  Electron interactions reinforce minigaps induced in metallic nanotubes by an
external field and turn the gap field dependence into a universal power law. An
exactly solvable Gross-Neveau model with an SU(4) symmetry is derived for
neutral excitations near half-filling. Charge excitations, described by a
sin-Gordon perturbation of Luttinger liquid theory, are composite solitons
formed by the charged and neutral fields with two separate length scales.
Charge compressibility at finite density, evaluated in terms of inter-soliton
interaction, exhibits a crossover from overlapping to non-overlapping soliton
state. Implications for the Coulomb blockade measurements are discussed.
