Quantum valley Hall effect and valleytronics in bilayer graphene

The advent of two-dimensional materials with hexagonal crystal symmetry offers a new electronic degree of freedom, i.e. valley, the manipulation of which could potentially be exploited to form new many-body ground states as well as new paradigms of electronic applications. In high-quality bilayer graphene, the application of a perpendicular electric field opens a tunable band gap, the sign of which can be reversed by reversing the polarity of the applied E-field. In analogy to the quantum spin Hall effect, valley-momentum locked quantum valley Hall kink states arise at the internal line junction of two oppositely gated bilayer graphene regions [1]. Moreover, the helicity of the kink states can take values of ± 1, by changing the polarities of the applied E-field. In this talk, I will describe our experiments in realizing the kink states, which exhibit conductance close to 4e^2/h at zero magnetic field and nice quantization at a few Teslas [2][3]. We also demonstrate, in a “cross” device consisting of four kink channels, the electrically controlled operations of a valley valve, a waveguide and a tunable electron beam splitter [3]. The on/off ratio of the valley valve is about 800% at B=0 and T=1.5K. These novel valleytronic operations exploit unique properties of the kink states. The high quality and versatile controls of this new helical 1D platform open the door to many exciting opportunities in valleytronics and topological physics.
[1] Ivar Martin, Ya. M. Blanter, and A. F. Morpurgo, Topological Confinement in Bilayer Graphene, PRL 100, 036804 (2008)
[2] J. Li, K. Wang, K. J. McFaul, Z. Zern, Y. Ren, K. Watanabe, T. Taniguchi, Z. Qiao, and J. Zhu, Gate-controlled topological conducting channels in bilayer graphene, Nature Nanotechnology 11, 1060 (2016).
[3] J. Li, R.-X. Zhang, Z. Yin, J. Zhang, K. Watanabe, T. Taniguchi, C. Liu, and J. Zhu, A valley valve and electron beam splitter in bilayer graphene, arXiv:1708.02311v1 (2017).