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 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. The combination of nanotechnology based on carbon nanotubes (NTs) [1,2] and spintronics [3] is thought to be extremely promising for future technological innovations. Tsukagoshi et al. have demonstrated efficient spin injection from cobalt electrodes in NTs. [4] However, diffusion of magnetic atoms from the electrode into the NT results in an increase in uncontrollable spin flips due to the large spin-orbit interactions of ferromagnetic atoms. Since long spin relaxation times are needed, spin filters composed of atoms with small spin-orbit interactions are desirable. In this Letter, a spin filter composed entirely of carbon atoms, arranged as a semiconducting NT between two metallic NTs is proposed. Schottky barriers at the interfaces form a quantum well within the semiconducting NT, while the metallic NTs function as leads. [5,6] When Coulomb interactions lift spin degeneracy in the quantum