  The relativistic nature of charge carriers in graphene is expected to lead to
an angle- dependent transmission through a potential barrier, where Klein
tunneling involves annihilation of an electron and a hole at the edges of the
barrier. The signatures of Klein tunneling have been observed in gated graphene
devices, but the angle dependence of the transmission probability has not been
directly observed. Here we show measurements of the angle-dependent
transmission through quasi-ballistic graphene heterojunctions with straight and
angled leads, in which the barrier height is controlled by a shared gate
electrode. Using a balanced differential measurement technique, we isolate the
angle-dependent contribution to the resistance from other angle-insensitive,
gate-dependent and device-dependent effects. We find large oscillations in the
transmission as a function of the barrier height in the case of Klein tunneling
at a 45 deg angle, as compared to normal incidence. Our results are consistent
with the model that predicts oscillations of the transmission probability due
to interference of chiral carriers in a ballistic barrier. The observed angle
dependence is the key element behind focusing of electrons and the realization
of a Veselago lens in graphene.
