Proposal for realizing the quantum spin Hall phase in a gapped graphene bilayer

Quantum spin Hall (QSH) insulators with gapless edge states have potential applications in designing low-dissipation devices. In spite of many predictions, to verify the QSH phase in graphene layered materials experimentally is still difficult due to the obstacle in achieving spin-orbit coupling strong enough. We propose a Rashba system of graphene bilayer gapped by dielectric layers and show it can host a valley-polarized QSH phase even when the Rashba interaction approached zero. Such a system exhibits asymmetric topological quantum phase transitions under opposite interlayer biases, due to the dielectric-potential induced inversion asymmetry in the absence of interlayer bias. Specifically, the quantum valley Hall phase exists in zigzag-edged nanoribbons under the bias in one direction but is absent under the reverse bias. These topological phenomena can be well understood by the competition among the dielectric-induced potential, Rashba interaction, and interlayer bias in modulating the bulk band gap. Moreover, the phase diagram is given and the corresponding phase boundaries are derived analytically. Our findings provide a possible way to detect the QSH-related asymmetric topological quantum phenomena in graphene bilayer based on the current experimental technology.