Reproducibility and quantization in quantum spin Hall platforms is a persisting challenge, limiting their use in hybrid realizations of topological superconductivity. We report robust and reproducible quantized transport in a graphene quantum Hall topological insulator, stabilized at low magnetic fields by screening long-range Coulomb interactions with a metallic Bi2Se3 back gate. Beyond quantized resistance plateaus, we demonstrate mode-resolved control via gate-defined chiral-helical junctions that selectively transmit or backscatter a single helical channel, a capability inaccessible in time-reversal symmetric quantum spin Hall systems. Targeted experiments and simulations identify contact-induced doping, effectively creating unintended chiral-helical interfaces, as a generic mechanism for quantization breakdown, which is mitigated by large area contacts that enhance edge-channel equilibration. Our findings establish metal screened graphene as a gate-tunable, interaction-driven helical system with quantized transport, spatially separable helical channels, and compatibility with superconducting proximity for topological devices.
@article{arxiv.2512.13256,
title = {Chiral-helical junctions in screened graphene},
author = {Bilal Kousar and Selma Franca and David Perconte and Anton Khvalyuk and Wenmin Yang and Hadrien Vignaud and Frédéric Gay and Kenji Watanabe and Takashi Taniguchi and Clemens B. Winkelmann and Yangtao Zhou and Zheng Vitto Han and Alexandre Assouline and Jens H. Bardarson and Adolfo G. Grushin and Hermann Sellier and Benjamin Sacépé},
journal= {arXiv preprint arXiv:2512.13256},
year = {2025}
}
Comments
5 figures and 6 SI figures; fixed some typos and fig S6