High temperature quantum valley Hall effect with quantized resistance in bilayer graphene

The resistance quantization is the hallmark of a two-dimensional topological insulator. However, realizing ballistic transport at zero magnetic field in helical edge states that preserve time reversal symmetry remains challenging due to the spatial overlap of the counter-propagating electrons. In this talk, I’ll present our recent efforts in achieving resistance quantization of helical edge states, known as the kink states, in a dual-split-gate-defined junction in Bernal-stacked bilayer graphene (Huang et al., Science 385, 657 (2024)). The quantization at h/4e² is robust across a wide range of gate voltage, dc bias, and in different devices, and is weakly temperature dependent, only a 1% change at 12 K and a 3% change at 46 K. I’ll also show our demonstration of a topological switch, which is controlled by turning on and off the kink states. At last, I’ll talk about the lifting of the four-fold degeneracy of the kink states in the quantum Hall regime by unbalancing the electric displacement fields on the two sides of the junction. A single-pair spin-momentum-locking helical edge state is created, pointing to a new way of realizing synthetic helical edge states at integer and fractional quantum Hall states. Our results establish the kink states as a versatile platform for both applications such as quantum interconnection networks and fundamental physics exploration such as topological superconductivity.