# Two-dimensional second-order topological insulator in graphdiyne

## TL;DR

This paper identifies graphdiyne as the first realistic two-dimensional second-order topological insulator with protected corner states, revealing its hidden topological nature through combined computational and theoretical analysis.

## Contribution

It demonstrates that the experimentally synthesized 2D material graphdiyne is a 2D SOTI with protected corner states, expanding the material platform for higher-order topological phases.

## Key findings

- Graphdiyne hosts topologically protected 0D corner states.
- Crystalline symmetry plays a crucial role in the topological protection.
- The topological features are robust against symmetry-breaking perturbations.

## Abstract

A second-order topological insulator (SOTI) in $d$ spatial dimensions features topologically protected gapless states at its $(d-2)$-dimensional boundary at the intersection of two crystal faces, but is gapped otherwise. As a novel topological state, it has been attracting great interest, but it remains a challenge to identify a realistic SOTI material in two dimensions (2D). Here, based on combined first-principles calculations and theoretical analysis, we reveal the already experimentally synthesized 2D material graphdiyne as the first realistic example of a 2D SOTI, with topologically protected 0D corner states. The role of crystalline symmetry, the robustness against symmetry-breaking, and the possible experimental characterization are discussed. Our results uncover a hidden topological character of graphdiyne and promote it as a concrete material platform for exploring the intriguing physics of higher-order topological phases.