Theoretical studies of spin-dependent electrical transport through carbon nanotubes

Spin-dependent coherent quantum transport through carbon nanotubes (CNT) is studied theoretically within a tight-binding model and the Green function partitioning technique. End-contacted metal/nanotube/metal systems are modelled and next studied in the magnetic context, i.e. either with ferromagnetic electrodes or in external magnetic fields. The former case shows that quite a substantial giant magnetoresistance (GMR) effect occurs (±20%) for disorder-free CNTs. Anderson-disorder averaged GMR, in turn, is positive and reduced down to several per cent in the vicinity of the charge neutrality point. In parallel magnetic fields, characteristic Aharonov–Bohm-type oscillations are revealed with pronounced features due to a combined effect of: length-to-perimeter ratio, unintentional electrode-induced doping, Zeeman splitting and energy-level broadening. In particular, a CNT is predicted to lose its ability to serve as a magneto-electrical switch when its length and perimeter become comparable. In the case of perpendicular geometry, there are conductance oscillations approaching asymptotically the upper theoretical limit to the conductance, 4e2/h. Moreover in the ballistic transport regime, initially the conductance increases only slightly with the magnetic field or remains nearly constant because spin up- and spin down-contributions to the total magnetoresistance partially compensate each other.