# Higher-Order Topological Insulator in Twisted Bilayer Graphene

## TL;DR

This paper proposes that twisted bilayer graphene at large angles can act as a higher-order topological insulator, exhibiting robust corner states due to bulk topology and symmetry considerations.

## Contribution

It introduces twisted bilayer graphene as a new material platform for higher-order topological insulators with angle-independent corner states.

## Key findings

- Corner states occur at half filling in twisted bilayer graphene.
- Bulk gap opens due to intervalley scattering at large angles.
- Corner states are robust as long as symmetries are preserved.

## Abstract

Higher-order topological insulators are newly proposed topological phases of matter, whose bulk topology manifests as localized modes at two- or higher-dimensional lower boundaries. In this work, we propose the twisted bilayer graphenes with large angles as higher-order topological insulators, hosting topological corner charges. At large commensurate angles, the intervalley scattering opens up the bulk gap and the corner states occur at half filling. Based on both first-principles calculations and analytic analysis, we show the striking results that the emergence of the corner states do not depend on the choice of the specific angles as long as the underlying symmetries are intact. Our results show that the twisted bilayer graphene can serve as a robust candidate material of two-dimensional higher-order topological insulator.