  Electron properties of Carbon nanotubes can change qualitatively in a
transverse electric field. In metallic tubes the sign of Fermi velocity can be
reversed in a sufficiently strong field, while in semiconducting tubes the
effective mass can change sign. These changes in the spectrum manifest
themselves in a breakup of the Fermi surface and in the energy gap suppression,
respectively. The effect is controlled by the field inside the tube which is
screened due to the polarization induced on the tube. The theory of screening
links it with the chiral anomaly for 1D fermions and obtains a universal
screening function determined solely by the Carbon pi electron band.
