Counter-propagating broken-symmetry quantum Hall edge states in a bilayer graphene line junction

Helical edge states are excellent platforms to advance 1D physics and quantum information science. The coupling of helical edge states to s-wave superconductors, for instance, may lead to new forms of superconductivity and excitations. Past efforts have explored natural helical edge states in quantum spin Hall insulators and synthetic systems created using chiral edge states in e.g. electron-hole bilayers. In this talk, we demonstrate a new approach to engineering synthetic helical edge states using a dual-split-gate-defined line junction in bilayer graphene (BLG). Using this structure, we have previously demonstrated the precise quantization of the quantum valley Hall effect at zero magnetic field (Huang et al., Science 385, 657 (2024)). In a magnetic field, the bulk BLG bands evolve into broken-symmetry Landau levels. By controlling the LL ordering on the two sides of the junction using a D-field, we lift the four-fold (spin and layer) degeneracy of the zero-field quantum valley Hall kink states, achieving additional conductance plateaus at 1, 2, 3 e²/h. The 1 e²/h plateau corresponds to a single pair of counter-propagating edge states with opposite spins inside the 70nm wide junction. Our experiment points to a promising direction of realizing synthetic helical edge states at integers and fractions.