Edge state in AB-stacked bilayer graphene and its correspondence with SSH ladder

TL;DR

This paper investigates edge states in AB-stacked bilayer graphene ribbons, revealing their topological nature, dependence on stacking and edge termination, and correspondence with SSH ladder models, with implications for realistic material behavior.

Contribution

It demonstrates the topological origin of edge states in bilayer graphene via SSH ladder analogy and explores effects of stacking, edge types, and interlayer interactions.

Findings

Edge states are protected by chiral symmetry and described by SSH ladder models.

Stacking configuration significantly influences the existence of edge states.

Interlayer interactions and bias affect the robustness of topological edge states.

Abstract

We study edge states in AB-stacked bilayer graphene (BLG) ribbon where the Chern number of the corresponding two-dimensional (2D) bulk Hamiltonian is zero. The existence and topological features of edge states when two layers ended with the same or different edge terminations (zigzag, bearded, armchair) are discussed. The edge states (non-dispersive bands near the Fermi level) are states localized at the edge of graphene nanoribbon that only exists in certain range of momentum $k_y$. Their existence near the Fermi level are protected by the chiral symmetry with topology well described by coupled Su-Schrieffer-Heeger (SSH) chains model, i.e., SSH ladder, based on the bulk-edge correspondence of one-dimensional (1D) systems. These zero-energy edge states can exist in the whole $k_y$ region when two layers have zigzag and bearded edges, respectively. Winding number calculation shows a topological phase transition between two distinct non-trivial topological phases when crossing the Dirac points. Interestingly, we find the stacking configuration of BLG ribbon is important since they can lead to unexpected edge states without protection from the chiral symmetry both near the Fermi level in armchair-armchair case and in the gap within bulk bands that are away from Fermi level in the general case. The influence of interlayer next nearest neighbor (NNN) interaction and interlayer bias are also discussed to fit the realistic graphene materials, which suggest the robust topological features of edge states in BLG systems.