dashed line represents $Δ=0$. (a) Isotropic $s±$ state. (b) Anisotropic $s±$ state with nodes on $β$ pockets. (c) Same as (b) but for a state with marginal (“kissing”) nodes. (d) Same as (b) but for deep gap minima.