Spin and charge transport through carbon based systems

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2007

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Abstract

In this thesis, we investigate spin-dependent transport through ferromagnet-contacted single-walled carbon nanotubes (SWCNTs), in which charge transport shows the Fabry-Perot (FP) interference effect, the Kondo effect and the Coulomb blockade effect at low temperatures. Hysteric magnetoresistance (MR) is observed in all three transport regimes, which can be controlled by both the external magnetic field and the gate voltage. The MR in the FP interference regime can be well understood by a model considering the intrinsic electronic structure of SWCNTs and the quantum interference effect. In the strongly interacting Kondo regime, the Kondo effect is not suppressed by the presence of nearby ferromagnetism. Several observed MR features including the non-splitted zero-bias Kondo peak and positive MR switching can be explained by the strong Kondo effect and weak ferromagnetism in the leads. In the Coulomb blockade regime, several effects that can be associated with the magneto-Coulomb effect have been observed, and isolated spin accumulation and transport through the SWCNT quantum dot have been realized by a four-probe non-local measurements. We also studied charge transport behavior through organic semiconductor pentacene thin film transistors (OTFTs) in the limit of single- or a few molecular layers of pentacene films. The charge transport in these devices can be well explained by the multiple trapping and release model. The structural disorders induced by the physical and chemical causes, such as grain boundaries, interactions with gate insulator, metal contacts and ambient conditions can be responsible for the localized trap states in the ultrathin layer OTFTs, which are further confirmed by the electric force microscopy (EFM) measurements.

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