  We use a cooled Scanning Probe Microscope (SPM) to electron motion in
nanoscale devices. The charged tip of the SPM is raster scanned at a constant
height above the surface as the conductance of the device is measured. The
image charge scatters electrons away, changing the path of electrons through
the sample.1-3 Using this technique, we have imaged cyclotron orbits3 for
ballistic hBN-graphene-hBN devices that flow between two narrow contacts in the
magnetic focusing regime. Here we present an analysis of our magnetic focusing
imaging results based on the effects of the tip-created charge density dip on
the motion of ballistic electrons. The density dip locally reduces the Fermi
energy, creating a force that pushes electrons away from the tip. When the tip
is above the cyclotron orbit, electrons are deflected away from the receiving
contact, creating an image by reducing the transmission between contacts. The
data and our analysis suggest that graphene edge is rather rough, and electrons
scattering off the edge bounce in random directions. However, when the tip is
close to the edge it can enhance transmission by bouncing electrons away from
the edge, toward the receiving contact. Our results demonstrate that a cooled
SPM is a promising tool to investigate the motion of electrons in ballistic
graphene devices.
