Nexus and Dirac lines in topological materials

TL;DR

This paper explores the $Z_2$ topological properties of Dirac lines in graphite, revealing how multiple lines merge and annihilate at a nexus point, analogous to Dirac monopoles.

Contribution

It introduces the concept of a nexus point in topological materials where Dirac lines with different charges merge and annihilate, expanding understanding of topological band contacts.

Findings

Four Dirac lines in graphite with specific topological charges

Merger and annihilation of lines at the H-point

Analogy between nexus point and Dirac monopole

Abstract

We consider the $Z_2$ 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 $Z_2$ strings terminate.