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Exchange effects in spin polarized transport through carbon nanotube quantum dots

We investigate linear and nonlinear transport across single-walled carbon nanotube quantum dots weakly coupled to spin-polarized leads. We consider tubes of finite length and small diameter, where not only forward scattering contributions of the Coulomb potential, but also short-ranged processes play an important role. In particular, they induce exchange effects leading for electron fillings 4n+2 either to a non-degenerate groundstate of spin S=0 or to a triplet groundstate. In the linear regime we present analytical results for the conductance - for both the S=0 and the triplet groundstate - and demonstrate that an external magnetic field is crucial to reveal the spin nature of the groundstates. In the nonlinear regime we show stability diagrams that clearly distinguish between the different groundstates. We observe a negative differential conductance (NDC) effect in the S=0 groundstate for antiparallel lead magnetization. In presence of an external magnetic field spin blockade effects can be detected, again leading to NDC effects for both groundstates.