Second-order and real Chern topological insulator in twisted bilayer $\alpha$-graphyne

TL;DR

This paper identifies twisted bilayer $ ext{α}$-graphyne as a second-order and real Chern topological insulator with stable properties at large twisting angles, expanding topological material understanding beyond twisted bilayer graphene.

Contribution

It demonstrates that twisted bilayer $ ext{α}$-graphyne exhibits second-order and real topological phases, providing first-principles and tight-binding evidence for its topological properties and phase transitions.

Findings

Twisted bilayer $ ext{α}$-graphyne is a stable real Chern insulator at 21.78°.

The material exhibits in-gap corner states indicating second-order topology.

A topological phase transition occurs when symmetry is broken, reducing the twisting angle.

Abstract

The study of higher-order and real topological states as well as the material realization have become a research forefront of topological condensed matter physics in recent years. Twisted bilayer graphene (tbG) is proved to have higher-order and real topology. However whether this conclusion can be extended to other two-dimensional twisted bilayer carbon materials and the mechanism behind it lack explorations. In this paper, we identify the twisted bilayer $\alpha$-graphyne (tbGPY) at large twisting angle as a real Chern insulator (also known as Stiefel-Whitney insulator) and a second-order topological insulator. Our first-principles calculations suggest that the tbGPY at 21.78$^\circ$ is stable at 100 K with a larger bulk gap than the tbG. The non-trivial topological indicators, including the real Chern number and a fractional charge, and the localized in-gap corner states are demonstrated from first-principles and tight-binding calculations. Moreover, with $\mathcal C_{6z}$ symmetry, we prove the equivalence between the two indicators, and explain the existence of the corner states. To decipher the real and higher-order topology inherited from the Moir\'e heterostructure, we construct an effective four band tight-binding model capturing the topology and dispersion of the tbGPY at large twisting angle. A topological phase transition to a trivial insulator is demonstrated by breaking the $\mathcal C_{2y}$ symmetry of the effective model, which gives insights on the trivialization of the tbGPY as reducing the twisting angle to 9.43$^\circ$ suggested by our first-principles calculations.