  Zero bias conductance per spin of nanotube double junction (NTDJ) is
investigated theoretically using the tight binding model, unrestricted
Hartree-Fock approximation and non-equilibrium Green's functions.
  NTDJ consists of two metallic nanotubes joined by a piece of semiconducting
nanotube, with the transition between the nanotubes made up of sets of 5 and 7
member carbon rings.
  A quantum well forms in the central semiconducting NT region, bounded by
Schottky barriers.
  Spin current occurs when Coulomb interactions raise the spin degeneracy of
resonant levels in the quantum well.
  As long as an appropriate semiconducting NT length is chosen, spin direction
can be controlled by gate voltage, i.e., NTDJ functions as a nano spin filter.
