Nexus and Dirac lines in topological materials

We consider the Z₂ topology of the Dirac lines, i.e., lines of band contacts, on an example of graphite. Four lines—three with topological charge {N}₁=1 each and one with {N}₁=-1—merge together near the H-point and annihilate due to summation law 1+1+1-1=0. The merging point is similar to the real-space nexus, an analog of the Dirac monopole at which the Z₂ strings terminate.