Ballistic Electron Source with Magnetically Controlled Valley Polarization in Bilayer Graphene

The achievement of valley-polarized electron currents is a cornerstone for the realization of valleytronic devices. Here, we report on ballistic coherent transport experiments where two opposite quantum point contacts (QPCs) are defined by electrostatic gating in a bilayer graphene (BLG) channel. By steering the ballistic currents with an out-of-plane magnetic field we observe two current jets, a consequence of valley-dependent trigonal warping. Tuning the BLG carrier density and number of QPC modes (m) with a gate voltage we find that the two jets are present for m=1 and up to m=6, indicating the robustness of the effect. Semiclassical simulations confirm the origin of the signals by quantitatively reproducing the jet separations without fitting parameters. In addition, our model shows that the ballistic current jets have opposite valley polarization. As a consequence, by steering each jet toward the detector using a magnetic field, we achieve full control over the valley polarization of the collected currents, envisioning such devices as ballistic current sources with tunable valley polarization. We also show that collimation experiments are a sensitive probe to the trigonal warping of the Fermi surface.