  Based on the Cluster Perturbation solution of the Hubbard hamiltonian for a
2-D honeycomb lattice we present quasiparticle band structures of nanoribbons
at half filling as a function of the on-site electron-electron repulsion. We
show that at moderate values of e-e interaction ribbons with armachair shaped
edges exhibit an unexpected semimetallic behavior, recovering the original
insulating character only at larger values of $U$.

Strongly Correlated Electrons