  We predict and analyze a novel spin filter in semiconducting carbon
nanotubes. By using local electrostatic gates, the conduction and valence bands
can be modulated to form a double-barrier structure. The confined region below
the valence band defines a Zener quantum dot, which exhibits resonant
tunneling. The resonances split in a magnetic field to make a bipolar spin
filter for applications in spintronics and quantum information processing. We
model this using k*p envelope function theory and show that this is in
excellent agreement with a corresponding tight-binding calculation.
