Second-Order Real Nodal-Line Semimetal in Three-Dimensional Graphdiyne

TL;DR

This paper identifies 3D graphdiyne as the first realistic material exhibiting second-order real nodal-line semimetal behavior, with unique topological boundary modes protected by multiple topological charges.

Contribution

It demonstrates that experimentally synthesized 3D graphdiyne is a second-order real nodal-line semimetal with distinct topological boundary modes, supported by first-principles calculations and theoretical analysis.

Findings

Presence of real nodal rings protected by topological charges

Distinct boundary modes including hinge Fermi arcs and double drumhead surface bands

Potential topological transition to a 3D real Chern insulator

Abstract

Real topological phases featuring real Chern numbers and second-order boundary modes have been a focus of current research, but finding their material realization remains a challenge. Here, based on first-principles calculations and theoretical analysis, we reveal the already experimentally synthesized three-dimensional (3D) graphdiyne as the first realistic example of the recently proposed second-order real nodal-line semimetal. We show that the material hosts a pair of real nodal rings, each protected by two topological charges: a real Chern number and a 1D winding number. The two charges generate distinct topological boundary modes at distinct boundaries. The real Chern number leads to a pair of hinge Fermi arcs, whereas the winding number protects a double drumhead surface bands. We develop a low-energy model for 3D graphdiyne which captures the essential topological physics. Experimental aspects and possible topological transition to a 3D real Chern insulator phase are discussed.