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Understanding how the orbital motion of electrons is coupled to the spin degree of freedom in nanoscale systems is central for applications in spin-based electronics and quantum computation. Here we demonstrate such spin-orbit coupling in a carbon-nanotube quantum dot in the general multielectron regime and in the presence of finite disorder. Also, we find a systematic dependence of the spin-orbit coupling on the electron occupation of the quantum dot. Such a dependence has not been seen in any other system and follows from the curvature-induced spin-orbit-split Dirac spectrum of the underlying graphene lattice. Our findings suggest that the spin-orbit coupling is a general property of carbon-nanotube quantum dots, which should provide a unique platform for the study of spin-orbit effects and their applications. [ABSTRACT FROM AUTHOR]

Structural defects greatly affect the electronic properties of single wall carbon nanotubes (CNT’s), for instance by increasing the sensitivity to their environment; an effect which can be utilized for better performance of CNT based chemical sensors. Here we show that electrostatic force microscopy (EFM) can be used as a non-invasive technique for probing defects in individual CNT’s supported on insulating substrates. The technique is demonstrated by monitoring the change in EFM signal upon intentionally inducing defect by an oxygen plasma etch, and is applied to assess the quality of as-grown CNT samples and to study the effect of exposing CNT’s to the low energy electron irradiation of a scanning electron microscope (SEM). [ABSTRACT FROM AUTHOR]

We investigate how the orbital magnetic moments of electron and hole states in a carbon nanotube quantum dot depend on the number of carriers on the dot. Low temperature transport measurements are carried out in a setup where the device can be rotated in an applied magnetic field, thus enabling accurate alignment with the nanotube axis. The field dependence of the level structure is measured by excited state spectroscopy and excellent correspondence with a single-particle calculation is found. In agreement with band structure calculations we find a decrease of the orbital magnetic moment with increasing electron or hole occupation of the dot, with a scale given by the band gap of the nanotube. [ABSTRACT FROM AUTHOR]