Momentum-space structure of surface states in a topological semimetal with a nexus point of Dirac lines

TL;DR

This paper explores a complex topological defect in momentum space where multiple Dirac lines connect, forming a nexus that influences surface states in a topological semimetal with implications for surface electronic properties.

Contribution

It introduces a model of interconnected Dirac lines forming a nexus in a topological semimetal, highlighting the role of mirror symmetries and their impact on surface states.

Findings

Identification of a nexus of Dirac lines in a topological semimetal

Demonstration of transition from type I to type II Dirac lines near the nexus

Calculation of drumhead surface states affected by the nexus

Abstract

Three-dimensional topological semimetals come in different variants, either containing Weyl points or Dirac lines. Here we describe a more complicated momentum-space topological defect where several separate Dirac lines connect with each other, forming a momentum-space equivalent of the real-space nexus considered before for Helium-3. Close to the nexus the Dirac lines exhibit a transition from type I to type II lines. We consider a general model of stacked honeycomb lattices with the symmetry of Bernal (AB) stacked graphite and show that the structural mirror symmetries in such systems protect the presence of the Dirac lines, and also naturally lead to the formation of the nexus. By the bulk-boundary correspondence of topological media, the presence of Dirac lines lead to the formation of drumhead surface states at the side surfaces of the system. We calculate the surface state spectrum, and especially illustrate the effect of the nexus on these states.