  We study the interaction effects on the ground state of nanographite ribbons
with zigzag edges. Within the mean-field approximation, we found that there are
two possible phases: the superconducting (SC) phase and the excitonic insulator
(EI). The two phases are separated by a first-order transition point. After
taking into account the low-lying fluctuations around the mean-field solutions,
the SC phase becomes a spin liquid phase with one gapless charge mode.
  On the other hand, all excitations in the EI phase, especially the spin
excitations, are gapped.
