Two-Dimensional Topological Insulator State and Topological Phase Transition in Bilayer Graphene

TL;DR

This paper demonstrates that gated bilayer graphene can host a strong topological insulator phase with unique edge modes, transitioning from a quantum valley Hall insulator as Rashba spin-orbit coupling increases, and remains robust under certain conditions.

Contribution

It reveals the conditions for topological phase transition in bilayer graphene and characterizes the novel edge modes with combined spin and valley filtering.

Findings

Gated bilayer graphene hosts a strong TI phase with Rashba SO coupling.

Transition from quantum valley Hall to strong TI occurs when Rashba coupling exceeds a threshold.

Edge modes exhibit both spin and valley filtering, combining properties of quantum spin and valley Hall insulators.

Abstract

We show that gated bilayer graphene hosts a strong topological insulator (TI) phase in the presence of Rashba spin-orbit (SO) coupling. We find that gated bilayer graphene under preserved time-reversal symmetry is a quantum valley Hall insulator for small Rashba SO coupling $\lambda_{\mathrm{R}}$, and transitions to a strong TI when $\lambda_{\mathrm{R}} > \sqrt{U^2+t_\bot^2}$, where $U$ and $t_\bot$ are respectively the interlayer potential and tunneling energy. Different from a conventional quantum spin Hall state, the edge modes of our strong TI phase exhibit both spin and valley filtering, and thus share the properties of both quantum spin Hall and quantum valley Hall insulators. The strong TI phase remains robust in the presence of weak graphene intrinsic SO coupling.