Second-order topology in two-dimensional azulenoid kekulene carbon lattices

TL;DR

This paper demonstrates the existence of higher-order topological insulator phases in 2D azulenoid-kekulene carbon lattices, revealing robust corner states and topological invariants through first-principles calculations.

Contribution

It introduces a new class of organic 2D carbon materials exhibiting higher-order topological phases confirmed by topological invariants and corner charges.

Findings

Confirmed HOTI phase with fractional corner charge of e/3

Identified robust corner states in nanoflakes

Structural modifications preserve topological corner states

Abstract

The discovery of higher-order topological insulator (HOTI) has established a new paradigm for understanding symmetry-constrained boundary electronic states. Here, based on first-principles calculations, we demonstrate the emergence of HOTI phase in organic lattices of two-dimensional azulenoid-kekulene-type carbon allotropes, namely AKC-[3,3] and AKC-[6,0]. Enabled by the $C_6$ rotational symmetry, the nontrivial bulk topology is confirmed through the topological invariant and fractionally quantized corner charge, giving $\{[M^{(I)}_{2}],[K^{(3)}_{2}]\}$ = $\{0,2\}$ and $Q_{\mathrm{corner}} = e/3$, respectively, accompanied by the emergence of exotic corner states in nanoflakes. Notably, the structural modifications are explored, revealing that in the derived structure PAK-[6,0], whose corner-localized states are preserved, highlighting the robustness of the higher-order topological phase. These findings highlight azulenoid-kekulene-based carbon allotropes as a promising platform to explore the interplay between structural design, crystalline symmetry, and higher-order topological boundary responses in two dimensional carbon systems.