  Massless Dirac fermions in graphene provide unprecedented opportunities to
realize the Klein paradox, which is one of the most exotic and striking
properties of relativistic particles. In the seminal theoretical work
[Katsnelson et al., Nat. Phys. 2 620 (2006)], it was predicted that the
massless Dirac fermions can pass through one-dimensional (1D) potential
barriers unimpededly at normal incidence. Such a result seems to preclude
confinement of the massless Dirac fermions in graphene by using 1D potential
barriers. Here, we demonstrate, both experimentally and theoretically, that
massless Dirac fermions can be trapped in quasi-1D npn junction of a continuous
graphene monolayer. Because of highly anisotropic transmission of the massless
Dirac fermions at n-p junction boundaries (the so-called Klein tunneling in
graphene), charge carries incident at large oblique angles will be reflected
from one edge of the junction with high probability and continue to bounce from
the opposite edge. Consequently, these electrons are trapped for a finite time
to form quasi-bound states in the quasi-1D npn junction. The quasi-bound states
seen as pronounced resonances are probed and the quantum interference patterns
arising from these states are directly visualized in our scanning tunneling
microscope measurements.
