Figure 2: SPT phase in 3D with
symmetry.
From: Symmetry-protected topological phases from decorated domain walls

Red (blue) surfaces represent domain walls of the Z2 (
) symmetry. They intersect along curves (black lines). The ground state wave function is a superposition of all domain wall configurations that differ by a sign depending on whether there is an even or odd number of intersection curves. This automatically implies a protected edge state when a domain wall intersects the surface of the sample (grey dashed line).
symmetry.