Ground state of graphite ribbons with zigzag edges

We study the interaction effects on the ground state of nanographite ribbons with zigzag edges. Within the mean-field approximation, we find that there are two possible phases: the singlet superconducting (SS) phase and the excitonic insulator (EI) phase. 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 SS 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.