Imaging the Collimation and Magnetic Focusing of Dirac Electrons

Van der Waals heterostructures have emerged as a new playground for exploring electron-optics - the solid-state analogues of optical devices, such as lenses, beam splitters and interferometers. A fundamental building block for constructing these electron-optic devices is an electron source that can provide a narrow and collimated beam. In this work, we observe that the naturally occuring P-N junction formed at a metal-graphene interface leads to a strongly collimated beam of electrons, which can be turned on/off by simply changing the doping of the graphene. We non-invasively study this collimation by imaging magnetic focusing in real-space using a scanning single-electron transistor as an electrostatic potential probe.We observe a drastic transition of the potential landscape when the graphene doping changes sign, from featureless for electron-doped (no P-N junction) to oscillating for hole-doped. Additionally, we show that there is a universal relation between the measured contact resistance of the metal-graphene interface to the degree of collimation, which should be considered in future ballistic experiments.