Observation of magnetic-field-modulated energy gap in carbon nanotubes

We have measured the low-bias four-probe conductance of carbon nanotubes as a function of magnetic field parallel to the tube axis. The measured data showed a clear energy gap near the Fermi level whose magnitude changed with the magnetic field. The electrical transport properties of the nanotube turned out to be critically dependent on the change of energy gap. The periodic oscillation of the magnetoresistance of a carbon nanotube can be explained by the periodic modulation of the energy gap with the magnetic field.