Nexus and Dirac lines in topological materials

T. T. Heikkila, G. E. Volovik

Published 2015-05-13Version 1

We consider the $Z_2$ topology of the Dirac lines -- lines of band contacts -- on 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, the analog of Dirac monopole at which the $Z_2$ strings terminate.