h/e magnetic flux modulation of the energy gap in nanotube quantum dots
We have studied the energy spectrum of multiwall carbon nanotubes using differential conductance measurements. The carbon nanotubes act as the Coulomb island of a single electron tunneling transistor. Temperature dependence of the conductivity curves shows that the electron wave functions extend between electrodes, and the energy levels are well-defined so they can be traced. Differential conductivity maps of the tubes are obtained as functions of the coaxial magnetic field, as well as of the perpendicular field. The maps for coaxial magnetic field show that a conductivity gap is induced as a function of the magnetic field, and the gap oscillates with a period of h/e, similar to that of Ajiki-Ando gap modulations. Ajiki and Ando predicted that a magnetic field can cause inter-conversion between metallic and semiconducting behavior of carbon nanotubes. Similarly, the measured density of states of the tubes also has a period of h/e. Tight binding calculations of carbon nanotube band structure are compared to the data.
multiwall carbon nanotubes, conductivity, Ajiki-Ando gap modulations, carbon nanotube band structure
University of Illinois - Urbana-Champaign
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