Resonant spin current in nanotube double junctions

Zero-bias conductance per spin of nanotube double junction (NTDJ) is investigated theoretically using the tight-binding model, unrestricted Hartree-Fock approximation, and nonequilibrium 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 five and seven 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.