  We investigate the role of pseudospin structure of few-layer black phosphorus
(BP) in interband tunneling properties in lateral BP junctions. We find that
interband tunneling is critically dependent on junction directions because of
the anisotropic pseudospin structure of BP. When the armchair direction of BP
is normal to the interface, pseudospins of incident and transmitted carriers
are nearly aligned so that interband tunneling is highly effective, analogous
to the Klein tunneling in graphene. However, when the zigzag direction is
normal to the interface, interband tunneling is suppressed by misaligned
pseudospins. We also study junctions of band-gap inverted BP where the
electronic structure is characterized by two Dirac cones. In this case,
intervalley tunneling is prohibited either by momentum conservation or by
pseudospin mismatch while intravalley tunneling is Klein-like irrespective of
the junction direction. These results provide a foundation for developing
high-performance devices from BP and other pseudospin materials.
