Topological valleytronics using kink states in bilayer graphene: a valley valve and beam splitter

Two-dimensional honeycomb lattices possess a valley degree of freedom, the control of which offers opportunities to realize topologically protected edge modes to transport electrons without dissipation, and potentially contribute to new paradigms of electronics, namely valleytronics. In this talk, I will describe how we realize valley-momentum locked topological 1D channels, i.e. quantum valley Hall kink states, at internal line boundaries of two artificial domains of Bernal stacked bilayer graphene with opposite band gaps via asymmetrical gating [1]. These kink states possess mean free path of a few µm at B=0, and their helicity (±1) can be conveniently controlled by changing the gating polarity. In an all electrically defined “cross” device with 4 channels, I will demonstrate reconfigurable ballistic waveguides of the kink states, and valleytronic operations such as valley valve and electron beam splitter that exploit topological protection [2]. Kink states can ballistically transmit through two channels with aligned helicity (on state), while get blocked between channels with opposite helicity (off state). The on/off ratio reaches 800% at B=0, and it doesn’t require valley-polarized current to operate, instead relying on the control of topology. In a B-field, Fermi energy becomes a knob to continuously tune the current splitting ratio of kink states in two propagation directions, resembling the function of a quantum point contact for quantum Hall edge states. The versatile control and potential scalability of the kink states open the door to many exciting possibilities in low dimensional topological applications.
[1] J. Li, K. Wang, K. J. McFaul, Z. Zern, Y. F. Ren, K. Watanabe, T. Taniguchi, Z. H. Qiao, J. Zhu, “Gate-controlled topological conducting channels in bilayer graphene”, Nat. Nano. 11, 1060 (2016)
[2] J. Li, R. Zhang, Z. Yin, J. Zhang, K. Watanabe, T. Taniguchi, C. Liu, J. Zhu, “A valley valve and electron beam splitter", Science 362, 1149 (2018)