Low Magnetic Field Regime of a Gate-Defined Quantum Point Contact in High-Mobility Graphene

Louis Veyrat, Anna Jordan, Katrin Zimmermann, Frederic Gay, Kenji Watanabe, Takeshi Taniguchi, Hermann Sellier, Benjamin Sacépé

Published 2018-06-26Version 1

Gate-defined quantum point contacts (QPC's) are widely used on semiconductor heterostructures to control electronic transport at the nanoscale in two-dimensional electron gases. In graphene, however, tailoring gate-defined QPC's remains challenging due to the Dirac band structure that inevitably induces gapless, conductive npn junctions beneath gate electrodes. For narrow split-gate geometry, conductance quantization can only develop in the graphene quantum Hall regime when the broken symmetry states open a gap between electron and hole states. Yet, the transition between the low-field and high-field quantum Hall regimes in the split-gate geometry remains unexplored. Here, we report on the evolution of the coherent electronic transport through a gate-defined QPC in a high-mobility graphene device from ballistic transport to quantum Hall regime upon increasing the magnetic field. At low field, the conductance exhibits Fabry-P\'{e}rot resonances resulting from the npn cavities formed beneath the top-gated regions, with evidence of the graphene's Berry phase by magnetic steering of electron trajectories. Our findings show that, above a critical field $B^*$ corresponding to the cyclotron radius equal to the npn cavity length, Fabry-P\'{e}rot resonances vanish and snake trajectories are guided through the constriction, marking the emergence of the QPC contribution to the conductance. At higher field, transport is done by quantum Hall edge channels, with the characteristic signature of current equilibration in a QPC geometry.