Quantum valley Hall effect with quantized conductance in bilayer graphene kink states

Ballistic edge states arising from the bulk-edge correspondence of a topological insulator are interesting one-dimension systems with potential applications in quantum electronics. Helical edge states hosted by the quantum spin Hall effect and quantum valley Hall effect preserve the time reversal symmetry and enable the construction of novel state of matter such as topological superconductivity. Previous work has demonstrated the existence of quantum valley Hall kink states at the electrically created line junction of two oppositely biased bilayer graphene regions and the operations of a valley valve, waveguide and tunable beam splitter [1][2]. However, the resistance of the kink states was not quantized to the expected value of h/4e² at zero magnetic field. By improving device quality, here we show the attainment of very flat resistance plateaus to within 50omega of the expected value. The quantization is robust in a wide range of temperatures (20mK-10 K), Fermi energies, and dc biases. The gate-defined nature of the kink states enables in situ electrical operations. We demonstrate the on/off switching of the kink states with the speed of a few ms and an on/off ratio of greater than 100.

[1] Li. et al., Nat. Nano. 11, 1060 (2016), [2] Li et al., Science 362, 1149 (2018)