Speaker
Description
In the quantum Hall (QH) regime, charge carriers flow in one dimensional quantum Hall edge channels (QHECs) and are topologically protected against backscattering. Due to its intrinsic two-dimensionality and the massless nature of its charge carriers, graphene constitutes a fascinating platform for studying QH effect. However, graphene edges are known to strongly harm the topological protection of QHECs. Local probe measurements have recently helped understand why. They highlighted that both up- and downstream QHECs coexist along the same device edge and that impurities, located between these counterpropagating QHECs, couple them and allow backscattering of charge carriers, normally prohibited. Nevertheless, the origin of counterpropagating QHECs remains largely debated and it is unclear whether they exist both for holes and electrons.
To answer these open questions, we performed scanning gate microscopy (SGM) in the quantum Hall regime on a graphene constriction. SGM, consisting in scanning a biased AFM tip, used as a local top gate, while recording the device resistance, gives access to the locations where backscattering occurs.
Our results suggest different mechanisms of topological breakdown for holes and electrons. While backscattering occurs along the edges for holes, in line with previous studies, it takes place in the vicinity of the constriction for electrons.
Supported by simulation results, we find that counterpropagating QHECs exist for both types of carriers but the metallic contacts cause the observed contrasting behaviour. By inducing a local doping in their vicinity, contacts lead to a decoupling between counterpropagating holes QHECs while they cause a perfect equilibration of electrons QHECs. Our results highlight the importance of contacts when designing a QHECs-based device and apply to other two-dimensionnal materials, especially for those presenting topological properties.
| Speaker Country | Belgium |
|---|